Recent studies of merging black holes suggest that the cosmological mass fraction of primordial black holes (PBHs) is not completely negligible. The mass of a PBH can be as massive as an intermediate mass black hole (IMBH). In this article, we assume that dark matter particles are thermal relic particles and they can self-annihilate. The dark matter around an IMBH may have formed a density spike so that the rate of dark matter annihilation would be greatly enhanced. Using the gamma-ray data of the Fornax cluster and assuming merging events in the cluster are not important, we show that the amount of IMBHs (including PBHs) is very sub-dominant compared with the amount of baryons and dark matter. The upper limit on the IMBH-to-baryon ratio is $\sim 7\times 10^{-4}$ for $m_{\rm DM} \le 10$ TeV.
We report the Fermi Large Area Telescope detection of extended gamma-ray emission from the lobes of the radio galaxy Fornax A using 6.1 years of Pass 8 data. After Centaurus A, this is now the second example of an extended gamma-ray source attributed to a radio galaxy. Both an extended flat disk morphology and a morphology following the extended radio lobes were preferred over a point-source description, and the core contribution was constrained to be < 14% of the total gamma-ray flux. A preferred alignment of the gamma-ray elongation with the radio lobes was demonstrated by rotating the radio…
We present new low-frequency observations of the nearby radio galaxy Fornax A at 154 MHz with the Murchison Widefield Array, microwave flux-density measurements obtained from WMAP and Planck data, and gamma-ray flux densities obtained from Fermi data. We also compile a comprehensive list of previously published images and flux-density measurements at radio, microwave and X-ray energies. A detailed analysis of the spectrum of Fornax A between 154 MHz and 1510 MHz reveals that both radio lobes have a similar spatially-averaged spectral index, and that there exists a steep-spectrum bridge of diff…
The HAWC Gamma-Ray Observatory has reported the discovery of TeV gamma-ray emission extending several degrees around the positions of Geminga and B0656+14 pulsars. Assuming these gamma rays are produced by inverse Compton scattering off low-energy photons in electron halos around the pulsars, we determine the diffusion of electrons and positrons in the local interstellar medium. We will present the morphological and spectral studies of these two VHE gamma-ray sources and the derived positron spectrum at Earth.
The angular dependence of emission in gamma-ray bursts (GRB) is of fundamental importance in understanding the underlying physical mechanisms, as well as in multimessenger search efforts. We examine the prospects of using reconstructed GRB jet opening angles and off-axis observer angles in determining the jet structure. We show that the reconstructed angles by Ryan et al. (2015) are inconsistent with uniform jet structure. We further calculate the number of GRBs with accurately reconstructed opening and observer angles necessary to differentiate between some phenomenological non-uniform struct…
The angular extension of the gamma-ray image of 3C 279 may be constrained by studying its solar occultations as suggested by Fairbairn et al. (2010). We perform this kind of analysis for seven occultations observed by Fermi-LAT in 2008-2014, using the Fermi-LAT Solar System tools. The results are interpreted in terms of models with extended gamma-ray halo of 3C 279; first constraints on the size and the flux of the halo are reported.
