Gamma-ray Energy Band Research Articles

Probing Orbital Parameters of Gamma-Ray Binaries with TeV Light Curves

Gamma-ray binaries are binary systems in which the energy flux peaks in the gamma-ray energy band. They harbor a compact object (a neutron star or a black hole) orbiting around a massive star, which provides a strong radiation field. It is believed that the gamma-ray emission from such objects can be strongly attenuated through the electron-positron pair production in gamma–gamma interactions. The importance of gamma–gamma absorption depends on the orbital phase and on the geometry of the system. In this work, we propose a method showing how the orbital parameters of gamma-ray binaries could be probed with TeV light curves that have imprinted features of gamma–gamma absorption.

The Formation of Hard Very High Energy Spectra from Gamma-ray Burst Afterglows via Two-zone Synchrotron Self-Compton Emission

Electron Compton scattering of target photons into the gamma-ray energy band (inverse Compton scattering; IC) is commonly expected to dominate the very high energy (VHE) spectra in gamma-ray bursts (GRBs) especially during the afterglow phase. For sufficiently large center-of-mass energies in these collisions, the effect of the electron recoil starts reducing the scattering cross-section (the Klein–Nishina regime). The IC spectra generated in the Klein–Nishina regime is softer and has a smaller flux level compared to the synchrotron spectra produced by the same electrons. The detection of afterglow emission from nearby GRB190829A in the VHE domain with H.E.S.S. has revealed an unexpected feature: the slope of the VHE spectrum matches well the slope of the X-ray spectra, despite expectations that, for the IC production process, the impact of the Klein–Nishina effect should be strong. The multi-wavelength spectral energy distribution appears to be inconsistent with predictions of one-zone synchrotron–self-Compton models. We study the possible impact of two-zone configuration on the properties of IC emission when the magnetic field strength differs considerably between the two zones. Synchrotron photons from the strong magnetic field zone provide the dominant target for cooling of the electrons in the weak magnetic field zone, which results in a formation of hard electron distribution and consequently of a hard IC emission. We show that the two-zone model can provide a good description of the Swift's X-ray Telescope and VHE H.E.S.S. data.

Rotation-powered pulsars in the soft- and medium energy gamma-ray bands

Rotation-powered pulsars in the soft- and medium energy gamma-ray bands

Search of the pair echo signatures in the high-energy light curve of GRB190114C

A model of the time delayed electromagnetic cascade "echo" is applied to the bright gamma-ray burst GRB190114C - the first gamma-ray burst to be contemporaneously detected in high and very high energy gamma-ray bands. It is shown that the internal spread of the cascade in the absence of the intervening magnetic fields dilutes the "echo" emission over $10^3-10^5$ seconds depending on the energy. Accounting for the measured source flux in the $0.3-1$ TeV gamma-ray band, the prediction of the "echo" model is shown to match the detected lower-energy gamma-ray emission $10^4$ seconds after the burst. However, the "echo" emission remains indistinguishable from the intrinsic GRB190114C flux within the measurement uncertainties. Implications of this in the context of the intergalactic magnetic field measurement are discussed.

Development of a Compton telescope calorimeter module for MeV-range gamma-ray astronomy

Gamma-ray astronomy in the MeV energy range shows a lack of sensitivity compared to other gamma-ray energy bands. Several space observatory projects are proposed to space agencies to fill this gap. The COMCUBE CubeSat project will focus on the measurement of the polarization of the prompt emission of gamma-ray bursts (GRBs) which can provide an understanding of the physics of ultra-relativistic jets. Furthermore, the linear polarization of distant gamma-ray sources could be a probe to study the fundamental physics related to the Lorentz invariance violation. A Compton telescope is particularly well-suited for that purpose. We develop an instrument including double-sided silicon strip detectors and scintillation crystals coupled to a pixelated photodetector. The incoming photon undergoes an inelastic scattering in one or several layers of a position-sensitive silicon strip detector before being absorbed in a position-sensitive calorimeter based on inorganic scintillators. The measurement of both positions and energy deposits enables the determination of the photon’s source direction. It also enables measurement of the linear polarization of the incident gamma-rays. In this contribution, we present the results of the extensive work we have carried out to develop the calorimeter and its integration into a Compton telescope prototype. We conclude by showing the first image and polarization measurements.

All-sky Medium Energy Gamma-ray Observatory eXplorer mission concept

The All-sky Medium Energy Gamma-ray Observatory eXplorer (AMEGO-X) is designed to identify and characterize gamma rays from extreme explosions and accelerators. The main science themes include: supermassive black holes and their connections to neutrinos and cosmic rays; binary neutron star mergers and the relativistic jets they produce; cosmic ray particle acceleration sources including Galactic supernovae; and continuous monitoring of other astrophysical events and sources over the full sky in this important energy range. AMEGO-X will probe the medium energy gamma-ray band using a single instrument with sensitivity up to an order of magnitude greater than previous telescopes in the energy range 100 keV to 1 GeV that can be only realized in space. During its three-year baseline mission, AMEGO-X will observe nearly the entire sky every two orbits, building up a sensitive all-sky map of gamma-ray sources and emission. AMEGO-X was submitted in the recent 2021 NASA MIDEX Announcement of Opportunity.

Observation of the Gamma-Ray Binary HESS J0632+057 with the H.E.S.S., MAGIC, and VERITAS Telescopes

Abstract The results of gamma-ray observations of the binary system HESS J0632 + 057 collected during 450 hr over 15 yr, between 2004 and 2019, are presented. Data taken with the atmospheric Cherenkov telescopes H.E.S.S., MAGIC, and VERITAS at energies above 350 GeV were used together with observations at X-ray energies obtained with Swift-XRT, Chandra, XMM-Newton, NuSTAR, and Suzaku. Some of these observations were accompanied by measurements of the Hα emission line. A significant detection of the modulation of the very high-energy gamma-ray fluxes with a period of 316.7 ± 4.4 days is reported, consistent with the period of 317.3 ± 0.7 days obtained with a refined analysis of X-ray data. The analysis of data from four orbital cycles with dense observational coverage reveals short-timescale variability, with flux-decay timescales of less than 20 days at very high energies. Flux variations observed over a timescale of several years indicate orbit-to-orbit variability. The analysis confirms the previously reported correlation of X-ray and gamma-ray emission from the system at very high significance, but cannot find any correlation of optical Hα parameters with fluxes at X-ray or gamma-ray energies in simultaneous observations. The key finding is that the emission of HESS J0632 + 057 in the X-ray and gamma-ray energy bands is highly variable on different timescales. The ratio of gamma-ray to X-ray flux shows the equality or even dominance of the gamma-ray energy range. This wealth of new data is interpreted taking into account the insufficient knowledge of the ephemeris of the system, and discussed in the context of results reported on other gamma-ray binary systems.

Constraints on Lorentz Invariance Violation with Multiwavelength Polarized Astrophysical Sources

Possible violations of Lorentz invariance (LIV) can produce vacuum birefringence, which results in a frequency-dependent rotation of the polarization plane of linearly polarized light from distant sources. In this paper, we try to search for a frequency-dependent change of the linear polarization angle arising from vacuum birefringence in the spectropolarimetric data of astrophysical sources. We collect five blazars with multiwavelength polarization measurements in different optical bands (UBVRI). Taking into account the observed polarization angle contributions from both the intrinsic polarization angle and the rotation angle induced by LIV, and assuming that the intrinsic polarization angle is an unknown constant, we obtain new constraints on LIV by directly fitting the multiwavelength polarimetric data of the five blazars. Here, we show that the birefringence parameter η quantifying the broken degree of Lorentz invariance is limited to be in the range of −9.63×10−8<η<6.55×10−6 at the 2σ confidence level, which is as good as or represents one order of magnitude improvement over the results previously obtained from ultraviolet/optical polarization observations. Much stronger limits can be obtained by future multiwavelength observations in the gamma-ray energy band.

New era of dark matter particle detection

The astronomical evidence of the existence of dark matter is overwhelming. However, the physical nature of dark matter is still unknown. It is widely believed that dark matter is some kind of new particles beyond the standard model of particle physics. Alternative candidates include the massive compact halo objects such as primordial black holes. Among all these candidates, the class of so-called weakly interacting massive particles (WIMPs) is the most natural and well theoretically motivated one. The WIMP scenario can naturally explain the abundance of dark matter in the universe via a canonical thermal production mechanism. It also gives a practical way of detecting those particles via various kinds of particle physics experiments. The direct detection method measures the momentum exchange when a dark matter particle scatters off a nucleus. The sensitivities of direct detection increase remarkably during the past 30 years since it was first proposed in the 1980s. China has operated two direct detection experiments, the CDEX and PandaX experiments, in the Jinping underground laboratory. Through the efforts of all the direct detection experiments, including CDEX and PandaX, the current upper limit about the spin-independent scattering cross section between the dark matter particle and the nucleon reaches the level of 10⁻⁴⁷ cm² which is only about two orders of magnitude higher than the neutrino-induced background. Although no convincing signal has been detected yet, such null results can constrain severely many kinds of theoretical models of dark matter. Several experiments are upgrading now, with the goal of a fiducial detector mass of several tons, which are expected to further improve the direct detection sensitivities. The indirect detection method is to search for the annihilation or decay products of dark matter in cosmic rays and gamma-rays, which is complimentary to the direct detection. The main indirect detection experiments in operation include the alpha magnetic spectrometer and the calorimeter electron telescope on the International Space Station, the Fermi gamma-ray space telescope, and China’s dark matter particle explorer. Several anomalies have been reported by the indirect detection experiments, such as the positron excess, the excess of the total electron plus positron spectrum, the gamma-ray excess in the Galactic center region. Interpretations of these excesses are still in debate. Nevertheless, these anomalies offer us targets for further investigation with increased statistics and improved understanding of the astrophysical backgrounds. Both the direct detection and indirect detection experiments will have significantly upgrading in the coming years. Particularly, China’s PandaX experiment is currently building a 4-ton scale detector which is expected to start to operate in 2021. The long-term plan of the CDEX experiment is also a ton-scale detector for multi-purpose studies including the dark matter searches. For the indirect detection, the HERD payload onboard the Chinese Space Station is in the prototype phase, and the next generation very large area gamma-ray space telescope is under design. On the ground we are building the large high-altitude air shower observatory for high-sensitivity sky survey in the very high energy gamma-ray band. All these efforts will make China play a significant or even leading role in the campaign of dark matter detection. We are looking forward to the breakthroughs in the exploration of the nature of dark matter.

On the Origin of High-energy Neutrinos from NGC 1068: The Role of Nonthermal Coronal Activity

Abstract NGC 1068, a nearby type-2 Seyfert galaxy, is reported as the hottest neutrino spot in the 10 yr survey data of IceCube. Although there are several different possibilities for the generation of high-energy neutrinos in astrophysical sources, feasible scenarios allowing such emission in NGC 1068 have not yet been firmly defined. We show that the flux level of GeV and neutrino emission observed from NGC 1068 implies that the neutrino emission can be produced only in the vicinity of the supermassive black hole in the center of the galaxy. The coronal parameters, such as magnetic field strength and corona size, that make this emission possible, are consistent with the spectral excess registered in the millimeter range. The suggested model and relevant physical parameters are similar to those revealed for several nearby Seyferts. Due to the internal gamma-ray attenuation, the suggested scenario cannot be verified by observations of NGC 1068 in the GeV and TeV gamma-ray energy bands. However, the optical depth is expected to become negligible for MeV gamma-rays, thus future observations in this band will be able to validate our model.

