Search

Pulsations from the Crab pulsar have been detected by the MAGIC telescopes at energies up to 1.5 TeV, and the pulsed emission from the Vela pulsar was detected by H.E.S.S. reaching tens of TeV. These discoveries, along with the proposed additional emission due to inverse Compton scattering at TeV energies, lead us to consider suitable candidates for detection with current and future extensive air show (EAS) experiments at very-high-energy (VHE; 0.1 |$-$| 100 TeV) ranges. Leveraging energy spectrum data from pulsars as observed by Fermi and Imaging Atmospheric Cherenkov Telescopes (IACTs) and considering the sensitivities of both LHAASO and SWGO, this study evaluates their detectability and estimates the time required for their significant detection. Our results indicate that LHAASO could detect the Crab's pulsed signal within six years, while SWGO might detect Vela's signal within one year. Observations of the most energetic Fermi pulsars with EAS experiments will provide insight into the nature of VHE pulsar emissions, helping us to clarify the primary characteristics of VHE pulsars. [ABSTRACT FROM AUTHOR]

The γ-ray emission from stars is induced by the interaction of cosmic rays with stellar atmospheres and photon fields. This emission is expected to come in two components: a stellar disc emission, where γ-rays are mainly produced in atmospheric showers generated by hadronic cosmic rays, and an extended halo emission, where the high density of soft photons in the surroundings of stars create a suitable environment for γ-ray production via inverse Compton (IC) scattering by cosmic ray electrons. Besides the Sun, no other disc or halo from single stars has ever been detected in γ-rays. However, by assuming a cosmic ray spectrum similar to that observed on Earth, the predicted γ-ray emission of superluminous stars, e.g. Betelgeuse and Rigel, could be high enough to be detected by the Fermi Large Area Telescope (LAT) after its first decade of operations. In this work, we use 12 yr of Fermi -LAT observations along with IC models to study nine superluminous nearby stars, both individually and via stacking analysis. Our results show no significant γ-ray emission, but allow us to restrict the stellar γ-ray fluxes to be on average <3.3 × 10−11 ph cm−2 s−1 at a 3σ confidence level, which translates to an average local density of electrons in the surroundings of our targets to be less than twice of that observed for the Solar system. [ABSTRACT FROM AUTHOR]

Synchrotron radio emission from non-relativistic jets powered by massive protostars has been reported, indicating the presence of relativistic electrons and magnetic fields of strength ∼0.3–5 mG. We study diffusive shock acceleration and magnetic field amplification in protostellar jets with speeds between 300 and 1500 km s−1. We show that the magnetic field in the synchrotron emitter can be amplified by the non-resonant hybrid (Bell) instability excited by the cosmic ray streaming. By combining the synchrotron data with basic theory of Bell instability we estimate the magnetic field in the synchrotron emitter and the maximum energy of protons. Protons can achieve maximum energies in the range 0.04–0.65 TeV and emit γ rays in their interaction with matter fields. We predict detectable levels of γ rays in IRAS 16547−5247 and IRAS 16848−4603. The γ ray flux can be significantly enhanced by the gas mixing due to Rayleigh–Taylor instability. The detection of this radiation by the Fermi satellite in the GeV domain and the forthcoming Cherenkov Telescope Array at higher energies may open a new window to study the formation of massive stars, as well as diffusive acceleration and magnetic field amplification in shocks with velocities of about 1000 km s−1. [ABSTRACT FROM AUTHOR]

Deep observation of the High Energy Stereoscopic System (HESS) on the most extended pulsar wind nebula HESS J1825−137 reveals an enhanced energy-dependent morphology, providing useful information on the particle transport mechanism in the nebula. We find that the energy-dependent morphology is consistent with a diffusion-dominated transport of electrons/positrons. It provides an alternative possible interpretation for the unusually large spatial extent (i.e. |${\gtrsim} 100$|  pc) of the nebula, which could then be attributed to the diffusion of escaping electrons/positrons from a compact plerion. The influence of various model parameters on the energy-dependent extent of the nebula is studied in the diffusion-dominated scenario. We also show that the energy-dependent morphology of the nebula may also be used to study the spin-down history of the pulsar. [ABSTRACT FROM AUTHOR]

The next era of ground-based Cherenkov telescope development will see a great increase in both quantity and quality of γ-ray morphological data. This initiated the development of a spatiotemporal leptonic transport code to model pulsar wind nebulae. We present results from this code that predicts the evolution of the leptonic particle spectrum and radiation at different radii in a spherically symmetric source. We simultaneously fit the overall broad-band spectral energy distribution, the surface brightness profile, and the X-ray photon index versus radius for PWN 3C 58, PWN G21.5 − 0.9, and PWN G0.9 + 0.1. Such concurrent fitting of disparate data sets is non-trivial and we thus investigate the utility of different goodness-of-fit statistics, specifically the traditional χ2 test statistic and a newly developed scaled-flux-normalized test statistic to obtain best-fitting parameters. We find reasonable fits to the spatial and spectral data of all three sources, but note some remaining degeneracies that will have to be broken by future observations. [ABSTRACT FROM AUTHOR]

The Crab Nebula is an extreme particle accelerator that boosts the energy of electrons up to a few PeV (⁠|$10^{15} \ \rm eV$|⁠), close to the maximum energy allowed theoretically. The physical conditions in the acceleration site and the nature of the acceleration process itself remain highly uncertain. The key information about the highest-energy accelerated particles is contained in the synchrotron and inverse Compton (IC) channels of radiation at energies above 1 MeV and 100 TeV, respectively. A recent report of the detection of an ultra-high-energy gamma-ray signal from the Crab Nebula up to 300 TeV allows us to determine the energy distribution of the highest-energy electrons and to derive the magnetic field strength in the acceleration region, |$B\le 120\rm \, \mu G$|⁠ , in a parameter-free way. This estimate brings new constraints on the properties of non-thermal particle distributions and places important constraints on the magnetohydrodynamic models for the Crab Nebula, in particular on the feasible magnetization and anisotropy of the pulsar wind. The calculations of synchrotron and IC emission show that future observations with instruments that allow detection of the Crab Nebula above 300 TeV and above 1 MeV will clarify the conditions that allow acceleration of electrons beyond PeV energies in the Crab Nebula. In particular, we will be able to verify the hypothetical multicomponent composition of the electron energy distribution, and we will determine the magnetic field strength in the regions responsible for the acceleration of PeV electrons. [ABSTRACT FROM AUTHOR]

