TeV Γ-ray Emission Research Articles

Detailed spectral and morphological analysis of the shell type supernova remnant RCW 86

Aim. We aim for an understanding of the morphological and spectral properties of the supernova remnant RCW 86 and for insights into the production mechanism leading to the RCW 86 very high-energy γ-ray emission.Methods. We analyzed High Energy Spectroscopic System (H.E.S.S.) data that had increased sensitivity compared to the observations presented in the RCW 86 H.E.S.S. discovery publication. Studies of the morphological correlation between the 0.5–1 keV X-ray band, the 2–5 keV X-ray band, radio, and γ-ray emissions have been performed as well as broadband modeling of the spectral energy distribution with two different emission models.Results. We present the first conclusive evidence that the TeV γ-ray emission region is shell-like based on our morphological studies. The comparison with 2–5 keV X-ray data reveals a correlation with the 0.4–50 TeV γ-ray emission. The spectrum of RCW 86 is best described by a power law with an exponential cutoff at Ecut = (3.5 ± 1.2stat) TeV and a spectral index of Γ ≈ 1.6 ± 0.2. A static leptonic one-zone model adequately describes the measured spectral energy distribution of RCW 86, with the resultant total kinetic energy of the electrons above 1 GeV being equivalent to ~0.1% of the initial kinetic energy of a Type Ia supernova explosion (1051 erg). When using a hadronic model, a magnetic field of B ≈ 100 μG is needed to represent the measured data. Although this is comparable to formerly published estimates, a standard E−2 spectrum for the proton distribution cannot describe the γ-ray data. Instead, a spectral index of Γp ≈ 1.7 would be required, which implies that ∼7 × 1049/ncm−3 has been transferred into high-energy protons with the effective density ncm−3 = n/1 cm−3. This is about 10% of the kinetic energy of a typical Type Ia supernova under the assumption of a density of 1 cm−3.

Gamma-rays from the binary system containing PSR J2032+4127 during its periastron passage

The energetic pulsar, PSR J2032+4127, has recently been discovered in the direction of the unidentified HEGRA TeV γ-ray source (TeV J2032+4130). It is proposed that this pulsar forms a binary system with the Be type star, MT91 213, expected to reach periastron late in 2017. We performed detailed calculations of the γ-ray emission produced close to the binary system’s periastron passage by applying a simple geometrical model. Electrons accelerated at the collision region of pulsar and stellar winds initiate anisotropic inverse Compton pair cascades by scattering soft radiation from the massive companion. The γ-ray spectra, from such a comptonization process, are compared with the measurements of the extended TeV γ-ray emission from the HEGRA TeV γ-ray source. We discuss conditions within the binary system, at the periastron passage of the pulsar, for which the γ-ray emission from the binary can overcome the extended, steady TeV γ-ray emission from the HEGRA TeV γ-ray source.

Revisiting SNR Puppis A with Seven Years of Fermi Large Area Telescope Observations

Abstract Puppis A is a very famous and extensively studied supernova remnant that shows strong evidence of shock-cloud interaction. We reanalyze its GeV γ-ray emission using seven years of Pass 8 data recorded by the Fermi Large Area Telescope. The morphology of the γ-ray emission is more compatible with that of the thermal X-ray and IR emissions than the radio image, which suggests a possible correlation between the gamma-ray-emitting region and dense clouds. The γ-ray spectrum in the energy range of 1–500 GeV shows a break at 7.92 ± 1.91 GeV, with photon indices of 1.81 ± 0.08 below the break and 2.53 ± 0.12 above the break, which can naturally explain the lack of TeV γ-ray emission from Puppis A. The multi-wavelength observations favor a hadronic origin for the γ-ray emission.

Origin of very high energy emission in galaxy clusters

The Perseus cluster of galaxies with the central galaxy NGC 1275 is ideally suitable for studying both the physics of relativistic jets from Active Galactic Nuclei and for revealing the feedback role of the central galaxy. We present the results of fifteen-year-long observations of the AGN NGC 1275 at energies 800 GeV–40 TeV discovered by the SHALON telescope in 1996. The data obtained at very high energies by SHALON, namely the images of the galaxy and its surroundings, and the flux variability indicate that TeV γ-ray emission is produced by a number of processes: in particular, part of this emission is generated by relativistic jets in the nucleus of NGC 1275 itself. Unique data on GK Per(Nova 1901) and the IC 310 radio galaxy TeV γ-ray emission were obtained with the SHALON experiment for the first time.

