Very high energy γ-rays from galactic sources

The field of Very High Energy (VHE) gamma ray astronomy using the Atmospheric Cerenkov Technique has entered an interesting phase with detection of various galactic and extragalactic sources. Among galactic sources, only the Crab nebula has been established as a standard candle. Most observations on pulsars are in agreement as to the necessity for the GeV spectra to steepen at < 200 GeV. While the Imaging method for increase of sensitivity has been successful with many results, an alternate technique —Wavefront Sampling Technique- is also being used by an increasing number of experiments. The recently commissioned experiment at Pachmarhi (PACT) in India is presented as an example of this technique. Preliminary results from this experiment show detection of VHE γ-ray emission from (a) the Crab nebula at a high significance and (b) Crab and Geminga pulsars at > 1.5 TeV which could be the second component of the Outer Gap pulsar models.     

Search for gamma-ray bursts with energies above 1 TEV

Some models of gamma-ray bursts suggest that their spectrum may extend into the energy range above 10 GeV, i.e. above energies accessible with CGRO. Data taken with the HEGRA extensive air shower array are used to search for very high energetic gamma-ray burst counterparts above 1 TeV. First results from the search for BATSE correlated and BATSE independent bursts are presented.     

Analysis of the activity of the blazar BL Lacertae over the period 1998–2011

In 1998–2011 the blazar (active galactic nucleus) BL Lacertae was observed at Crimean Astrophysical Observatory (CrAO) with the second-generation GT-48 Cherenkov telescope at energies >1 TeV with a total significance of 11.8σ. More than 20 flares and a fourfold change in yearly mean fluxes (>1 TeV) were recorded. The optical (B band) data obtained at CrAO and the TeV data are shown to correlate in some time intervals. The optical data are also compared with the X-ray RXTE/ASM (2–10 keV) data. In addition, the data from GT-48 are compared with the gamma-ray fluxes recorded by the Fermi LAT space telescope (0.1–300 GeV). The 2009 flare at TeV and Fermi energies has been studied. As a result, it has been found that as the activity rises the increase in flux at high energies exceeds its increase at low energies. This conclusion may be related to the conversion mechanism of particle acceleration. This is consistent with the results of studies for a similar object, 1ES 1426+428.     

Milagro observations of potential TeV emitters

This paper reports the results from three targeted searches of Milagro TeV sky maps: two extragalactic point source lists and one pulsar source list. The first extragalactic candidate list consists of 709 candidates selected from the Fermi-LAT 2FGL catalog. The second extragalactic candidate list contains 31 candidates selected from the TeVCat source catalog that have been detected by imaging atmospheric Cherenkov telescopes (IACTs). In both extragalactic candidate lists Mkn 421 was the only source detected by Milagro. This paper presents the Milagro TeV flux for Mkn 421 and flux limits for the brighter Fermi-LAT extragalactic sources and for all TeVCat candidates. The pulsar list extends a previously published Milagro targeted search for Galactic sources. With the 32 new gamma-ray pulsars identified in 2FGL, the number of pulsars that are studied by both Fermi-LAT and Milagro is increased to 52. In this sample, we find that the probability of Milagro detecting a TeV emission coincident with a pulsar increases with the GeV flux observed by the Fermi-LAT in the energy range from 0.1 GeV to 100 GeV.     

Gamma-ray bursts, ultra-high-energy cosmic rays, and cosmic gamma-ray background

We argue that gamma-ray bursts (GRBs) may be the origin of the cosmic gamma-ray background radiation observed in the GeV range. It has theoretically been discussed that protons may carry a much larger amount of energy than electrons in GRBs, and this large energy can be radiated in the TeV range by synchrotron radiation of ultra-high-energy protons ( 10²⁰ eV). The possible detection of GRBs above 10 TeV suggested by the Tibet and HEGRA groups also supports this idea. If this is the case, most of TeV gamma-rays from GRBs are absorbed in intergalactic fields and eventually form GeV gamma-ray background, whose flux is in good agreement with the recent observation.     