We report on results of a multiband monitoring campaign from radio to γ-rays of the high-redshift flat spectrum radio quasar S5 0836 + 710 during a high-activity period detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. Two major flares were detected, in 2015 August and November. In both episodes, the apparent isotropic γ-ray luminosity exceeds 1050 erg s−1, with a doubling time-scale of about 3 h. The high γ-ray activity may be related to a superluminal knot that emerged from the core in 2015 April at the peak of the radio activity and is moving downstream along the jet. The low variability observed in X-rays may indicate that X-ray emission is produced by the low-energy tail of the same electron population that produces the γ-ray emission. The analysis of full-polarization pc-scale radio observations suggests the presence of a limb-brightened polarization structure at about 1 mas from the core in which a rotation measure gradient with a sign change is observed transverse to the jet direction. These characteristics are consistent with a scenario in which Faraday rotation is produced by a sheath of thermal electrons with a toroidal magnetic field surrounding the emitting jet. [ABSTRACT FROM AUTHOR]

We present a multiwavelength study of the flat-spectrum radio quasar CTA 102 during 2013–2017. We use radio-to-optical data obtained by the Whole Earth Blazar Telescope, 15 GHz data from the Owens Valley Radio Observatory, 91 and 103 GHz data from the Atacama Large Millimeter Array, near-infrared data from the Rapid Eye Monitor telescope, as well as data from the Swift (optical-UV and X-rays) and Fermi (γ-rays) satellites to study flux and spectral variability and the correlation between flux changes at different wavelengths. Unprecedented γ-ray flaring activity was observed during 2016 November–2017 February, with four major outbursts. A peak flux of (2158 ± 63) × 10−8 ph cm−2 s−1, corresponding to a luminosity of (2.2 ± 0.1) × 1050 erg s−1, was reached on 2016 December 28. These four γ-ray outbursts have corresponding events in the near-infrared, optical, and UV bands, with the peaks observed at the same time. A general agreement between X-ray and γ-ray activity is found. The γ-ray flux variations show a general, strong correlation with the optical ones with no time lag between the two bands and a comparable variability amplitude. This γ-ray/optical relationship is in agreement with the geometrical model that has successfully explained the low-energy flux and spectral behaviour, suggesting that the long-term flux variations are mainly due to changes in the Doppler factor produced by variations of the viewing angle of the emitting regions. The difference in behaviour between radio and higher energy emission would be ascribed to different viewing angles of the jet regions producing their emission. [ABSTRACT FROM AUTHOR]

In this paper, we discuss an artificial intelligence based approach to constrain the redshift of blazars using combined γ–ray observations from the Fermi Large Area Telescope (LAT) and ground based atmospheric Cherenkov telescopes (ACTs) in GeV and sub TeV energy regimes respectively. The spectral measurements in GeV and TeV energy bands show a redshift dependent spectral break in the γ–ray spectra of blazars. We use this observational feature of blazars to constrain their redshift. The observed spectral information of blazars with known redshifts reported in the Fermi catalogs (3FGL and 1FHL) and TeV catalog are used to train an Artificial Neural Network (ANN) based algorithm. The training of the ANN methodology is optimized using Levenberg - Marquardt algorithm with γ–ray spectral indices and redshifts of 35 well observed blazars as input and output parameters respectively. After training, we use only observed spectral indices in GeV and sub TeV regimes for 10 blazars as inputs to predict their redshifts. The comparison of predicted redshifts by the ANN with the known redshift suggests that both the values are consistent within ∼ 18% uncertainty. The method proposed in the present work would be helpful in future for constraining or predicting the redshifts of the blazars using only observational γ–ray spectral informations obtained from the Fermi-LAT and current generation IACTs as well as from the next generation Cherenkov Telescope Array (CTA) with improved source statistics. [ABSTRACT FROM AUTHOR]

Observations from the radio to the gamma-ray wavelengths indicate that supernova remnant (SNR) shocks are sites of effective particle acceleration. It has been proposed that the presence of dense clumps in the environment where supernovae explode might have a strong impact on the shape of the hadronic gamma-ray spectrum. Here we present a detailed numerical study of the penetration of relativistic protons into clumps that are engulfed by a SNR shock, taking into account the magneto-hydrodynamical properties of the background plasma. We show that the spectrum of protons inside clumps is much harder than that in the diffuse inter-clump medium, and we discuss the implications for the formation of the spectrum of hadronic gamma-rays, which no longer reflects the acceleration spectrum of protons, resulting substantially modified inside the clumps owing to propagation effects. For the Galactic SNR RX J1713.7 – 3946, we show that a hadronic scenario including dense clumps inside the remnant shell is able to reproduce the broadband gamma-ray spectrum from GeV to TeV energies. Moreover, we argue that small clumps crossed by the shock could provide a natural explanation for the non-thermal X-ray variability observed in some hotspots of RX J1713.7 – 3946. Finally, we discuss the detectability of gamma-ray emission from clumps with the upcoming Cherenkov Telescope Array, and the possible detection of the clumps themselves through molecular lines. [ABSTRACT FROM AUTHOR]

Fast radio bursts (FRBs) are bright flashes observed typically at GHz frequencies with millisecond duration, whose origin is likely extragalactic. Their nature remains mysterious, motivating searches for counterparts at other wavelengths. FRB 121102 is so far the only source known to repeatedly emit FRBs and is associated with a host galaxy at redshift |$z$| ≃ 0.193. We conducted simultaneous observations of FRB 121102 with the Arecibo and MAGIC telescopes during several epochs in 2016–2017. This allowed searches for millisecond time-scale burst emission in very-high-energy (VHE) gamma-rays as well as the optical band. While a total of five FRBs were detected during these observations, no VHE emission was detected, neither of a persistent nature nor burst-like associated with the FRBs. The average integral flux upper limits above 100 GeV at 95 per cent confidence level are 6.6 × 10−12photons cm−2s−1(corresponding to luminosity L VHE≲ 1045erg s−1) over the entire observation period, and 1.2 × 10−7photons cm−2s−1(L VHE≲ 1049erg s−1) over the total duration of the five FRBs. We constrain the optical U -band flux to be below 8.6 mJy at 5σ level for 1-ms intervals around the FRB arrival times. A bright burst with U -band flux 29 mJy and duration ∼12 ms was detected 4.3 s before the arrival of one FRB. However, the probability of spuriously detecting such a signal within the sampled time space is 1.5 per cent (2.2, post-trial), i.e. consistent with the expected background. We discuss the implications of the obtained upper limits for constraining FRB models. [ABSTRACT FROM AUTHOR]

The bulk Lorentz factor (Γ) of the outflow is an essential parameter for understanding the physics of gamma-ray burst (GRB). Informations about the Lorentz factors of some individual GRBs have been obtained from the spectral features of the high-energy gamma-ray emissions (>100 MeV), assuming that the spectral breaks or cutoffs are due to the pair-production attenuation (i.e. γγ → e+e−). In this paper, we attempt to interpret the dependence of the Large Area Telescope (LAT) detection rate of GRBs on the number of high-energy gamma-rays, taking into account the attenuation effect. We first simulate a long-GRB sample with Monte Carlo method using the luminosity function, rate distribution with redshift, and properties of the GRB spectrum. To characterize the distribution of the Lorentz factors, we assume that the Lorentz factors follow the relation $\Gamma =\Gamma _0E_{\rm iso, 52}^{k}$ , where Eiso,52 is the isotropic photon energy in unit of 1052 erg. After taking into account the attenuation effect related with the above Lorentz factor distribution, we are able to reproduce the LAT-detected rate of GRBs as the function of the number of gamma-rays for suitable choice of the values of Γ0 and k. The result suggests that the distribution of the bulk Lorentz factor for the majority of GRBs is in the range of 50−250. [ABSTRACT FROM AUTHOR]

We present results from a leptonic emission code that models the spectral energy distribution of a pulsar wind nebula by solving a Fokker–Planck-type transport equation and calculating inverse Compton and synchrotron emissivities. We have created this time-dependent, multizone model to investigate changes in the particle spectrum as they traverse the pulsar wind nebula, by considering a time and spatially dependent B-field, spatially dependent bulk particle speed implying convection and adiabatic losses, diffusion, as well as radiative losses. Our code predicts the radiation spectrum at different positions in the nebula, yielding the surface brightness versus radius and the nebular size as function of energy. We compare our new model against more basic models using the observed spectrum of pulsar wind nebula G0.9+0.1, incorporating data from H.E.S.S. as well as radio and X-ray experiments. We show that simultaneously fitting the spectral energy distribution and the energy-dependent source size leads to more stringent constraints on several model parameters. [ABSTRACT FROM AUTHOR]

We report the detection of the pulsed signal of the radio-quiet magnetar-like pulsar PSR J1846-0258 in the high-energy γ -ray data of the Fermi Large Area Telescope (Fermi LAT). We produced phase-coherent timing models exploiting RXTE PCA and Swift XRT monitoring data for the post- (magnetar-like) outburst period from 2007 August 28 to 2016 September 4, with independent verification using INTEGRAL ISGRI and Fermi GBM data. Phase-folding barycentric arrival times of selected Fermi LAT events from PSR J1846-0258 resulted in a 4.2σ detection (30-100 MeV) of a broad pulse consistent in shape and aligned in phase with the profiles that we measured with Swift XRT (2.5-10 keV), INTEGRAL ISGRI (20-150 keV), and Fermi GBM (20-300 keV). The pulsed flux (30-100 MeV) is (3.91 ± 0.97) × 10-9 photons cm-2 s-1 MeV-1. Declining significances of the INTEGRAL ISGRI 20-150 keV pulse profiles suggest fading of the pulsed hard X-ray emission during the post-outburst epochs. We revisited with greatly improved statistics the timing and spectral characteristics of PSR B1509-58 as measured with the Fermi LAT. The broadband pulsed emission spectra (from 2 keV up to GeV energies) of PSR J1846-0258 and PSR B1509-58 can be accurately described with similarly curved shapes, with maximum luminosities at 3.5 ± 1.1 MeV (PSR J1846-0258) and 2.23 ± 0.11 MeV (PSR B1509-58). We discuss possible explanations for observational differences between Fermi LAT detected pulsars that reach maximum luminosities at GeV energies, like the second magnetar-like pulsar PSR J1119-6127, and pulsars with maximum luminosities at MeV energies, which might be due to geometric differences rather than exotic physics in high-B fields. [ABSTRACT FROM AUTHOR]

A systematic search for cyclical sources of γ -ray emission on the period range from days to years in the Fermi-LAT sky is performed. Looking for cyclical emission, the sky is binned into equal-area pixels and the generalized Lomb–Scargle periodogram is computed for each of these pixels. The search on the period range between 2.5 and 30 d in the Galactic plane confirms periodicities of three binaries, LSI +61°303, LS 5039 and 1FGL J1018.6-5856. The all-sky search on the period range between 30 d and 2.5 yr confirm quasi-periodicities of three blazars, PG 1553+113, PKS 2155-304 and BL Lacertae. Evidence for quasi-periodic behaviours of four blazars, 4C +01.28, S5 0716+71, PKS 0805-07 and PKS 2052-47, is presented. Three of these blazars, 4C+01.28, PKS 0805-07 and PKS 2052-47, are located at high redshifts. These three sources are potential candidates to binary systems of supermassive black holes, provided that major galaxy mergers are more frequent and that galaxies are more gas-rich at high redshifts. [ABSTRACT FROM AUTHOR]

B1957+20 is a millisecond pulsar located in a black-widow-type compact binary system with a low-mass stellar companion. The interaction of the pulsar wind with the companion star wind and/or the interstellar plasma is expected to create plausible conditions for acceleration of electrons to TeV energies and subsequent production of very high-energy γ -rays in the inverse Compton process. We performed extensive observations with the Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) telescopes of B1957+20. We interpret results in the framework of a few different models, namely emission from the vicinity of the millisecond pulsar, the interaction of the pulsar and stellar companion wind region or bow shock nebula. No significant steady very high-energy γ -ray emission was found. We derived a 95 per cent confidence level upper limit of 3.0 × 10-12 cm-2 s-1 on the average γ -ray emission from the binary system above 200 GeV. The upper limits obtained with the MAGIC constrain, for the first time, different models of the high-energy emission in B1957+20. In particular, in the inner mixed wind nebula model with mono-energetic injection of electrons, the acceleration efficiency of electrons is constrained to be below ~2-10 per cent of the pulsar spin-down power. For the pulsar emission, the obtained upper limits for each emission peak are well above the exponential cut-off fits to the Fermi-LAT data, extrapolated to energies above 50 GeV. The MAGIC upper limits can rule out a simple power-law tail extension through the sub-TeV energy range for the main peak seen at radio frequencies. [ABSTRACT FROM AUTHOR]

It is largely recognized that gamma-ray burst (GRB) jets involve ultrarelativistic motion. However, the value of the Lorentz factor Γ0 is still not clear and only lower limits are known for most bursts. We suggest here a new method to obtain upper limits on Γ0. The early high-energy synchrotron afterglow flux depends strongly on Γ0. Upper limits on GeV emission therefore provide upper limits on Γ0. Applying this method to 190 Fermi GRBs which have not been detected by the Fermi-LAT, we place upper limits on the high-energy afterglow flux, and in turn on Γ0. For bursts at a typical redshift z = 2, we find values of the order of 200 (and above) for a homogeneous density medium, and in the range 100-400 for a wind-like medium. These upper limits are consistent with (and are very close to) lower limits and direct estimates inferred using other methods, suggesting that the typical Lorentz factors of GRB jets are of the order of a few hundred. [ABSTRACT FROM AUTHOR]

The Crab Pulsar has been recently detected at very high energies (VHE) with its pulsed VHE emission reaching up to 1.5 TeV. The VHE peaks appear synchronized with the peaks at GeV energies and show VHE spectra following hard power-law functions. These new findings have been interpreted as evidence for a γ-ray production that happens very close to the light cylinder. Motivated by these experimental results, we consider the efficiency of magnetocentrifugal particle acceleration in the magnetosphere of the Crab Pulsar, reexamining and extending results obtained in a previous work. It is shown that efficient magnetocentrifugal acceleration close to the light cylinder could provide the required electron Lorentz factors of 5 × 106 and that the resulting inverse Compton (IC) scattering off thermal photons might explain the enigmatic TeV emission of the pulsar. We estimate the corresponding VHE luminosity and provide a derivation of its spectral characteristics that appear remarkably close to the observational results to encourage further studies. [ABSTRACT FROM AUTHOR]

The energy dissipation mechanism within gamma-ray burst (GRB) outflows, driving their extremely luminous prompt γ-ray emission is still uncertain. The leading candidates are internal shocks and magnetic reconnection. While the emission from internal shocks has been extensively studied, that from reconnection still has few quantitative predictions. We study the expected prompt-GRB emission from magnetic reconnection and compare its temporal and spectral properties to observations. The main difference from internal shocks is that for reconnection one expects relativistic bulk motions with Lorentz factors Γ′≳ a few in the jet's bulk frame. We consider such motions of the emitting material in two antiparallel directions (e.g. of the reconnecting magnetic-field lines) within an ultrarelativistic (with Γ ≫ 1) thin spherical reconnection layer. The emission's relativistic beaming in the jet's frame greatly affects the light curves. For emission at radii R0 < R < R0 + ΔR (with Γ = const), the observed pulse width is ΔT ∼ (R0/2cΓ2) max (1/Γ′, ΔR/R0), i.e. up to ∼Γ′ times shorter than for isotropic emission in the jet's frame. We consider two possible magnetic reconnection modes: a quasi-steady state with continuous plasma flow into and out of the reconnection layer, and sporadic reconnection in relativistic turbulence that produces relativistic plasmoids. Both of these modes can account for many observed prompt-GRB properties: variability, pulse asymmetry, the very rapid declines at their end and pulse evolutions that are either hard to soft (for Γ′ ≲ 2) or intensity tracking (for Γ′ > 2). However, the relativistic turbulence mode is more likely to be relevant for the prompt sub-MeV emission and can naturally account also for the peak luminosity – peak frequency correlation. [ABSTRACT FROM AUTHOR]

The TeV gamma-ray point source HESS J1832-093 remains unidentified despite extensive multiwavelength studies. The gamma-ray emission could originate in a very compact pulsar wind nebula or an X-ray binary system composed of the X-ray source XMMU J183245-0921539, and a companion star (2MASS J18324516-0921545). To unveil the nature of XMMU J183245-0921539 and its relation to HESS J1832-093, we performed deeper follow-up observations in X-rays with Chandra and Swift to improve source localization and to investigate time variability. We observed an increase of the X-ray flux by a factor of ~6 in the Chandra data compared to previous observations. The source is point-like for Chandra and its updated position is only 0.3 arcsec offset from 2MASS J18324516-0921545, confirming the association with this infrared source. Subsequent Swift target of opportunity observations resulted in a lower flux, again compatible with the one previously measured with XMM-Newton, indicating a variability time-scale of the order of two months or shorter. The now-established association of XMMU J183245-0921539 and 2MASS J18324516-0921545, and the observed variability in X-rays are strong evidence for binary nature of HESS J1832-093. Furthermore, observations to characterize the optical counterpart as well as to search for orbital periodicity are needed to confirm this scenario. [ABSTRACT FROM AUTHOR]

We studied time variability and spectral evolution of the Galactic black hole transient Swift J174510.8-262411 during the first phase of its outburst. INTEGRAL and Swift observations collected from 2012 September 16 until October 30 have been used. The total squared fractional rms values did not drop below 5 per cent and quasi-periodic oscillations (QPOs), when present, were type-C, indicating that the source never made the transition to the soft-intermediate state. Even though the source was very bright (up to 1 Crab in hard X-rays), it showed a so called failed outburst as it never reached the soft state. XRT and IBIS broad-band spectra, well represented by a hybrid thermal/non-thermal Comptonization model, showed physical parameters characteristic of the hard and intermediate states. In particular, the derived temperature of the geometrically thin disc blackbody was about 0.6 keV at maximum. We found a clear decline of the optical depth of the corona electrons (close to values of 0.1), as well as of the total compactness ratio lh/ls. The hard-to-hard/intermediate state spectral transition is mainly driven by the increase in the soft photon flux in the corona, rather than small variations of the electron heating. This, associated with the increasing of the disc temperature, is consistent with a disc moving towards the compact object scenario, i.e. the truncated-disc model. Moreover, this scenario is consistent with the decreasing fractional squared rms and increasing of the noise and QPO frequency. In our final group of observations, we found that the contribution from the non-thermal Comptonization to the total power supplied to the plasma is 0.59+0.02 -0.05 and that the thermal electrons cool to kTe < 26 keV. [ABSTRACT FROM AUTHOR]

The inverse Compton catastrophe is defined as a dramatic rise in the luminosity of inverse Compton scattered photons. It is described by a non-linear loop of radiative processes that sets in for high values of the electron compactness and is responsible for the efficient transfer of energy from electrons to photons, predominantly through inverse Compton scatterings. We search for the conditions that drive a magnetized non-thermal source to the inverse Compton catastrophe regime and study its multiwavelength (MW) photon spectrum. We develop a generic analytical framework and use numerical calculations as a backup to the analytical predictions. We find that the escaping radiation from a source in the Compton catastrophe regime bears some unique features. The MW photon spectrum is a broken power law with a break at ~mec² due to the onset of the Klein Nishina suppression. The spectral index below the break energy depends on the electron and magnetic compactnesses logarithmically, while it is independent of the electron power-law index (s). The maximum radiating power emerges typically in the γ-ray regime, at energies ~mec² (~γmaxmec²) for s > 2 (s ≲ 2), where γmax is the maximum Lorentz factor of the injected electron distribution. We apply the principles of the inverse Compton catastrophe to blazars and γ-ray bursts using the analytical framework we developed, and show how these can be used to impose robust constraints on the source parameters. [ABSTRACT FROM AUTHOR]

By statistically analysing a large sample that includes Fermi-detected blazars (FBs) and non-Fermi-detected blazars (NFBs), we find that there are significant differences between FBs and NFBs in redshift, black hole mass, jet kinetic power from cavity power, broad-line luminosity and the ratio of core luminosity to absolute V-band magnitude (Rv), but not in the ratio of the radio core to extended flux (Rc) and the Eddington ratio. Compared with NFBs, FBs have larger mean jet power, Rc and Rv, while having smaller mean redshift, black hole mass and broad-line luminosity. These results support the fact that the beaming effect is the main reason for differences between FBs and NFBs, and that FBs are likely to have a more powerful jet. For both FBs and NFBs, there are significant correlations between the jet power and the accretion rate (traced by the broad-emission line luminosity), and between the jet power and the black hole mass. For FBs, the black hole mass does not have a significant influence on jet power, while for NFBs, both the accretion rate and black hole mass contribute to the jet power. Our results support the ‘blazar sequence’ and show that the synchrotron peak frequency (νpeak) is associated with the accretion rate but not with the black hole mass. [ABSTRACT FROM AUTHOR]

We consider the conditions within a Poynting-flux-dominated gamma-ray burst (GRB) emission region. Because of the enormous magnetic energy density, relativistic electrons will cool in such a region extremely rapidly via synchrotron. As there is no known mechanism that can compete in these magnetic environments with synchrotron it must be the source of the prompt sub-MeV emission. This sets strong limits on the size and Lorentz factor of the outflow. Furthermore, synchrotron cooling is too efficient. It overproduces optical and X-ray as compared with the observations. This overproduction of low-energy emission can be avoided if the electrons are re-accelerated many times (≳ 5 × 104) during each pulse (or are continuously heated) or if they escape the emitting region before cooling down. We explore the limitations of both models practically ruling out the later and demonstrating that the former requires two different acceleration mechanisms as well as an extremely large magnetic energy to Baryonic energy ratio. To be viable, any GRB model based on an emission region that is Poynting flux dominated must demonstrate how these conditions are met. We conclude that if GRB jets are launched magnetically dominated they must dissipate somehow most of their magnetic energy before they reach the emission region. [ABSTRACT FROM AUTHOR]

