Star-forming galaxies are huge reservoirs of cosmic rays (CRs) and these CRs convert a significant fraction of their energy into $γ$-rays by colliding with the interstellar medium (ISM). Several nearby star-forming galaxies have been detected in GeV-TeV $γ$-rays. It is also found that the $γ$-ray luminosities in 0.1-100 GeV correlate well with indicators of star formation rates of the galaxies, such as the total infrared (IR) luminosity. In this paper, we report a systematic search for possible $γ$-ray emission from galaxies in the IRAS Revised Bright Galaxies Sample, using 11.4 years of $γ$-ray data taken by the Fermi Large Area Telescope (LAT). Two new galaxies, M33 and Arp 299, are detected significantly. The two galaxies are consistent with the empirical correlation between the $γ$-ray luminosity and total infrared luminosity, suggesting that their $γ$-ray emissions should mainly originate from CRs interacting with ISM. Nevertheless, there is a tentative evidence that the flux of the $γ$-ray emission from Arp~299 is variable. If the variability is true, part of the emission from Arp 299 should originate from the obscured AGN in this interacting galaxy system. In addition, we find that the $γ$-ray excess from M33 is located at the northeast region of the galaxy, where a supergiant H II region, NGC604, resides. This indicates that some bright star-forming regions in spiral galaxies could play a dominant role in the galaxy in producing $γ$-ray emission.
Cosmic rays (CRs) in starburst galaxies produce high energy gamma-rays by colliding with the dense interstellar medium (ISM). Arp 220 is the nearest ultra luminous infrared galaxy (ULIRG) that has star-formation at extreme levels, so it has long been predicted to emit high-energy gamma-rays. However, no evidence of gamma-ray emission was found despite intense efforts of search. Here we report the discovery of high-energy gamma-ray emission above 200 MeV from Arp 220 at a confidence level of $\sim 6.3 σ$ using 7.5 years of \textsl {Fermi} Large Area Telescope observations. The gamma-ray emissio…
Extragalactic cosmic ray populations are important diagnostic tools for tracking the distribution of energy in nuclei and for distinguishing between activity powered by star formation versus active galactic nuclei (AGNs). Here, we compare different diagnostics of the cosmic ray populations of the nuclei of Arp 220 based on radio synchrotron observations and the recent gamma-ray detection. We find the gamma-ray and radio emission to be incompatible; a joint solution requires at minimum a factor of 4 - 8 times more energy coming from supernovae and a factor of 40 - 70 more mass in molecular gas …
The cores of ultra-luminous infrared galaxies (ULIRGs) are very dense environments, with a high rate of star formation and supernova explosions. They are thought to be sites of cosmic-ray acceleration, and are predicted to emit $γ$-rays in the GeV to TeV range. So far, no ULIRG has been detected in $γ$-rays. Arp 220, the closest ULIRG to Earth, has been well studied, and detailed models of $γ$-ray production in this galaxy are available. They predict a rather hard $γ$-ray spectrum up to several TeV. Due to its large rate of star formation, high gas density, and its close proximity to Earth, Ar…
We report on simultaneous observations of the local starburst system Arp 299 with NuSTAR and Chandra, which provides the first resolved images of this galaxy up to energies of ~ 45 keV. Fitting the 3-40 keV spectrum reveals a column density of $N_{\rm H}$ ~ 4 x10^{24} cm^{-2}, characteristic of a Compton-thick AGN, and a 10-30 keV luminosity of 1.2x 10^{43} ergs s^{-1}. The hard X-rays detected by NuSTAR above 10 keV are centered on the western nucleus, Arp 299-B, which previous X-ray observations have shown to be the primary source of neutral Fe-K emission. Other X-ray sources, including Arp …
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…
We used the Z-transformed Discrete Correlation Function (ZDCF) and the Stochastic Process Estimation for AGN Reverberation (SPEAR) methods for the time series analysis of the continuum and the H$α$ and H$β$ line fluxes of a sample of well known type 1 active galactic nuclei (AGNs): Arp 102B, 3C 390.3, NGC 5548, and NGC 4051, where the first two objects are showing double-peaked emission line profiles. The aim of this work is to compare the time lag measurements from these two methods, and check if there is a connection with other emission line properties. We found that the obtained time lags f…
