Intrinsic constraints on very high energy emission in gamma-ray loud blazars
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Photons with very high energy (VHE) up to TeV emitted from active galactic nuclei (AGNs) provide some invaluable information on the origin of gamma-ray emission. Although 66 blazars have been detected by EGRET, only three low-redshift X-ray-selected BL Lac objects (Mrk 421, Mrk 501, and 1ES 2344+514) are conclusive TeV emitters (PKS 2155-304 is a potential TeV emitter), since VHE photons may be absorbed by cosmological background infrared photons (external absorption). Based on the mirror effect of clouds in the broad-line region, we argue that there is an intrinsic mechanism for the deficiency of TeV emission in blazars. Employing the observable quantities, we derive the pair production optical depth tau(gamma gamma)(epsilon(obs)) due to the interaction of VHE photons with the reflected synchrotron photons by electron Thomson scattering in the broad-line region. This sets stronger constraints on very high energy emission, and provides a sensitive upper limit on the Doppler factor of the relativistic bulk motion. It has been suggested that the intrinsic absorption be distinguished from the external by the observation on variation of the multiwavelength continuum.Keywords:
BL Lac object
Extragalactic background light
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The last years have seen a revolution in ground-based gamma-ray detectors. We can now detect the spectra of nearby TeV blazars like Mrk 421 and 501 out to approximately 20 TeV, and during the strongest flares, we can now follow fluctuations in these spectra on timescales close to the shortest ones likely in these objects. We point out that this represents a unique opportunity. Using these and future detectors in combination with broadband X-ray satellites like SAX and RXTE, we will be able to simultaneously follow all significant X-ray/gamma-ray variations in a blazar's emission. This will provide the most stringent test yet of the synchrotron-Compton emission model for these objects. In preparation for the data to come, we present sample SSC model calculations using a fully self-consistent, accurate code to illustrate the variability behavior one might see (the range of behavior is wider than many expect) and to show how good timing information can probe physical conditions in the source. If the model works, i.e., if X-ray/TeV variations are consistent with being produced by a common electron distribution, then we show it is possible to robustly estimate the blazar's intrinsic TeV spectrum from its X-ray spectrum. Knowing this spectrum, we can then determine the level of absorption in the observed spectrum. Constraining this absorption, due to gamma-ray pair production on diffuse radiation, provides an important constraint on the infrared extragalactic background intensity. Without the intrinsic spectrum, we show that detecting absorption is very difficult and argue that Mrk 421 and 501, as close as they are, may already be absorbed by a factor 2 at approximately 3 TeV. This should not be ignored when fitting emission models to the spectra of these objects.
Spectral shape analysis
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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 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 a special form of SED of EBL and unknown intrinsic spectra of these blazars. Here we show how the proposed and existing experiments, VERITAS, HESS, MAGIC, STACEE and CELESTE may be able to detect or severely limit the EBL SED by extension of spectral measurements into the critical 100–300 GeV regime.
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H1426+428 is one of blazars that are observed by $γ$-rays in the TeV region. Because TeV $γ$-rays from distant sources are subject to attenuation by the extragalactic background light (EBL) via electron-positron pair production, the intrinsic spectrum of the TeV $γ$-rays should be inferred by using the models of radiation processes and EBL spectrum. We set constraints on the physical condition of H1426+428 with the synchrotron-self-Compton model applying several EBL models. We find that the emission region of H1426+428 is moving toward us with the bulk Lorentz factor of $\sim 20$ and that its magnetic field strength is $\sim 0.1$ G. These properties are similar to other TeV blazars such as Mrk 421 and Mrk 501. However, the ratio of the energy density of nonthermal electrons to that of the magnetic fields is about 190 and fairly larger than those of Mrk 421 and Mrk 501, which are about 5 -- 20. It is also found that the intensity of EBL in the middle and near infrared wavelengths should be low, i.e., the intensity at 10 $μ$m is about 1 nW m$^{-2}$ sr$^{-1}$ to account for the observed TeV $γ$-ray flux. Because the spectral data of H1426+428 in X-rays and $γ$-rays used in our analysis were not obtained simultaneously, further observations of TeV blazars are necessary to make the constraints on EBL more stringent.
