共查询到20条相似文献，搜索用时 31 毫秒
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Charles D. Dermer 《Astrophysics and Space Science》2007,309(1-4):127-137
Our knowledge of the high-energy universe is undergoing a period of rapid change as new astronomical detectors of high-energy radiation start to operate at their design sensitivities. Now is a boomtime for high-energy astrophysics, with new discoveries from Swift and HESS, results from MAGIC and VERITAS starting to be reported, the upcoming launches of the γ-ray space telescopes GLAST and AGILE, and anticipated data releases from IceCube and Auger. A formalism for calculating statistical properties of cosmological γ-ray sources is presented. Application is made to model calculations of the statistical distributions of γ-ray and neutrino emission from (i) beamed sources, specifically, long-duration GRBs, blazars, and extragalactic microquasars, and (ii) unbeamed sources, including normal galaxies, starburst galaxies and clusters. Expressions for the integrated intensities of faint beamed and unbeamed high-energy radiation sources are also derived. A toy model for the background intensity of radiation from dark-matter annihilation taking place in the early universe is constructed. Estimates for the γ-ray fluxes of local group galaxies, starburst, and infrared luminous galaxies are briefly reviewed. Because the brightest extragalactic γ-ray sources are flaring sources, and these are the best targets for sources of PeV–EeV neutrinos and ultra-high energy cosmic rays, rapidly slewing all-sky telescopes like MAGIC and an all-sky γ-ray observatory beyond Milagro will be crucial for optimal science return in the multi-messenger age. 相似文献
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The EGRET observations have confirmed and proposed the new isotropically distributed γ-ray background, but the known objects radiating γ-rays can not supply so much radiation. Meanwhile, EGRET also reveals a population of γ-ray sources with no radio counterparts which are isotropically distributed in the sky, indicating their possible cosmological origins. Wang et al.[13] proposed a new γ-ray radiation process driven by the radiation feedback of AGNs. The energy of the radiation peaks around 1 GeV—0.1 TeV with the typical luminosity of 1042—1043 ergs · s?1. This kind of radiation process in the radio quiet quasars make them the potential γ-ray radiation sources as well as the contributors to the γ-ray background. We consider two cases in which the seed photons in the inverse-Compton processes are from the accretion disks of quasars and cosmic microwave background (CMB), respectively. We find that the former contributes 78%—92% of the background radiation around 1 GeV, while the contribution from the latter is negligible. The radio quiet quasars are highly likely to become the objects which contribute the most energy to the γ-ray background around 1 GeV. 相似文献
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Based upon the Fermi blazars sample, the radio and γ-ray emissions are compiled for a sample of 74 γ-ray loud blazars to calculate the radio to γ-ray effective spectrum index α R γ. The correlations between α R γ and γ-ray luminosity, and between radio and γ-ray luminosity are also investigated. 相似文献
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V. V. Vassiliev 《Astroparticle Physics》2000,12(4)
Recent observations of the TeV γ-ray spectra of the two closest active galactic nuclei (AGNs), Markarian 501 (Mrk 501) and Markarian 421 (Mrk 421), by the Whipple and HEGRA collaborations have stimulated efforts to estimate or limit the spectral energy density (SED) of extragalactic background light (EBL) which causes attenuation of TeV photons via pair-production when they travel cosmological distances. In spite of the lack of any distinct cutoff-like feature in the spectra of Mrk 501 and Mrk 421 (in the interval 0.26–10 TeV) which could clearly indicate the presence of such a photon absorption mechanism, we demonstrate that a strong EBL attenuation signal (survival probability of 10 TeV photon <10−2) may still be present in the spectra of these AGNs. This attenuation could escape detection due to ambiguity of spectra interpretation between intrinsic properties of the sources and absorption by EBL. By estimating the minimal and maximal opacity of the universe to TeV γ-ray photons, we calculate the visibility range for current and future γ-ray observatories, and show that the Whipple γ-ray telescope should be able to detect (in 10 hours at a 5σ confidence level) a BL Lac object with properties similar to Mrk 501 during its peak activity located at distances up to z=0.12. The proposed atmospheric Cherenkov telescope array VERITAS should be able to see such an object at least as far as z=0.3. Finally, we show that the proposed experiments, VERITAS, HESS, and MAGIC, may even be able to actually measure the EBL SED because their observations extend to the critical 75–150 GeV regime. In this transition region a distinct “knee-like” feature should exist in the spectra of blazars, which is invariant with respect to their intrinsic properties. The change of the spectral index and flux amplitude across this knee, if observed for several blazars, will provide missing pieces of information needed to measure EBL in the wavelength range 0.1–30 μm. 相似文献