Recent analyses of the gamma-ray spectrum from the ultra-luminous infrared galaxy Arp 220 have revealed a discrepancy in the cosmic ray energy injection rates derived from the gamma-rays versus the radio emission. While the observed radio emission is consistent with the star formation rate inferred from infrared observations, a significantly higher cosmic ray population is necessary to accurately model the measured gamma-ray flux. To resolve this discrepancy between the radio and gamma-ray observations, we find that we must increase the cosmic ray energy injection rate and account for an infra…
We present results from a 90 ks Chandra ACIS-S observation of the X-ray luminous interacting galaxy system Arp 299 (NGC 3690/IC 694). We detect 25 discrete X-ray sources with luminosities above 4.0x10^38 erg s^-1 covering the entire Ultra Luminous X-ray source (ULX) regime. Based on the hard X-ray spectra of the non-nuclear discrete sources identified in Arp 299, and their association with young, actively star-forming region of Arp 299 we identify them as HMXBs. We find in total 20 off-nuclear sources with luminosities above the ULX limit, 14 of which are point-like sources. Furthermore we obs…
The unknown $γ$-ray excess in the northwest region of Arp 220 was revisited by analyzing $\sim$11.8 years of the \textit{Fermi} Large Area Telescope (\textit{Fermi}-LAT) data in this study. We found that its photon flux was approximately three times higher than that of the previous study in the 0.2-100 GeV band, and the corresponding significance level$\sim8.15σ$ was approximately four times higher than before. The light curves of 15 and 45 time bins from the whole time all showed two active periods, and the variability of the second period was more significant than that of the first period. T…
The cores of Arp 220, the closest ultra-luminous infrared starburst galaxy, provide an opportunity to study interactions of cosmic rays under extreme conditions. In this paper, we model the populations of cosmic rays produced by supernovae in the central molecular zones of both starburst nuclei. We find that ~65 - 100% of cosmic rays are absorbed in these regions due to their huge molecular gas contents, and thus, the nuclei of Arp 220 nearly complete proton calorimeters. As the cosmic ray protons collide with the interstellar medium, they produce secondary electrons that are also contained wi…
The GeV and TeV emission from M82 and NGC 253 observed by Fermi, HESS, and VERITAS constrains the physics of cosmic rays (CRs) in these dense starbursts. We argue that the gamma rays are predominantly hadronic in origin, as expected by previous studies. The measured fluxes imply that pionic losses are efficient for CR protons in both galaxies: we show that a fraction F_cal ~ 0.2 - 0.4 of the energy injected in high energy primary CR protons is lost to inelastic proton-proton collisions (pion production) before escape, producing gamma rays, neutrinos, and secondary electrons and positrons. We d…
Recently, Fermi-LAT detected GeV emission during the X-ray flaring activity in GRB 100728A. We study various scenarios for its origin. The hard spectrum of the GeV emission favors the external inverse-Compton origin in which X-ray flare photons are up-scattered by relativistic electrons in the external forward shock. This external IC scenario, with anisotropic scattering effect taken into account, can reproduce the temporal and spectral properties of the GeV emission in GRB 100728A.
We report the detection of GeV $γ$-ray emission from supernova remnant HESS J1731-347 using 9 years of {\it Fermi} Large Area Telescope data. We find a slightly extended GeV source in the direction of HESS J1731-347. The spectrum above 1 GeV can be fitted by a power-law with an index of $Γ= 1.77\pm0.14$, and the GeV spectrum connects smoothly with the TeV spectrum of HESS J1731-347. Either a hadronic-leptonic or a pure leptonic model can fit the multi-wavelength spectral energy distribution of the source. However, the hard GeV $γ$-ray spectrum is more naturally produced in a leptonic (inverse …
An issue currently under debate in the literature is how far from the black hole is the Fermi-observed GeV emission of powerful blazars emitted. Here we present a clear diagnostic tool for testing whether the GeV emission site is located within the sub-pc broad emission line (BLR) region or further out in the few pc scale molecular torus (MT) environment. Within the BLR the scattering takes place at the onset of the Klein-Nishina regime, causing the electron cooling time to become almost energy independent and as a result, the variation of high-energy emission is expected to be achromatic. Con…
