Posts Tagged gamma ray

Recent Postings from gamma ray

Extending LHC Coverage to Light Pseudoscalar Mediators and Coy Dark Sectors [Cross-Listing]

Many dark matter models involving weakly interacting massive particles (WIMPs) feature new, relatively light pseudoscalars that mediate dark matter pair annihilation into Standard Model fermions. In particular, simple models of this type can explain the gamma ray excess originating in the Galactic Center as observed by the Fermi Large Area Telescope. In many cases the pseudoscalar’s branching ratio into WIMPs is suppressed, making these states challenging to detect at colliders through standard dark matter searches. Here, we study the prospects for observing these light mediator states at the LHC without exploiting missing energy techniques. While existing searches effectively probe pseudoscalars with masses between 5 – 14 GeV and above 90 GeV, the LHC reach can be extended to cover much of the interesting parameter space in the intermediate 20 – 80 GeV mass range in which the mediator can have appreciable Yukawa-like couplings to Standard Model fermions but would have escaped detection by LEP and other experiments. Models explaining the Galactic Center excess via a light pseudoscalar mediator can give rise to a promising signal in this regime through the associated production of the mediator with bottom quarks while satisfying all other existing constraints. We perform an analysis of the backgrounds and trigger efficiencies, detailing the cuts that can be used to extract the signal. A significant portion of the otherwise unconstrained parameter space of these models can be conclusively tested at the 13 TeV LHC with 100 fb$^{-1}$, and we encourage the ATLAS and CMS collaborations to extend their existing searches to this mass range.

Extending LHC Coverage to Light Pseudoscalar Mediators and Coy Dark Sectors

Many dark matter models involving weakly interacting massive particles (WIMPs) feature new, relatively light pseudoscalars that mediate dark matter pair annihilation into Standard Model fermions. In particular, simple models of this type can explain the gamma ray excess originating in the Galactic Center as observed by the Fermi Large Area Telescope. In many cases the pseudoscalar’s branching ratio into WIMPs is suppressed, making these states challenging to detect at colliders through standard dark matter searches. Here, we study the prospects for observing these light mediator states at the LHC without exploiting missing energy techniques. While existing searches effectively probe pseudoscalars with masses between 5 – 14 GeV and above 90 GeV, the LHC reach can be extended to cover much of the interesting parameter space in the intermediate 20 – 80 GeV mass range in which the mediator can have appreciable Yukawa-like couplings to Standard Model fermions but would have escaped detection by LEP and other experiments. Models explaining the Galactic Center excess via a light pseudoscalar mediator can give rise to a promising signal in this regime through the associated production of the mediator with bottom quarks while satisfying all other existing constraints. We perform an analysis of the backgrounds and trigger efficiencies, detailing the cuts that can be used to extract the signal. A significant portion of the otherwise unconstrained parameter space of these models can be conclusively tested at the 13 TeV LHC with 100 fb$^{-1}$, and we encourage the ATLAS and CMS collaborations to extend their existing searches to this mass range.

Limits on Dark Matter Annihilation Signals from the Fermi LAT 4-year Measurement of the Isotropic Gamma-Ray Background

We search for evidence of dark matter (DM) annihilation in the isotropic gamma-ray background (IGRB) measured with 50 months of Fermi Large Area Telescope (LAT) observations. An improved theoretical description of the cosmological DM annihilation signal, based on two complementary techniques and assuming generic weakly interacting massive particle (WIMP) properties, renders more precise predictions compared to previous work. More specifically, we estimate the cosmologically-induced gamma-ray intensity to have an uncertainty of a factor ~20 in canonical setups. We consistently include both the Galactic and extragalactic signals under the same theoretical framework, and study the impact of the former on the IGRB spectrum derivation. We find no evidence for a DM signal and we set limits on the DM-induced isotropic gamma-ray signal. Our limits are competitive for DM particle masses up to tens of TeV and, indeed, are the strongest limits derived from Fermi LAT data at TeV energies. This is possible thanks to the new Fermi LAT IGRB measurement, which now extends up to an energy of 820 GeV. We quantify uncertainties in detail and show the potential this type of search offers for testing the WIMP paradigm with a complementary and truly cosmological probe of DM particle signals.

Propagation of quantum particles in Brans-Dicke spacetime. The case of Gamma Ray Bursts

The propagation of boson particles in a gravitational field described by the Brans-Dicke theory of gravity is analyzed. We derive the wave function of the scalar particles, and the effective potential experienced by the quantum particles considering the role of the varying gravitational coupling. Besides, we calculate the probability to find the scalar particles near the region where a naked singularity is present. The extremely high energy radiated in such a situation could account for the huge emitted power observed in Gamma Ray Bursts.

Propagation of quantum particles in Brans-Dicke spacetime. The case of Gamma Ray Bursts [Cross-Listing]

The propagation of boson particles in a gravitational field described by the Brans-Dicke theory of gravity is analyzed. We derive the wave function of the scalar particles, and the effective potential experienced by the quantum particles considering the role of the varying gravitational coupling. Besides, we calculate the probability to find the scalar particles near the region where a naked singularity is present. The extremely high energy radiated in such a situation could account for the huge emitted power observed in Gamma Ray Bursts.

Propagation of quantum particles in Brans-Dicke spacetime. The case of Gamma Ray Bursts [Cross-Listing]

The propagation of boson particles in a gravitational field described by the Brans-Dicke theory of gravity is analyzed. We derive the wave function of the scalar particles, and the effective potential experienced by the quantum particles considering the role of the varying gravitational coupling. Besides, we calculate the probability to find the scalar particles near the region where a naked singularity is present. The extremely high energy radiated in such a situation could account for the huge emitted power observed in Gamma Ray Bursts.

Evidence of coupling between the thermal and nonthermal emission in the gamma-ray binary LS I +61 303

The gamma-ray binary LS I +61 303 is composed of a Be star and a compact companion orbiting in an eccentric orbit. Variable flux modulated with the orbital period of ~26.5 d has been detected from radio to very high-energy gamma rays. In addition, the system presents a superorbital variability of the phase and amplitude of the radio outburst with a period of ~4.6 yr. We present optical photometric observations of LS I +61 303 spanning ~1.5 yr and contemporaneous Halpha equivalent width (EW Halpha) data. The optical photometry shows, for the first time, that the known orbital modulation suffers a positive orbital phase shift and an increase in flux for data obtained 1-yr apart. This behavior is similar to that already known at radio wavelengths, indicating that the optical flux follows the superorbital variability as well. The orbital modulation of the EW Halpha presents the already known superorbital flux variability but shows, also for the first time, a positive orbital phase shift. In addition, the optical photometry exhibits a lag of ~0.1-0.2 in orbital phase with respect to the EW Halpha measurements at similar superorbital phases, and presents a lag of ~0.1 and ~0.3 orbital phases with respect noncontemperaneous radio and X-ray outbursts, respectively. The phase shifts detected in the orbital modulation of thermal indicators, such as the optical flux and the EW Halpha, are in line with the observed behavior for nonthermal indicators, such as X-ray or radio emission. This shows that there is a strong coupling between the thermal and nonthermal emission processes in the gamma-ray binary LS I +61 303. The orbital phase lag between the optical flux and the EW Halpha is naturally explained considering different emitting regions in the circumstellar disk, whereas the secular evolution might be caused by the presence of a moving one-armed spiral density wave in the disk.