Extragalactic background light
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Throughout this thesis, the spectral and temporal long-term behavior of the Active Galactic Nucleus (AGN) 1ES1959+650 is studied. Being a blazar, i.e. an AGN with a highly relativistic plasma outflow pointing towards the Earth, and having shown strong hints of hadronic emission processes in the past, 1ES1959+650 is a primary candidate for a source of cosmic rays. The origin of these ultra high energetic charged particles is the central question of astroparticle physics. Therefore, 1ES1959+650 has been monitored with the MAGIC telescope since 2004, measuring very high energy (VHE) gamma-rays. The resulting time-integrated energy spectrum reported here is by far the most exact one determined for this source up to now. Also the temporal variability for different wavelength bands, which should be connected in the case of most non-hadronic emission processes, is examined in this thesis. While strong variations occur in the optical and X-rays, the VHE gamma-ray emission is compatible with a constant flux. Furthermore, the time-integrated spectral energy distribution is compiled from radio to VHE gamma-rays and modeled for hadronic and leptonic emission scenarios to unveil the character of the emitting particles. Unexpected results are obtained on the spectral shape of the observed blazar, especially in high energy gamma-rays. By this, 1ES1959+650 is one of the very first high frequency peaked blazars whose non-thermal emission cannot be explained with a single-zone SSC model.Additionally, in this thesis the idea of long-term monitoring of blazars at VHE gamma-rays is pursued further. The FACT telescope, the first application of G-APDs as photosensors in an imaging atmospheric Cherenkov telescope, has recently been constructed and already during its commissioning phase reported on very promising first results. After the commissioning, it will be devoted to monitoring observations of bright blazars. For the construction of this telescope, the completely reworked aluminum mirrors have been characterized, including measurements of the focal lengths and the spot sizes of a reflected point-like light source.Finally, it is presented that DWARF, a global network of a small number of Cherenkov telescopes dedicated to monitoring observations of TeV-bright blazars, will be well suited to obtain comprehensive, well sampled lightcurves which are needed to study possible temporal correlations between VHE gamma-rays and other wavelengths.
Astroparticle physics
Spectral energy distribution
Extragalactic background light
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We summarize broadband observations of the TeV-emitting blazar 1ES 1959+650, including optical R-band observations by the robotic telescopes Super-LOTIS and iTelescope, UV observations by Swift UVOT, X-ray observations by the Swift X-ray Telescope (XRT), high-energy gamma-ray observations with the Fermi Large Area Telescope (LAT) and very-high-energy (VHE) gamma-ray observations by VERITAS above 315 GeV, all taken between 17 April 2012 and 1 June 2012 (MJD 56034 and 56079). The contemporaneous variability of the broadband spectral energy distribution is explored in the context of a simple synchrotron self Compton (SSC) model. In the SSC emission scenario, we find that the parameters required to represent the high state are significantly different than those in the low state. Motivated by possible evidence of gas in the vicinity of the blazar, we also investigate a reflected-emission model to describe the observed variability pattern. This model assumes that the non-thermal emission from the jet is reflected by a nearby cloud of gas, allowing the reflected emission to re-enter the blob and produce an elevated gamma-ray state with no simultaneous elevated synchrotron flux. The model applied here, although not required to explain the observed variability pattern, represents one possible scenario which can describe the observations. As applied to an elevated VHE state of 66% of the Crab Nebula flux, observed on a single night during the observation period, the reflected-emission scenario does not support a purely leptonic non-thermal emission mechanism. The reflected emission model does, however, predict a reflected photon field with sufficient energy to enable elevated gamma-ray emission via pion production with protons of energies between 10 and 100 TeV.
BL Lac object
Spectral energy distribution
Cherenkov Telescope Array
Crab Nebula
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Observations of very-high-energy (VHE; E > 250 GeV) γ-ray emission from several blazars at z > 0.1 have placed stringent constraints on the elusive spectrum and intensity of the intergalactic infrared background radiation (IIBR). Correcting the observed VHE spectrum for γγ absorption, even by the lowest plausible level of the IIBR, provides evidence for a very hard (photon spectral index Γph < 2) intrinsic source spectrum out to TeV energies. Such a hard VHE γ-ray spectrum poses a serious challenge to the conventional synchrotron self-Compton (SSC) interpretation of the VHE emission of TeV blazars and suggests the emergence of a separate emission component beyond a few hundred GeV. Here we propose that such a very hard, slowly variable VHE emission component in TeV blazars may be produced via Compton upscattering of cosmic microwave background (CMB) photons by shock-accelerated electrons in an extended jet. For the case of 1ES 1101–232, this component could dominate the bolometric luminosity of the extended jet if the magnetic fields are of the order of typical intergalactic magnetic fields (B ∼ 10 μG) and if electrons are still being accelerated out to TeV energies (γ ≳ 4 × 106) on kiloparsec scales along the jet.
Extragalactic background light
Cosmic infrared background
Cosmic background radiation
Spectral index
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Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation James H. Buckley; Very high energy gamma rays from blazars. AIP Conf. Proc. 7 October 2001; 587 (1): 235–245. https://doi.org/10.1063/1.1419406 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioAIP Conference Proceedings Search Advanced Search |Citation Search
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Extragalactic background light
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