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Post Info TOPIC: PKS 2155--304
Blobrana


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ATel 9579: The increasing X-Ray Activity of PKS 2155-304



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Blobrana


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An international team of astrophysicists, using telescopes on the ground and in space, has uncovered surprising changes in radiation emitted by an active galaxy classified as a Blazar.
The picture that emerges from these first-ever simultaneous observations with optical, X-ray and new-generation gamma-ray telescopes is much more complex than scientists expected and challenges current theories of how the radiation is generated.
The galaxy in question is PKS 2155-304, a type of object known as a "blazar".

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Blobrana


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PKS 2155-304 is one of the brightest active galaxies in the sky.
It is a strong emitter from radio to high energy frequencies. PKS 2155-305 is at redshift z = 0.116 (Falomo, Pesce & Treves 1993) and it is one of the brightest and most studied BL Lacs and is often considered the prototype of X-ray selected BL Lacs.

Source

Position(2000): RA = 21:58:52.07, Dec= -30:13:32.1,
z = 0.116

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Blobrana


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Title: XMM-Newton observations of the TeV BL Lac object PKS 2155--304 in 2006: signature of inverse Compton X-ray emission?
Authors: Y.H. Zhang (Tsinghua)

This paper reports the first discovery of possible inverse Compton (IC) X-ray emission below 10 keV in the typical high-energy peaked BL Lac object (HBL) PKS 2155--304. Two XMM-Newton observations performed in 2006 reveal that the 0.6--10 keV X-ray spectra of the source harden (\Delta\Gamma ~ 0.1--0.3) at break energies of about 4 keV. The concave X-ray spectra of the source could be easily interpreted by a mixture of a steep component (i.e., the high energy tail of the synchrotron emission) and a flat one (i.e., the low energy side of the IC emission). However, the steep spectra (\Gamma ~ 2.3) in the hard X-rays indicate that the synchrotron emission still dominates over the IC one, while the latter is effectively present and perceived as flattening the synchrotron spectrum in this energy range. The quasi-simultaneous optical-UV-X-ray spectral energy distributions (SEDs) obtained with XMM-Newton suggest that the concave X-ray spectra of the source could be the result of down-ward shift of the synchrotron peak frequency to the optical band, incurring the IC emission to become more important in the hard (~ 4$--10 keV) X-ray band with respective to other cases in which the synchrotron emission peaks in the UV-soft-X-ray range. This discovery provides a new clue for a smooth transition between HBLs and the low-energy peaked BL Lac objects (LBLs).

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