GeV-TeV Gamma-ray Emission Research Articles

Time-dependent gamma-ray emission from quasi-spherical explosions: the case of novae

Abstract We consider a general model for a quasi-spherical explosion in which a part of the explosion energy is thermalized, forming an expanding photosphere around a compact object, and the second part of energy is taken by the expanding shell of material which forms a shock wave in the surrounding medium. Different types of particles (electrons, hadrons) can, in principle, be accelerated at the shock. They interact with the thermal radiation from the photosphere and also with the material at the shell. We determine the equilibrium spectra of particles in the shell as a function of time after explosion and calculate the time-dependent γ-ray spectra by taking into account the effects which are due to the anisotropy of the photo-spheric radiation field on the Inverse Compton (IC) process; we also include the absorption of IC γ-rays in the photosphere radiation. We conclude that, in principle, both leptonic and hadronic models can explain the GeV-TeV gamma-ray emission recently detected from the Nova RS Oph. However, the hadronic model, in comparison to the leptonic model, is more energetically demanding and requires much stronger magnetization of the nova shell.

Deciphering the Origin of the GeV–TeV Gamma-Ray Emission from SS 433

Abstract We investigate hadronic and leptonic scenarios for the GeV–TeV gamma-ray emission from jets of the microquasar SS 433. The emission region of the TeV photons coincides with the X-ray knots, where electrons are efficiently accelerated. On the other hand, the optical high-density filaments are also located close to the X-ray knots, which may support a hadronic scenario. We calculate multiwavelength photon spectra of the extended jet region by solving the transport equations for the electrons and protons. We find that both hadronic and leptonic models can account for the observational data, including the latest Fermi Large Area Telescope result. The hadronic scenarios predict higher-energy photons than the leptonic scenarios, and future observations such as with the Cherenkov Telescope Array, the Large High-Altitude Air Shower Observatory, and the Southern Wide-field Gamma-ray Observatory may distinguish between these scenarios and unravel the emission mechanism of GeV–TeV gamma rays. Based on our hadronic scenario, the analogy between microquasars and radio galaxies implies that the X-ray knot region of the radio-galaxy jets may accelerate heavy nuclei up to ultrahigh energies.

A Serendipitous Discovery of GeV Gamma-Ray Emission from Supernova 2004dj in a Survey of Nearby Star-forming Galaxies with Fermi-LAT

Abstract The interaction between a supernova ejecta and the circumstellar medium drives a strong shock wave that accelerates particles (i.e., electrons and protons). The radio and X-ray emission observed after the supernova explosion can be interpreted as synchrotron emission from accelerated electrons. The accelerated protons are expected to produce GeV–TeV gamma-ray emission via proton–proton collisions, but the flux is usually low since only a small fraction of the supernova kinetic energy is converted into the shock energy at the very early time. The low gamma-ray flux of the nearest supernova explosion, SN 1987A, agrees with this picture. Here we report a serendipitous discovery of a fading GeV gamma-ray source in spatial coincidence with one of the nearest and brightest supernova—SN 2004dj from our gamma-ray survey of nearby star-forming galaxies with Fermi-LAT. The total gamma-ray energy released by SN 2004dj is about 6 × 1047 erg. We interpret this gamma-ray emission arising from the supernova ejecta interacting with a surrounding high-density shell, which decelerates the ejecta and converts ∼1% of the SN kinetic energy to relativistic protons.

GeV–TeV γ-rays produced by electrons in the kpc-scale jet as a result of Comptonization of the inner jet emission

A few radio galaxies have been discovered to emit GeV-TeV gamma-ray emission in spite of relatively large viewing angles of their jets. We consider a scenario in which such large angle gamma-ray emission is preferentially produced by isotropically distributed relativistic electrons in the kpc scale jet. These electrons comptonize soft, non-thermal radiation from the inner jet which is beamed due to its relativistic motion. In terms of such a model, we interpret recently discovered TeV gamma-ray emission from the kpc scale jets in Cen~A. Interestingly, due to the geometry of the inverse Compton scattering process, intensity of the gamma-ray emission from the counter-jet is predicted to be on a higher level than the emission from the jet. This is true provided that both, a jet and a counter-jet, are twins, i.e. the physical processes in the jet and counter-jet result in the acceleration of particles in a similar way. However, in general, the jet and counter-jet can differ significantly. Therefore, their relative contribution to the gamma-ray emission from the jets in Cen~A can become more complicated. Investigation of the distribution of the TeV gamma-ray emission along jets of nearby radio galaxies with the next generation telescopes should provide constraints on the considered model and the content of the kpc-scale jets.