The Origin of Non-Thermal Emission from FSRQs

The observations of astrophysical sources in a large frequency range (from radio to very high energy gamma-ray bands) provide complete information on the non-thermal processes taking place in different objects. Here, the origin of broadband emission from the jets of at-spectrum radio quasars are discussed. For the current study the blazars detected above 100 GeV: PKS 1441+25, 3C 279, PKS 1222+216, PKS 1510-089, as well as CTA 102, which was in flaring state in optcal/UV, X-ray and high energy gamma-ray bands, are selected. The publicly available data of Fermi LAT, Swift UVOT/XRT, Nustar telescopes have been analyzed, which enables to identify the prominent flaring and quiescent states for those sources, as well as, study the spectral properties, constrain the size and location of the emitting region. The multiwavelength emission spectra of those sources, in different states, are modeled, which is crucial for understanding the particle acceleration and emission processes in their jets. For this purpose, a new code that can derive the model free parameters which statistically better describe the observed data is used. It derives the best-fit parameters and their uncertainties through Markov Chain Monte Carlo sampling of the likelihood distributions. By means of the detailed theoretical modeling of acquired data, it was possible to derive or at least constrain some crucial parameters such as the magnetic field, jet energetic, electron energy density etc.

High resolution gamma-ray spectrometer with MHz capabilities for runaway electron studies at ASDEX Upgrade.

A new gamma-ray spectrometer with MHz capabilities has been developed to measure the bremsstrahlung emission spectrum in the gamma-ray energy band generated by MeV range runaway electrons in disruption experiments at ASDEX Upgrade. Properties of the runaway electrons are inferred from the measured bremsstrahlung spectrum by a deconvolution technique, particularly with regard to their maximum energy. Changes induced to the runaway electron velocity space are unambiguously observed both in massive gas injection and resonant magnetic perturbation experiments with the detector.

Prospects for gamma-ray observations of narrow-line Seyfert 1 galaxies with the Cherenkov Telescope Array

Gamma-ray emitting narrow-line Seyfert 1 ($\gamma$-NLSy1) galaxies are thought to harbour relatively low-mass black holes (10$^6$-10$^8$ M$_{\odot}$) accreting close to the Eddington limit. They show characteristics similar to those of blazars, such as flux and spectral variability in the gamma-ray energy band and radio properties which point toward the presence of a relativistic jet. These characteristics make them an intriguing class of sources to be investigated with the Cherenkov Telescope Array (CTA), the next-generation ground-based gamma-ray observatory. We present our extensive set of simulations of all currently known $\gamma$-ray emitters identified as NLS1s (20 sources),investigating their detections and spectral properties, taking into account the effect of both the extra-galactic background light in the propagation of gamma-rays and intrinsic absorption components.We find that the prospects for observations of $\gamma$-NLSy1 with CTA are promising. In particular, the brightest sources of our sample, SBS 0846+513, PMN J0948+0022, and PKS 1502+036 can be detected during high/flaring states, the former two even in the case in which the emission occurs within the highly opaque central regions, which prevent $\gamma$ rays above few tens of GeV to escape. In this case the low-energy threshold of CTA will play a key role. If, on the other hand, high-energy emission occurs outside the broad line region, we can detect the sources up to several hundreds of GeV-depending on the intrinsic shape of the emitted spectrum. Therefore, CTA observations will provide valuable information on the physical conditions and emission properties of their jets.