Very high energy gamma-ray emission of Perseus Cluster and NGC 1275

The Perseus cluster of galaxies with the central galaxy NGC 1275 is ideally suitable both for studying the physics of relativistic jets from Active Galactic Nuclei and for revealing the feedback role of the central galaxy. The data obtained at very high energies by SHALON, namely the images of the galaxy and its surroundings, and the flux variability indicate that the TeV γ-ray emission is produced by a number of processes: in partraular, part of this emission is generated by relativistic jets in the nucleus of NGC 1275 itself. Unique data on GK Per(Nova 1901) TeV γ-ray emission were obtained with SHALON experiment for the first time.

A young supernova remnant illuminating nearby molecular clouds with cosmic rays

The supernova remnant (SNR) HESS J1731-347 displays strong nonthermal TeV γ-ray and X-ray emission, thus the object is presently accelerating particles to very high energies. A distinctive feature of this young SNR is the nearby (~30 pc in projection) extended source HESS J1729-345, which is currently unidentified but is in spatial projection coinciding with known molecular clouds (MC). We model the SNR evolution to explore whether the TeV emission from HESS J1729-345 can be explained as emission from runaway hadronic cosmic rays (CRs) that are illuminating these MCs. The observational data of HESS J1729-345 and HESS J1731-347 can be reproduced using core-collapse SN models for HESS J1731-347. Starting with different progenitor stars and their presupernova environment, we model potential SNR evolution histories along with the CR acceleration in the SNR and the diffusion of the CRs. A simplified three-dimensional structure of the MCs is introduced based on 12CO data of that region, adopting a distance of 3.2 kpc to the source. A Monte Carlo based diffusion model for the escaping CRs is developed to deal with the inhomogeneous environment. The fast SNR forward shock speed, as implied from the X-ray data, can easily be explained when employing scenarios with progenitor star masses between 20 M⊙ and 25 M⊙, where the SNR shock is still expanding inside the main-sequence (MS) bubble at present time. The TeV spectrum of HESS J1729-345 is satisfactorily fitted by the emission from the highest energy CRs that have escaped the SNR, using a standard Galactic CR diffusion coefficient in the interclump medium. The TeV image of HESS J1729-345 can be explained with a reasonable three-dimensional structure of MCs. The TeV emission from the SNR itself is dominated by leptonic emission in this model. We also explore scenarios where the shock is starting to encounter the dense MS progenitor wind bubble shell. The escaping hadronic CR hypothesis for the γ-ray emission of HESS J1729-345 can still hold,but even in this case our model cannot easily account for the TeV emission from HESS J1731-347 in a hadronic scenario.

Long-term studies of the Cygnus Region and its objects: Cyg X-3 and γCygni SNR

The long-term studies of the Cygnus Region at energies 800 GeV - 100 TeV by SHALON telescope are presented in this paper. The results of twenty-year-long observations of the Cyg X-3 binary at energies 800 GeV - 85 TeV, detected by SHALON in 1995y are presented with images and integral spectra. The 4.8 hour orbital modulation of TeV γ-ray emission of Cyg X-3 was found in SHALON data. A number of high activity period of Cyg X-3 were detected at energies > 800 GeV during the all observation time. The correlation soft X-ray and TeV energy γ-ray fluxes is traced. Additional to Cyg X-3 study data on the TeV sources of Cygnus Region, namely γCygni SNR was obtained with SHALON.

Multi-epoch study of the gamma-ray emission within the M87 magnetosphere model

M87 is a nearby radio galaxy that has been detected at energies ranging from radio to very high energy (VHE) gamma-rays. Its proximity and its jet, misaligned from the line of sight allow detailed morphological studies. The imaging atmospheric Cherenkov technique (from 100 GeV to 10 TeV) provides insufficient angular resolution (few arc-minutes) to resolve the M87 emission region. However, the short time scale variability observed by MAGIC, HESS and VERITAS suggests the TeV emission is coming from a very small region, most likely close to the core. We propose that the variable TeV emission may be produced in a pair-starved region of the central black hole (BH) magnetosphere, i.e. a region where the density of the electron-positron plasma is not sufficient to completely screen the accelerating electric field. The funnel, a low density and magnetically dominated region around the poles, appears as a favourable site of low-density where a Blandford-Znajek process may explain the main properties of the TeV γ-ray emission from M87. We produce a broadband spectral energy distribution (SED) of the resulting radiation and compare the model with the observed fluxes from the nucleus of M87, for both low and high γ-ray activities. We finish with a brief discussion on the connection between the accretion rate and the intermittence in the formation of gaps in the magnetosphere.

Search for TeV γ-ray emission from blazar 1ES1218+304 with TACTIC telescope during March–April 2013

In this paper, we present results of TeV γ-ray observations of the high synchrotron peaked BL Lac object 1ES 1218+304 (z=0.182) with the TACTIC (TeV Atmospheric Cherenkov Telescope with Imaging Camera). The observations are primarily motivated by the unusually hard GeV–TeV spectrum of the source despite its relatively large redshift. The source is observed in the TeV energy range with the TACTIC from March 1, 2013 to April 15, 2013 (MJD 56352–56397) for a total observation time of 39.62h and no evidence of TeV γ-ray activity is found from the source. The corresponding 99% confidence level upper limit on the integral flux above a threshold energy of 1.1TeV is estimated to be 3.41×10-12photonscm−2s−1 (i.e. <23% Crab Nebula flux) assuming a power law differential energy spectrum with photon index 3.0, as previously observed by the MAGIC and VERITAS telescopes. For the study of multi-wavelength emission from the source, we use nearly simultaneous optical, UV and X-ray data collected by the UVOT and XRT instruments on board the Swift satellite and high energy γ-ray data collected by the Large Area Telescope on board the Fermi satellite. We also use radio data at 15GHz from OVRO 40m telescope in the same period. No significant increase of activity is detected from radio to TeV γ-rays from 1ES1218+304 during the period from March 1, 2013 to April 15, 2013.

X-ray spectroscopy of the mixed morphology supernova remnant W 28 with XMM-Newton

Abstract We report on spatially resolved X-ray spectroscopy of the north-eastern part of the mixed morphology supernova remnant (SNR) W 28 with XMM-Newton. The observed field of view includes a prominent and twisted shell emission forming the edge of this SNR as well as part of the center-filled X-ray emission brightening toward the south-west edge of the field of view. The shell region spectra are in general represented by an optically thin thermal plasma emission in collisional ionization equilibrium with a temperature of ∼ 0.3 keV and a density of ∼ 10 cm−3, which is much higher than the density obtained for inner parts. In contrast, we detected no significant X-ray flux from one of the TeV γ-ray peaks with an upper-limit flux of 2.1 × 10−14 erg cm−2 s−1 in the 2–10 keV band. The large flux ratio of TeV to X-ray, larger than 16, and the spatial coincidence of the molecular cloud and the TeV γ-ray emission site indicate that the TeV γ-ray of W 28 is π0-decay emission originating from collisions between accelerated protons and molecular cloud protons. Comparing the spectrum in the TeV band and the X-ray upper limit, we obtained a weak upper limit on the magnetic field strength B ≲ 1500 μG.

Long term performance evaluation of the TACTIC imaging telescope using ∼400 h Crab Nebula observation during 2003–2010

The TeV atmospheric Cherenkov telescope with imaging camera (TACTIC) γ-ray telescope has been in operation at Mt. Abu, India since 2001 to study TeV γ-ray emission from celestial sources. During the last 10 years, apart from consistently detecting a steady signal from the Crab Nebula above ∼1.2 TeV energy, at a sensitivity level of ∼5.0 σ in ∼25 h, the telescope has also detected flaring activity from Mrk 421 and Mrk 501 on several occasions. Although we used Crab Nebula data partially, in some of the reported results, primarily for testing the validity of the full data analysis chain, the main aim of this work is to study the long term performance of the TACTIC telescope by using consolidated data collected between 2003 and 2010. The total on-source data, comprising ∼402 h, yields an excess of ∼(3742±192) γ-ray events with a statistical significance of ∼19.9 σ. The off-source data, comprising ∼107 h of observation, is found to be consistent with a no-emission hypothesis, as expected. The resulting γ-ray rate for the on-source data is determined to be ∼(9.31±0.48) h −1. A power law fit (dΦ/dE=f 0 E −Γ ) with f 0 ∼ (2.66±0.29) × 10−11 cm−2 s−1 TeV−1 and Γ ∼ 2.56±0.10 is found to provide reasonable fit to the inferred differential spectrum within statistical uncertainties. The spectrum matches reasonably well with that obtained by other groups. A brief summary of the improvements in the various subsystems of the telescope carried out recently, which has resulted in a substantial improvement in its detection sensitivity (viz., ∼5 σ in an observation period of ∼13 h as compared to ∼25 h earlier) are also presented in this paper. Encouraged by the detection of strong γ-ray signals from Mrk 501 and Mrk 421 on several occasions, there is considerable scope for the TACTIC telescope to monitor similar TeV γ-ray emission activity from other active galactic nuclei on a long-term basis.