Globular clusters (GCs) contain huge number of low-mass stars and also large number of millisecond pulsars (MSPs). Due to the number of stars, the stellar and MSP winds mix efficiently within the GC. Such mixture of winds leaves GC and interacts with the galactic medium creating a bow shock nebula around GC. The bow shock nebula is filled with relativistic leptons accelerated in the pulsar magnetospheres and/or wind regions. We argue that nebulae around GCs, immersed in relatively dense medium close to the galactic plane, should have complicated morphology due to interaction with the surrounding gas. Therefore, TeV γ-ray sources, related to these nebulae, are expected to be misaligned in respect to GC cores, as observed in the case of GC Ter 5. On the other hand, GCs in low-density medium, i.e. far away from the galactic disc, should produce bow shocks at large distances from the GC cores. The TeV γ-ray sources around such nebulae are expected to be almost spherical and centred on the GC cores. We perform numerical calculations of the TeV γ-ray emission produced by leptons escaping from the GC Ter 5. It is shown that TeV γ-ray source related to Ter 5 should be misaligned in respect to the core of GC as observed by the HESS Collaboration. [ABSTRACT FROM AUTHOR]

High redshift blazars are among the most powerful objects in the Universe. Although they represent a significant fraction of the extragalactic hard X-ray sky, they are not commonly detected in γ-rays. High redshift ( z > 2 ) objects represent < 10 per cent of the active galactic nuclei (AGN) population observed by Fermi so far, and γ-ray flaring activity from these sources is even more uncommon. The characterization of the radio-to-γ-ray properties of high redshift blazars represents a powerful tool for the study of the energetics of such extreme objects and the Extragalactic Background Light. This contribution will present results of multi-band campaigns, from radio to γ-rays, on PKS 0836+710, PKS 2149−306, and TXS 0536+145. The latter is the highest redshift detection of a flaring γ-ray blazar so far. At the peaks of their respective flares these sources reached an apparent isotropic gamma-ray luminosity of about 10 50 erg·s − 1 , which is comparable with the luminosity observed from the most powerful blazars. The physical properties derived from the multi-wavelength observations of these sources are then compared with those shown by the high redshift population.

This paper discusses the spectral energy distribution (SED) of all blazars with redshift detected by the Fermi satellite and listed in the 3LAC catalog. The so called “blazar sequence” from the phenomenological point of view will be updated, with no theory or modelling. It will be shown that: (i) pure data show that jet and accretion power are related; (ii) the updated blazar sequence maintains the properties of the old version, albeit with a less pronounced dominance of the γ-ray emission; (iii) at low bolometric luminosities, two different types of objects have the same high energy power: low black hole mass flat spectrum radio quasars and high mass BL Lacs. Therefore, at low luminosities, there is a very large dispersion of SED shapes; (iv) in low power BL Lacs, the contribution of the host galaxy is important. Remarkably, the luminosity distribution of the host galaxies of BL Lacs are spread in a very narrow range; (v) a simple sum of two smoothly joining power laws can describe the blazar SEDs very well.

We present our latest results on the connection between accretion rate and relativistic jet power in active galactic nuclei (AGN), by using a large sample which includes mostly blazars, but contains also some radio galaxies. The jet power can be traced by γ-ray luminosity in the case of blazars, and radio luminosity for both classes. The accretion-disc luminosity is instead traced by the broad emission lines. Among blazars, we find a correlation between broad line emission and the γ-ray or radio luminosities, suggesting a direct tight connection between jet power and accretion rate. We confirm that the observational differences between blazar subclasses reflect differences in the accretion regime, but with blazars only we cannot properly access the low-accretion regime. By introducing radio galaxies, we succeed in observing the fingerprint of the transition between radiatively efficient and inefficient accretion discs in the jetted AGN family. The transition occurs at the standard critical value Ld/LEdd ∼ 10−2 and it appears smooth. Below this value, the ionizing luminosity emitted by the accretion structure drops significantly. [ABSTRACT FROM PUBLISHER]

We report results of a multiband monitoring campaign of the flat spectrum radio quasar TXS 0536+145 at redshift 2.69. This source was detected during a very high γ-ray activity state in 2012 March by the Large Area Telescope on board Fermi, becoming the γ-ray flaring blazar at the highest redshift detected so far. At the peak of the flare the source reached an apparent isotropic γ-ray luminosity of 6.6 × 1049 erg s−1 which is comparable to the values achieved by the most luminous blazars. This activity triggered radio-to-X-rays monitoring observations by Swift, Very Long Baseline Array, European VLBI Network, and Medicina single-dish telescope. Significant variability was observed from radio to X-rays supporting the identification of the γ-ray source with TXS 0536+145. Both the radio and γ-ray light curves show a similar behaviour, with the γ-rays leading the radio variability with a time lag of about 4-6 months. The luminosity increase is associated with a flattening of the radio spectrum. No new superluminal component associated with the flare was detected in high-resolution parsec-scale radio images. During the flare the γ-ray spectrum seems to deviate from a power law, showing a curvature that was not present during the average activity state. The γ-ray properties of TXS 0536+145 are consistent with those shown by the high-redshift γ-ray blazar population. [ABSTRACT FROM AUTHOR]

The physical origin of the >0.1 GeV emission detected from gamma-ray bursts (GRBs) by the Fermi satellite has not yet been completely understood. In this work, we consider the GeV light curves of 10 GRBs with measured redshift detected by the Fermi Large Area Telescope (LAT). These light curves are characterized by a long-lived (≳102 seconds) emission, whose luminosity decays in time as a power law. While the decay rate is similar for all GRBs (i.e. LLAT ∝ t−1.2), the normalization spans about two orders of magnitude in luminosity. However, after re-normalizing the luminosities to the prompt energetics Eprompt the light curves overlap. We consider the scenario in which the temporally extended LAT emission is dominated by synchrotron radiation from electrons accelerated at the forward external shock. According to this model, at high energies (i.e. above the typical synchrotron frequencies) a small dispersion of the Eprompt-normalized light curves is expected. The fact that the LAT temporally extended emission follows this behaviour reinforces its interpretation in terms of afterglow radiation from external shocks. Assuming this scenario, we argue that the parameters ϵe and ηγ (i.e. the fraction of shock-dissipated energy gained by the electrons, and the efficiency of the mechanism producing the prompt radiation, respectively) must be narrowly distributed. [ABSTRACT FROM PUBLISHER]