We analyze 2.8-yr data of 1-100 GeV photons for clusters of galaxies, collected with the Large Area Telescope onboard the Fermi satellite. By analyzing 49 nearby massive clusters located at high Galactic latitudes, we find no excess gamma-ray emission towards directions of the galaxy clusters. Using flux upper limits, we show that the Fornax cluster provides the most stringent constraints on the dark matter annihilation cross section. Stacking a large sample of nearby clusters does not help improve the limit for most dark matter models. This suggests that a detailed modeling of the Fornax clus…
Dwarf spheroidal galaxies are compact stellar objects with small or negligible astrophysical backgrounds, widely considered as promising targets to search for a signal from the dark matter decay and annihilation. We present constraints on the lifetime of the superheavy decaying dark matter branching to the $q\bar{q}$ channel in the mass range $10^{19} - 10^{25}$ eV based on the directional limits on the ultra-high-energy (UHE) gamma rays obtained by the Pierre Auger Observatory and the Telescope Array experiment. Attenuation effects during the propagation of UHE photons towards Earth are taken…
I consider here acceleration and heating of relativistic outflow by local magnetic energy dissipation process in Poynting flux dominated outflow. Adopting the standard assumption that the reconnection rate scales with the Alfven speed, I show here that the fraction of energy dissipated as thermal photons cannot exceed (13 $\hat γ-14)^{-1} = 30$% (for adiabatic index $\hat γ= 4/3$) of the kinetic energy at the photosphere. Even in the most radiatively efficient scenario, the energy released as non-thermal photons during the prompt phase is at most equal to the kinetic energy of the outflow. The…
Nearby electron/positron accelerators, mostly Pulsar Wind Nebulae (PWNe), have been proposed as potential origins of the positron excess above 10 GeV. The HAWC Observatory reveals two very extended sources spatially coincident with two nearby middle-aged pulsars: Geminga and PSR B0656+14, suggesting ultrarelativistic electrons/positrons accelerated in our backyard. Morphological studies on these two PWNe provide a constraint on the diffusion coefficient at HAWC energies. In this poster, we will present the model development and morphological studies on these PWNe, and the derived diffusion coe…
The Fermi Gamma-Ray Space Telescope has discovered many gamma-ray pulsars, both as radio-loud objects and radio-quiet or radio-weak pulsars that have been identified through blind period searches. The latter presumably have gamma-ray beams originating from high altitudes in the magnetosphere, resulting in little or no overlap with the radio beam. The exponential cut-off of the emission at high energy also points to a medium- or high-altitude origin of the gamma rays. Population synthesis offers means to study two complementary aspects of the gamma-ray pulsar population: energetics and light cu…
We use the recent measurement of the isotropic $γ$-ray background (IGRB) by Fermi LAT and analysis of the contribution of unresolved point $γ$-ray sources to IGRB to build constraints on the models of ultra-high cosmic rays (UHECR) origin. We also calculate the minimal expected diffuse $γ$-ray flux produced by UHECR interactions with an interstellar photon background. Finally, for the subclass of dark matter (DM) models with decaying weakly interacting massive particles (WIMP), we build constraints on the particle decay time using minimal expected contributions to the IGRB from unresolved poin…
We derive new bounds on decaying Dark Matter from the gamma ray measurements of (i) the isotropic residual (extragalactic) background by Fermi and (ii) the Fornax galaxy cluster by HESS. We find that those from (i) are among the most stringent constraints currently available, for a large range of DM masses and a variety of decay modes, excluding half-lives up to ~10^26 to few 10^27 seconds. In particular, they rule out the interpretation in terms of decaying DM of the e^\pm spectral features in Pamela, Fermi and HESS, unless very conservative choices are adopted. We also discuss future prospec…
We demonstrate a method for quantitatively comparing gamma-ray pulsar light curves with magnetosphere beaming models. With the Fermi LAT providing many pulsar discoveries and high quality pulsar light curves for the brighter objects, such comparison allows greatly improved constraints on the emission zone geometry and the magnetospheric physics. Here we apply the method to Fermi LAT light curves of a set of bright pulsars known since EGRET or before. We test three approximate models for the magnetosphere structure and two popular schemes for the location of the emission zone, the Two Pole Caus…
We propose a direct and model-independent method to constrain the Lorentz factor of a relativistically expanding object, like gamma-ray bursts. Only the measurements, such as thermal component of the emission, the distance and the variable time scale of the light curve, are used. If the uncertainties are considered, we will obtain lower limits of the Lorentz factor instead. We apply this method to GRB 090618 and get a lower limit of the Lorentz factor to be 22. The method can be used to any relativistically moving object, such as gamma-ray bursts, blazars, and soft gamma-ray repeaters, providi…