A synchrotron self-Compton scenario for the very high energyγ-ray emission of the intermediate BL Lacertae object W Comae

W Comae has significant variability in multi-wavelengthes, from radio to gamma-ray bands. A bright outburst in optical and X-ray bands was observed in 1998, and most recently, a strong TeV flare was detected by VERITAS in 2008. It is the first TeV intermediate-frequency-peaked BL Lacertae source. I find that both the broadband spectral energy distributions (SEDs) which were quasi-simultaneously obtained during the TeV flare and during the optical/X-ray outburst are well fit by using a single-zone synchrotron + synchrotron-self-Compton model. The satisfactory fitting requires a large beaming factor, i.e., δ～25 and δ～20 for the TeV flare and the optical/X-ray outburst, respectively, suggesting that both the optical/X-ray outburst and the TeV flare are from a relativistic jet. The size of the emission region of the TeV flare is three times larger than that of the optical/X-ray outburst, and the strength of the magnetic field for the TeV flare is～14 times smaller than that of the X-ray/optical outburst, likely indicating that the region of the TeV flare is more distant from the core than that of the X-ray/optical outburst. The inverse Compton component of the TeV flare peaks around 1.3 GeV, but it is around 0 MeV for the X-ray/optical outburst, lower than that for the TeV flare by two orders of magnitude. The model predicts that the optical/X-ray outburst might be accompanied by a strong MeV/GeV emission, but the TeV flare may be not associated with the X-ray/optical outburst. The GeV emission is critical for characterizing the SEDs of the optical/X-ray outburst and the TeV flare. The predicted GeV flux is above the sensitivity of Fermi/LAT, and it could be verified with the observations by Fermi/LAT in the near future.     

Photon-photon absorption above a molecular cloud torus in blazars

Gamma rays have been observed from two blazars at TeV energies. One of these, Markarian 421, has been observed also at GeV energies and has roughly equal luminosity per decade at GeV and TeV energies. Photon-photon pair production on the infrared background radiation is expected to prevent observation above 1 TeV. However, the infrared background is not well known and it may be possible to observe the nearest blazars up to energies somewhat below 100 TeV where absorption on the cosmic microwave background will give a sharp cut-off. Blazars are commonly believed to correspond to low power radio galaxies, seen down along a relativistic jet; as such they are all expected to have the nuclear activity encircled by a dusty molecular torus, which subtends an angle of 90 degrees or more in width as seen from the central source. Photon-photon pair production can also take place on the infrared radiation produced at the AGN by this molecular torus and surrounding outer disk. We calculate the optical depth for escaping γ-rays produced near the central black hole and at various points along the jet axis for the case of blazars where the radiation is observed in a direction closely aligned with the jet. We find that the TeV emission site must be well above the top of the torus. For example, if the torus has an inner radius of 0.1 pc and an outer radius of 0.2 pc, then the emission site in Mrk 421 would have be at least 0.25 pc above the upper surface of the torus, and if Mrk 421 is observed above 50 TeV in the future, the emission site would have to be at least 0.5 pc above the upper surface. This has important implications for models of γ-ray emission in active galactic nuclei.     

Milagro limits and HAWC sensitivity for the rate-density of evaporating Primordial Black Holes

Primordial Black Holes (PBHs) are gravitationally collapsed objects that may have been created by density fluctuations in the early universe and could have arbitrarily small masses down to the Planck scale. Hawking showed that due to quantum effects, a black hole has a temperature inversely proportional to its mass and will emit all species of fundamental particles thermally. PBHs with initial masses of ∼5.0 × 10¹⁴ g should be expiring in the present epoch with bursts of high-energy particles, including gamma radiation in the GeV–TeV energy range. The Milagro high energy observatory, which operated from 2000 to 2008, is sensitive to the high end of the PBH evaporation gamma-ray spectrum. Due to its large field-of-view, more than 90% duty cycle and sensitivity up to 100 TeV gamma rays, the Milagro observatory is well suited to perform a search for PBH bursts. Based on a search on the Milagro data, we report new PBH burst rate density upper limits over a range of PBH observation times. In addition, we report the sensitivity of the Milagro successor, the High Altitude Water Cherenkov (HAWC) observatory, to PBH evaporation events.     

Inverse Compton e± pair cascade model for the γ-ray production in massive binary LSI +61° 303

We apply an inverse Compton   e ^±  pair cascade model for γ-ray production in the massive binary system LSI +61° 303 assuming that electrons are accelerated already inside the inner part of the jet launched by the compact object. γ-ray spectra, affected by the cascade process, and lower energy spectra, from the synchrotron cooling of the highest energy electrons in the jet, are calculated as a function of the phase of this binary system. γ-ray spectra expected in such a model have different shape than those ones produced by electrons in the jet directly to observer. Moreover, the model predicts clear anticorrelation between γ-ray fluxes in the GeV (1–10 GeV) and TeV (>200 GeV) energy ranges with the peak of the TeV emission at the phase ∼0.5 (the peak half-width ranges between the phases ∼0.4–0.9, for the inclination of the binary system equal to 60°, and ∼0.4–0.1 for 30°). The fine features of TeV γ-ray emission (fluxes and spectral shapes) as a function of the phase of the binary system are consistent with recent observations reported by the MAGIC collaboration. Future simultaneous observations in the GeV energies (by the GLAST and AGILE telescopes) and in the TeV energies (by the MAGIC and VERITAS telescopes) should test other predictions of the considered model supporting or disproving the hypothesis of acceleration of electrons already in the inner part of the microquasar jets.     

TeV γ-ray fluxes from the long campaigns on Mrk 421 as constraints on the emission of TeV–PeV neutrinos and UHECRs

Long TeV γ-ray campaigns have been carried out to study the spectrum, variability and duty cycle of the BL Lac object Markarian 421. These campaigns have given some evidence of the presence of protons in the jet: (i) Its spectral energy distribution which shows two main peaks; one at low energies (∼1 keV) and the other at high energies (hundreds of GeV), has been described by using synchrotron proton blazar model. (ii) The study of the variability at GeV γ-rays and X-rays has indicated no significant correlation. (iii) TeV γ-ray detections without activity in X-rays, called “orphan flares” have been observed in this object.Recently, The Telescope Array Collaboration reported the arrival of 72 ultra-high-energy cosmic rays with some of them possibly related to the direction of Markarian 421. The IceCube Collaboration reported the detection of 37 extraterrestrial neutrinos in the TeV–PeV energy range collected during three consecutive years. In particular, no neutrino track events were associated with this source. In this paper, we consider the proton–photon interactions to correlate the TeV γ-ray fluxes reported by long campaigns with the neutrino and ultra-high-energy cosmic ray observations around this blazar. Considering the results reported by The IceCube and Telescope Array Collaborations, we found that only from ∼25% to 70% of TeV fluxes described with a power law function with exponential cutoff can come from the proton–photon interactions.     

Particle acceleration in supernova-remnant shocks

It has been known for over 50 years that the radio emission from shell supernova remnants (SNRs) indicates the presence of electrons with energies in the GeV range emitting synchrotron radiation. The discovery of nonthermal X-ray emission from supernova remnants is now 30 years old, and its interpretation as the extension of the radio synchrotron spectrum requires electrons with energies of up to 100 TeV. SNRs are now detected at GeV and TeV photon energies as well. Strong suggestions of the presence of energetic ions exist, but conclusive evidence remains elusive. Several arguments suggest that magnetic fields in SNRs are amplified by orders of magnitude from their values in the ambient interstellar medium. Supernova remnants are thus an excellent laboratory in which to study processes taking place in very high Mach-number shocks. I review the observations of high-energy emission from SNRs, and the theoretical framework in which those observations are interpreted.     

Comparison of Fermi-LAT and CTA in the region between 10–100 GeV

The past decade has seen a dramatic improvement in the quality of data available at both high (HE: 100 MeV to 100 GeV) and very high (VHE: 100 GeV to 100 TeV) gamma-ray energies. With three years of data from the Fermi Large Area Telescope (LAT) and deep pointed observations with arrays of Cherenkov telescope, continuous spectral coverage from 100 MeV to ∼10 TeV exists for the first time for the brightest gamma-ray sources. The Fermi-LAT is likely to continue for several years, resulting in significant improvements in high energy sensitivity. On the same timescale, the Cherenkov Telescope Array (CTA) will be constructed providing unprecedented VHE capabilities. The optimisation of CTA must take into account competition and complementarity with Fermi, in particularly in the overlapping energy range 10–100 GeV. Here we compare the performance of Fermi-LAT and the current baseline CTA design for steady and transient, point-like and extended sources.     

Search for GRB counterparts: non-optical

The present status of the search for counterparts of gamma-ray bursts is given. Particular emphasis will be put on the search for flaring counterparts, i.e. emission during or shortly after the gamma-ray emission. In view of a separate review on optical counterparts I will concentrate on GeV/TeV, X-ray, radio and neutrino searches.     

Discovery of GeV gamma-ray emission from the LMC B0443-6657 with the <Emphasis Type="Italic">Fermi</Emphasis> Large Area Telescope

We report the discovery of gamma-ray detection from the Large Magellanic Cloud (LMC) B0443-6657 using the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. LMC B0443-6657 is a flat-spectrum radio source, possibly associated with a supernova remnant in the Large Magellanic Cloud (LMC N4). Employing the LAT data of 8 years, our results show a significant excess (\(>9.4\sigma \)) of gamma rays in the range of 0.2–100 GeV above the gamma-ray background. A power-law function is found to adequately describe the 0.2–\(100\mbox{ GeV}\)\(\gamma \)-ray spectrum, which yields a photon flux of \(3.27\pm 0.53\ \text{photon}\,\mbox{cm}^{2}\,\mbox{s}^{-1}\) with a photon index of \(2.35\pm 0.11\), corresponding to an isotropic gamma-ray luminosity of \(5.3\times 10^{40}~\mbox{erg}\,\mbox{s}^{-1}\). The hadronic model predicts a low X-ray and TeV flux while the leptonic model predicts an observable flux in these two energy bands. The follow-up observations of the LMC B0443-6657 in X-ray or TeV band would distinguish the radiation models of gamma rays from this region.     

High energy γ-radiation from the core of radio galaxy Centaurus A

The results of analysis of approximately 3 years of gamma-ray observations (August 2008-July 2011) of the radio galaxy Centaurus A with the Fermi Large Area Telescope (Fermi LAT) are presented. By modeling the surrounding (background) sources including the giant lobes of Centaurus A, and using the standard binned likelihood analysis method, the energy spectrum of the core is derived. In the energy range below several GeV it is described by a single power-law with photon index Γ = 2.73 ± 0.06 in agreement with the report of the Fermi LAT collaboration based on the first 10 months observations of the source. However, at higher energies the new data show significant excess above the extrapolation of the energy spectrum from low energies. The total flux between 200 MeV to 50 GeV is estimated to be (1.63 ± 0.14) × 10⁻⁷ ph cm⁻² s⁻¹. The comparison of the corresponding Spectral Energy Distribution (SED) at GeV energies with the SED in the TeV energy band reported by the H.E.S.S. collaboration shows that we deal with two or perhaps even three components of gamma-radiation originating from different regions located within the central 10 kpc of Centaurus A.     

河外TeV能段γ射线背景下限的估算

Fermi卫星对GeV能段的河外伽马射线背景(Extragalactic Gamma-ray Background, EGB)进行了较为精确的测量, 极大提高了对高能伽马射线背景的认识, 但是在TeV能段, 使用空间探测器进行观测非常困难, 只能依赖地面伽马射线探测器, 如成像大气切伦科夫望远镜. 目前, 对于TeV能段的河外伽马射线背景的认识还不完善. 使用有低活跃状态能谱的61个TeV源(包含2个星暴星系、6个射电星系以及53个耀变体)的累计流量给出河外TeV伽马射线背景的下限. 结果显示, 低能段(0.5--4.5TeV)流量由两个临近的耀变体Mrk 421和Mrk 501主导, 贡献了大约58%的累计背景流量; 而大于4.5TeV的能段, 由3个已观测到10TeV以上能段流量的极端耀变体H 1426+428、1ES 1959+650以及1ES 0229+200主导. 最后分别探究了星暴星系、射电星系以及耀变体对河外TeV伽马射线背景的贡献, 不同耀变体子类对河外TeV伽马射线背景的贡献以及不同红移区间TeV源对河外伽马射线背景的贡献.     

Morphological and spectral studies of the shell-type supernova remnants RX J1713.7–3946 and RX J0852.0–4622 with H.E.S.S.

In 2004 and 2005, the shell-type supernova remnants RX J1713.7–3946 and RX J0852.0–4622 were observed and detected with the complete H.E.S.S. array, a system of four Imaging Cherenkov Telescopes located in Namibia and dedicated to the observations of γ-rays above 100 GeV. The energy spectra of these two sources have been measured over a wide energy range and revealed an integral flux above 1 TeV similar to that of the Crab Nebula. Their morphologies were resolved with high accuracy with H.E.S.S. and exhibit a striking correlation with the X-ray images, thereby pioneering a technique of unambiguously identifying spatially extended γ-ray sources. The results of the observations will be presented. Similarities and differences between these two sources will be pointed out as well as possible implications. M. Lemoine-Goumard, F. Aharonian, D. Berge, B. Degrange, D. Hauser, N. Komin, O. Reimer, U. Schwanke for the H.E.S.S. Collaboration     

Implications of the detection of pulsed TeV gamma-rays from Geminga

The implications of the recently reported 237 ms pulsed TeV gamma-ray signal from Geminga with respect to system energetics and gamma-ray emission mechanisms are discussed. Using the reported flux values in the MeV-GeV range from the EGRET and COMPTEL experiments and the published results on TeV emission, the derived emission spectrum of Geminga in the MeV-TeV energy range is shown to suggest a spectral break at 2 GeV, with the differential exponent changing from a value of –1.50 upto 2 GeV to a value of –2.7 in the TeV range.