X-Ray and GeV Gamma-Ray Variability of the Radio Galaxy NGC 1275

Abstract We analyzed Suzaku/XIS data (2006–2015) and Fermi/LAT data (2008–2015) of the gamma-ray emitting radio galaxy NGC 1275. Correlated brightening of the nucleus in both the X-ray and GeV gamma-ray energy bands was found for the period 2013–2015. This is the first evidence of correlated variability between these two energy bands for NGC 1275. We also analyzed Swift/XRT data and found that the X-ray flux increased over several days in 2010, coincidentally with the GeV gamma-ray flare. During the flare, the X-ray spectra were softer, with a photon index of ∼2 compared with 1.5–2.1 of the other periods, suggesting the brightening of a synchrotron component. The GeV gamma-ray band also showed a higher flux with a harder spectrum during the 2010 flare. Simultaneous X-ray and GeV gamma-ray flux increase in the flare could be explained by the shock-in-jet scenario. On the other hand, a long-term gradual brightening of radio, X-ray, and GeV gamma-ray flux with a larger gamma-ray amplitude could have an origin other than internal shocks, and some of these possibilities are discussed.

LEPTONIC AND LEPTO-HADRONIC MODELING OF THE 2010 NOVEMBER FLARE FROM 3C 454.3

In this study, we use a one-zone leptonic and a lepto-hadronic model to investigate the multi-wavelength emission and the prominent flare of the flat spectrum radio quasar 3C 454.3 in Nov 2010. We perform a parameter study with both models to obtain broadband fits to the spectral energy distribution of 3C 454.3. Starting with the baseline parameters obtained from the fits, we then investigate different flaring scenarios for both models to explain an extreme outburst and spectral hardening of 3C 454.3 that occurred in Nov 2010. We find that the one zone lepto-hadronic model can successfully explain both the broadband multi-wavelength spectral energy distribution and light curves in the optical R, Swift XRT and Fermi gamma-ray bandpasses for 3C 454.3 during quiescence and the peak of the Nov. 2010 flare. We also find that the one-zone leptonic model produces poor fits to the broadband spectra in the X-ray and high energy gamma-ray band passes for the Nov. 2010 flare.

Operating performance of the gamma-ray Cherenkov telescope: An end-to-end Schwarzschild–Couder telescope prototype for the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) consortium aims to build the next-generation ground-based very-high-energy gamma-ray observatory. The array will feature different sizes of telescopes allowing it to cover a wide gamma-ray energy band from about 20GeV to above 100TeV. The highest energies, above 5TeV, will be covered by a large number of Small-Sized Telescopes (SSTs) with a field-of-view of around 9°. The Gamma-ray Cherenkov Telescope (GCT), based on Schwarzschild–Couder dual-mirror optics, is one of the three proposed SST designs. The GCT is described in this contribution and the first images of Cherenkov showers obtained using the telescope and its camera are presented. These were obtained in November 2015 in Meudon, 2http://www.cta-observatory.orgFrance.

Channeling Radiation Characteristics of Positrons Channeled in Perfect Single-Walled Carbon Nanotubes

We investigate the channeling radiation characteristics of positrons channeled in single-walled carbon nanotubes (SWCNTs). The channeled positrons move along the nanotube axis, (z-axis), with energies (10-500) MeV. This study covers SWCNTs with different chiral indices (n,m). The energy eigenvalues, in a plane normal to the nanotube axis (xy-plane), of relativistic positron channeled through SWCNTs has been used to calculate the emitted photon energy in the forward direction, i.e., channeling radiation. Also, the calculations covered both the wave length of the emitted radiation and the maximum number of bound states of the channeled positrons as a function of the nanotube radius at different incident energies. The results of the calculations showed that, in this energy range, the emitted radiation energy lies in the X-ray band and as the energy of the incident positron increase, the emitted radiation energy increase towards the gamma-ray energy band and accordingly, the wave length decrease towards the gamma-ray band for the same transition

The observed radio/gamma-ray emission correlation for blazars with the Fermi-LAT and the RATAN-600 data