New bounds on decaying Dark Matter are derived from the gamma-ray measurements of (i) the isotropic residual (extragalactic) background by Fermi and (ii) the Fornax galaxy cluster by H.E.S.S. We find that those from (i) are among the most stringent constraints currently available, for a large range of dark matter masses and a variety of decay modes, excluding half-lives up to about 10^26 to few 10^27 seconds. In particular, they rule out the interpretation in terms of decaying dark matter of the e+/- spectral features in PAMELA, Fermi and H.E.S.S., unless very conservative choices are adopted.…
One of the cornerstones of modern physics is Einstein's special relativity, with its constant speed of light and zero photon mass assumptions. Constraint on the rest mass m_γ of photons is a fundamental way to test Einstein's theory, as well as other essential electromagnetic and particle theories. Since non-zero photon mass can give rise to frequency-(or energy-) dependent dispersions, measuring the time delay of photons with different frequencies emitted from explosive astrophysical events is an important and model-independent method to put such a constraint. The cosmological gamma-ray burst…
Since the launch of the Large Area Telescope (LAT) on board the Fermi spacecraft in June 2008, the number of observed gamma-ray pulsars has increased dramatically. A large number of these are also observed at radio frequencies. Constraints on the viewing geometries of 5 of 6 gamma-ray pulsars exhibiting single-peaked gamma-ray profiles were derived using high-quality radio polarization data (Weltevrede et al., 2010). We obtain independent constraints on the viewing geometries of 6 by using a geometric emission code to model the Fermi LAT and radio light curves (LCs). We find fits for the magne…
Since the launch of the Fermi Gamma-Ray Space Telescope, several hundred gamma-ray pulsars have been discovered, some being radio-loud and some radio-quiet with time-aligned radio and gamma-ray light curves. In the second Fermi Pulsar Catalogue, 117 new gamma-ray pulsars have been reported based on three years of data collected by the Large Area Telescope on the Fermi satellite, providing a wealth of information such as the peak separation~$Δ$ of the gamma-ray pulsations and the radio lag~$δ$ between the gamma-ray and radio pulses. We selected several radio-loud millisecond gamma-ray pulsars w…
Gamma-ray searches for new physics such as dark matter are often driven by investigating the composition of the extragalactic gamma-ray background (EGB). Classic approaches to EGB decomposition manifest in resolving individual point sources and dissecting the intensity spectrum of the remaining unresolved component. Furthermore, statistical methods have recently been proven to outperform the sensitivity of classic source detection algorithms in finding point-source populations in the unresolved flux regime. In this article, we employ the 1-point photon count statistics of eight years of Fermi-…
In the past decade, the properties of annihilating dark matter models were examined by various kinds of data, including the data of gamma rays, radio waves, X-ray, positrons, electrons, antiprotons and neutrinos. In particular, most of the studies focus on the data of our Galaxy, nearby galaxies (e.g. M31 galaxy) or nearby galaxy clusters (e.g. Fornax cluster). In this article, we examine the archival radio continuum spectral data of a relatively high-redshift galaxy cluster (A697 cluster) to constrain the properties of annihilating dark matter. We find that leptophilic annihilation channels (…
One of the possible sources of hadronic cosmic rays (CRs) are newborn pulsars. If this is indeed the case, they should feature diffusive gamma-ray halos produced by interactions of CRs with interstellar gas. In this paper we try to identify extended gamma-ray emission around young pulsars, making use of the 7-year Fermi-LAT data. For this purpose we select and analyze a set of eight pulsars that are most likely to possess detectable gamma-ray halos. We find extended emission that might be interpreted as a gamma-ray halo only in the case of PSR J0007+7303. Its luminosity accords with the total …
Deviation of the gamma-ray energy spectra of Flat Spectrum Radio Quasars (FSRQs) from a simple power law has been previously observed but the cause of this remains unidentified. If the gamma-ray emission region is close to the central black hole then absorption of gamma-rays with photons from the broad line region is predicted to produce two spectral breaks in the gamma-ray spectra at fixed energies. We examine 9 bright FSRQs for evidence of breaks and curvature. Although we confirm deviation from a simple power law, break energies are usually not where predicted by the double-absorber model. …
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 partic…
We consider decaying dark matter with masses $10^{7} \lesssim M \lesssim 10^{16}$ GeV, as a source of ultra-high energy (UHE) gamma rays. Using recent limits on UHE gamma-ray flux for energies $E_γ> 2 \cdot 10^{14}$ eV, provided by extensive air shower observatories, we put limits on masses and lifetimes of the dark matter. We also discuss possible dark matter decay origin of tentative 100 PeV photon flux detected with EAS-MSU experiment.
We analyze 9 years of PASS 8 $\textit{Fermi}$-LAT data in the 60$-$500 MeV range and determine flux upper limits (UL) for 17 gamma-ray dark pulsars as a probe of axions produced by nucleon-nucleon Bremsstrahlung in the pulsar core. Using a previously published axion decay gamma-ray photon flux model for pulsars which relies on a high core temperature of 20 MeV, we improve the determination of the UL axion mass ($m_a$), at 95 percent confidence level, to 9.6 $\times$ 10$^{-3}$ eV, which is a factor of 8 improvement on previous results. We show that the axion emissivity (energy loss rate per vol…
We calculate the effects of an asymmetric $^{56}\mathrm{Ni}$ distribution in Type Ia supernova (SN Ia) ejecta on the gamma-ray escape timescale ($t_0$) that characterizes the late light curve ($>40$ days after peak) and find the effect is modest compared to other possible variations in ejecta structure. We parameterize asymmetry in the $^{56}\mathrm{Ni}$ distribution and calculate $t_0$ for a grid of SN ejecta models spanning a large volume of the asymmetry parameter space. The models have spherical density profiles while the $^{56}\mathrm{Ni}$ distribution in them has various levels of asymme…
Star-forming galaxies produce gamma-rays primarily via pion production, resulting from inelastic collisions between cosmic ray protons and the interstellar medium (ISM). The dense ISM and high star formation rates of luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs) imply that they should be strong gamma-ray emitters, but so far only two LIRGs have been detected. Theoretical models for their emission depend on the unknown fraction of cosmic ray protons that escape these galaxies before interacting. We analyze Fermi-LAT data for 82 of the brightest IRAS LIRGs and ULIRGs. We exami…
Fast variations of gamma-ray flux from Active Galactic Nuclei and Gamma-Ray Bursts can constrain Lorentz Invariance Violation (LIV) because of the delayed (or advanced) arrival of photons with higher energies: this approach has lead to the current world-best limits on the energy scale of Quantum Gravity. Here we report on constraints on LIV studying the gamma-ray emission up to TeV energies from the Galactic Crab pulsar, recently discovered by the MAGIC collaboration. A likelihood analysis of the pulsar events reconstructed for energies above 400 GeV finds no significant variation of energy-de…
High-energy cosmic rays (CRs) can be accelerated in the relativistic jets of Active Galactic Nuclei (AGNs) powered by supermassive black holes. The baryon loading efficiency onto relativistic CR baryons from the accreting black holes is poorly constrained by observations so far. In this paper, we suggest that the $γ$-ray emission of galaxy clusters can be used to study the baryon loading factor of AGN jets, since CRs injected by AGN jets are completely confined in the galaxy clusters and sufficiently interact with intra-cluster medium via hadronic process, producing diffuse $γ$-rays. We study …
Studying the propagation of gamma rays on cosmological distances encompasses a variety of scientific fields, focusing on diffuse radiation fields such as the extragalactic background light, on the probe of the magnetism of the Universe on large scales, and on physics beyond the standard models of cosmology and particle physics. The measurements, constraints and hints from observations of gamma-ray blazars by airborne and ground-based instruments are briefly reviewed. These observations point to gamma-ray cosmology as one of the major science cases of the Cherenkov Telescope Array, CTA.
Some approaches to Quantum Gravity (QG) predict a modification of photon dispersion relations due to a breaking of Lorentz invariance. The effect is expected to affect photons near an effective QG energy scale. This scale has been constrained by observing gamma rays emitted from variable astrophysical sources such as gamma-ray bursts and flaring active galactic nuclei. Pulsars exhibit a periodic emission of possibly ms time scale. In 2014, the H.E.S.S. experiment reported the detection down to 20 GeV of gamma rays from the Vela pulsar having a periodicity of 89 ms. Using a likelihood analysis,…
Observations of short gamma-ray bursts indicate ongoing energy injection following the prompt emission, with the most likely candidate being the birth of a rapidly rotating, highly magnetised neutron star. We utilise X-ray observations of the burst remnant to constrain properties of the nascent neutron star, including its magnetic field-induced ellipticity and the saturation amplitude of various oscillation modes. Moreover, we derive strict upper limits on the gravitational wave emission from these objects by looking only at the X-ray light curve, showing the burst remnants are unlikely to be …
The joint detection of GW 170817 and GRB 170817A indicated that at least a fraction of short gamma ray bursts (SGRBs) originate from binary neutron star (BNS) mergers. One possible remnant of a BNS merger is a rapidly rotating, strongly magnetized neutron star, which has been discussed as one possible central engine for GRBs. For a rapidly rotating magnetar central engine, the deposition of the rotation energy into the ejecta launched from the merger could lead to bright radio emission. The brightness of radio emission years after an SGRB would provide an estimation of the kinetic energy of ej…
The positive-to-negative transition of spectral lag is an uncommon feature reported in a small number of GRBs. An application of such a feature has been made to constrain the critical quantum gravity energy ($E_{\rm QG}$) of the light photons under the hypothesis that the Lorentz invariance might be violated. Motivated by previous case studies, this paper systematically examined the up-to-date Fermi/GBM GRB sample for the lag transition feature to establish a comprehensive physical limit on the Lorentz Invariance Violation (LIV). This search resulted in 32 GRBs with redshift available, which e…
With the large sample of young gamma-ray pulsars discovered by the Fermi Large Area Telescope (LAT), population synthesis has become a powerful tool for comparing their collective properties with model predictions. We synthesised a pulsar population based on a radio emission model and four gamma-ray gap models (Polar Cap, Slot Gap, Outer Gap, and One Pole Caustic) normalizing to the number of detected radio pulsars in select group of surveys. The luminosity and the wide beams from the outer gaps can easily account for the number of Fermi detections in 2 years of observations. The wide slot-gap…
Ultra-high cosmic rays (UHECRs) with energies >10^19 eV emitted at cosmological distances will be attenuated by cosmic microwave and infrared background radiation through photohadronic processes. Lower energy extra-galactic cosmic rays (~10^18-10^19 eV) can only travel a linear distance smaller than ~Gpc in a Hubble time due to the diffusion if the extra-galactic magnetic fields are as strong as nano Gauss. These prevent us from directly observing most of the UHECRs in the universe, and thus the observed UHECR intensity reflects only the emissivity in the nearby universe within hundreds of Mpc…
We compute the intrinsic isotropic peak luminosity function (LF) and formation rate of long gamma-ray bursts (LGRBs) using a novel approach. We complement a standard log\,$N$\,--\,log\,$P$ brightness distribution and $V_{\mathrm{max}}$ estimations with two observation-time relations: a redshift--observation-time relation (log\,$z$\,--\,log\,$T$) and a new luminosity--observation-time relation (log\,$L$\,--\,log\,$T$). We show that this approach reduces degeneracies that exist between the rate and LF of a brightness distribution. To account for the complex triggering algorithm employed by \emph…
We study the possibility for detecting gamma-ray emission from galaxy clusters. We consider 1) leptophilic models of dark matter (DM) annihilation that include a Sommerfeld enhancement (SFE), 2) different representative benchmark models of supersymmetric DM, and 3) cosmic ray (CR) induced pion decay. Among all clusters/groups of a flux-limited X-ray sample, we predict Virgo, Fornax and M49 to be the brightest DM sources and find a particularly low CR-induced background for Fornax. For a minimum substructure mass given by the DM free-streaming scale, cluster halos maximize the substructure boos…
We study the connection between low-luminosity gamma-ray bursts (llGRBs) and ultra-high-energy cosmic rays (UHECRs) using the canonical low-luminosity GRB 060218 as a proxy. We focus on the consequential synchrotron emission from electrons that are coaccelerated in the UHECR acceleration region, comparing this emission to observations. Both the prompt and afterglow phases are considered. For the prompt phase, we assume the coaccelerated electrons are injected with a power law distribution instantaneously (without additional heating or reacceleration), which results in bright optical-UV emissio…
The High-Altitude Water Cherenkov Observatory (HAWC) is a large field of view (~2 sr) continuously operating experiment sensitive to very-high energy (VHE) gamma rays (~0.3-100 TeV). These characteristics make it well suited for observing or constraining the VHE emission of rapid transients such as some gravitational waves progenitors. Of special interest are the events at low redshift where the attenuation due to the extragalactic background light is minimal. This is the case for binary neutron star mergers in the horizon of the LIGO and Virgo experiments, for which HAWC can either detect or …
Constraining Cosmic-Ray Acceleration and Escape in Middle-Aged Supernova Remnants with GeV-TeV Gamma-Ray Observations
Southwest Jiaotong University（西南交通大学）；Institute of High Energy Physics, Chinese Academy of Sciences（中国科学院高能物理研究所）；Shanghai Astronomical Observatory, Chinese Academy of Sciences（中国科学院上海天文台）
Cold dark matter models predict the existence of a large number of substructures within dark matter halos. If the cold dark matter consists of weakly interacting massive particles, their annihilation within these substructures could lead to diffuse GeV emission that would dominate over the annihilation signal of the host halo. In this work we search for GeV emission from three nearby galaxy clusters: Coma, Virgo and Fornax. We first remove known extragalactic and galactic diffuse gamma-ray backgrounds and point sources from the Fermi 2-year catalog and find a significant residual diffuse emiss…
We study the capabilities of the Fermi-LAT instrument on board of the Fermi mission to constrain particle dark matter properties, as annihilation cross section, mass and branching ratio into dominant annihilation channels, with gamma-ray observations from the galactic center. Besides the prompt gamma-ray flux, we also take into account the contribution from the electrons/positrons produced in dark matter annihilations to the gamma-ray signal via inverse Compton scattering off the interstellar photon background, which turns out to be crucial in the case of dark matter annihilations into mu+mu- …
Primordial black holes can represent all or most of the dark matter in the window $10^{17}-10^{22}\,$g. Here we present an extension of the constraints on PBHs of masses $10^{13}-10^{18}\,$g arising from the isotropic diffuse gamma ray background. Primordial black holes evaporate by emitting Hawking radiation that should not exceed the observed background. Generalizing from monochromatic distributions of Schwarzschild black holes to extended mass functions of Kerr rotating black holes, we show that the lower part of this mass window can be closed for near-extremal black holes.
GRB 130821A was detected by Fermi-GBM/LAT, Konus-Wind, SPI-ACS/INTEGRAL, RHESSI and Mars Odyssey-HEND. Although the data of GRB 130821A are very limited, we show in this work that the high energy gamma-ray emission (i.e., above 100 MeV) alone imposes tight constraint on the density profile of the circum-burst medium as well as the initial Lorentz factor of the outflow. The temporal behavior of the high energy gamma-ray emission is consistent with the forward shock synchrotron radiation model and the circum-burst medium likely has a constant-density profile. The Lorentz factor is about a few hu…
Observations of extended gamma-ray emission around Galactic cosmic-ray (CR) sources can be used as novel probes of interstellar magnetic fields. Using very-high-energy gamma-ray data from the HAWC Observatory, we place constraints on the properties of the magnetic turbulence within $\approx 25$ pc from Geminga. We inject and propagate individual CR electrons in 3D realizations of turbulent magnetic fields, calculate the resulting gamma-ray emission, and compare with HAWC measurements of this region. We find that HAWC data is compatible with expectations for Kolmogorov or Kraichnan turbulence, …
The X-ray plateau emission observed in many Long gamma-ray bursts (LGRBs) has been usually interpreted as the spin-down luminosity of a rapidly spinning, highly magnetized neutron star (millisecond magnetar). If this is true, then the magnetar may emit extended gravitational wave (GW) emission associated with the X-ray plateau due to non-axisymmetric deformation or various stellar oscillations. The advanced LIGO and Virgo detectors have searched for long-duration GW transients for several years, no evidence of GWs from any magnetar has been found until now. In this work, we attempt to search f…
