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BeppoSAX Observations of 1 Jy BL Lacertae Objects
P. Padovani 1,2
Space Telescope Science Institute, Baltimore, MD, USA
P. Giommi
BeppoSAX Science Data Center, Rome, Italy
A. Comastri
Osservatorio Astronomico di Bologna, Italy
A. Wolter, G. Ghisellini, T. Maccacaro, L. Maraschi, G. Tagliaferri
Osservatorio Astronomico di Brera/Merate, Italy
F. Mantovani, C. Stanghellini
Istituto di Radioastronomia, Bologna, Italy
C. M. Urry
Space Telescope Science Institute, Baltimore, MD, USA
1. Observations and Preliminary Results
We present preliminary results on BeppoSAX observations of seven BL Lacertae
objects selected from the 1 Jy sample. These observations are part of a Core
Program we are currently conducting which includes all 1 Jy BL Lacs with
estimated 0.1 - 10 keV flux larger than 2 â 10 -12 erg cm -2 s -1 .
The X­ray astronomy satellite BeppoSAX is a project of the Italian Space
Agency (ASI) with a participation of the Netherlands Agency for Aerospace Pro­
grams (NIVR). The scientific payload comprises four Narrow Field Instruments
(NFI): the Low Energy Concentrator Spectrometer (LECS), the Medium En­
ergy Concentrator Spectrometer (MECS), the High Pressure Gas Scintillation
Proportional Counter (HPGSPC), and the Phoswich Detector System (PDS),
all pointing in the same direction, and two Wide Field Cameras (WFC), point­
ing in opposite directions perpendicular to the NFI common axis. A detailed
description of the entire BeppoSAX mission can be found in Boella et al. (1997).
Our main scientific objectives are to: 1. study inverse Compton emission,
thought to be the dominant X­ray component in this BL Lac subclass, which is
1 A#liated to the Astrophysics Division, Space Science Department, European Space Agency
2 On leave from Dipartimento di Fisica, II Universit‘a di Roma, Italy
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characterized by a peak in the multifrequency spectra in the IR/Optical band; 2.
compare the BeppoSAX spectra with the far­ to near­infrared (1 -- 200µ) spectra
for the objects observed by ISO as part of a GO program (P.I. P. Padovani); 3.
check for any di#erences between the X­ray spectra of BL Lacs with the peak of
the emission in the IR/Optical band (LBL) and in the UV/X­ray band (HBL;
Padovani & Giommi 1995). Di#erences exist in the ROSAT band (e.g., Padovani
& Giommi 1996). A parallel BeppoSAX Core Program, in fact, deals with HBL
(Wolter et al. 1998).
Details on the observations, data analysis and reduction can be found in
Padovani et al. (1998), who presented results for three of the seven sources
described here. Spectral fits for all sources with LECS (0.1 -- 10 keV) data show
that the fitted NH values are consistent with Galactic absorption, which was
then assumed in all composite (LECS and MECS) fits.
The preliminary results of the combined LECS and MECS fits are as follows.
The mean value of the energy index # x is # 1, but the 6 LBL in the sample
(BL Lacs with the peak of the emission in the IR/Optical band) have a mean
value ## x # = 0.92±0.07, while the only HBL in the sample (PKS 2005-489) has
the steepest X­ray index (# x = 1.32 ± 0.04). Inclusion of the 10 HBL studied
by Wolter et al. (1988) gives a mean HBL value in the 0.1 - 10 keV range of
## x # = 1.31 ± 0.05, clearly steeper than the 1 Jy LBL mean. PKS 1144-379
is the only object which shows a significant departure from a single power law
(although the verdict on PKS 1519-273 is still open given its low signal to noise
ratio), i.e., a flattening above 2 keV. Detection at high energies by the PDS (E
> 10 keV) is certain for BL Lac and possible for OJ 287.
A comparison with ROSAT results (Urry et al. 1996) shows that some ob­
jects exhibit spectral and flux variability. In particular, both OJ 287, in a lower
state as compared to the ROSAT observations (taken in November 1991), and
PKS 2005-489, which was in a higher state, have a harder BeppoSAX spectrum.
Although BeppoSAX spectra cover a wider energy range than the ROSAT ones,
this trend agrees with the observed di#erent behaviors of X­ray variability of
LBL and HBL, interpreted by Padovani & Giommi (1996) as an e#ect of the
change of the position of the peak of the emission (which moves into the X­ray
band from lower energies and out of the X­ray band to higher energies for LBL
and HBL respectively).
Overall, the di#erent BeppoSAX spectra for LBL and HBL are consistent
with a dominance of synchrotron emission in HBL and an inverse Compton
contribution in LBL. More specific conclusions on the hard X­ray properties of
the 1 Jy sample will have to wait completion of the BeppoSAX observations.
References
Boella, G., et al. 1997, A&AS, 122, 299
Padovani, P. & Giommi, P. 1995, ApJ, 444, 567
Padovani, P. & Giommi, P. 1996, MNRAS, 279, 526
Padovani, P., et al. 1998, Nuclear Physics B (Proc. Suppl.), 69/1--3, 431
Urry, C. M., et al. 1996, ApJ, 463, 424
Wolter, A., et al. 1998, A&A, 335, 899
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