A strong radio brightening at the jet base of M87 during the elevated very-high-energy gamma-ray state in 2012

The nearby radio galaxy M87 offers a unique opportunity for exploring the connection between gamma-ray production and jet formation at an unprecedented linear resolution. However, the origin and location of the gamma-rays in this source is still elusive. Based on previous radio/TeV correlation events, the unresolved jet base (radio core) and the peculiar knot HST-1 at >120 pc from the nucleus are proposed as candidate site(s) of gamma-ray production. Here we report our intensive, high-resolution radio monitoring observations of the M87 jet with the VLBI Exploration of Radio Astrometry (VERA) and the European VLBI Network (EVN) from February 2011 to October 2012. During this period, an elevated level of the M87 flux is reported at TeV with VERITAS. We detected a remarkable flux increase in the radio core with VERA at 22/43 GHz coincident with the VHE activity. Meanwhile, HST-1 remained quiescent in terms of its flux density and structure at radio. These results strongly suggest that the TeV gamma-ray activity in 2012 originates in the jet base within 0.03 pc (projected) from the central supermassive black hole.

Selection effects in Gamma Ray Bursts correlations: consequences on the ratio between GRB and star formation rates [Replacement]

Gamma Ray Bursts (GRBs) visible up to very high redshift have become attractive targets as potential new distance indicators. It is still not clear whether the relations proposed so far originate from an unknown GRB physics or result from selection effects. We investigate this issue in the case of the $L_X-T^*_a$ correlation (hereafter LT) between the X-ray luminosity $L_X (T_a)$ at the end of the plateau phase, $T_a$, and the rest frame time $T^{*}_a$. We devise a general method to build mock data sets starting from a GRB world model and taking into account selection effects on both time and luminosity. This method shows how not knowing the efficiency function could influence the evaluation of the intrinsic slope of any correlation and the GRB density rate. We investigate biases (small offsets in slope or normalization) that would occur in the LT relation as a result of truncations, possibly present in the intrinsic distributions of $L_X$ and $T^*_a$. We compare these results with the ones in Dainotti et al. (2013) showing that in both cases the intrinsic slope of the LT correlation is $\approx -1.0$. This method is general, therefore relevant to investigate if any other GRB correlation is generated by the biases themselves. Moreover, because the farthest GRBs and star-forming galaxies probe the reionization epoch, we evaluate the redshift-dependent ratio $\Psi(z)=(1+z)^{\alpha}$ of the GRB rate to star formation rate (SFR). We found a modest evolution $-0.2\leq \alpha \leq 0.5$ consistent with Swift GRB afterglow plateau in the redshift range $0.99<z<9.4$.

Selection effects in Gamma Ray Bursts correlations: consequences on the ratio between GRB and star formation rates [Replacement]

Gamma Ray Bursts (GRBs) visible up to very high redshift have become attractive targets as potential new distance indicators. It is still not clear whether the relations proposed so far originate from an unknown GRB physics or result from selection effects. We investigate this issue in the case of the $L_X-T^*_a$ correlation (hereafter LT) between the X-ray luminosity $L_X (T_a)$ at the end of the plateau phase, $T_a$, and the rest frame time $T^{*}_a$. We devise a general method to build mock data sets starting from a GRB world model and taking into account selection effects on both time and luminosity. This method shows how not knowing the efficiency function could influence the evaluation of the intrinsic slope of any correlation and the GRB density rate. We investigate biases (small offsets in slope or normalization) that would occur in the LT relation as a result of truncations, possibly present in the intrinsic distributions of $L_X$ and $T^*_a$. We compare these results with the ones in Dainotti et al. (2013) showing that in both cases the intrinsic slope of the LT correlation is $\approx -1.0$. This method is general, therefore relevant to investigate if any other GRB correlation is generated by the biases themselves. Moreover, because the farthest GRBs and star-forming galaxies probe the reionization epoch, we evaluate the redshift-dependent ratio $\Psi(z)=(1+z)^{\alpha}$ of the GRB rate to star formation rate (SFR). We found a modest evolution $-0.2\leq \alpha \leq 0.5$ consistent with Swift GRB afterglow plateau in the redshift range $0.99<z<9.4$.

Selection effects in Gamma Ray Bursts correlations: consequences on the ratio between GRB and star formation rates

Gamma Ray Bursts (GRBs) visible up to very high redshift have become attractive targets as potential new distance indicators. It is still not clear whether the relations proposed so far originate from an unknown GRB physics or result from selection effects. We investigate this issue in the case of the $L_X-T^*_a$ correlation (hereafter LT) between the X-ray luminosity $L_X (T_a)$ at the end of the plateau phase, $T_a$, and the rest frame time $T^{*}_a$. We devise a general method to build mock data sets starting from a GRB world model and taking into account selection effects on both time and luminosity. This method shows how not knowing the efficiency function could influence the evaluation of the intrinsic slope of any correlation and the GRB density rate. We investigate biases (small offsets in slope or normalization) that would occur in the LT relation as a result of truncations, possibly present in the intrinsic distributions of $L_X$ and $T^*_a$. We compare these results with the ones in Dainotti et al. (2013) showing that in both cases the intrinsic slope of the LT correlation is $\approx -1.0$. This method is general, therefore relevant to investigate if any other GRB correlation is generated by the biases themselves. Moreover, because the farthest GRBs and star-forming galaxies probe the reionization epoch, we evaluate the redshift-dependent ratio $\Psi(z)=(1+z)^{\alpha}$ of the GRB rate to star formation rate (SFR). We found a modest evolution $-0.2\leq \alpha \leq 0.5$ consistent with Swift GRB afterglow plateau in the redshift range $0.99<z<9.4$.

Performance of a new electron-tracking Compton camera under intense radiations from a water target irradiated with a proton beam

We have developed an electron-tracking Compton camera (ETCC) for use in next-generation MeV gamma ray telescopes. An ETCC consists of a gaseous time projection chamber (TPC) and pixel scintillator arrays (PSAs). Since the TPC measures the three dimensional tracks of Compton-recoil electrons, the ETCC can completely reconstruct the incident gamma rays. Moreover, the ETCC demonstrates efficient background rejection power in Compton-kinematics tests, identifies particle from the energy deposit rate (dE/dX) registered in the TPC, and provides high quality imaging by completely reconstructing the Compton scattering process. We are planning the "Sub-MeV gamma ray Imaging Loaded-on-balloon Experiment" (SMILE) for our proposed all-sky survey satellite. Performance tests of a mid-sized 30 cm-cubic ETCC, constructed for observing the Crab nebula, are ongoing. However, observations at balloon altitudes or satellite orbits are obstructed by radiation background from the atmosphere and the detector itself. The background rejection power was checked using proton accelerator experiments conducted at the Research Center for Nuclear Physics, Osaka University. To create the intense radiation fields encountered in space, which comprise gamma rays, neutrons, protons, and other energetic entities, we irradiated a water target with a 140 MeV proton beam and placed a SMILE-II ETCC near the target. In this situation, the counting rate was five times than that expected at the balloon altitude. Nonetheless, the ETCC stably operated and identified particles sufficiently to obtain a clear gamma ray image of the checking source. Here, we report the performance of our detector and demonstrate its effective background rejection based in electron tracking experiments.