An issue currently under debate in the literature is how far from the black hole is the Fermi-observed GeV emission of powerful blazars emitted. Here we present a diagnostic tool for testing whether the GeV emission site is located within the sub-pc broad emission line (BLR) region or further out in the pc scale molecular torus (MT) environment. Within the BLR the scattering takes place at the onset of the Klein-Nishina regime, causing the electron cooling time to become almost energy independent and as a result, the variation of high-energy emission is expected to be achromatic. Contrarily, i…
The high energy GeV emission of gamma-ray bursts (GRBs), detected by \emph{Fermi}/LAT, has a significantly different morphology compared to the lower energy MeV emission, detected by \emph{Fermi}/GBM. Though the late time GeV emission is believed to be synchrotron radiation produced via an external shock, this emission as early as the prompt phase is puzzling. Meaningful connection between these two emissions can be drawn only by an accurate description of the prompt MeV spectrum. We perform a time-resolved spectroscopy of the GBM data of long GRBs having significant GeV emission, using a mode…
The Fermi-LAT revealed that the census of the gamma-ray sky is dominated by blazars. Looking for a possible connection between radio and gamma-ray emission is a central issue for understanding the blazar physics, and various works were dedicated to this topic. However, while a strong and significant correlation was found between radio and gamma-ray emission in the 0.1-100 GeV energy range, the connection between radio and very high energy (VHE, E>0.1 TeV) emission is still elusive. The main reason is the lack of a homogeneous VHE sky coverage, due to the operational mode of the imaging atmosph…
We analyze the emission in the 0.3-30 GeV energy range of Gamma-Ray Bursts detected with the Fermi Gamma-ray Space Telescope. We concentrate on bursts that were previously only detected with the Gamma-Ray Burst Monitor in the keV energy range. These bursts will then be compared to the bursts that were individually detected with the Large Area Telescope at higher energies. To estimate the emission of faint GRBs we use non-standard analysis methods and sum over many GRBs to find an average signal which is significantly above background level. We use a subsample of 99 GRBs listed in the Burst Cat…
HESS J1356-645 is considered to be a pulsar wind nebula (PWN) associated with the pulsar PSR J1357-6429. We reanalyze the GeV gamma-ray emission in the direction of HESS J1356-645 with more than 13 years of Fermi Large Area Telescope (LAT) data. The extended gamma-ray emission above 5 GeV is found to be spatially coincident with HESS J1356-645. The spectrum in the energy range of 1 GeV-1 TeV can be described by a power law with an index of $Γ=1.51\pm0.10$. The broadband spectrum of HESS J1356-645 can be reproduced by a leptonic model with a broken power-law electronic spectrum. In addition, we…
One of the most prominent features of the $γ$-ray sky is the emission from our own Galaxy. The Galactic plane has been observed by Fermi-LAT in GeV and H.E.S.S. in TeV light. Fermi has modeled the Galactic emission as the sum of a complex 'diffuse' emission model with the predominately point source catalogs of 1FHL and 2FGL, while H.E.S.S. has primarily detected extended TeV sources. At GeV energies, Galactic diffuse emission dominates the $γ$-ray Milky Way but, as sources have hard spectra, it is likely their emission dominates at TeV energies. Generally the spatial shape and fraction of sour…
Recently, the GeV radiation during the X-ray flare activity in GRB 100728A was detected by Feimi/LAT. Here we study the dynamics and emission properties of collision between two homogeneous shells based on the late internal shock model. The GeV photons can be produced from X-ray flare photons up-scattered by relativistic electrons accelerated by forward-reverse shocks, where involved radiative processes include synchrotron self-Compton and crossing inverse-Compton scattering. By analytical and numerical calculations, the observed spectral properties in GRB 100728A can be well explained.