Wolf-Rayet stars have been identified as objects in their final phase of massive star evolution. It has been suggested that Wolf-Rayet stars are the progenitors of long-duration gamma ray bursts in low metallicity environments. However, this deduction has yet to be proven. Here we report on our initial results from a VLT/FORS linear spectropolarimetry survey of Wolf-Rayet stars in the Magellanic Clouds, which is intended to constrain the physical criteria - such as weaker stellar winds, rapid rotation, and associated asymmetry - of the collapsar model. Finally, we provide an outlook for polari…
To investigate parsec-scale jet flow conditions during GeV gamma-ray flares detected by the Fermi Large Angle Telescope, we obtained centimeter-band total flux density and linear polarization monitoring observations from 2009.5 through 2012.5 with the 26-meter Michigan radio telescope for a sample of core-dominated blazars. We use these data to constrain radiative transfer simulations incorporating propagating shocks oriented at an arbitrary angle to the flow direction in order to set limits on the jet flow and shock parameters during flares temporally associated with gamma-ray flares in 0420-…
Recently, some very tight constraints of annihilating dark matter have been obtained from gamma-ray data of the Milky Way and Milky Way dwarf spheroidal satellite galaxies. In this article, we report that there are two excellent galaxy clusters (A2877 and Fornax) which can provide interesting constraints for annihilating dark matter. The lower limits of the dark matter mass for the thermal relic annihilation cross section are 25 GeV, 6 GeV, 130 GeV and 100 GeV respectively for the $e^+e^-$, $μ^+μ^-$, $τ^+τ^-$ and $b\bar{b}$ channels. For some configuration of our working assumptions, our resul…
Primordial black holes (PBHs) are thought to have been formed as a result of density fluctuations in the very early Universe. It is suggested that PBHs of mass $\sim 5 \times 10^{14} \mathrm{\ g}$ or less have evaporated through the release of Hawking radiation by the present day. However, PBHs of initial mass $10^{15} \mathrm{\ g}$ should still be evaporating at the present epoch. Over the past few years, very high-energy (VHE; E $>$ 100 GeV) gamma-ray emission from PBHs in the form of a burst has been searched for using ground-based gamma-ray instruments. However, no observational evidence h…
Starlight in the Universe impedes the passage of high energy (e.g. TeV) gamma rays due to positron-electron pair production. The history of this stellar radiation field depends upon observations of star formation rate which themselves can only be interpreted in the context of a particular cosmology. For different equations of state of dark energy, the star formation rate data suggests a different density of stellar photons at a particular redshift and a different probability of arrival of gamma rays from distant sources. In this work we aim to show that this effect can be used to constrain the…
In quantum gravity, a foamy structure of space-time leads to Lorentz invariance violation (LIV). As the most energetic astrophysical processes in the Universe, gamma-ray bursts (GRBs) provide an effective way to probe quantum gravity effects. We use continuous spectra of 20 short GRBs detected by the Swift satellite to give a conservative lower limit of quantum gravity energy scale $M_\textrm{QG} $. Due to the LIV effect, photons with different energy have different velocities. This will lead to the delayed arrival of high energy photons relative to the low energy ones. Based on the fact that …
Long gamma-ray bursts (GRBs) are considered to be originated from core collapse of massive stars. It is believed that the afterglow property is determined by the density of the material in the surrounding interstellar medium. Therefore, the circumburst density can be used to distinguish between an interstellar wind $n(R) \propto R^{\rm -k}$, and a constant density medium (ISM), $n(R) = const$. Previous studies with different afterglow samples, show that the circumburst medium of GRBs is neither simply supported by an interstellar wind, nor completely favored by an ISM. In this work, our new sa…
In the past few decades, observations indicated that an unexplained high production rate of positrons (the strong 511 keV line) exists in the Milky Way center. By using the fact that a large amount of high density gas used to exist near the Milky Way center million years ago, we model the rate of positrons produced due to dark matter annihilation. We consider the effect of adiabatic contraction of dark matter density due to the supermassive black hole at the Milky Way center and perform a detailed calculation to constrain the possible annihilation channel and dark matter mass range. We find th…