Performance of a new electron-tracking Compton camera under intense radiations from a water target irradiated with a proton beam [Replacement]

We have developed an electron-tracking Compton camera (ETCC) for use in next-generation MeV gamma ray telescopes. An ETCC consists of a gaseous time projection chamber (TPC) and pixel scintillator arrays (PSAs). Since the TPC measures the three dimensional tracks of Compton-recoil electrons, the ETCC can completely reconstruct the incident gamma rays. Moreover, the ETCC demonstrates efficient background rejection power in Compton-kinematics tests, identifies particle from the energy deposit rate (dE/dX) registered in the TPC, and provides high quality imaging by completely reconstructing the Compton scattering process. We are planning the "Sub-MeV gamma ray Imaging Loaded-on-balloon Experiment" (SMILE) for our proposed all-sky survey satellite. Performance tests of a mid-sized 30 cm-cubic ETCC, constructed for observing the Crab nebula, are ongoing. However, observations at balloon altitudes or satellite orbits are obstructed by radiation background from the atmosphere and the detector itself. The background rejection power was checked using proton accelerator experiments conducted at the Research Center for Nuclear Physics, Osaka University. To create the intense radiation fields encountered in space, which comprise gamma rays, neutrons, protons, and other energetic entities, we irradiated a water target with a 140 MeV proton beam and placed a SMILE-II ETCC near the target. In this situation, the counting rate was five times than that expected at the balloon altitude. Nonetheless, the ETCC stably operated and identified particles sufficiently to obtain a clear gamma ray image of the checking source. Here, we report the performance of our detector and demonstrate its effective background rejection based in electron tracking experiments.

Constraining the Physical Conditions in the Jets of Gamma-ray Flaring Blazars using Centimeter-Band Polarimetry and Radiative Transfer Simulations. II. Exploring Parameter Space and Implications

We analyze the shock-in-jet models for the gamma-ray flaring blazars 0420-014, OJ 287, and 1156+295 presented in Aller et al. (2014, Paper I), quantifying how well the modeling constrains internal properties of the flow (low energy spectral cutoff, partition between random and ordered magnetic field), the flow dynamics (quiescent flow speed and orientation), and the number and strength of the shocks responsible for radio-band flaring. We conclude that well-sampled, multifrequency polarized flux light curves are crucial for defining source properties. We argue for few, if any, low energy particles in these flows, suggesting no entrainment and efficient energization of jet material, and for approximate energy equipartition between the random and ordered magnetic field components, suggesting that ordered field is built by non-trivial dynamo action from the random component, or that the latter arises from a jet instability that preserves the larger-scale, ordered flow. We present evidence that the difference between orphan radio-band (no gamma-ray counterpart) and non-orphan flares is due to more complex shock interactions in the latter case.

H.E.S.S. detection of TeV emission from the interaction region between the supernova remnant G349.7+0.2 and a molecular cloud [Replacement]

G349.7+0.2 is a young Galactic supernova remnant (SNR) located at the distance of 11.5 kpc and observed across the entire electromagnetic spectrum from radio to high energy (HE) Gamma-rays. Radio and infrared observations indicate that the remnant is interacting with a molecular cloud. In this paper, the detection of very high energy (VHE) Gamma-ray emission coincident with this SNR with the High Energy Stereoscopic System (H.E.S.S.) is reported. An integral flux F(E>400GeV)=(6.5 +-1.1stat +-1.3syst) x 10^{-13} ph/cm/s corresponding to 0.7% of that of the Crab Nebula and to a luminosity of 10^34 erg/s above the same energy threshold, and a steep photon index Gamma_VHE = 2.8 +-0.27stat +-0.20syst are measured. The analysis of more than 5 yr of Fermi-LAT data towards this source shows a power-law like spectrum with a best-fit photon index Gamma_HE = 2.2 +-0.04stat +0.13-0.31syst. The combined Gamma-ray spectrum of G349.7+0.2 can be described by either a broken power-law (BPL) or a power-law with exponential (or sub-exponential) cutoff (PLC). In the former case, the photon break energy is found at E_br,gamma = 55 +70-30 GeV, slightly higher than what is usually observed in the HE/VHE Gamma-ray emitting middle-aged SNRs known to be interacting with molecular clouds. In the latter case, the exponential (respectively sub-exponential) cutoff energy is measured at E_cut,gamma = 1.4 +1.6-0.55 (respectively 0.35 +0.75-0.21) TeV. A pion-decay process resulting from the interaction of the accelerated protons and nuclei with the dense surrounding medium is clearly the preferred scenario to explain the Gamma-ray emission. The BPL with a spectral steepening of 0.5-1 and the PLC provide equally good fits to the data. The product of the average gas density and the total energy content of accelerated protons and nuclei amounts to nH Wp ~ 5 x 10^51 erg/cm3.

H.E.S.S. detection of TeV emission from the interaction region between the supernova remnant G349.7+0.2 and a molecular cloud

G349.7+0.2 is a young Galactic supernova remnant (SNR) located at the distance of 11.5 kpc and observed across the entire electromagnetic spectrum from radio to high energy (HE) Gamma-rays. Radio and infrared observations indicate that the remnant is interacting with a molecular cloud. In this paper, the detection of very high energy (VHE) Gamma-ray emission coincident with this SNR with the High Energy Stereoscopic System (H.E.S.S.) is reported. An integral flux F(E>400GeV)=(6.5 +-1.1stat +-1.3syst) x 10^{-13} ph/cm/s corresponding to 0.7% of that of the Crab Nebula and to a luminosity of 10^34 erg/s above the same energy threshold, and a steep photon index Gamma_VHE = 2.8 +-0.27stat +-0.20syst are measured. The analysis of more than 5 yr of Fermi-LAT data towards this source shows a power-law like spectrum with a best-fit photon index Gamma_HE = 2.2 +-0.04stat +0.13-0.31syst. The combined Gamma-ray spectrum of G349.7+0.2 can be described by either a broken power-law (BPL) or a power-law with exponential (or sub-exponential) cutoff (PLC). In the former case, the photon break energy is found at E_br,gamma = 55 +70-30 GeV, slightly higher than what is usually observed in the HE/VHE Gamma-ray emitting middle-aged SNRs known to be interacting with molecular clouds. In the latter case, the exponential (respectively sub-exponential) cutoff energy is measured at E_cut,gamma = 1.4 +1.6-0.55 (respectively 0.35 +0.75-0.21) TeV. A pion-decay process resulting from the interaction of the accelerated protons and nuclei with the dense surrounding medium is clearly the preferred scenario to explain the Gamma-ray emission. The BPL with a spectral steepening of 0.5-1 and the PLC provide equally good fits to the data. The product of the average gas density and the total energy content of accelerated protons and nuclei amounts to nH Wp ~ 5 x 10^51 erg/cm3.

