ON THE X-RAY-SPECTRA OF BL LACERTAE OBJECTS

We present results from an analysis of 311 X-ray spectra of 42 BL Lac objects recently published in a catalog. We have studied the mean spectrum of both X-ray and radio selected BL Lac objects (XBLs and RBLs, respectively) in two energy bands (soft and medium energies) and compare our results with those obtained by a similar analysis of spectral data of other classes of active galaxies. X-ray selected objects are characterized by an average slope of 2.23 +/- 0.17 and 2.55 +/- 0.30 at soft and medium energies, respectively; the observed dispersion is consistent with being entirely due to spectral changes. For radio selected objects we obtain a mean slope of 2.52 +/- 0.73 in the soft band and of 2.03 +/- 0.22 in the medium band; in this case, our data are insufficient to determine whether the dispersion is due to spectral variability only. There are strong indications in our analysis that XBLs and RBLs have different spectra: while XBLs are characterized by a convex spectrum (the spectrum steepens at higher frequencies), RBLs show instead a concave shape (the spectrum flattens at higher energies). Finally, there seems to be a trend of increasing photon index value from radio-loud QSOs, Seyfert galaxies, radio-quiet QSOs, and BL Lac objects. Analysis of the column density data indicates that the amount of X-ray absorbing material in BL Lac objects is negligible. The majority of our sample sources (similar to 90%) are variable in intensity. The variability parameter, nu, has a broad range of values up to 2.8 and shows no difference between X-ray and radio selected sources. Our data indicates that there is no common pattern in the long-term flux variability, although we can divide our sources into strongly and weakly variable objects. Spectral variability is present in five sources (3C 66A, Mrk 421, Mrk 501, Mrk 180, PKS 2005-489) out of a subsample of 14 objects for which we have sufficient data: apart from 3C 66A whose spectrum was found to be variable below a few keV, all other sources were characterized by spectral changes at medium energies (i.e., > 2 keV). Of the six objects with both soft- and medium-energy data, only Mrk 501 is variable both at soft and medium energies and shows a correlation between slope and brightness, which goes in the opposite sense in the two energy bands. Individual source data further allow us to quantify the dispersion due to spectral variability.