GAMMA-RAYS FROM MILLISECOND PULSAR POPULATION IN THE GALACTIC CENTRE

The central dense stellar cluster in the Galactic Center (GC) contains the mass of ~4 times larger than that of the central black hole. It is expected to be formed as a result of a merging process of several massive globular clusters which provided a large number of millisecond pulsars (MSPs). We propose that the GeV-TeV γ-ray emission observed from GC is in fact a cumulative effect of the emission from several globular clusters captured by the GC black hole. We calculate the expected TeV γ-ray emission produced by leptons, injected by pulsars, by the Inverse Compton Scattering process in the diffusive radiation field. It is shown that this emission can be responsible for the multi-TeV γ-rays observed by the Cherenkov telescopes from GC if about a thousand of MSPs are present in the central cluster in GC.

Gamma-rays from millisecond pulsar population within the central stellar cluster in the Galactic Centre

Abstract It was proposed that the central dense stellar cluster in the Galactic Centre, containing the mass of ∼4 times larger than that of the central black hole, had been formed as a result of a merging of several massive globular clusters. These globular clusters are expected to provide a large number of millisecond pulsars (MSPs) within the central parsec of the Galactic Centre. We propose that the GeV γ-ray emission observed from the Galactic Centre is in fact a cumulative effect of the emission from several globular clusters captured by the Galactic Centre black hole. Moreover, the MSPs in globular clusters accelerate leptons in their wind zones to energies of the order of a few tens of TeV injecting them into the dense infrared and optical radiation region present within the central parsec. We calculate the expected TeV γ-ray emission produced by these leptons by the inverse Compton scattering process in the soft radiation field. It is shown that this emission can be responsible for the multi-TeV γ-rays observed by the Cherenkov telescopes from the Galactic Centre for reasonable densities of the soft radiation, diffusion models for the propagation of leptons, their injection parameters (fluxes and spectral proprieties). If the energy conversion efficiency from the pulsars to the relativistic leptons is of the order of 10 per cent, then about a thousand of MSPs have to be present in the central cluster in the Galactic Centre.

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

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

Multi-band Observation of TeV Supernova Remnants

AbstractWe study several TeV Supernova Remnants (SNRs W51C, CTB 37A, CTB 37B and G353.6-0.7) by radio and X-ray observations. We utilize neutral hydrogen (\HI) 21 cm line data to measure their kinematic distances, and use the CO line survey sensitive to molecular hydrogen clouds to validate these distance measurements and understand their relation to the TeV SNRs. Our study show that the TeV γ-ray emission from W51C should not be associated with the high-velocity HI clouds; CTB 37A and CTB 37B are at different distances and are only by chance nearby each other on the sky; the extended TeV emission from G353.6-0.7 possibly originates from the interaction between the SNR shock and the adjacent CO clouds.

ν production in Centaurus A and M87 from γ ray interactions with the gas and dust at the sources

Centaurus A and M87 are the closest galaxies with active galactic nuclei and TeV γ-ray emission. The existence of such TeV radiation suggests the production of a neutrino flux from the photo-hadronic interactions of the γ-photons and the gas and dust content of the active galaxies. Using a simple model of Centaurus A and M87, the corresponding ν luminosities at source and their fluxes at Earth were calculated. The neutrino fluxes associated with the aforementioned process resulted to be E2Φν+ν̄ ≲ 10−13 s−1 GeVcm−2, more than 6 orders of magnitude below the modern upper limits from neutrino telescopes. It will be shown that, at high-energies, relevant for neutrino astronomy, these ν fluxes are not competitive with those fluxes derived from astrophysical scenarios involving cosmic ray acceleration.

Modelling TeV γ-ray emission from the kiloparsec-scale jets of Centaurus A and M87

The widespread detection of synchrotron X-ray emission from the jets of low-power, nearby radio galaxies implies the presence of electrons at and above TeV energies. In this paper we explore the possibility that the TeV gamma-rays detected from the radio galaxies Cen A and M87, which both have bright, well-studied X-ray jets, are produced at least in part by inverse-Compton scattering of various photon fields by the high-energy electrons responsible for the synchrotron X-rays on kiloparsec scales. We describe a new numerical code that we have developed to carry out inverse-Compton calculations taking account of all the relevant physics and using detailed models of the jets and the photon fields in which they are embedded, and show that existing constraints on the very high-energy (VHE) gamma-ray fluxes of these two objects already place significant constraints on the magnetic field strengths in the jet in Cen A. Finally, we discuss the prospects for constraints on radio galaxy jet physics that may be obtained from observations with the Cerenkov Telescope Array (CTA).

Gamma-rays and neutrinos from accreating neutron stars

AbstractDense wind of a massive star can be partially captured by a companion neutron star (NS) creating a very turbulent and magnetized transition region at some distance from the NS surface. We consider the consequences of electron and hadron acceleration at such a transition region. Electrons lose energy on the synchrotron process and the inverse Compton (IC) scattering of thermal radiation from the NS surface and/or the massive star. We calculate the synchrotron spectra (from X-rays to soft γ-rays) and IC spectra in the case of sources accreting the matter under the accretor and propeller scenarios. It is argued that a population of accreting massive binaries, recently discovered by the INTEGRAL observatory, can be detectable by the Fermi LAT telescope. On the other hand, TeV γ-ray emission from other class of massive binaries can be interpreted in terms of a magnetar accreting matter in the propeller scenario. We also calculate the expected neutrino event rates in a km2 detector produced by relativistic hadrons accelerated in such scenario.

Injected spectrum for TeV γ-ray emission from the galactic center

The detection of very high energy γ-ray emission from the Galactic center has been reported by four independent groups. One of these γ-ray sources, the 10 TeV γ-ray radiation reported by HESS, has been suggested as having a hadronic origin when relativistic protons are injected into and interact with the dense ambient gas. Assuming that such relativistic protons required by the hadronic model come from the tidal disruption of a star by the massive black hole of Sgr A*, we explore the spectrum of the relativistic protons. In the calculations, we investigate cases where different types of stars are tidally disrupted by the black hole of Sgr A*, and we consider that different diffusion mechanisms are used for the propagation of protons. The initial energy distribution of the injected spectrum of protons is assumed to follow a power-law with an exponential cut-off, and we derive the different indices of the injected spectra for the tidal disruption of different types of stars. For the best fit to the spectrum of photons detected by HESS, the spectral index of the injected relativistic protons is about 2.05 when a red giant is tidally disrupted by the black hole of Sgr A* and the diffusion mechanism is the Effective Confinement of Protons.

Search for Pulsar Wind Nebula Associations with Unidentified TeV γ‐Ray Sources

Many of the recently discovered TeV γ-ray sources are associated with pulsar wind nebulae (PWNe). In fact, they represent the most populous class of Galactic sources at TeV energies. In addition, HESS has also discovered, in a survey of the Galactic plane, a population of TeV γ-ray sources that are still without definitive counterparts at longer wavelengths. For a number of these sources, a pulsar is an evident association, which is often located within an extended region of the TeV γ-ray emission. These particular HESS sources are promising candidates for yet not resolved PWNe. Here we have undertaken a systematic search for X-ray counterparts of the sources, using the archival Chandra data, within the spatial bounds of the unidentified HESS sources. A number of X-ray sources have been detected in the Chandra fields. Two of them, CXOU J161729.3–505512 and CXOU J170252.4–412848, are of a special interest because of their excellent positional coincidence with the pulsars PSR J1617–5055 and PSR J1702–4128, respectively. The first source is extended, with a bright core of 2.6'' (FWHM) in radius but the emission can be seen up to roughly 20''. The second one is much fainter and detected only with marginal significance (4.6 σ). It might also be slightly extended, although the situation is quite uncertain due to very limited statistics. The analysis of the archival Chandra data for a middle-aged pulsar (PSR J1913+1011) does not reveal any statistically significant excess at and around the position of the pulsar or the center of gravity of its plausible TeV γ-ray counterpart (HESS J1912+101). We discuss the implications of the results.