The dominant component of the (100 MeV - 50 GeV) GRB emission detected by LAT starts with a delay relative to the prompt soft (sub-MeV) gamma-rays and lasts long after the soft component fades. This has lead to the intriguing suggestion that this high energy emission is generated via synchrotron emission of relativistic electrons accelerated by the external shock. Moreover, the limits on the MeV afterglow emission lead to the suggestion that, at least in bright GeV bursts the field is not amplified beyond compression in the shock. We show here that considerations of confinement (within the dec…
The origin of GeV emission from the early epoch of gamma-ray bursts (GRBs) is a widely discussed issue. The long gamma-ray burst GRB 170405A, observed by the Fermi Gamma-ray Space Telescope showed high-energy emission delayed by $\sim$20 s with respect to the X-ray emission, followed by temporally fading gamma-ray emission lasting for $\sim$1,000 s, as commonly observed in high-energy GRBs. In addition, a high-energy spectral cutoff at $\sim$50 MeV was detected in the prompt emission phase. If this feature is caused by pair-production opacity, the bulk Lorentz factor of the GRB ejecta can be e…
We analyze 10 years of Fermi-LAT data towards the SS433/W50 region. With the latest source catalog and diffuse background models, the gamma-ray excess from SS433/W50 is detected with a significance of 6σ in the photon energy range of 500 MeV - 10 GeV. Our analysis indicates that an extended flat disk morphology is preferred over a point-source description, suggesting that the GeV emission region is much larger than that of the TeV emission detected by HAWC. The size of the GeV emission is instead consistent with the extent of the radio nebula W50, a supernova remnant being distorted by the jet…
It is well-known that the $γ$-ray emission in blazars originate in the relativistic jet pointed at the observers. However, it is not clear whether the exact location of the GeV emission is less than a pc from the central engine, such that it may receive sufficient amount of photons from the broad line region (BLR) or farther out at 1-100 pc range. The former assumption has been successfully used to model the spectral energy distribution of many blazars. However, simultaneous detection of TeV $γ$-rays along with GeV outbursts in some cases indicate that the emission region must be outside the B…
We try to infer the location of the GeV emission region for 21 flat spectrum radio quasars (FSRQs) with quasi-simultaneous spectral energy distributions (SEDs), in which the SEDs of 21 FSRQs are reproduced by the one-zone leptonic model including the synchrotron-self Compton (SSC) and external Compton (EC) processes. We suggest that the X-ray emission could be produced by the SSC process and the GeV emission could come from the EC process. The EC emission could originate from the inverse Compton (IC) scattering of photons from the broad line region (BLR) and accretion disk or dust torus by the…
HESS J1825-137 is one of the most powerful and luminous TeV gamma-ray pulsar wind nebulae (PWN). To the south of HESS J1825-137, Fermi-LAT observation revealed a new region of GeV gamma-ray emission with three apparent peaks (termed here, GeV-ABC). This study presents interstellar medium (ISM) data and spectral energy distribution (SED) modelling towards the GeV emission to understand the underlying particle acceleration. We considered several particle accelerator scenarios - the PWN associated with HESS J1825-137, the progenitor SNR also associated with HESS J1825-137, plus the gamma-ray bina…
We study GeV emission from gamma-ray binaries by assuming that the compact object is a young pulsar. We assume that the relativistic unshocked pulsar wind with Lorentz factor of 10^4~5 can produce the GeV emission by the inverse-Compton scattering process in the dense soft-photon field of the companion star. The travel distance of the unshocked pulsar wind that moves toward the observer depends on the orbital phase of the pulsar. We discuss that the orbital modulation of the GeV emission is a result of combination of the effects of the travel distance of the unshocked pulsar wind and of the an…
The binary system HESS J0632+057 was recently detected by {Fermi} to possess orbital modulated GeV emission. In this paper, we study the possibility that the compact companion of HESS J0632+057 is a pulsar. Under such a presumption, we focus on the high energy emission mechanism of this system, which is as follows. The pulsar companion travels through the circumstellar disc of the main sequence star twice in each orbit, when some of the matter is gravity-captured. The captured matter develops an accretion disc around the pulsar, and the soft photons from which are inverse Compton scattered by …
A cluster of galaxies is a huge system bounded by gravitation, and cosmic rays are thought to be confined in the system, thus it should contain much non-thermal components. Many theories predict significant gamma-ray emission that could be detectable by state-of-the-art gamma-ray telescopes. Some clusters have already been studied by using Fermi gamma-ray space telescope in the GeV band and Cherenkov telescopes in the TeV band, but most clusters are not studied in both energy bands. Here I present results on GeV gamma-ray emission from clusters of galaxies which have been given upper limits by…
Magnetic fields may play a dominant role in gamma-ray bursts, and recent observations by the Fermi satellite indicate that GeV radiation, when detected, arrives delayed by seconds from the onset of the MeV component. Motivated by this, we discuss a magnetically dominated jet model where both magnetic dissipation and nuclear collisions are important. We show that, for parameters typical of the observed bursts, such a model involving a realistic jet structure can reproduce the general features of the MeV and a separate GeV radiation component, including the time delay between the two. The model …
The observations of high energy $γ$-ray emission from the Galactic center (GC) by HESS, and recently by Fermi, suggest the cosmic ray acceleration in the GC and possibly around the supermassive black hole. In this work we propose a lepton-hadron hybrid model to explain simultaneously the GeV-TeV $γ$-ray emission. Both electrons and hadronic cosmic rays were accelerated during the past activity of the GC. Then these particles would diffuse outwards and interact with the interstellar gas and background radiation field. The collisions between hadronic cosmic rays with gas is responsible to the Te…
