Multi-wavelength monitoring of the BL Lacertae Object PKS 2155-304 II. The ROSAT Observations

Details Multi-wavelength monitoring of the BL Lacertae Object PKS 2155-304 II. The ROSAT Observations Multi-wavelength monitoring of the BL Lacertae Object PKS 2155-304 II. The ROSAT Observations

Aug 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...288..433b&link_type=abstract

Astronomy and Astrophysics 288, 433-447 (1994)

Astronomy and Astrophysics

Astrophysics

62

Bl Lac Objects: Individual (Pks 2155-304 ), X-Rays: Galaxies, Galaxies: Active

Scientific paper

Extensive ROSAT PSPC observations of the BL Lac object PKS 2155-304 carried out between November 12 and November 15, 1991 revealed the source to be in a bright state with a soft X-ray flux of F_x_(0.1-2.4keV)=~7x10^-10^erg/cm^2^/s. Rapid variations in the X-ray flux of up to 30% within a day were apparent but at the sampling rate of ~96 min all of the gross features in the X-ray light curve of this source are time resolved. The soft X-ray flux shows a close correlation with the simultaneous UV flux recorded by IUE although at the beginning and at the end of the PSPC observations there is some deviation from a strict linkage of the X-ray and UV light curves; detailed analysis has shown that the maximal lag of the UV with respect to the soft X-ray flux is <~3 hours. The soft X-ray spectrum of PKS 2155-304 remained essentially unchanged over the full duration of the observations. Individual PSPC spectra are consistent with a simple single power-law form with photon index {GAMMA}~-2.65 plus galactic absorption. More complex spectral fits do not improve the quality of the fits significantly. We cannot confirm previous claims of absorption structures at energies ~600eV, but the presence of a narrow absorption trough with width <~40eV or an absorption edge with moderate optical depth (τ~0.25 ) cannot be ruled out from the ROSAT data. The optical to X-ray spectrum must have a break in the unaccessible energy range between the IUE and ROSAT energy windows. The high energy properties of PKS 2155-304 are consistent with synchrotron emission from a relativistic jet. However, intensity variations without spectral changes are hard to accommodate in existing jet models. More refined, self-consistent time-dependent models are required to explain the details of the physical processes operating in this source.

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