We discuss the γ-ray emission of the radiogalaxy NGC 1275 (the central galaxy of the Perseus Cluster), detected by Fermi-LAT (very-high energies) and MAGIC, in the framework of the ‘spine-layer’ scenario, in which the jet is assumed to be characterized by a velocity structure, with a fast spine surrounded by a slower layer. The existence of such a structure in the parsec scale jet of NGC 1275 has been recently proved through VLBI observations. We discuss the constraints that the observed spectral energy distribution imposes to the parameters and we present three alternative models, corresponding to three different choices of the angles between the jet and the line of sight (θv = 6°, 18° and 25°). While for the case with θv = 6° we obtain an excellent fit, we consider this solution unlikely, since such small angles seem to be excluded by radio observations of the large-scale jet. For θv = 25° the required large intrinsic luminosity of the soft (IR-optical) component of the spine determines a large optical depth for γ-rays through the pair production scattering γγ → e+e−, implying a narrow cut-off at ∼50 GeV. We conclude that intermediate angles are required. In this case the low frequency and the high-energy emissions are produced by two separate regions and, in principle, a full variety of correlations is expected. The correlation observed between the optical and the γ-ray flux, close to linearity, is likely linked to variations of the emissivity of the spine. [ABSTRACT FROM PUBLISHER]

We have analysed a large sample of clean blazars detected by Fermi Large Area Telescope (LAT). Using the literature and calculation, we obtained intrinsic γ-ray luminosity excluding beaming effect, black hole mass, broad-line luminosity (used as a proxy for disc luminosity), jet kinetic power from ‘cavity’ power and bulk Lorentz factor for parsec-scale radio emission, and studied the distributions of these parameters and relations between them. Our main results are as follows. (i) After excluding beaming effect and redshift effect, intrinsic γ-ray luminosity with broad-line luminosity, black hole mass and Eddington ratio have significant correlations. Our results confirm the physical distinction between BL Lacs and FSRQs. (ii) The correlation between broad-line luminosity and jet power is significant which supports that jet power has a close link with accretion. Jet power depends on both the Eddington ratio and black hole mass. We also obtain logLBLR ∼ (0.98 ± 0.07)logPjet for all blazars, which is consistent with the theoretical predicted coefficient. These results support that jets are powered by energy extraction from both accretion and black hole spin (i.e. not by accretion only). (iii) For almost all BL Lacs, Pjet > Ldisc; for most of FSRQs, Pjet < Ldisc. The ‘jet-dominance’ (parametrized as $\frac{P_{\rm jet}}{L_{\rm disc}}$) is mainly controlled by the bolometric luminosity. Finally, the radiative efficiency of γ-ray and properties of TeV blazars detected by Fermi LAT were discussed. [ABSTRACT FROM AUTHOR]

The broad-line region (BLR) is an important component of blazars, especially for flat spectrum radio quasars. The soft photons arising from the BLR will substantially affect the transparency of the γ-ray photons produced in the relativistic jet. We study the effect of the geometrical structure of the BLR on the absorption of γ-rays. We find that the γ-ray optical depth strongly depends on the geometrical structure of the BLR. For a “flat” BLR geometry, γ-ray photons with specified energies could escape transparently even if their emission region is located inside the cavity of the BLR. [ABSTRACT FROM AUTHOR]

Context. HD 195592 is an O-type super-giant star, known as a well-established runaway. Recently, a Fermi γ-ray source (2FGL J2030.7+4417) with a position compatible with that of HD 195592 has been reported. Aims. Our goal is to explore the scenario where HD 195592 is the counterpart of the Fermi γ-ray source, modeling the non-thermal emission produced in the bowshock of the runaway star. Methods. We calculated the spectral energy distribution of the radiation produced in the bowshock of HD 195592 and we compared it with Fermi observations of the 2FGL J2030.7+4417. Results. We present relativistic particle losses and the resulting radiation of the bowshock of HD 195592 and show that the latter is compatible with the detected γ-ray emission. Conclusions. We conclude that the Fermi source 2FGL J2030.7+4417 might be produced, under some energetic assumptions, by inverse Compton up-scattering of photons from the heated dust in the bowshock of the runaway star. Therefore, HD 195592 might be the very first object detected belonging to the category of γ-ray emitting runaway massive stars, whose existence has been recently predicted. [ABSTRACT FROM AUTHOR]

ABSTRACT Intriguing sub-TeV tails in the pulsed γ-ray emission from the Crab pulsar have recently been discovered by the Major Atmospheric Gamma-Ray Imaging Cherenkov (MAGIC) telescope and Very Energetic Radiation Imaging Telescope Array System (VERITAS) collaborations. These were not clearly predicted by any pulsar model. At present, it is argued that this emission is produced by electrons in the inverse Compton process that occur either in the outer gap of the pulsar magnetosphere or in the pulsar wind region at some distance from the light cylinder. We analyse another scenario, which is consistent with the basic features of this enigmatic emission. It is proposed that this emission is caused by electrons accelerated very close to the light cylinder where the e± plasma cannot saturate the induced huge electric fields. Electrons reach energies sufficient for the production of hard γ-ray spectra in the curvature radiation process. Because of different curvature radii of the leading and trailing magnetic field lines, the γ-ray spectra from separate pulses should extend to different maximum energies. The scenario can also explain the lower-level γ-ray emission from the interpulse region (between P1 and P2) observed in the light curve of the Crab pulsar. Moreover, we argue that pulsars with parameters close to the Vela pulsar should also show pulsed emission, with the cut-off clearly at lower energies (∼50 GeV) than that observed for the Crab pulsar. However, such tail emission is not expected in pulsars with parameters close to the Geminga pulsar. The model also predicts the tail γ-ray emission extending up to ∼50 GeV from some millisecond pulsars with extreme parameters, such as PSR J0218+4243 and PSR J1823-3021A. [ABSTRACT FROM AUTHOR]