A Scientific Trigger Unit for Space-Based Real-Time Gamma Ray Burst Detection, II - Data Processing Model and Benchmarks

The Scientific Trigger Unit (UTS) is a satellite equipment designed to detect Gamma Ray Bursts (GRBs) observed by the onboard 6400 pixels camera ECLAIRs. It is foreseen to equip the low-Earth orbit French-Chinese satellite SVOM and acts as the GRB trigger unit for the mission. The UTS analyses in real-time and in great details the onboard camera data in order to select the GRBs, to trigger a spacecraft slew re-centering each GRB for the narrow field-of-view instruments, and to alert the ground telescope network for GRB follow-up observations. A few GRBs per week are expected to be observed by the camera; the UTS targets a close to 100% trigger efficiency, while being selective enough to avoid fake alerts. This is achieved by running the complex scientific algorithms on a radiation tolerant hardware, based on a FPGA data pre-processor and a CPU with a Real-Time Operating System. The UTS is a scientific software, firmware and hardware co-development. A Data Processing Model (DPM) has been developed to fully validate all the technical choices deeply impacted by the ITAR restriction applied to the development. The DPM permits to evaluate the processing power and the memory bandwidth, and to adjust the balance load between software and firmware. This paper presents the UTS DPM functionalities and architecture. It highlights the results obtained with the full GRB trigger algorithms implemented on a rad-tolerant ITAR-free processor.

A Scientific Trigger Unit for Space-Based Real-Time Gamma Ray Burst Detection, I - Scientific Software Model and Simulations

The on-board Scientific Trigger Unit (UTS) is designed to detect Gamma Ray Bursts (GRBs) in real-time, using the data produced by the ECLAIRs camera, foreseen to equip the future French-Chinese satellite mission SVOM (Space-based Variable Objects Monitor). The UTS produces GRB alerts, sent to the ground for GRB follow-up observations, and requests the spacecraft slew to repoint its narrow field instruments onto the GRB afterglow. Because of the diversity of GRBs in duration and variability, two simultaneously running GRB trigger algorithms are implemented in the UTS, the so called Image Trigger performing systematic sky image reconstruction on time scales above 20 s, and the Count-Rate Trigger, selecting a time scale from 10 ms to 20 s showing an excess in count-rate over background estimate, prior to imaging the excess for localization on the sky. This paper describes both trigger algorithms and their implementation in a library, compiled for the Scientific Software Model (SSM) running on standard Linux machines, and which can also be cross-compiled for the Data Processing Model (DPM), in order to have the same algorithms running on both platforms. While the DPM permits to validate the hardware concept and benchmark the algorithms (see paper II), the SSM allows to optimize the algorithms and estimate the GRB trigger-rate of ECLAIRs/UTS. The result of running on the SSM a dynamic photon by photon simulation based on the BATSE GRB catalog is presented.

Multi-wavelength Emission from the Fermi Bubble II. Secondary Electrons and the Hadronic Model of the Bubble

We analyse the origin of the gamma-ray flux from the Fermi Bubbles (FBs) in the framework of the hadronic model in which gamma-rays are produced by collisions of relativistic protons with the protons of background plasma in the Galactic halo. It is assumed in this model that the observed radio emission from the FBs is due to synchrotron radiation of secondary electrons produced by $pp$ collisions. However, if these electrons loose their energy by the synchrotron and inverse-Compton, the spectrum of secondary electrons is too soft, and an additional arbitrary component of primary electrons is necessary in order to reproduce the radio data. Thus, a mixture of the hadronic and leptonic models is required for the observed radio flux. It was shown that if the spectrum of primary electrons is $\propto E_e^{-2}$, the permitted range of the magnetic field strength is within 2 – 7 $\mu$G region. The fraction of gamma-rays produced by $pp$ collisions can reach about 80% of the total gamma-ray flux from the FBs. If magnetic field is <2 $\mu$G or >7 $\mu$G the model is unable to reproduce the data. Alternatively, the electrons in the FBs may lose their energy by adiabatic energy losses if there is a strong plasma outflow in the GC. Then, the pure hadronic model is able to reproduce characteristics of the radio and gamma-ray flux from the FBs. However, in this case the required magnetic field strength in the FBs and the power of CR sources are much higher than those followed from observations.

Gamma-ray flaring activity from the gravitationally lensed blazar PKS 1830-211 observed by Fermi LAT [Replacement]

The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope routinely detects the highly dust-absorbed, reddened, and MeV-peaked flat spectrum radio quasar PKS 1830-211 (z=2.507). Its apparent isotropic gamma-ray luminosity (E>100 MeV) averaged over $\sim$ 3 years of observations and peaking on 2010 October 14/15 at 2.9 X 10^{50} erg s^{-1}, makes it among the brightest high-redshift Fermi blazars. No published model with a single lens can account for all of the observed characteristics of this complex system. Based on radio observations, one expects time delayed variability to follow about 25 days after a primary flare, with flux about a factor 1.5 less. Two large gamma-ray flares of PKS 1830-211 have been detected by the LAT in the considered period and no substantial evidence for such a delayed activity was found. This allows us to place a lower limit of about 6 on the gamma rays flux ratio between the two lensed images. Swift XRT observations from a dedicated Target of Opportunity program indicate a hard spectrum and with no significant correlation of X-ray flux with the gamma-ray variability. The spectral energy distribution can be modeled with inverse Compton scattering of thermal photons from the dusty torus. The implications of the LAT data in terms of variability, the lack of evident delayed flare events, and different radio and gamma-ray flux ratios are discussed. Microlensing effects, absorption, size and location of the emitting regions, the complex mass distribution of the system, an energy-dependent inner structure of the source, and flux suppression by the lens galaxy for one image path may be considered as hypotheses for understanding our results.

Gamma-ray flaring activity from the gravitationally lensed blazar PKS 1830-211 observed by Fermi LAT

The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope routinely detects the highly dust-absorbed, reddened, and MeV-peaked flat spectrum radio quasar PKS 1830-211 (z=2.507). Its apparent isotropic gamma-ray luminosity (E>100 MeV) averaged over $\sim$ 3 years of observations and peaking on 2010 October 14/15 at 2.9 X 10^{50} erg s^{-1}, makes it among the brightest high-redshift Fermi blazars. No published model with a single lens can account for all of the observed characteristics of this complex system. Based on radio observations, one expects time delayed variability to follow about 25 days after a primary flare, with flux about a factor 1.5 less. Two large gamma-ray flares of PKS 1830-211 have been detected by the LAT in the considered period and no substantial evidence for such a delayed activity was found. This allows us to place a lower limit of about 6 on the gamma rays flux ratio between the two lensed images. Swift XRT observations from a dedicated Target of Opportunity program indicate a hard spectrum and with no significant correlation of X-ray flux with the gamma-ray variability. The spectral energy distribution can be modeled with inverse Compton scattering of thermal photons from the dusty torus. The implications of the LAT data in terms of variability, the lack of evident delayed flare events, and different radio and gamma-ray flux ratios are discussed. Microlensing effects, absorption, size and location of the emitting regions, the complex mass distribution of the system, an energy-dependent inner structure of the source, and flux suppression by the lens galaxy for one image path may be considered as hypotheses for understanding our results.