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In this paper, we review the prospects for studies of active galactic nuclei (AGN) using the envisioned future Cherenkov Telescope Array (CTA). This review focuses on jetted AGN, which constitute the vast majority of AGN detected at gamma-ray energies. Future progress will be driven by the planned lower energy threshold for very high energy (VHE) gamma-ray detections to ∼10 GeV and improved flux sensitivity compared to current-generation Cherenkov Telescope facilities. We argue that CTA will enable substantial progress on gamma-ray population studies by deepening existing surveys both through increased flux sensitivity and by improving the chances of detecting a larger number of low-frequency peaked blazars because of the lower energy threshold. More detailed studies of the VHE gamma-ray spectral shape and variability might furthermore yield insight into unsolved questions concerning jet formation and composition, the acceleration of particles within relativistic jets, and the microphysics of the radiation mechanisms leading to the observable high-energy emission. The broad energy range covered by CTA includes energies where gamma-rays are unaffected from absorption while propagating in the extragalactic background light (EBL), and extends to an energy regime where VHE spectra are strongly distorted. This will help to reduce systematic effects in the spectra from different instruments, leading to a more reliable EBL determination, and hence will make it possible to constrain blazar models up to the highest energies with less ambiguity. 相似文献
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We present a calculation of the blazar contribution to the extragalactic diffuse γ -ray background (EGRB) in the EGRET energy range. Our model is based on inverse-Compton scattering as the dominant γ -ray production process in the jets of flat spectrum radio quasars (FSRQs) and BL Lac objects, and on the unification scheme of radio-loud AGN. According to this picture, blazars represent the beamed fraction of the Fanaroff–Riley radio galaxies (FR galaxies).
The observed log N –log S distribution and redshift distribution of both FSRQs and BL Lacs constrain our model. Depending slightly on the evolutionary behaviour of blazars, we find that unresolved AGN underproduce the intensity of the extragalactic background radiation. With our model only 20–40 per cent of the extragalactic background emission can be explained by unresolved blazars if we integrate to a maximum redshift of Zmax =3. For Z max =5, blazars could account for 40–80 per cent of the EGRB. Roughly 70–90 per cent of the AGN contribution to the EGRB would result from BL Lacs. While the systematic uncertainties in our estimate for the FSRQ contribution appear small, in the case of BL Lacs our model parameters are not consistent with the results from studies in other wavelength regimes, and therefore may have larger systematic uncertainties. Thus we end up with two possibilities, depending on whether we underpredict or overpredict the BL Lac contribution: either unresolved AGN cannot account for the entire EGRB, or unresolved BL Lacs produce the observed background.
We predict a significant flattening of the γ -ray log N –log S function in the next two decades of flux below the EGRET threshold. 相似文献
The observed log N –log S distribution and redshift distribution of both FSRQs and BL Lacs constrain our model. Depending slightly on the evolutionary behaviour of blazars, we find that unresolved AGN underproduce the intensity of the extragalactic background radiation. With our model only 20–40 per cent of the extragalactic background emission can be explained by unresolved blazars if we integrate to a maximum redshift of Z
We predict a significant flattening of the γ -ray log N –log S function in the next two decades of flux below the EGRET threshold. 相似文献
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The multiband nonthermal emissions in radio,X-ray,and very high-energy (VHE)γ-ray bands from two distant blazars,H 2356-309 and 1ES 1218+304,have been detected,and,especially from recent observations with the Suzaku,MAGIC and VERITAS telescopes,clearly reveal nonthermal power-law spectra. We study the broadband nonthermal spectra of the two sources by using a combination of a onezone homogeneous synchrotron self-Compton (SSC) model and an inhomogeneous conical jet model,where the new external background light (EBL) model is taken into account. The results show that (1) the nonthermal emissions of the two blazars,ranging from X-rays to VHE γ-rays,are from the homogeneous zone whereas the emissions in the radio bands can be explained as the radiation from the inhomogeneous conical jet; (2) a strict lower-limit EBL model can be used to explain their observed spectra well. 相似文献
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《New Astronomy》2015
Non extensive statistical physics has been applied to various problems in physics including astrophysics. In this paper we explore the possibility of using non-extensive approach to explain the recently observed pulsed γ-ray from Crab pulsar above 100 GeV observed by VERITAS γ-ray telescope. 相似文献
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We discuss the contribution of the blazar population to the extragalactic background radiation across the electromagnetic (e.m.) spectrum with particular reference to the microwave, hard-X-ray and γ-ray bands. Our estimates are based on a recently derived blazar radio LogN-LogS that was built by combining several radio and multi-frequency surveys. We show that blazar emission integrated over cosmic time gives rise to a considerable broad-band non-thermal cosmic background that dominates the extragalactic brightness in the high-energy part of the e.m. spectrum. We also estimate the number of blazars that are expected to be detected by future planned or hypothetical missions operating in the X-ray and γ-ray energy bands. 相似文献
