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Astron. Astrophys. 335, 899-911 (1998)

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BeppoSAX spectral survey of soft X-ray selected BL Lacertae objects

A. Wolter 1, A. Comastri 2, G. Ghisellini 1, P. Giommi 3, M. Guainazzi 3, T. Maccacaro 1, L. Maraschi 1, P. Padovani; ** 4, C.M. Raiteri 5, G. Tagliaferri 1, C.M. Urry 4 and M. Villata 5

1 Osservatorio Astronomico di Brera Via Brera, 28 I-20121 Milano, Italy
2 Osservatorio Astronomico di Bologna, Via Zamboni 33, I-40126 Bologna, Italy
3 A.S.I., Beppo-SAX Science Data Center, Via Corcolle 19, I-00131 Roma, Italy
4 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD. 21218, USA
5 Osservatorio Astronomico di Torino, Strada Osservatorio 20, I-10025 Pino Torinese (TO), Italy

Received 19 February 1998 / Accepted 24 April 1998


We present X-ray spectra obtained with BeppoSAX (Satellite per Astronomia X) of 10 BL Lac objects, selected from the Einstein Medium Sensitivity and Slew Surveys. We find that in about half of the objects a fit in the 0.1-10 keV range with a single power law and free absorption yields values of [FORMULA] larger than the Galactic ones. In most of these cases, however, broken power law fits with [FORMULA] fixed at the Galactic values yield an alternative, better description of the data and indicate a steepening of the spectrum with increasing energy. One object (1ES1101-232) is detected up to [FORMULA] 100 keV. Its spectral energy distribution (SED) peaks in the medium energy X-ray band. For each object we compute the peak frequency of the SED from multifrequency data. The spectral indices [FORMULA] in the 2-10 keV band ([FORMULA]) are smaller (i.e. flatter spectrum) for objects with higher peak frequencies. We therefore confirm and extend to higher energies the behavior already known for X-ray selected BL Lac objects in the ROSAT band. We do not find spectral indices smaller than 1; however, the flat distribution of [FORMULA] and the correlation between [FORMULA] and peak frequency found from our data suggest that a number of objects may exist, which in the quiescent status have flatter spectrum and peak frequency in the hard X-ray range.

Key words: BL Lacertae objects: general – X-rays: galaxies

* Affiliated to the Astrophysics Division, Space Science Department, European Space Agency
** On leave from Dipartimento di Fisica, II Università di Roma "Tor Vergata", Italy

Send offprint requests to: A. Wolter, (anna@brera.mi.astro.it)

© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998