Search for the shortest variability at gamma rays in flat-spectrum radio quasars

📝 Abstract

We report about the search for short-term variability in the high-energy gamma-ray energy band of three flat-spectrum radio quasars (3C 454.3, 3C 273, PKS B1222+216), whose flux at E > 100 MeV exceeded the value of 10^-5 ph cm^-2 s^-1 for at least one day. Although, the statistics was not yet sufficient to effectively measure the characteristic time scale, it allowed us to set tight upper limits on the observed doubling time scale (< 2-3 hours) – the smallest measured to date at MeV energies –, which can constrain the size of the gamma-ray emitting region. The results obtained in the present work favor the hypothesis that gamma rays are generated inside the broad-line region.

💡 Analysis

We report about the search for short-term variability in the high-energy gamma-ray energy band of three flat-spectrum radio quasars (3C 454.3, 3C 273, PKS B1222+216), whose flux at E > 100 MeV exceeded the value of 10^-5 ph cm^-2 s^-1 for at least one day. Although, the statistics was not yet sufficient to effectively measure the characteristic time scale, it allowed us to set tight upper limits on the observed doubling time scale (< 2-3 hours) – the smallest measured to date at MeV energies –, which can constrain the size of the gamma-ray emitting region. The results obtained in the present work favor the hypothesis that gamma rays are generated inside the broad-line region.

📄 Content

arXiv:1101.1085v3 [astro-ph.HE] 26 Apr 2011 Astronomy & Astrophysics manuscript no. aa˙v2 c⃝ESO 2021 November 2, 2021 Search for the shortest variability at gamma rays in flat-spectrum radio quasars (Research Note) L. Foschini1, G. Ghisellini1, F. Tavecchio1, G. Bonnoli1, A. Stamerra2,3 1 INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807, Merate (LC), Italy e-mail: luigi.foschini@brera.inaf.it 2 Universit`a di Siena, via Roma 56, 53100, Siena, Italy 3 INFN Pisa, Via Buonarroti 2, 56127, Pisa, Italy Received –; accepted – ABSTRACT We report about the search for short-term variability in the high-energy γ-ray energy band of three flat-spectrum radio quasars (3C 454.3, 3C 273, PKS B1222+216), whose flux at E > 100 MeV exceeded the value of 10−5 ph cm−2 s−1 for at least one day. Although, the statistics was not yet sufficient to effectively measure the characteristic time scale, it allowed us to set tight upper limits on the observed doubling time scale (< 2−3 hours) – the smallest measured to date at MeV energies –, which can constrain the size of the γ-ray emitting region. The results obtained in the present work favor the hypothesis that γ rays are generated inside the broad-line region. Key words. galaxies: quasars: individual: 3C 454.3, 3C 273, PKS B1222+216 – galaxies: jets – gamma-rays: galaxies

1. Introduction The latest generation of ground-based Cerenkov telescope, like HESS and MAGIC, has detected flux changes on a few minutes time scale in the BL Lac Objects PKS 2155−304 (Aharonian et al. 2007) and Mkn 501 (Albert et al. 2007). These episodes, al- though exceptional, have severely challenged the common as- sumptions about the size and location of the emitting region. According to these models, the emitting region of size r > rg = GM/c2 (being rg the gravitational radius of the central black hole with mass M) is located at a distance R ∼r/ψ from the central spacetime singularity, where ψ is the aperture of the jet (gener- ally ψ ∼0.1 −0.25, see Ghisellini & Tavecchio 2009, Dermer et al. 2009). Changes in the emitted radiation from this blob occur on a characteristic observed time τ/(1+z) > r/cδ, where δ is the Doppler factor of the jet. It is worth noting that the dissipation zone should be suffi- ciently far from the central black hole to avoid pair production due to the interaction with the photons emitted mainly by the accretion disc and the broad-line region (BLR). BL Lac Objects are known to have a photon-starved environment: the accretion disc in these sources is weak, likely an advection-dominated disc (ADAF), which is also inefficient in ionizing the BLR. Therefore, the variability of a few minutes observed in these BL Lac Objects can be explained by assuming very small blobs at small R: the paucity of soft photons makes it possible for GeV photons to escape without pair production. This is no more possible in the case of FSRQs, where discs with accretion rates of a few tens of percent of the Eddington rate are present, which in turn efficiently photoionize the BLR, generating broad and strong optical emission lines. This rich en- vironment (disc, BLR) makes the cooling of relativistic electrons more efficient through the external Compton (EC) processes, but, on the other hand, if the emitting blob is too close to the accretion disc to be sufficiently small for the short variability, then the pair production could severely absorb the GeV photons. Some researchers have proposed different solutions to this problem, which are basically relying on the increase of the Doppler boosting or some special jet structure (Begelman et al. 2008, Finke et al. 2008, Ghisellini & Tavecchio 2008, Giannios et al. 2009). The latter, based on the hypothesis that there are very compact relativistic blobs within the jet (“nee- dle/jet” model, Ghisellini & Tavecchio 2008; “jet-in-jet” model, Giannios et al. 2009), can be adapted to both BL Lac Objects and FSRQs, although it seems an ad hoc solution at the present stage and, perhaps, it is necessary to better assess this type of solution by comparing with more and more observations. Therefore, the most critical aspect of these theories would be the discovery of variability of a few minutes in the case of FSRQs. The shortest time scales measured in FSRQs is of about half an hour, as observed in the hard X-ray emission (20−40 keV) of NRAO 530 in 2004 February (Foschini et al. 2006). However, given the energy range, it would be possible that this short flare could have been generated by the X-ray corona, although the emitted power (∼8 × 1047 erg s−1) seems a bit large to be due to an unbeamed component. In addition, an increase of the radio polarization around the time of the flare suggests that it was in- deed related to the jet, although the coarse observation sequence did not allow to set tight constraints and doubts shadow the con- clusions. Anyway, if short time scales could be observed in FSRQs at hard X-rays, which are sampling low energy part of the inverse- Comp

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