Double Neutron Star Mergers and Short Gamma-ray Bursts: Long-lasting High-energy Signatures and Remnant Dichotomy

Abstract The recent detection of gravitational waves and electromagnetic counterparts from the double neutron star merger event GW+EM170817 supports the standard paradigm of short gamma-ray bursts (SGRBs) and kilonovae/macronovae. It is important to reveal the nature of the compact remnant left after the merger, either a black hole or neutron star, and their physical link to the origin of the long-lasting emission observed in SGRBs. The diversity of the merger remnants may also lead to different kinds of transients that can be detected in future. Here we study the high-energy emission from the long-lasting central engine left after the coalescence, under certain assumptions. In particular, we consider the X-ray emission from a remnant disk and the nonthermal nebular emission from disk-driven outflows or pulsar winds. We demonstrate that late-time X-ray and high-frequency radio emission can provide useful constraints on properties of the hidden compact remnants and their connections to long-lasting SGRB emission, and we discuss the detectability of nearby merger events through late-time observations at ∼30–100 days after the coalescence. We also investigate the GeV–TeV gamma-ray emission that occurs in the presence of long-lasting central engines and show the importance of external inverse Compton radiation due to upscattering of X-ray photons by relativistic electrons in the jet. We also search for high-energy gamma rays from GW170817 in the Fermi-LAT data and report upper limits on such long-lasting emission. Finally, we consider the implications of GW+EM170817 and discuss the constraints placed by X-ray and high-frequency radio observations.

Exploring the connection between radio and GeV-TeV γ-ray emission in the 1FHL and 2FHL AGN samples

The Fermi Large Area Telescope (LAT) revealed that blazars, representing the most extreme radio-loud active galactic nuclei (AGN) population, dominate the census of the gamma-ray sky, and a significant correlation was found between radio and gamma-ray emission in the 0.1-100 GeV energy range. However, the possible connection between radio and very high energy (VHE, E>0.1 TeV) emission still remains elusive, owing to the lack of a homogeneous coverage of the VHE sky. The main goal of this work is to quantify and assess the significance of a possible connection between the radio emission on parsec scale measured by the very long baseline interferometry (VLBI) and GeV-TeV gamma-ray emission in blazars, which is a central issue for understanding the blazar physics and the emission processes. We investigate the radio VLBI and high energy gamma-ray emission by using two large and unbiased AGN samples extracted from the first and second Fermi-LAT catalogs of hard gamma-ray sources detected above 10 GeV (1FHL) and 50 GeV (2FHL). For comparison, we perform the same correlation analysis by using the 0.1-300 GeV gamma-ray energy flux provided by the third Fermi-LAT source catalog. We find that the correlation strength and significance depend on the gamma-ray energy range with a different behavior among the blazar sub-classes. Overall, the radio and gamma-ray emission above 10 GeV turns out to be uncorrelated for the full samples and for all of the blazar sub-classes with the exception of high synchrotron peaked (HSP) objects, which show a strong and significant correlation. On the contrary, when 0.1-300 GeV gamma-ray energies are considered, a strong and significant correlation is found for the full blazar sample as well as for all of the blazar sub-classes. We interpret and explain this correlation behavior within the framework of the blazar spectral energy distribution properties.

TeV gamma-ray emission initiated by the population or individual millisecond pulsars within globular clusters

Two energetic millisecond pulsars (MSPs) within globular clusters (GC), J1823-3021A in NGC 6624 and PSR B1821-24 in M28, have been recently discovered to emit pulsed GeV gamma-rays. These MSPs are expected to eject energetic leptons. Therefore, GCs have been proposed to produce GeV-TeV gamma-rays as a result of the comptonization process of the background radiation within a GC. We develop this general scenario by taking into account not only the diffusion process of leptons within a GC but also their advection with the wind from the GC. Moreover, we consider distribution of MSP within a GC and the effects related to the non-central location of the dominating, energetic MSP. Such more complete scenario is considered for the modelling of the GeV-TeV gamma-ray emission from the core collapsed GC M15 and also for GCs which contain recently discovered energetic MSPs within NGC 6624 and M28. The confrontation of the modelling of the gamma-ray emission with the observations with the present Cherenkov telescopes and the future Cherenkov Telescope Array (CTA) allows to constrain more reliably the efficiency of lepton production within the inner magnetosphere of the MSPs and re-accelerated in their vicinity. We discuss the expected limits on this parameter in the context of expectations from the pulsar models. we conclude that deep observations of GCs, even with the present sensitivity of Cherenkov telescopes (H.E.S.S., MAGIC, VERITAS), should start to constrain the models for the acceleration and radiation processes of leptons within the inner pulsar magnetosphere and its surrounding.

A hybrid model of GeV–TeV gamma ray emission from the Galactic center

The observations of high energy γ-ray emission from the Galactic center (GC) by HESS, and recently by Fermi, suggest cosmic ray acceleration in the GC and possibly around the supermassive black hole. In this work we propose a lepton–hadron hybrid model to simultaneously explain the GeV–TeV γ-ray emission. Both electrons and hadronic cosmic rays were accelerated during the previous activity of the GC. These particles would then diffuse outwards and interact with the interstellar gas and background radiation field. The collisions between hadronic cosmic rays with gas are responsible for the TeV γ-ray emission detected by HESS. With fast cooling in the strong radiation field, the electrons would cool down and radiate GeV photons through inverse Compton scattering off the soft background photons. This scenario provides a natural explanation for the observed GeV–TeV spectral shape of γ-rays.

OBSERVATION OF SUPERNOVA REMNANT IC 443 WITH THE FERMI LARGE AREA TELESCOPE

ABSTRACT We report observation of the supernova remnant (SNR) IC 443 (G189.1+3.0) with the Fermi Gamma-ray Space Telescope Large Area Telescope (LAT) in the energy band between 200 MeV and 50 GeV. IC 443 is a shell-type SNR with mixed morphology located off the outer Galactic plane where high-energy emission has been detected in the X-ray, GeV and TeV gamma-ray bands. Past observations suggest IC 443 has been interacting with surrounding interstellar matter. Proximity between dense shocked molecular clouds and GeV–TeV gamma-ray emission regions detected by EGRET, MAGIC, and VERITAS suggests an interpretation that cosmic-ray (CR) particles are accelerated by the SNR. With the high gamma-ray statistics and broad energy coverage provided by the LAT, we accurately characterize the gamma-ray emission produced by the CRs accelerated at IC 443. The emission region is extended in the energy band with θ68 = 0.°27 ± 0.°01(stat) ± 0.°03(sys) for an assumed two-dimensional Gaussian profile and overlaps almost completely with the extended source region of VERITAS. Its centroid is displaced significantly from the known pulsar wind nebula (PWN) which suggests the PWN is not the major contributor in the present energy band. The observed spectrum changes its power-law slope continuously and continues smoothly to the MAGIC and VERITAS data points. The combined gamma-ray spectrum (200 MeV <E< 2 TeV) is reproduced well by decays of neutral pions produced by a broken power-law proton spectrum with a break around 70 GeV.

Upper limits on GEV-TEV gamma-ray emission for follow-up observations with the magic telescope

During the first observation cycle of the MAGIC telescope, 9 successful GRB follow-up observations were performed. For 2 bursts, the observation started even while the prompt emission was still ongoing. In none of these observations a significant excess over background could be detected. We present a direct determination of the MAGIC sensitivity in GRB observation mode from the Crab Nebula and the obtained upper limits for the 9 observations. At energies around 100GeV, MAGIC is currently the fastest and most sensitive GRB detector in the world.