Swift observations of IBL and LBL objects

¹ Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca scientifica 1, 00133 Roma, Italy e-mail: fmassaro@cfa.harvard.edu
² Harvard, Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 92138, UK
³ ASI Science Data Center, ESRIN, 00044 Frascati, Italy
⁴ Dipartimento di Fisica, Universitá di Perugia, Viale A. Pascoli 1, 06123 Perugia, Italy
⁵ Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale A. Moro 2, 00185 Roma, Italy
⁶ Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
⁷ NASA NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

Received: 20 March 2008
Accepted: 16 July 2008

Abstract

Context. BL Lacs are an enigmatic class of active galactic nuclei (AGNs), characterized by the non-thermal continuum typically attributed to synchrotron and inverse Compton emission. Depending on the frequency location of the maxima of these components, they are subdivided into three subclasses LBLs, IBLs, and HBLs. We present the results of a set of observations of eight BL Lac objects of LBL and IBL type performed by the XRT and UVOT detectors onboard the Swift satellite between January 2005 and November 2006.

Aims. We are mainly interested in measuring the spectral parameters, and particularly the steepness between the UV and the X-ray band, useful for determining the classification of these sources. We compare the behavior of these sources with previous XMM-Newton, BeppoSAX observations and with historical data in the X-ray and in the optical band. We are also interested in classifying the sources in our sample on the basis of the Swift observations and comparing them with their classification presented in literature.

Methods. We performed X-ray spectral analysis of observed BL Lac objects using a simple powerlaw and in a few cases the log-parabolic model. We also combined the UV emission with the low energy X-ray data to describe their spectral energy distribution.

Results. We used Swift observational data to classify sources in our sample and derived parameters of their spectral energy distribution.

Conclusions. We found that for the IBLs X-rays low states show features of the high energy component, usually interpreted as due to inverse Compton emission. Sources in our sample exhibit a range of temporal UV and X-ray behaviors, some objects having clear and neat correlated UV and X-ray variations (e.g. ON231) and other objects showing no clear (e.g. AO 0235+164) UV and X-ray correlation. Finally, we also note that our estimates of spectral curvature are in the range of that measured for the High frequency peaked BL Lac objects (HBLs).