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Different X-ray Spectra
Are the X-ray Spectra of Flat-spectrum Radio Quasars and BL Lacertae
Objects different?
Paolo Padovani, II Università di Roma,
Paolo Giommi, ASI, SDC and Fabrizio Fiore, ASI, SDC and Osservatorio
Astronomico di Roma
Original Version: MNRAS, (1997), 284, 569
Text copyright (c) 1997. Royal Astronomical Society.
Abstract
We study the X-ray spectra of 114 flat-spectrum radio quasars (FSRQ) using the
hardness ratios as given in the WGA catalogue of ROSAT sources. This sample
includes all WGA FSRQ with high-quality data and comprises about 20 per cent of
presently known such objects, which makes this the largest FSRQ sample ever
studied in the X-ray band. We find that FSRQ have a distribution of energy
spectral indices ranging between 0 and 3 with a mean value alpha_x ~ 1. This is
consistent with that of low-energy cutoff BL Lacs (LBL; alpha_x ~ 1.1),
generally found in radio surveys, but significantly different from that of
high-energy cutoff BL Lacs (HBL), normally selected in the X-ray band, which
display steeper X-ray spectra (alpha_x ~ 1.5). The shape of the
optical-to-X-ray continuum is concave (that is alpha_x< alpha_ox) for the
majority of FSRQ, as found for LBL, supporting a dominance of inverse Compton
emission in the X-ray band in most objects. Our results are at odds with
previous studies of the X-ray spectra of FSRQ, which were however plagued by
low spectral resolution and/or small number statistics and selection effects,
and have important implications for the proposed connections between FSRQ and
BL Lacs.
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