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VHE gamma-ray emission of PKS 2155-304: spectral and temporal variability
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2010 (Engelska)Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 520, s. A83-Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Context. Observations of very high-energy.-rays from blazars provide information about acceleration mechanisms occurring in their innermost regions. Studies of variability in these objects lead to a better understanding of the mechanisms in play. Aims. To investigate the spectral and temporal variability of VHE (>100 GeV) gamma-rays of the well-known high-frequency-peaked BL Lac object PKS 2155-304 with the HESS imaging atmospheric Cherenkov telescopes over a wide range of flux states. Methods. Data collected from 2005 to 2007 were analyzed. Spectra were derived on time scales ranging from 3 years to 4 min. Light curve variability was studied through doubling timescales and structure functions and compared with red noise process simulations. Results. The source was found to be in a low state from 2005 to 2007, except for a set of exceptional flares that occurred in July 2006. The quiescent state of the source is characterized by an associated mean flux level of (4.32 +/- 0.09(stat) +/- 0.86(syst)) x 10(-11) cm(-2) s(-1) above 200 GeV, or approximately 15% of the Crab Nebula, and a power-law photon index of Gamma = 3.53 +/- 0.06(stat) +/- 0.10(syst). During the flares of July 2006, doubling timescales of similar to 2 min are found. The spectral index variation is examined over two orders of magnitude in flux, yielding different behavior at low and high fluxes, which is a new phenomenon in VHE gamma-ray emitting blazars. The variability amplitude characterized by the fractional rms F(var) is strongly energy-dependent and is proportional to E(0.19 +/- 0.01). The light curve rms correlates with the flux. This is the signature of a multiplicative process that can be accounted for as a red noise with a Fourier index of similar to 2. Conclusions. This unique data set shows evidence of a low-level.-ray emission state from PKS 2155-304 that possibly has a different origin than the outbursts. The discovery of the light curve ognormal behavior might be an indicator of the origin of aperiodic variability in blazars.

Ort, förlag, år, upplaga, sidor
2010. Vol. 520, s. A83-
Nyckelord [en]
gamma rays: general, galaxies: active, galaxies: jets, BL Lacertae objects: individual: PKS 2155-304
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Forskningsämne
Fysik, Astropartikelfysik
Identifikatorer
URN: urn:nbn:se:lnu:diva-30250DOI: 10.1051/0004-6361/201014484ISI: 000283064200047OAI: oai:DiVA.org:lnu-30250DiVA, id: diva2:706039
Tillgänglig från: 2014-03-18 Skapad: 2013-11-08 Senast uppdaterad: 2017-12-05Bibliografiskt granskad

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Becherini, Yvonne

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Becherini, YvonnePunch, M.
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Astronomy and Astrophysics
Astronomi, astrofysik och kosmologi

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