Effect of Degenerated Particles on Internal Bremsstrahlung of Majorana Dark Matter [Cross-Listing]

Gamma-ray generated by annihilation or decay of dark matter can be its smoking gun signature. In particular, gamma-ray coming from internal bremsstrahlung of dark matter is promising since it can be a leading emission of sharp gamma-ray. However if thermal production of Majorana dark matter is considered, the derived cross section for internal bremsstrahlung becomes too small to be observed by future gamma-ray experiments. We consider a framework to achieve an enhancement of the cross section by taking into account degenerated particles with dark matter. We find that the enhancement of about order one is possible without conflict with the dark matter relic density. Due to the enhancement, it would be tested by the future experiments such as GAMMA-400 and CTA.

Effect of Degenerated Particles on Internal Bremsstrahlung of Majorana Dark Matter

Gamma-ray generated by annihilation or decay of dark matter can be its smoking gun signature. In particular, gamma-ray coming from internal bremsstrahlung of dark matter is promising since it can be a leading emission of sharp gamma-ray. However if thermal production of Majorana dark matter is considered, the derived cross section for internal bremsstrahlung becomes too small to be observed by future gamma-ray experiments. We consider a framework to achieve an enhancement of the cross section by taking into account degenerated particles with dark matter. We find that the enhancement of about order one is possible without conflict with the dark matter relic density. Due to the enhancement, it would be tested by the future experiments such as GAMMA-400 and CTA.

High-energy gamma-ray and neutrino backgrounds from clusters of galaxies and radio constraints

Cosmic-ray protons accumulate for cosmological times in clusters of galaxies as their typical radiative and diffusive escape times are longer than the Hubble time. Their hadronic interactions with protons of the intra-cluster medium generate secondary electrons, gamma-rays and neutrinos. We here estimate the contribution from clusters to the diffuse gamma-ray and neutrino backgrounds. We model the cluster population by means of their mass function, using a phenomenological luminosity-mass relation applied to all clusters, as well as a detailed semi-analytical model. Additionally, we consider observationally-motivated values for the cluster magnetic field. This is a crucial parameter since the observed radio counts due to synchrotron emission by secondary electrons need to be respected. For a choice of parameters respecting all current constraints, and assuming a spectral index of -2, we find that hadronic interactions in clusters contribute by less than 10% to the extragalactic gamma-ray background observed by Fermi and to the IceCube flux. They account for less than 1% for spectral indices <=-2. The neutrino flux observed by IceCube can be reproduced without violating constraints only if a very hard (and speculative) spectral index >-2 is adopted. However, this scenario is in tension with the IceCube data, which seem to suggest a spectral energy distribution of the neutrino flux that decreases with energy. In the case of proton-photon interactions, we find that very likely protons do not reach sufficiently high energies to produce neutrinos in clusters. We argue that our results are optimistic due to our assumptions, and that clusters of galaxies cannot give any relevant contribution to the extragalactic gamma-ray and neutrino backgrounds. Finally, we find that the cluster contribution to the angular fluctuations in the gamma-ray background is subdominant, less than 10%. [abridged]

Discovery of a New Galactic Center Excess Consistent with Upscattered Starlight

We present a new extended gamma ray excess toward the Galactic Center that traces the 3.4 micron infrared emission morphology. Combined with its measured spectrum, this new extended source is consistent with inverse Compton emission from a high-energy electron-positron population with energies up to about 10 GeV. Previously detected emissions tracing the 20 cm radio, interpreted as bremsstrahlung radiation, and the Galactic Center Extended emission tracing a spherical distribution and peaking at 2 GeV, are also detected. We show that the inverse Compton and bremsstrahlung emissions are likely due to the same source of electrons and positrons. All three extended emissions may be explained within the framework of a model where the dark matter annihilates to leptons or a model with unresolved millisecond pulsars in the Galactic Center.

Discovery of a New Galactic Center Excess Consistent with Upscattered Starlight [Cross-Listing]

We present a new extended gamma ray excess toward the Galactic Center that traces the 3.4 micron infrared emission morphology. Combined with its measured spectrum, this new extended source is consistent with inverse Compton emission from a high-energy electron-positron population with energies up to about 10 GeV. Previously detected emissions tracing the 20 cm radio, interpreted as bremsstrahlung radiation, and the Galactic Center Extended emission tracing a spherical distribution and peaking at 2 GeV, are also detected. We show that the inverse Compton and bremsstrahlung emissions are likely due to the same source of electrons and positrons. All three extended emissions may be explained within the framework of a model where the dark matter annihilates to leptons or a model with unresolved millisecond pulsars in the Galactic Center.

Galactic Centre GeV Photons from Dark Technicolor

We present a classically scale-invariant model with a confining dark sector, which is coupled to the Standard Model through the Higgs portal. The galactic centre gamma ray excess can be explained in this model by collision-induced dark matter decays to b-quarks. We discuss the possibility to obtain the dark matter relic density through thermal freeze-out, which however requires excessive fine-tuning. We then instead focus on a freeze-in scenario and perform detailed calculations and a parameter scan. We find that the observed relic density and the gamma ray excess can be explained by a wide range of parameters in this model.

Galactic Centre GeV Photons from Dark Technicolor [Replacement]

We present a classically scale-invariant model with a confining dark sector, which is coupled to the Standard Model through the Higgs portal. The galactic centre gamma ray excess can be explained in this model by collision-induced dark matter decays to b-quarks. We discuss the possibility to obtain the dark matter relic density through thermal freeze-out, which however requires excessive fine-tuning. We then instead focus on a freeze-in scenario and perform detailed calculations and a parameter scan. We find that the observed relic density and the gamma ray excess can be explained by a wide range of parameters in this model.

Galactic Centre GeV Photons from Dark Technicolor [Replacement]

We present a classically scale-invariant model with a confining dark sector, which is coupled to the Standard Model through the Higgs portal. The galactic centre gamma ray excess can be explained in this model by collision-induced dark matter decays to b-quarks. We discuss the possibility to obtain the dark matter relic density through thermal freeze-out, which however requires excessive fine-tuning. We then instead focus on a freeze-in scenario and perform detailed calculations and a parameter scan. We find that the observed relic density and the gamma ray excess can be explained by a wide range of parameters in this model.

Revisiting the SN1987A gamma-ray limit on ultralight axion-like particles [Replacement]

We revise the bound from the supernova SN1987A on the coupling of ultralight axion-like particles (ALPs) to photons. In a core-collapse supernova, ALPs would be emitted via the Primakoff process, and eventually convert into gamma rays in the magnetic field of the Milky Way. The lack of a gamma-ray signal in the GRS instrument of the SMM satellite in coincidence with the observation of the neutrinos emitted from SN1987A therefore provides a strong bound on their coupling to photons. Due to the large uncertainty associated with the current bound, we revise this argument, based on state-of-the-art physical inputs both for the supernova models and for the Milky-Way magnetic field. Furthermore, we provide major amendments, such as the consistent treatment of nucleon-degeneracy effects and of the reduction of the nuclear masses in the hot and dense nuclear medium of the supernova. With these improvements, we obtain a new upper limit on the photon-ALP coupling: g_{a\gamma} < 5.3 x 10^{-12} GeV^{-1}, for m_a < 4.4 x 10^{-10} eV, and we also give its dependence at larger ALP masses. Moreover, we discuss how much the Fermi-LAT satellite experiment could improve this bound, should a close-enough supernova explode in the near future.

