High-energy Gamma-ray Emission Research Articles

Features of development of sustained fluxes of high-energy gamma-ray emission at different stages of solar flares

We have studied properties of sustained gamma fluxes having quantum energies of >100 MeV at different stages of flares with 1-min temporal resolution (Fermi/LAT). 
 The most probable process of emergence of high-energy gamma-quanta during the impulsive phase of flares (6 events) has been confirmed. Acceleration of particles, produced by flare energy release (at dissipation of current sheet), occurs when they interact with a shock front of a coronal mass ejection (CME), which develops in the same active region at the same time. Nuclear interactions of accelerated protons (>500 MeV) with plasma ions lead further to the emergence of high-energy gamma-quanta. We have established that the interaction between a flare flux and a high-speed CME during the flare impulsive phase occurs within fairly limited periods — from 2 to 16 min. In the events considered, we have found a direct connection between maximum gamma flux F max (γ > 100 MeV) and CME velocity.
 High maximum values of gamma fluxes are typical of the flare impulsive phase: 3.5·10⁻⁴ cm⁻²s⁻¹ ≤ F max (γ > 100 MeV) ≤ 1.3·10⁻² cm⁻² s⁻¹. At the same time, the value F max (γ > 100 MeV) = 0.013 cm⁻²s⁻¹ was the highest for the events observed by Fermi/LAT from 2008 to 2017.
 During the development of CMEs moving with a supersonic speed, shock waves are formed which are the major power source of accelerated particles during the main phase of gradual flares. In some cases, however, the impact of shock waves on particle acceleration is the greatest in the short impulsive phase.
 To reveal parameters most effectively influencing the generation of high-energy gamma-ray emission, we have compared 17 flare events. The most significant parameter proved to be the time interval of joint action of flare process and CME shocks.
 We have established that during simultaneous development of flare process and CME attendant on the flare, the most efficient particle acceleration occurs which gives rise to maximum fluxes of high-energy gamma-quanta.

H.E.S.S. observations of the flaring gravitationally lensed galaxy PKS 1830–211

PKS 1830-211 is a known macrolensed quasar located at a redshift of z=2.5. Its high-energy gamma-ray emission has been detected with the Fermi-LAT instrument and evidence for lensing was obtained by several authors from its high-energy data. Observations of PKS 1830-211 were taken with the H.E.S.S. array of Imaging Atmospheric Cherenkov Telescopes in August 2014, following a flare alert by the Fermi- LAT collaboration. The H.E.S.S observations were aimed at detecting a gamma-ray flare delayed by 20-27 days from the alert flare, as expected from observations at other wavelengths. More than twelve hours of good quality data were taken with an analysis threshold of $\sim67$ GeV. The significance of a potential signal is computed as a function of the date as well as the average significance over the whole period. Data are compared to simultaneous observations by Fermi-LAT. No photon excess or significant signal is detected. An upper limit on PKS 1830-211 flux above 67 GeV is computed and compared to the extrapolation of the Fermi-LAT flare spectrum.

Gas phase SiO in the circumstellar environment of the recurrent nova T Coronae Borealis

We report the discovery of the diatomic molecule SiO in the gas phase in the environment of the recurrent nova T Coronae Borealis. While some of the SiO is photospheric, a substantial portion must arise in the wind from the red giant component of T CrB. A simple fit to the SiO feature, assuming local thermodynamic equilibrium, suggests a SiO column density of 2.8x10^17 /cm2 and temperature ~1000K; the SiO column density is similar to that present in the winds of field red giants. A search for SiO maser emission is encouraged both before and after the next anticipated eruption. We find that the 12C/13C ratio in the red giant is <9, with a best fit value of ~5, a factor ~18 times lower than the solar value of 89. We find no convincing evidence for the presence of dust in the environment of T CrB, which we attribute to the destructive effects on nucleation sites of hard X-ray emission. When the next eruption of T CrB occurs, the ejected material will shock the wind, producing X-ray and coronal line emission, as is the case for the recurrent nova RS Oph. T CrB is also a good candidate for very high energy gamma-ray emission, as first observed during the 2010 outburst of V407 Cyg. We include in the paper a wide variety of infrared spectroscopic and photometric data.