We study the correlation between gamma-ray and radio band radiation for 123 blazars, using the Fermi-LAT first source catalogue (1FGL) and the RATAN-600 data obtained at the same period of time (within a few months). We found an apparent positive correlation for BL Lac and flat-spectrum radio quasar (FSRQ) sources from our sample through testing the value of the Pearson product-moment correlation coefficient. The BL Lac objects show higher values of the correlation coefficient than FSRQs at all frequencies, except 21.7 GHz, and at all bands, except 10–100 GeV, typically at high confidence level (>99 per cent). At higher gamma-ray energies the correlation weakens and even becomes negative for BL Lacs and FSRQs. For BL Lac blazars, the correlation of the fluxes appeared to be more sensitive to the considered gamma-ray energy band, than to the frequency, while for FSRQ sources the correlation changed notably both with the considered radio frequency and gamma-ray energy band. We used a data randomization method to quantify the significance of the computed correlation coefficients. We find that the statistical significance of the correlations we obtained between the flux densities at all frequencies and the photon flux in all gamma-ray bands below 3 GeV is high for BL Lacs (chance probability ∼10−3–10−7). The correlation coefficient is high and significant for the 0.1–0.3 GeV band and low and insignificant for the 10–100 GeV band for both types of blazars for all considered frequencies.

EXPERIMENTAL CONSTRAINTS ONγ-RAY PULSAR GAP MODELS AND THE PULSAR GeV TO PULSAR WIND NEBULA TeV CONNECTION

The pulsar emission mechanism in the gamma-ray energy band is poorly understood. Currently, there are several models under discussion in the pulsar community. These models can be constrained by studying the collective properties of a sample of pulsars, which became possible with the large sample of gamma-ray pulsars discovered by the Fermi Large Area Telescope (Fermi-LAT). In this paper we develop a new experimental multi-wavelength technique to determine the beaming factor $\left( f_\Omega \right)$ dependance on spin-down luminosity of a set of GeV pulsars. This technique requires three input parameters: pulsar spin-down luminosity, pulsar phase-averaged GeV flux and TeV or X-ray flux from the associated Pulsar Wind Nebula (PWN). The analysis presented in this paper uses the PWN TeV flux measurements to study the correlation between $f_\Omega$ and $\dot{E}$. The measured correlation has some features that favor the Outer Gap model over the Polar Cap, Slot Gap and One Pole Caustic models for pulsar emission in the energy range of 0.1 to 100 GeV, but one must keep in mind that these simulated models failed to explain many of the most important pulsar population characteristics. A tight correlation between the pulsar GeV emission and PWN TeV emission was also observed, which suggests the possibility of a linear relationship between the two emission mechanisms. In this paper we also discuss a possible mechanism to explain this correlation.

Effect of geodetic precession on the evolution of pulsar high-energy pulse profiles as derived with the striped-wind model

Geodetic precession has been observed directly in the double-pulsar system PSR J0737-3039. Its rate has even been measured and agrees with predictions of general relativity. Very recently, the double pulsar has been detected in X-rays and gamma-rays. This fuels the hope observing geodetic precession in the high-energy pulse profile of this system. Unfortunately, the geometric configuration of the binary renders any detection of such an effect unlikely. Nevertheless, this precession is probably present in other relativistic binaries or double neutron star systems containing at least one X-ray or gamma-ray pulsar.}{We compute the variation of the high-energy pulse profile expected from this geodetic motion according to the striped-wind model. We compare our results with two-pole caustic and outer gap emission patterns.}{For a sufficient misalignment between the orbital angular momentum and the spin angular momentum, a significant change in the pulse profile as a result of geodetic precession is expected in the X-ray and gamma-ray energy band.}{The essential features of the striped wind are indicated in several plots showing the evolution of the maximum of the pulsed intensity, the separation of both peaks, if present, and the variation in the width of each peak. We highlight the main differences with other competing high-energy models.}{We make some predictions about possible future detection of high-energy emission from double neutron star systems with the highest spin precession rate. Such observations will definitely favour some pulsed high-energy emission scenarios.