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标题：H.E.S.S. detection of very high-energy γ-ray emission from the quasar PKS 0736+017
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作者：H. Abdalla ; R. Adam ; F. Aharonian 等
期刊名称：Astronomy & Astrophysics
印刷版ISSN：0004-6361
电子版ISSN：1432-0746
出版年度：2020
卷号：633
页码：1-11
DOI：10.1051/0004-6361/201935906
语种：English
出版社：EDP Sciences
摘要：Context.Flat-spectrum radio-quasars (FSRQs) are rarely detected at very high energies (E ≥ 100 GeV) due to their low-frequency-peaked spectral energy distributions. At present, only six FSRQs are known to emit very high-energy (VHE) photons, representing only 7% of the VHE extragalactic catalog, which is largely dominated by high-frequency-peaked BL Lacertae objects.Aims.Following the detection of MeV–GeVγ-ray flaring activity from the FSRQ PKS 0736+017 (z = 0.189) withFermi-LAT, the H.E.S.S. array of Cherenkov telescopes triggered target-of-opportunity (ToO) observations on February 18, 2015, with the goal of studying theγ-ray emission in the VHE band.Methods.H.E.S.S. ToO observations were carried out during the nights of February 18, 19, 21, and 24, 2015. Together withFermi-LAT, the multi-wavelength coverage of the flare includesSwiftobservations in soft X-ray and optical-UV bands, and optical monitoring (photometry and spectro-polarimetry) by theStewardObservatory, and the ATOM, the KAIT, and the ASAS-SN telescopes.Results.VHE emission from PKS 0736+017 was detected with H.E.S.S. only during the night of February 19, 2015.Fermi-LAT data indicate the presence of aγ-ray flare, peaking at the time of the H.E.S.S. detection, with a flux doubling timescale of around six hours. Theγ-ray flare was accompanied by at least a 1 mag brightening of the non-thermal optical continuum. No simultaneous observations at longer wavelengths are available for the night of the H.E.S.S. detection. Theγ-ray observations with H.E.S.S. andFermi-LAT are used to put constraints on the location of theγ-ray emitting region during the flare: it is constrained to be just outside the radius of the broad-line regionrBLRwith a bulk Lorentz factor Γ ≃ 20, or at the level of the radius of the dusty torusrtoruswith Γ ≃ 60.Conclusions.PKS 0736+017 is the seventh FSRQ known to emit VHE photons, and atz = 0.189 is the nearest so far. The location of theγ-ray emitting region during the flare can be tightly constrained thanks to opacity, variability, and collimation arguments.
关键词：engamma rays: generalastroparticle physicsrelativistic processesquasars: individual: PKS 0736+017