Constraints on very high energy gamma-ray emission from the Fermi bubbles with future ground-based experiments

The origin of sub-TeV gamma rays detected by the \textit{Fermi}-Large Area Telescope (LAT) from the \textit{Fermi} Bubbles (FBs) at the Galactic center is still uncertain. In a hadronic model, acceleration of protons and/or nuclei and their subsequent interactions with gas in the bubble volume can produce the observed gamma rays. Recently the High Altitude Water Cherenkov (HAWC) observatory reported an absence of gamma-ray excess from the Northern FB at $b\gtrsim 6^\circ$ Galactic latitude, which resulted in flux upper limits in the energy range of $1.2-126$ TeV. These upper limits are consistent with the gamma-ray spectrum measured by \textit{Fermi}-LAT at $|b|\ge 10^\circ$, where an exponential cutoff at energies $\gtrsim 100$ GeV is evident. However, the FB gamma-ray spectrum at $|b|\le 10^\circ$, without showing any sign of cutoff up to around 1 TeV in the latest results, remains unconstrained. The upcoming Cherenkov Telescope Array (CTA) will perform a Galactic center survey with unprecedented sensitivity in the energy between 20 GeV and 300 TeV. In this work, we perform both morphological and classic on/off analyses with planned CTA deep central and extended survey and estimate the sensitivity of CTA to the FB hadronic gamma-ray flux models that best fit the spectrum at $|b|\le 10^\circ$ and whose counterpart neutrino flux model best fits the optimistic neutrino spectrum from IceCube Neutrino Observatory. We also perform sensitivity analysis with a future ground-based Cherenkov detector the Large High Altitude Air Shower Observatory (LHAASO). We find that CTA will be able to discover or constrain the FB gamma-ray flux at $|b|\le 10^\circ$ in the $\approx$ 200 GeV -- 100 TeV range with planned observation strategy, while LHAASO may constrain emission in the $\approx 100$~GeV -- 100~TeV range if $\lesssim 10\%$ systematic uncertainties can be achieved.

Is Supernova Remnant Cassiopeia A a PeVatron?

Abstract Cassiopeia A, a well-observed young core-collapse supernova remnant (SNR), is considered to be one of the best candidates for studying very high-energy particle acceleration up to PeV via the diffusive shock mechanism. Recently, MAGIC observations revealed a γ-ray spectral cutoff at ∼ 3.5 TeV , suggesting that if the TeV γ-rays have a hadronic origin, SNR Cas A can only accelerate particles to tens of TeV. Here, we propose a two-zone emission model for regions associated with the forward (zone 1) and inward/reverse shocks (zone 2). Given the low density in zone 1, it dominates the high-frequency radio emission, soft X-ray rim via the synchrotron process, and TeV γ-ray via the inverse Comptonization. With a relatively softer particle distribution and a higher cutoff energy for electrons, emissions from zone 2 dominate the low-frequency radio, hard X-ray via the synchrotron process and GeV γ-ray via hadronic processes. There is no evidence for high-energy cutoffs in the proton distributions implying that Cas A can still be a PeVatron. Hadronic processes from zone 1 dominate very high-energy gamma-ray emission. Future observations in the hundreds of TeV range can test this model.

Fermi-LAT observations of extreme spectral variability in IC 310

Abstract We investigate the physical mechanisms of high-energy gamma-ray emission from the TeV-emitting misaligned active galactic nucleus IC 310. 8 yr of data from the Fermi Large Area Telescope (Fermi-LAT) between 100 MeV and 500 GeV are reduced and analysed to study the temporal and spectral characteristics of IC 310. Point spread function-partitioned instrument response functions are used to improve the resolvability of IC 310 from nearby NGC 1275. Systematic effects due to this choice of instrument response functions and the proximity of NGC 1275 are investigated. We find strong spectral variability and detect the hard flaring state of IC 310 along with a previously undiscovered soft state in quiescent periods, and the first detection with Fermi-LAT below 1 GeV. This represents a shift in peak Compton energy of more than 5 orders of magnitude. Possible interpretations are discussed, but we lack the instantaneous sensitivity with Fermi to probe the underlying physics.

Complex gamma-ray behavior of the radio galaxy M 87

Context. In recent years, non-blazar active galactic nuclei (AGN) such as radio galaxies have emerged as a highly instructive source class providing unique insights into high energy acceleration and radiation mechanisms. Aims. Here we aim to produce a detailed characterization of the high-energy (HE; &gt;100 MeV) gamma-ray emission from the prominent radio galaxy M 87. Methods. We analyzed approximately eight years of Fermi-LAT data and derived the spectral energy distribution between 100 MeV and 300 GeV. We extracted lightcurves and investigated the variability behavior for the entire energy range as well as below and above 10 GeV. Results. Our analysis provides (i) evidence for HE gamma-ray flux variability and (ii) indications for a possible excess over the standard power-law model above Eb ∼ 10 GeV, similar to the earlier indications in the case of Cen A. When viewed in HE–VHE context, this is most naturally explained by an additional component dominating the highest-energy part of the spectrum. Investigation of the γ-ray lightcurves suggests that the lower-energy (&lt;10 GeV) component is variable on timescales of (at least) a few months. The statistics of the high energy component (&gt;10 GeV) does not allow significant constraints on variability. We do, however, find indications for spectral changes with time that support variability of the putative additional component and seem to favor jet-related scenarios for its origin capable of accommodating month-type variability. Conclusions. The current findings suggest that both the high-energy (&gt;Eb) and the very high energy (VHE; &gt;100 GeV) emission in M 87 are compatible with originating from the same physical component. The variability behavior at VHE then allows further constraints on the location and the nature of the second component. In particular, these considerations suggest that the VHE emission during the quiescent state originates in a similar region as during the flare.

Galactic Cosmic Rays: The first detection of TeV gamma-rays from Red Dwarfs

The present point of view on the sources of cosmic rays in Galaxy considers explosions of supernovae as sources of these particles up to energies of 1017 eV. However, the experimental data obtained with Pamela, Fermi, AMS-02 spectrometers requires the existence of nearby sources of cosmic rays at distances less then 1 kpc from the solar system. These sources could explain such experimental data as the growth of the ratio of galactic positrons to electrons with increasing energy, the complex dependence of the exponent of the proton and alpha spectra from the energy of these particles, the appearance of an anomaly component in cosmic rays. We consider active dwarf stars as possible sources of galactic cosmic rays in the energy range up to 1014 eV. These stars produce powerful stellar flares. The generation of high-energy cosmic rays has to be accompanied by high-energy gamma-ray emission. Here we present the SHALON long-term observation data aimed at searching for gamma-ray emission above 800 GeV from active red dwarf stars. The data obtained during more than 10 years observations of the dwarf stars V962 Tau, V780 Tau, V388 Cas and V1589 Cyg were analyzed. The high-energy gamma-ray emission in the TeV energy range, mostly of the flaring type from the sources mentioned above, was detected. This result confirms that active dwarf stars are also the sources of high-energy galactic cosmic rays.

Detection of gamma-ray emission from the Coma cluster with Fermi Large Area Telescope and tentative evidence for an extended spatial structure

