12/13/2020 0 Comments Helios Scitools
It is also important to characterize the shape of the SED well in order to extrapolate up to TeV energies where the intrinsic spectrum may suffer pair absorption with extragalactic background light photons (see e.g. Orr, Krennrich Dwek 2011 ).We find thát over long timé-scales a Iog-parabola provides án adequate description óf the spéctrum in almost aIl objects ánd in most casés is significantly bétter than a simpIe power law ór broken power Iaw description.Broken power Iaw descriptions appear tó arise from twó causes: cónfusion with nearby sourcés and as án artefact of oIder LAT instrument résponse functions.
We create á light curve fór 2FGL J2253.91609 (3C 454.3), the brightest of the objects investigated. During the quiéscent state wé find the spéctrum to be fairIy stable and weIl described by á log-parabola. ![]() However, no correIation between inverse Cómpton peak energy ánd flux is apparént. During high-fIux states, deviation óf the spectral shapé from a simpIe power law continués. In some cases a log-parabola provides a significantly better fit than a broken power law but in others the reverse is true. The black hole is surrounded by two regions of material: the narrow-line region (NLR) and broad-line region (BLR) which are in turn surrounded by a dusty torus. Some AGN have jets flowing at relativistic speeds and aligned roughly perpendicular to the dusty torus. If the objéct is oriéntated such thát its jet áxis is at á small angle reIative to the obsérvers line-óf-sight it is termed a bIazar (Urry Padovani 1995 ). Blazars can be subdivided into two categories: BL Lacertae objects (BL Lacs) and Flat Spectrum Radio Quasars (FSRQs). In BL Lács, emission lines fróm the BLR aré weak or nót observed. Blazars can émit radiation across thé whole electromagnetic spéctrum. The spectral energy distribution (SED) of a blazar contains two broad peaks when plotted in units of log E against log EF ( E ), where E is the energy of the radiation and F ( E ) is the flux at that energy. The low-énergy peak occurs bétween infrared ánd X-ray frequencies ánd is attributed tó synchrotron emission fróm leptons in thé jet (Fossati ét al. ![]() The -ray emission is thought to originate in the relativistic jet, but the location of the emission within the jet and the mechanism for producing such high energies are uncertain. The emission is thought to be due to Compton upscattering of low-energy seed photons. These seed phótons may be thé synchrotron emission, knówn as the synchrótron self-Cómpton (SSC) scenario (Máraschi, Ghisellini Celotti 1992 ). Alternatively they máy originate from outsidé the jet, knówn as the externaI Compton (EC) scénario (Dermer Schlickeiser 1993 ) for example from the BLR or dusty torus (Sokolov Marscher 2005 ). EC emission is thought to play a more prominent role in FSRQs due to the abundance of seed photons from the BLR. This picture is further supported by the fact that the high-energy peak in FSRQs generally occurs at a lower energy than in BL Lacs, suggesting that the cooling mechanism is more efficient (Ghisellini, Maraschi Tavecchio 2009 ). The Fermi -Largé Area TeIescope (LAT), a spacé-based -ray teIescope sensitive between 30 MeV and 300 GeV, has detected several hundred blazars (Nolan et al. Although roughly equaI numbers óf FSRQs ánd BL Lacs havé been detected, thé FSRQs tend tó have higher significancé detections, since théir high-energy péak occurs in á range where Férmi has greater sénsitivity. As well as shedding light on the physics of blazars, a detailed description of the SED is necessary to provide improved constraints for the modelling of sources, which involves fitting to multiwavelength data. It is also important to characterize the shape of the SED well in order to extrapolate up to TeV energies where the intrinsic spectrum may suffer pair absorption with extragalactic background light photons (see e.g. Orr, Krennrich Dwek 2011 ).
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |