The ROSAT X-ray Spectra of BL Lacertae Objects
Paolo Padovani, II Università di Roma and
Paolo Giommi, ASI, SDC
Original Version: MNRAS, 279, 526 (1996)
Text copyright (c) 1996. Royal Astronomical Society
Abstract
We study the X-ray spectra of 85 BL Lacertae objects using the
hardness ratios as given in the WGA catalogue of ROSAT sources. This
sample includes all WGA BL Lacs with high-quality data and comprises about 50
per cent of presently known BL Lacs. We find that BL Lacs have energy
power-law spectral indices between 0 and 3 with a mean value alpha_x ~ 1.4.
Significant differences, however, are present between high-energy cutoff BL
Lacs (HBLs), normally selected in the X-ray band, and low-energy cutoff BL
Lacs (LBLs), generally found in radio surveys. HBLs have steeper X-ray
spectral slopes (alpha_x ~ 1.5) well correlated with alpha_ox, and
anti-correlated with the X-ray-to-radio flux ratio and cutoff frequency, with
convex overall broad-band spectra. LBLs, on the other hand, have flatter X-ray
spectra (alpha_x ~ 1.1) and concave optical-X-ray continuum. We interpret
these results in terms of different mechanisms being responsible for the X-ray
emission in the two classes, namely synchrotron and inverse Compton for HBLs
and LBLs respectively. The observed differences are consistent with the
hypothesis that HBLs and LBLs are powered by essentially the same non-thermal
engines differing mainly in their synchrotron cutoff energy.
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