Many galaxy clusters have giant halos of non-thermal radio emission, indicating the presence of relativistic electrons in the clusters. Relativistic protons may also be accelerated by merger and/or accretion shocks in galaxy clusters. These cosmic-ray (CR) electrons and/or protons are expected to produce gamma-rays through inverse-Compton scatterings or inelastic $pp$ collisions respectively. Despite of intense efforts in searching for high-energy gamma-ray emission from galaxy clusters, conclusive evidence is still missing so far. Here we report the discovery of $\ge 200$ MeV gamma-ray emission from the Coma cluster direction with an unbinned likelihood analysis of the 9 years of {\it Fermi}-LAT Pass 8 data. The gamma-ray emission shows a spatial morphology roughly coincident with the giant radio halo, with an apparent excess at the southwest of the cluster. Using the test statistic analysis, we further find tentative evidence that the gamma-ray emission at the Coma center is spatially extended. The extended component has an integral energy flux of $\sim 2\times 10^{-12}{\rm \ erg\ cm^{-2}\ s^{-1}}$ in the energy range of 0.2 - 300 GeV and the spectrum is soft with a photon index of $\simeq-2.7$. Interpreting the gamma-ray emission as arising from CR proton interaction, we find that the volume-averaged value of the CR to thermal pressure ratio in the Coma cluster is about $\sim 2\%$. Our results show that galaxy clusters are likely a new type of GeV gamma-ray sources, and they are probably also giant reservoirs of CR protons.

Two Blobs in a Jet Model for the γ-Ray Emission in Radio Galaxies

Abstract In the unified scheme, FR I type radio galaxies are identified with the blazar type active galaxies for which jets are aligned at large angles to the line of sight. A few radio galaxies of this type have been discovered to emit GeV–TeV gamma-rays. We consider a scenario that naturally explains the very high energy gamma-ray emission at large angles to the jet axis. It is proposed that two emission regions are present in the jet at this same moment. The inner region (blob I) moves with the large Lorentz factor, producing radiation strongly collimated along the jet axis, as observed in BL Lac type blazars. On the other hand, the outer region (blob II), which moves with the mild Lorentz factor, contains isotropically distributed relativistic electrons in the blob reference frame. These electrons upscatter monodirectional soft radiation from blob I preferentially in the direction opposite to the jet motion. Therefore, gamma-rays, produced in blob II, can be emitted at relatively large angles to the jet axis in the observer’s reference frame. We analyze the basic emission features of such an external blob radiation model. The example modeling of the emission from the FR I type radio galaxy, NGC 1275, is presented.

Search for GeV Gamma-Ray Counterparts of Gravitational Wave Events by CALET

Abstract We present the results of searches for gamma-ray counterparts of the LIGO/Virgo gravitational wave events using CALorimetric Electron Telescope (CALET) observations. The main instrument of CALET, CALorimeter (CAL), observes gamma-rays from ∼1 GeV up to 10 TeV with a field of view (FOV) of nearly 2 sr. In addition, the CALET gamma-ray burst monitor views ∼3 sr and ∼2π sr of the sky in the 7 keV–1 MeV and the 40 keV–20 MeV bands, respectively, by using two different crystal scintillators. The CALET observations on the International Space Station started in 2015 October, and here we report analyses of events associated with the following gravitational wave events: GW151226, GW170104, GW170608, GW170814, and GW170817. Although only upper limits on gamma-ray emission are obtained, they correspond to a luminosity of 1049 ∼ 1053 erg s−1 in the GeV energy band depending on the distance and the assumed time duration of each event, which is approximately on the order of luminosity of typical short gamma-ray bursts. This implies that there will be a favorable opportunity to detect high-energy gamma-ray emission in further observations if additional gravitational wave events with favorable geometry will occur within our FOV. We also show the sensitivity of CALET for gamma-ray transient events, which is on the order of 10−7 erg cm−2 s−1 for an observation of 100 s in duration.

The broad-band properties of the intermediate synchrotron peaked BL Lac S2 0109+22 from radio to VHE gamma-rays

The MAGIC telescopes observed S2 0109+22 in 2015 July during its flaring activity in high energy gamma rays observed by Fermi-LAT. We analyse the MAGIC data to characterise the very high energy (VHE) gamma-ray emission of S2 0109+22, which belongs to the subclass of intermediate synchrotron peak (ISP) BL Lac objects. We study the multi-frequency emission in order to investigate the source classification. Finally, we compare the source long-term behaviour to other VHE gamma-ray emitting (TeV) blazars. We performed a temporal and spectral analysis of the data centred around the MAGIC interval of observation (MJD 57225-57231). Long-term radio and optical data have also been investigated using the discrete correlation function. The redshift of the source is estimated through optical host-galaxy imaging and also using the amount of VHE gamma-ray absorption. The quasi-simultaneous multi-frequency spectral energy distribution (SED) is modelled with the conventional one-zone synchrotron self-Compton (SSC) model. MAGIC observations resulted in the detection of the source at a significance level of $5.3\,\sigma$. The VHE gamma-ray emission of S2 0109+22 is variable on a daily time scale. VHE gamma-ray luminosity of the source is lower than the average of TeV BL Lacs. The optical polarization, and long-term optical/radio behaviour of the source are different from the general population of TeV blazars. All these findings agree with the classification of the source as an ISP BL Lac object. We estimate the source redshift as $z = 0.36 \pm 0.07$. The SSC parameters describing the SED are rather typical for blazars.

A comprehensive study of high-energy gamma-ray and radio emission from Cyg X-3

We study high-energy $\gamma$-rays observed from Cyg X-3 by the Fermi Large Area Telescope and the 15-GHz emission observed by the Ryle Telescope and the Arcminute Microkelvin Imager. We measure the $\gamma$-ray spectrum averaged over strong flares much more accurately than before, and find it well modelled by Compton scattering of stellar radiation by relativistic electrons with the power law index of $\simeq$3.5 and a low-energy cutoff at the Lorentz factor of $\sim\!10^3$. We find a weaker spectrum in the soft spectral state, but only upper limits in the hard and intermediate states. We measure strong orbital modulation during the flaring state, well modelled by anisotropic Compton scattering of blackbody photons from the donor by jet relativistic electrons. We discover a weaker orbital modulation of the 15 GHz radio emission, which is well modelled by free-free absorption by the stellar wind. We then study cross-correlations between radio, $\gamma$-ray and X-ray emissions. We find the cross-correlation between the radio and $\gamma$-ray emissions peaks at a lag less than 1 d, while we detect a distinct radio lag of $\sim$50 d with respect to the soft X-rays in the soft spectral state.

On the Anomalously Large Extension of the Pulsar Wind Nebula HESS J1825-137

Abstract The very high energy gamma-ray emission reported from a number of pulsar wind nebulae (PWNe) is naturally explained by the inverse Compton scattering of multi-TeV electrons. However, the physical dimensions of some gamma-ray-emitting PWNe significantly exceed the scales anticipated by the standard hydrodynamical paradigm of PWN formation. The most “disturbing” case in this regard is HESS J1825-137, which extends to distances of r ≈ 70 pc from the central pulsar PSR J1826−1334. If the gamma-ray emission is indeed produced inside the PWN, but not by electrons that escaped the nebula and diffuse in the interstellar medium (ISM), the formation of such an anomalously extended plerion could be realized, in a diluted environment with the hydrogen number density . In this paper, we explore an alternative scenario assuming that the pulsar responsible for the formation of the nebula initially had a very short rotation period. In this case, the sizes of both the PWN and the surrounding supernova remnant depend on the initial pulsar period, the braking index, and the ISM density. To check the feasibility of this scenario, we study the parameter space that would reproduce the size of HESS J1825-137. We show that this demand can be achieved if the braking index is small, , and the pulsar birth period is short, . This scenario can reproduce the wind termination position, which is expected at , only in a dense environment with . The requirement of the dense surrounding gas is supported by the presence of molecular clouds found in the source vicinity.

Gamma-ray emission from internal shocks in novae

Context. Gamma-ray emission at energies ≥100 MeV has been detected from nine novae using the Fermi Large Area Telescope (LAT), and can be explained by particle acceleration at shocks in these systems. Eight out of these nine objects are classical novae in which interaction of the ejecta with a tenuous circumbinary material is not expected to generate detectable gamma-ray emission. Aim. We examine whether particle acceleration at internal shocks can account for the gamma-ray emission from these novae. The shocks result from the interaction of a fast wind radiatively-driven by nuclear burning on the white dwarf with material ejected in the initial runaway stage of the nova outburst. Methods. We present a one-dimensional model for the dynamics of a forward and reverse shock system in a nova ejecta, and for the associated time-dependent particle acceleration and high-energy gamma-ray emission. Non-thermal proton and electron spectra are calculated by solving a time-dependent transport equation for particle injection, acceleration, losses, and escape from the shock region. The predicted emission is compared to LAT observations of V407 Cyg, V1324 Sco, V959 Mon, V339 Del, V1369 Cen, and V5668 Sgr. Results. The ≥100 MeV gamma-ray emission arises predominantly from particles accelerated up to ~100 GeV at the reverse shock and undergoing hadronic interactions in the dense cooling layer downstream of the shock. The emission rises within days after the onset of the wind, quickly reaches a maximum, and its subsequent decrease reflects mostly the time evolution of the wind properties. Comparison to gamma-ray data points to a typical scenario where an ejecta of mass 10−5–10−4 M⊙ expands in a homologous way with a maximum velocity of 1000–2000 km s−1, followed within a day by a wind with a velocity &lt;2000 km s−1 and a mass-loss rate of 10−4–10−3 M⊙ yr−1 declining over a time scale of a few days. Because of the large uncertainties in the measurements, many parameters of the problem are degenerate and/or poorly constrained except for the wind velocity, the relatively low values of which result in the majority of best-fit models having gamma-ray spectra with a high-energy turnover below ~10 GeV. Our typical model is able to account for the main features in the observations of the recent gamma-ray nova ASASSN-16ma. Conclusions. The internal shock model can account for the gamma-ray emission of the novae detected by Fermi LAT. Gamma-ray observations hold potential for probing the mechanism of mass ejection in novae, but should be combined to diagnostics of the thermal emission at lower energies to be more constraining.

The Three-dimensional Spatial Distribution of Interstellar Gas in the Milky Way: Implications for Cosmic Rays and High-energy Gamma-ray Emissions.

Direct measurements of cosmic ray (CR) species combined with observations of their associated γ-ray emissions can be used to constrain models of CR propagation, trace the structure of the Galaxy, and search for signatures of new physics. The spatial density distribution of interstellar gas is a vital element for all these studies. So far, models have employed the 2D cylindrically symmetric geometry, but their accuracy is well behind that of the available data. In this paper, 3D spatial density models for neutral and molecular hydrogen are constructed based on empirical model fitting to gas line-survey data. The developed density models incorporate spiral arms and account for the warping of the disk, and the increasing gas scale height with radial distance from the Galactic center. They are employed together with the GALPROP CR propagation code to investigate how the new 3D gas models affect calculations of CR propagation and high-energy γ-ray intensity maps. The calculations reveal non-trivial features that are directly related to the new gas models. The best-fit values for propagation model parameters employing 3D gas models are presented and they differ significantly from those derived with the 2D gas density models that have been widely used. The combination of 3D CR and gas density models provide a more realistic basis for the interpretation of non-thermal emissions from the Galaxy.

Performance evaluation of a direct-conversion flat-panel detector system in imaging and quality assurance for a high-dose-rate 192Ir source

In high-dose-rate (HDR) brachytherapy, a direct-conversion flat-panel detector (d-FPD) clearly depicts a 192Ir source without image halation, even under the emission of high-energy gamma rays. However, it was unknown why iridium is visible when using a d-FPD. The purpose of this study was to clarify the reasons for visibility of the source core based on physical imaging characteristics, including the modulation transfer functions (MTF), noise power spectral (NPS), contrast transfer functions, and linearity of d-FPD to high-energy gamma rays. The acquired data included: x-rays, [X]; gamma rays, [γ]; dual rays (X + γ), [D], and subtracted data for depicting the source ([D] − [γ]). In the quality assurance (QA) test for the positional accuracy of a source core, the coordinates of each dwelling point were compared between the planned and actual source core positions using a CT/MR-compatible ovoid applicator and a Fletcher–Williamson applicator. The profile curves of [X] and ([D] − [γ]) matched well on MTF and NPS. The contrast resolutions of [D] and [X] were equivalent. A strongly positive linear correlation was found between the output data of [γ] and source strength (r2 > 0.99). With regard to the accuracy of the source core position, the largest coordinate difference (3D distance) was noted at the maximum curvature of the CT/MR-compatible ovoid and Fletcher–Williamson applicators, showing 1.74 ± 0.02 mm and 1.01 ± 0.01 mm, respectively. A d-FPD system provides high-quality images of a source, even when high-energy gamma rays are emitted to the detector, and positional accuracy tests with clinical applicators are useful in identifying source positions (source movements) within the applicator for QA.