ABSTRACT The detections of X-ray emission from the kiloparsec-scale jets of blazars and radio galaxies could imply the existence of high-energy electrons in these extended jets, and these electrons could produce high-energy emission through the inverse Compton (IC) process. In this paper, we study the non-variable hard TeV emission from a blazar. The multiband emission consists of two components: (i) the traditional synchrotron self-Compton (SSC) emission from the inner jet; (ii) the emission produced via SSC and IC scattering of cosmic microwave background (CMB) photons (IC/CMB) and extragalactic background light (EBL) photons by relativistic electrons in the extended jet under the stochastic acceleration scenario. Such a model is applied to 1ES 1101-232. The results indicate the following. (i) The non-variable hard TeV emission of 1ES 1101-232, which is dominated by IC/CMB emission from the extended jet, can be reproduced well by using three characteristic values of the Doppler factor (δD = 5, 10 and 15) for the TeV-emitting region in the extended jet. (ii) In the cases of δD = 15 and 10, the physical parameters can achieve equipartition (or quasi-equipartition) between the relativistic electrons and the magnetic field. In contrast, the physical parameters largely deviate from equipartition for the case of δD = 5. Therefore, we conclude that the TeV emission region of 1ES 1101-232 in the extended jet should be moderately or highly beamed. [ABSTRACT FROM AUTHOR]

Context. Conceptually reconstructing the physical conditions in relativistic jets, given the observed electromagnetic spectrum, is a complex inverse problem. Aims. We aim to improve our understanding of the mechanisms operating in relativistic jets by modeling their broadband electromagnetic spectrum. Methods. We develop an inhomogeneous jet model where the injection of relativistic primary and secondary particles takes place in a spatially extended region. We calculate the contribution of all particles species to the jet emissivity by several radiative processes, and assess the effect of gamma-ray absorption in internal and external photon fields. A number of specific models with different parameters are computed to explore the possibilities of this scenario. Results. We obtain a variety of spectral shapes depending on the model parameters, some of them predicting significant gamma-ray emission. The observed broadband spectrum of the low-mass microquasar XTE J1118+480 can be satisfactorily reproduced by the model. Conclusions. Our results indicate that outbursts similar to those displayed in the past by XTE J1118+480 might be detected with present-day gamma-ray instruments. [ABSTRACT FROM AUTHOR]

We cross-correlate the Fermi 11-month survey (1FGL) catalogue with the 20-GHz Australia Telescope Compact Array (AT20G) radio survey catalogue composed of 5890 sources at declination . Among the 738 Fermi sources distributed in the southern sky, we find 230 highly probable candidate counterparts in the AT20G survey. Of these, 222 are already classified in the Fermi one-year Large Area Telescope (LAT) active galactic nucleus (AGN) catalogue (1LAC) as blazars [either flat spectrum radio quasars (FSRQs) or BL Lacertae objects (BL Lacs)], AGNs or sources of unknown class but with an associated counterpart, while eight are new associations. By studying the γ-ray and radio properties of these associations, we find a correlation between the γ-ray flux (above 100 MeV) and the 20-GHz flux density. This correlation is more than 3σ statistically significant, both for the population of BL Lacs and for FSRQs considered separately. We also find that the radio counterparts associated with the Fermi sources have, on average, flat radio spectra between 5 and 20 GHz and that Fermiγ-ray sources are not preferentially associated with ‘ultra-inverted spectrum’ radio sources. For two of the eight new associations, we build the broad-band spectral energy distribution combining Fermi, Swift and radio observations. One of these two sources is identified with the high-redshift FSRQ Swift J1656.3–3302 and we classify the other source as a candidate new FSRQ. We also study the brightest radio source of the 46 associations without an optical classification and classify it as a new BL Lac candidate ‘twin’ of the prototypical BL Lac OJ 287 if its redshift is larger, . [ABSTRACT FROM AUTHOR]

The compact X-ray binary system Cyg X-3 has been recently discovered as a source of GeV γ-rays by the AGILE and the Fermi satellites. It shows emission features in the GeV γ-rays similar to other γ-ray binaries which were also observed in the TeV γ-rays (LS 5039 and LSI +61 303). The question appears whether Cyg X-3 can be also detected in the TeV γ-rays by the Cherenkov telescopes. Here we discuss this problem in detail based on the anisotropic inverse Compton (IC) pair cascade model successfully applied to TeV γ-ray binaries. We calculate the γ-ray light curves and γ-ray spectra expected from the cascade process occurring inside the Cyg X-3 binary system. It is found that the γ-ray light curves at GeV energies can be consistent with the γ-ray light curve observed by the Fermi for reasonable parameters of the orbit of the injection source of relativistic electrons. Moreover, we show that in such a model the sub-TeV γ-ray emission (above 100 GeV) is expected to be below sensitivities of the present Cherenkov telescopes assuming that electrons are accelerated in Cyg X-3 to TeV energies. The next stage Cherenkov telescopes (MAGIC II, HESS II) should have the energy threshold in the range 20–30 GeV, in order to have a chance to detect the signal from Cyg X-3. Otherwise, the positive detection of γ-rays at energies above a few tens of GeV requires a telescope with the sensitivity of of Crab units. We conclude that detection of sub-TeV γ-rays from Cyg X-3 by on-ground telescopes has to probably wait for the construction of the Cherenkov Telescope Array. [ABSTRACT FROM AUTHOR]