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"Softer when brighter" behavior at gamma rays of the BL Lac Object PKS 2155-304

ATEL # 2947; Luigi Foschini (INAF OA Brera, Italy)
on 16 Oct 2010; 18:46 UT
Password Certification: Luigi Foschini (

Subjects: Gamma Ray, >GeV, A Comment, AGN

In the recent ATel #2944, Ciprini et al. reported about a bright outburst at gamma rays of the BL Lac Object PKS 2155-304 (z=0.116). They wrote: "A preliminary estimate of the photon index (2.60.3) points out a rather soft gamma-ray spectrum on this daily interval, unusual for this type of BL Lac objects." However, this is not so unusual in BL Lac Objects. In a poster presented at the 38th COSPAR Scientific Assembly (Bremen, Germany, 18-25 July 2010) at the event "E11 Time Variability at High Energies: A Probe of AGN Physics (including VLBI)", Foschini et al. analyzed the GeV spectral variability of a small sample of gamma-ray active galactic nuclei (AGN) and noted that the two BL Lac Objects (PKS 2155-304 and Mkn 421) displayed a "softer when brighter" behavior (see the link below). Specifically, in the case of PKS 2155-304, it is worth noting that over the whole studied period (4 Aug 2008 - 5 Apr 2010), the peak flux (occurred on 27 August 2009 - MJD 55070) reached a value of (0.900.28)10-6 ph cm-2s-1 (1 day integration; E > 100 MeV) with a photon index 2.90.3, which are consistent with the values measured in the outburst of a few days ago.

Poster presented at COSPAR E11

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