Very High Energy Emission from the Binary System Cyg X-3

Cyg X-3 is actively studied in the entire range of the electromagnetic spectrum from the radio band to ultrahigh energies. Based on the detection of ultrahigh-energy gamma-ray emission, it has been suggested that Cyg X-3 could be one of the most powerful sources of charged cosmic-ray particles in the Galaxy. We present the results of long-term observations of the Cygnus Х-3 region at energies 800 GeV–100 TeV by the SHALON mirror Cherenkov telescope. In 1995 the SHALON observations revealed a new Galactic source of very high energy gamma-ray emission coincident in its coordinates with the microquasar Cyg X-3. To reliably identify the detected source with Cyg X-3, an analysis has been performed and an orbital period of 4.8 h has been found, which is a signature of Cyg X-3. A series of flares in Cyg X-3 at energies >800 GeV and their correlation with the activity in the X-ray and radio bands have been observed. The results obtained in a wide energy range for Cyg X-3, including those during the periods of relativistic jet events, are needed to find the connection and to understand the different components of an accreting binary system.

Magnetospheric Gamma-Ray Emission in Active Galactic Nuclei

Abstract The rapidly variable, very high-energy (VHE) gamma-ray emission from active galactic nuclei (AGNs) has been frequently associated with non-thermal processes occurring in the magnetospheres of their supermassive black holes. The present work aims to explore the adequacy of different gap-type (unscreened electric field) models to account for the observed characteristics. Based on a phenomenological description of the gap potential, we estimate the maximum extractable gap power L gap for different magnetospheric setups, and study its dependence on the accretion state of the source. L gap is found in general to be proportional to the Blandford–Znajek jet power L BZ and a sensitive function of gap size h, i.e., , where the power index is dependent on the respective gap setup. The transparency of the vicinity of the black hole to VHE photons generally requires a radiatively inefficient accretion environment and thereby imposes constraints on possible accretion rates, and correspondingly on L BZ. Similarly, rapid variability, if observed, may allow one to constrain the gap size . Combining these constraints, we provide a general classification to assess the likelihood that the VHE gamma-ray emission observed from an AGN can be attributed to a magnetospheric origin. When applied to prominent candidate sources these considerations suggest that the variable (day-scale) VHE activity seen in the radio galaxy M87 could be compatible with a magnetospheric origin, while such an origin appears less likely for the (minute-scale) VHE activity in IC 310.

TeV Gamma-Ray Observations of the Binary Neutron Star Merger GW170817 with H.E.S.S.

Abstract We search for high-energy gamma-ray emission from the binary neutron star merger GW170817 with the H.E.S.S. Imaging Air Cherenkov Telescopes. The observations presented here have been obtained starting only 5.3 hr after GW170817. The H.E.S.S. target selection identified regions of high probability to find a counterpart of the gravitational-wave event. The first of these regions contained the counterpart SSS17a that has been identified in the optical range several hours after our observations. We can therefore present the first data obtained by a ground-based pointing instrument on this object. A subsequent monitoring campaign with the H.E.S.S. telescopes extended over several days, covering timescales from 0.22 to 5.2 days and energy ranges between to . No significant gamma-ray emission has been found. The derived upper limits on the very-high-energy gamma-ray flux for the first time constrain non-thermal, high-energy emission following the merger of a confirmed binary neutron star system.