Revisiting the SN1987A gamma-ray limit on ultralight axion-like particles [Replacement]

We revise the bound from the supernova SN1987A on the coupling of ultralight axion-like particles (ALPs) to photons. In a core-collapse supernova, ALPs would be emitted via the Primakoff process, and eventually convert into gamma rays in the magnetic field of the Milky Way. The lack of a gamma-ray signal in the GRS instrument of the SMM satellite in coincidence with the observation of the neutrinos emitted from SN1987A therefore provides a strong bound on their coupling to photons. Due to the large uncertainty associated with the current bound, we revise this argument, based on state-of-the-art physical inputs both for the supernova models and for the Milky-Way magnetic field. Furthermore, we provide major amendments, such as the consistent treatment of nucleon-degeneracy effects and of the reduction of the nuclear masses in the hot and dense nuclear medium of the supernova. With these improvements, we obtain a new upper limit on the photon-ALP coupling: g_{a\gamma} < 5.3 x 10^{-12} GeV^{-1}, for m_a < 4.4 x 10^{-10} eV, and we also give its dependence at larger ALP masses. Moreover, we discuss how much the Fermi-LAT satellite experiment could improve this bound, should a close-enough supernova explode in the near future.

Decoding the stellar fossils of the dusty Milky Way progenitors

We investigate the metallicity distribution function (MDF) in the Galactic halo and the relative fraction of Carbon-normal and Carbon-rich stars. To this aim, we use an improved version of the semi-analytical code GAlaxy MErger Tree and Evolution (GAMETE), that reconstructs the hierarchical merger tree of the MW, following the star formation history and the metal and dust evolution in individual progenitors. The predicted scaling relations between the dust, metal and gas masses for MW progenitors show a good agreement with observational data of local galaxies and of Gamma Ray Burst (GRB) host galaxies at 0.1 < z < 6.3. We find that in order to reproduce the observed tail of the MDF at [Fe/H] < -4, faint SN explosions have to dominate the metal yields produced by Pop III stars, disfavoring a Pop III IMF that extends to stellar masses > 140 M_{sun}, into the Pair-Instability SN progenitor mass range. The relative contribution of C-normal and C-enhanced stars to the MDF and its dependence on [Fe/H] points to a scenario where the Pop III/II transition is driven by dust-cooling and the first low-mass stars form when the dust-to-gas ratio in their parent clouds exceeds a critical value of D_crit = 4.4 x 10^{-9}.

Spectral and Timing Analysis of the Prompt Emission of Gamma Ray Bursts

A Thesis Submitted to the Tata Institute of Fundamental Research, Mumbai for the degree of Doctor of Philosophy in Physics (supervisor: Prof. A. R. Rao)

Luminosity function and jet structure of Gamma Ray Bursts

The structure of Gamma Ray Burst (GRB) jets impacts on their prompt and afterglow emission properties. The jet of GRBs could be uniform, with constant energy per unit solid angle within the jet aperture, or it could instead be structured, namely with energy and velocity that depend on the angular distance from the axis of the jet. We try to get some insight about the still unknown structure of GRBs by studying their luminosity function. We show that low (1e46-1e48 erg/s) and high (i.e. with L > 1e50 erg/s) luminosity GRBs can be described by a unique luminosity function, which is also consistent with current lower limits in the intermediate luminosity range (1e48-1e50} erg/s). We derive analytical expressions for the luminosity function of GRBs in uniform and structured jet models and compare them with the data. Uniform jets can reproduce the entire luminosity function with reasonable values of the free parameters. A structured jet can also fit adequately the current data, provided that the energy within the jet is relatively strongly structured, i.e. E propto theta^{-k} with k > 4. The classical E propto theta^{-2} structured jet model is excluded by the current data.

Low Energy Gamma Ray Excess Confronting a Singlet Scalar Extended Inert Doublet Dark Matter Model

Recent study of gamma rays originating from the region of galactic centre has confirmed an anomalous $\gamma$-ray excess within the energy range 1-3 GeV. This can be explained as the consequence of pair annihilation of a 31-40 GeV dark matter into $b \bar b$ with thermal annihilation cross-section $\sigma v \sim 1.4-2.0 \times 10^{-26}~\rm{cm^3/s}$. In this work we revisit the Inert Doublet Model (IDM) in order to explain this gamma ray excess. Taking the lightest inert particle (LIP) as a stable DM candidate we show that a 31-40 GeV dark matter derived from IDM will fail to satisfy experimental limits on dark matter direct detection cross-section obtained from ongoing direct detection experiments and is also inconsistent with LHC findings. We show that a singlet extended inert doublet model can easily explain the reported $\gamma$-ray excess which is as well in agreement with Higgs search results at LHC and other observed results like DM relic density and direct detection constraints.

Discovery of an ultracompact gamma-ray millisecond pulsar binary candidate

We report multi-wavelength observations of the unidentified Fermi object 2FGL J1653.6-0159. With the help of high-resolution X-ray observation, we have identified an X-ray and optical counterpart of 2FGL J1653.6-0159. The source exhibits a periodic modulation of 74.93 min in optical and possibly also in X-ray. We suggest that 2FGL J1653.6-0159 is a compact binary system with an orbital period of 74.93 min. Combining the gamma-ray and X-ray properties, 2FGL J1653.6-0159 is potentially a black widow/redback type gamma-ray millisecond pulsar (MSP). The optical and X-ray lightcurve profile shows that the companion is mildly heated by the high-energy emission and the X-rays are from intrabinary shock. Although no radio pulsation has been detected yet, we estimated that the spin period of the MSP is ~2 ms based on a theoretical model. If pulsation can be confirmed in the future, 2FGL J1653.6-0159 will become the first ultracompact rotation-powered MSP.

Discovery of an ultracompact gamma-ray millisecond pulsar binary candidate [Replacement]

We report multi-wavelength observations of the unidentified Fermi object 2FGL J1653.6-0159. With the help of high-resolution X-ray observation, we have identified an X-ray and optical counterpart of 2FGL J1653.6-0159. The source exhibits a periodic modulation of 75 min in optical and possibly also in X-ray. We suggest that 2FGL J1653.6-0159 is a compact binary system with an orbital period of 75 min. Combining the gamma-ray and X-ray properties, 2FGL J1653.6-0159 is potentially a black widow/redback type gamma-ray millisecond pulsar (MSP). The optical and X-ray lightcurve profiles show that the companion is mildly heated by the high-energy emission and the X-rays are from intrabinary shock. Although no radio pulsation has been detected yet, we estimated that the spin period of the MSP is ~2ms based on a theoretical model. If pulsation can be confirmed in the future, 2FGL J1653.6-0159 will become the first ultracompact rotation-powered MSP.

Galactic center gamma ray excess from two Higgs doublet portal dark matter [Cross-Listing]

We consider a simple extension of type-II two Higgs doublet model by introducing a real scalar being a candidate for the dark matter in the present Universe. The main annihilation mode of the dark matter particle with a mass of around $31-40$ GeV is into a $b\bar{b}$ pair, and this annihilation mode suitably explains the observed excess of the flux of gamma ray from the Galactic center. We identify the parameter region of the model which can fit the gamma ray excess and, at the same time, satisfy phenomenological constraints such as the observed dark matter relic density and the null results of the direct dark matter search experiments. Most of the parameter region is found to be within the search reach of the future direct dark matter detection experiments.

Galactic center gamma ray excess from two Higgs doublet portal dark matter

We consider a simple extension of type-II two Higgs doublet model by introducing a real scalar being a candidate for the dark matter in the present Universe. The main annihilation mode of the dark matter particle with a mass of around $31-40$ GeV is into a $b\bar{b}$ pair, and this annihilation mode suitably explains the observed excess of the flux of gamma ray from the Galactic center. We identify the parameter region of the model which can fit the gamma ray excess and, at the same time, satisfy phenomenological constraints such as the observed dark matter relic density and the null results of the direct dark matter search experiments. Most of the parameter region is found to be within the search reach of the future direct dark matter detection experiments.

From Engine to Afterglow: Collapsars Naturally Produce Top-Heavy Jets and Early-Time Plateaus in Gamma Ray Burst Afterglows [Replacement]

We demonstrate that the steep decay and long plateau in the early phases of gamma ray burst (GRB) X-ray afterglows are naturally produced in the collapsar model, by a means ultimately related to the dynamics of relativistic jet propagation through a massive star. We present two-dimensional axisymmetric hydrodynamical simulations which start from a collapsar engine and evolve all the way through the late afterglow phase. The resultant outflow includes a jet core which is highly relativistic after breaking out of the star, but becomes baryon-loaded after colliding with a massive outer shell, corresponding to mass from the stellar atmosphere of the progenitor star which became trapped in front of the jet core at breakout. The prompt emission produced before or during this collision would then have the signature of a high Lorentz factor jet, but the afterglow is produced by the amalgamated post-collision ejecta which has more inertia than the original highly relativistic jet core and thus has a delayed deceleration. This naturally explains the early light curve behavior discovered by Swift, including a steep decay and a long plateau, without invoking late-time energy injection from the central engine. The numerical simulation is performed continuously from engine to afterglow, covering a dynamic range of over ten orders of magnitude in radius. Light curves calculated from the numerical output demonstrate that this mechanism reproduces basic features seen in early afterglow data. Initial steep decays are produced by internal shocks, and the plateau corresponds to the coasting phase of the outflow.

From Engine to Afterglow: Collapsars Naturally Produce Top-Heavy Jets and Early-Time Plateaus in Gamma Ray Burst Afterglows

We demonstrate that the steep decay and long plateau in the early phases of gamma ray burst (GRB) afterglows are naturally produced in the collapsar model, by a means ultimately related to the dynamics of relativistic jet propagation through a massive star. We present hydrodynamical simulations which start from a collapsar engine and evolve all the way through the late afterglow phase. The resultant outflow includes a jet core which is highly relativistic after breaking out of the star, but becomes baryon-loaded and less relativistic after colliding with a massive outer shell, corresponding to mass from the stellar atmosphere of the progenitor star which became trapped in front of the jet core at breakout. The prompt emission produced before or during this collision would then have the signature of a high Lorentz factor jet, but the afterglow is produced by the amalgamated post-collision ejecta which has more inertia than the original highly relativistic jet core and thus has a delayed deceleration. This naturally explains the early light curve behavior discovered by Swift, including a steep decay and a long plateau, without invoking late-time energy injection from the central engine. The numerical simulation is performed continuously from engine to afterglow, covering a dynamic range of over ten orders of magnitude in radius as a relativistic jet propagates through a massive star, breaks out of the stellar surface and coasts, generating both internal and external shocks. Light curves calculated from the numerical output demonstrate that this mechanism reproduces basic features seen in early afterglow data. Initial steep decays are produced by internal shocks, and the plateau corresponds to the coasting phase of the outflow.

Discovery of GeV emission from the direction of the luminous infrared galaxy NGC 2146

Recent detection of high-energy gamma-ray emission from starburst galaxies M82 and NGC 253 suggests that starburst galaxies are huge revisors of cosmic rays and these cosmic rays convert a significant fraction of their energy into gamma-rays by colliding with the dense interstellar medium. In this {\it Letter}, we report the search for high-energy gamma-ray emission from several nearby star-forming and starburst galaxies using the 68 month data obtained with the Fermi Large Area Telescope. We found a $\sim5.5\sigma$ detection of gamma-ray emission above 200{\rm MeV} from a source spatially coincident with the location of the luminous infrared galaxy NGC~2146. Taking into account also the temporal and spectral properties of the gamma-ray emission, we suggest that the gamma-ray source is likely to be the counterpart of NGC~2146. The gamma-ray luminosity suggests that cosmic rays in NGC~2146 convert nearly half of their energy into secondary pions, so NGC~2146 is approximately a "proton calorimeter". It is also found that NGC~2146 obeys the quasi-linear scaling relation between the gamma-ray luminosity and total infrared luminosity for star-forming galaxies, strengthening the connection between massive star formation and gamma-ray emission in star-forming galaxies.

Discovery of GeV emission from the direction of the luminous infrared galaxy NGC 2146 [Replacement]

Recent detection of high-energy gamma-ray emission from starburst galaxies M82 and NGC 253 suggests that starburst galaxies are huge reservoirs of cosmic rays and these cosmic rays convert a significant fraction of their energy into gamma-rays by colliding with the dense interstellar medium. In this {\it Letter}, we report the search for high-energy gamma-ray emission from several nearby star-forming and starburst galaxies using the 68 month data obtained with the Fermi Large Area Telescope. We found a $\sim5.5\sigma$ detection of gamma-ray emission above 200{\rm MeV} from a source spatially coincident with the location of the luminous infrared galaxy NGC~2146. Taking into account also the temporal and spectral properties of the gamma-ray emission, we suggest that the gamma-ray source is likely to be the counterpart of NGC~2146. The gamma-ray luminosity suggests that cosmic rays in NGC~2146 convert nearly half of their energy into secondary pions, so NGC~2146 is approximately a "proton calorimeter". It is also found that NGC~2146 obeys the quasi-linear scaling relation between the gamma-ray luminosity and total infrared luminosity for star-forming galaxies, strengthening the connection between massive star formation and gamma-ray emission in star-forming galaxies.