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In this work, we report on the intense flaring activity from Mkn-421 in X-ray and γ-ray regimes simultaneously observed by Swift-XRT/BAT and Fermi-LAT satellite telescopes in February 2010. With the aim of understanding the underlying physics of the flaring state in Mkn-421, we have performed a detailed spectral analysis of Swift/XRT and Fermi/LAT observations of Mkn-421 during February 12–25, 2010 (MJD 55239–55252). Over this period, we study the daily light curves and spectral variability of the source in 1–10 keV, 0.1–1 GeV and 1–100 GeV energy bands. We have performed the spectral analysis of Swift-XRT and Fermi/LAT observations to study the spectral evolution in the X-ray and gamma-ray energy domains respectively. We also compute the fractional variability amplitude in both the energy bands during the above period. We study trends between spectral parameters and physical insights provided by the parameter responsible for X-ray and γ-ray emission from the source. We search for energetic features phenomenologically linked to the single zone SSC model for blazar emission. We also produce the broad band SED with a leptonic single zone SSC model for the source. 相似文献
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A number of groups have reported significant reduction in the flux of low energy (0.1–3 MeV) γ-rays in observations carried out during the past total solar eclipses. However, the contribution of the radon induced radioactivity to the overall γ-ray background can become substantial, especially during episodes of rain. Depending upon the pattern of the rainfall radon induced γ-ray background may vary significantly on time scales of ∼10 min, making the interpretation of the data in terms of an extraterrestrial effect such as a total solar eclipse rather difficult. A reliable estimate of the low energy terrestrial γ-ray (TGR) background is necessary before attempting to measure the possible contribution of any extraterrestrial phenomenon. The knowledge of the precise energies and branching ratios of radon and other radio-isotope induced γ-rays was exploited to accurately reproduce the TGR background, even in the presence of a large and variable contribution from radon induced radioactivity from fresh rain water. The measurement of the TGR background has paved the way for studying the variation of the soft γ-ray flux during the long duration total solar eclipse that occurred on 22 July 2009 in the middle of the Monsoon season in India. 相似文献
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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. 相似文献
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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. 相似文献
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Luigi Costamante 《Astrophysics and Space Science》2007,309(1-4):487-495
The study of the TeV emission from extragalactic sources is hindered by the uncertainties on the diffuse Extragalactic Background
Light (EBL). The recent H.E.S.S. results on the blazars 1ES 1101-232 and H 2356-309 represent a breakthrough on this issue.
Their unexpectedly hard spectra allow an upper limit to be derived on the EBL in the optical/near-infrared range, which is
very close to the lower limit given by the resolved galaxy counts. This result seems to exclude a large contribution to the
EBL from other sources (e.g. Population III stars) and indicates that the intergalactic space is more transparent to γ-rays than previously thought. A discussion of EBL absorption effects and further observational tests with Cherenkov telescopes
are presented.
For the H.E.S.S. collaboration. 相似文献
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《Astroparticle Physics》2010,32(6):431-436
We consider Vela Jr. as being the old Supernova Remnant (SNR) at the beginning of the transition from adiabatic to radiative stage of evolution. According to our model, Vela Jr. is situated outside Vela SNR at the distance of ∼600 pc and its age is 17500 yr. We model the high energy fluxes from Vela Jr. and its broadband spectrum. We find our results compatible with experimental data in radio waves, X- and γ-rays. Our hydrodynamical model of Vela Jr. explains the observed TeV γ-ray flux by hadronic mechanism. The proposed model does not contradict to the low density environment of the SNR and does not need extreme fraction of the explosion energy to be transferred to Cosmic Rays. 相似文献
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Felix A. Aharonian D. Khangulyan L. Costamante 《Monthly notices of the Royal Astronomical Society》2008,387(3):1206-1214
The energy spectra of TeV gamma-rays from blazars, after being corrected for intergalatic absorption in the extragalactic background light (EBL), appear unusually hard, a fact that poses challenges to the conventional models of particle acceleration in TeV blazars and/or to the EBL models. In this paper, we show that the internal absorption of gamma-rays caused by interactions with dense narrow-band radiation fields in the vicinity of compact gamma-ray production regions can lead to the formation of gamma-ray spectra of an almost arbitrary hardness. This allows significant relaxation of the current tight constraints on particle acceleration and radiation models, although at the expense of enhanced requirements to the available non-thermal energy budget. The latter, however, is not a critical issue, as long as it can be largely compensated by the Doppler boosting, assuming large (>10) Doppler factors of the relativistically moving gamma-ray production regions. The suggested scenario of formation of hard gamma-ray spectra predicts detectable synchrotron radiation of secondary electron–positron pairs which might require a revision of the current 'standard paradigm' of spectral energy distributions of gamma-ray blazars. If the primary gamma-rays are of hadronic origin related to pp or p γ interactions, the 'internal gamma-ray absorption' model predicts neutrino fluxes close to the detection threshold of the next generation high-energy neutrino detectors. 相似文献