Discovery of GeV emission from the direction of the luminous infrared galaxy NGC 2146 [Replacement]

Recent detection of high-energy gamma-ray emission from starburst galaxies M82 and NGC 253 suggests that starburst galaxies are huge reservoirs of cosmic rays and these cosmic rays convert a significant fraction of their energy into gamma-rays by colliding with the dense interstellar medium. In this paper, we report the search for high-energy gamma-ray emission from several nearby star-forming and starburst galaxies using the 68 month data obtained with the Fermi Large Area Telescope. We found a $\sim5.5\sigma$ detection of gamma-ray emission above 200{\rm MeV} from a source spatially coincident with the location of the luminous infrared galaxy NGC~2146. Taking into account also the temporal and spectral properties of the gamma-ray emission, we suggest that the gamma-ray source is likely to be the counterpart of NGC~2146. The gamma-ray luminosity suggests that cosmic rays in NGC~2146 convert most of their energy into secondary pions, so NGC~2146 is a "proton calorimeter". It is also found that NGC~2146 obeys the quasi-linear scaling relation between the gamma-ray luminosity and total infrared luminosity for star-forming galaxies, strengthening the connection between massive star formation and gamma-ray emission of star-forming galaxies. Possible TeV emission from NGC~2146 is predicted and the implications for high-energy neutrino emission from starburst galaxies are discussed.

Simplified Dark Matter Models Confront the Gamma Ray Excess

Inspired by the excess of gamma rays from the Galactic Center, we confront a number of simplified dark matter models with experimental data. Assuming a single dark matter particle coupled to standard matter via a spin-0 mediator, we compare model evidences for Majorana fermion, real scalar and real vector dark matter candidates. We consider dark matter annihilation into various fermionic final states contributing to the observed differential gamma ray flux. Our likelihood function also includes the dark matter relic density, its elastic scattering cross section with nuclei, and collider limits. Using Bayesian inference we confine the mass and couplings strengths of the dark matter and mediator particle. Our results show that, if the gamma ray excess is due to dark matter the above parameters are well constrained by the observations. We find that the Majorana fermion dark matter model is supported the most by the data.

Simplified Dark Matter Models Confront the Gamma Ray Excess [Replacement]

Inspired by the excess of gamma rays from the Galactic Center, we confront a number of simplified dark matter models with experimental data. Assuming a single dark matter particle coupled to standard matter via a spin-0 mediator, we compare model evidences for Majorana fermion, real scalar and real vector dark matter candidates. We consider dark matter annihilation into various fermionic final states contributing to the observed differential gamma ray flux. Our likelihood function also includes the dark matter relic density, its elastic scattering cross section with nuclei, and collider limits. Using Bayesian inference we confine the mass and couplings strengths of the dark matter and mediator particle. Our results show that, if the gamma ray excess is due to dark matter the above parameters are well constrained by the observations. We find that the Majorana fermion dark matter model is supported the most by the data.

Constraints on The Hadronic Content of Gamma Ray Bursts

The IceCube high-energy neutrino telescope has been collecting data since 2006. Conversely, hundreds of Gamma Ray Bursts (GRBs) have been detected by the GBM on board Fermi, since its launch in 2008. So far no neutrino event has been associated with a GRB, despite many models predicting the generation of high energy neutrinos through GRB photon interaction with PeV protons in the GRB jet. We use the non-detection of neutrinos to constrain the hadronic content of GRB jets independent of jet model parameters. Assuming a generic particle spectrum of $E^{-\alpha}$ with $\alpha = 2$, we find that the ratio of the energy carried by pions to that in electrons has to be small $f_\pi / f_e \lesssim 0.24$ at 95\% confidence level. A distribution of spectral slopes can lower $f_\pi / f_e$ by orders of magnitude. Another limit, independent of neutrinos, is obtained if one ascribes the measured Fermi/LAT GeV gamma-ray emission to pair-photon cascades of high-energy photons resulting from (the same photon-hadronic interactions and subsequent) neutral pion decays. Based on the generally observed MeV to GeV GRB fluence ratio of $\approx 10$, we show that $f_\pi / f_e \lesssim 0.3$. In some bursts, where this ratio is as low as unity, $f_\pi / f_e \lesssim 0.03$. These findings add to the mounting doubts regarding the presence of PeV protons in GRB jets.

On the Composition of GRBs' Collapsar Jets

The duration distribution of long Gamma Ray Bursts reveals a plateau at durations shorter than ~20 s (in the observer frame) and a power-law decline at longer durations (Bromberg et al., 2012). Such a plateau arises naturally in the Collapsar model. In this model the engine has to operate long enough to push the jet out of the stellar envelope and the observed duration of the burst is the difference between the engine’s operation time and the jet breakout time. We compare the jet breakout time inferred from the duration distribution (~10 s in the burst’s frame) to the breakout time of a hydrodynamic jet (~10 s for typical parameters) and of a Poynting flux dominated jet with the same overall energy (<~1 s). As only the former is compatible with the duration of the plateau in the GRB duration distribution, we conclude that the jet is hydrodynamic during most of the time that its head is within the envelope of the progenitor star and around the time when it emerges from the star. This would naturally arise if the jet forms as a hydrodynamic jet in the first place or if it forms Poynting flux dominated but dissipates most of its magnetic energy early on within the progenitor star and emerges as a hydrodynamic jet.

Recombining Plasma in the Gamma-ray Emitting Mixed-Morphology Supernova Remnant 3C 391

A group of middle-aged mixed-morphology (MM) supernova remnants (SNRs) interacting with molecular clouds (MC) has been discovered as strong GeV gamma-ray emitters by Large Area Telescope on board Fermi Gamma Ray Space Telescope (Fermi-LAT). The recent observations of the Suzaku X-ray satellite have revealed that some of these interacting gamma-ray emitting SNRs, such as IC443, W49B, W44, and G359.1-0.5, have overionized plasmas. 3C 391 (G31.9+0.0) is another Galactic MM SNR interacting with MC. It was observed in GeV gamma rays by Fermi-LAT as well as in the 0.3 $-$ 10.0 keV X-ray band by Suzaku. In this work, 3C 391 was detected in GeV gamma rays with a significance of $\sim$ 18 $\sigma$ and we showed that the GeV emission is point-like in nature. The GeV gamma-ray spectrum was shown to be best explained by the decay of neutral pions assuming that the protons follow a broken power-law distribution. We revealed radiative recombination structures of silicon and sulfur from 3C 391 using Suzaku data. In this paper we discuss the possible origin of this type of radiative plasma and hadronic gamma rays.

Bremsstrahlung and Gamma Ray Lines in 3 Scenarios of Dark Matter Annihilation

Gamma ray spectral features are of interest for indirect searches of dark matter (DM). Following Barger et al, we consider 3 simple scenarios of DM that annihilates into Standard Model (SM) fermion pairs. Scenario 1 is a Majorana DM candidate coupled to a charged scalar, scenario 2 is a Majorana DM coupled to a charged gauge boson and scenario 3 is a real scalar DM coupled a charged vector-like fermion. As shown by Barger et al, these 3 scenarios share precisely the same internal Bremsstrahlung spectral signature into gamma rays. Their phenomenology is however distinct. In particular for annihilation into light SM fermions, in the chiral limit, the 2-body annihilation cross section is p-wave suppressed for the Majorana candidates while it is d-wave suppressed for the real scalar. In the present work we study the annihilation into 2 gammas, showing that these three scenarios have distinct, and so potentially distinguishable, spectral signatures into gamma rays. In the case of the real scalar candidate we provide a new calculation of the amplitude for annihilation into 2 gammas.

 

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