©2000 BLAZAR Data, Vol. 2, n. 3

Absolute photometry of 49 X-ray selected BL Lacs

V. Beckmann

Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg, Germany

(Received: 2000 October 2; Accepted: 2000 November 20)

Absolute photometry of 49 X-ray selected BL Lac objects from the Hamburg/RASS X-ray Bright BL Lac Sample (HRX-BL Lac) has been carried out in the Johnson B band using the Calar Alto 1.23m telescope. Broad band overall spectral indices are computed between the optical and X-ray band and the radio and optical band. The objects fill the $alpha$_OX- $alpha$_RO plane in the radio weak and X-ray bright regime.

1. Introduction

Besides the measurement of redshift and spectral shape values of the optical fluxes are important to understand the nature of the BL Lac objects. Several results in the field of BL Lac physics are based on the spectral energy distribution, e.g. the overall spectral indices $alpha$_OX and $alpha$_RO. But accurate measurements of the optical flux, especially for faint BL Lac objects, are rare. The first glimps might give the impression that this is obsolete due to the variability of BL Lac objects. Additionally magnitudes with an accuracy of ~0.5 mag could be obtained by using the APM Sky Catalogue, the USNO data base, or the calibrated objective prism plates of the Hamburg Quasar Survey (Hagen et al. 1995). But the determination of brightnesses is only possible for objects with B < 18 mag. For fainter sources the uncertainty in the calibration increases dramatically. Values taken from literature are not satisfying for a statistical study of a larger sample of objects. The argument that photometry of BL Lac objects only makes sense if observations are carried out simultaneously (like combined campaigns with X-ray and optical telescopes for example) is only valid for the highly variable objects. On the other hand the variability of BL Lac objects strongly depends on the X-ray dominance $alpha$_OX. This has been shown by i.e. Heidt & Wagner 1998, Villata et al. 2000, Mujica et al. 1999, and Januzzi et al. 1994. The objects examined here are part of the Hamburg/ROSAT X-ray Bright BL Lac sample (HRX-BL, Beckmann 1999, Beckmann & Wolter 2000). This sample of in total more than 100 objects is based on optical identification of candidates derived from a radio/X-ray correlation (Bade et al. 1998). I present optical Johnson-B magnitudes for 49 HRX-BL Lacs. Additionally $alpha$_OX and $alpha$_RO values are derived using the X-ray flux at 1 keV and the radio flux at 1.4 GHz.

2. Observations and data reduction

The results presented here are based on an observation run in spring 2000. A total number of seven nights (28.4.-4.5.2000) was available at the Calar Alto 1.23m telescope. The detector was a CCD with a SITe#18b 2k x 2k chip, which covered a sky area of ~10' x 10'. Photometric B magnitudes have been derived by comparison with standard stars. For that purpose magnitudes of stars determined with the HST from the "Guide Star Photometric Catalog" (GSPC, Lasker et al. 1998) have been used. Directly before and/or after each exposure of a BL Lac the nearest GSPC star was observed to get an absolute calibration. In total it was possible to measure magnitudes for 49 BL Lac objects, especially the optically faintest of the HRX-BL sample. The direct images have been subtracted by a bias, determined on the overscan area of the CCD (the CCD was cooled with liquid nitrogen and no dark current subtraction is needed). After that the images were corrected with combined flat fields which had been taken in the dusk and dawn sky. The analysis of the direct images was done with the IRAF package (Tody 1993). Instrumental magnitudes were obtained in simulated aperture. The photometric radius was kept large enough (typically 6 arcsec or larger, if the objects appeared to be extended) to include all the light of the objects. Errors of magnitudes were estimated using standard IRAF procedures and including the uncertainties of the used reference stars from GSPC.
Data to compute the overall spectral indices were taken from the NVSS catalogue (Condon et al. 1998), and from the ROSAT Bright Source Catalogue (Voges et al. 1999), for the radio (1.4 GHz) and X-ray (1 keV) regime, respectively. For the computation of the spectral indices the fluxes within the source rest frame have been used. Therefore fluxes have been k-corrected (unknown redshifts have been set to z = 0.3 which is the mean value for the HRX BL Lac sample).

4. Results and Conclusion

The results are listed in Table 1. It is sorted for object position in right ascension (J2000.0). Object positions of new BL Lac objects can easily be told from the ROSAT catalogue names. The $alpha$_RO vs. $alpha$_OX plane for the BL Lacs investigated here is shown in Fig. 1.
Based on accurate measurements of the optical flux it is possible to determine overall spectral indices for X-ray dominated BL Lac objects, which show low variability in comparison to BL Lacs with a large $alpha$_OX value. The objects represent the radio weak and X-ray bright end of the BL Lac population.
Values for the overall spectral indices presented here are typical for RASS selected BL Lac objects (see e.g. Nass et al. (1996) who found for 34 RASS selected BL Lac objects 0.22 < $alpha$_RO < 0.64, and 0.59 < $alpha$_OX < 1.55). Compared to other samples the $alpha$_RO - $alpha$_OX plane of the objects of this work matches best the BL Lacs of the EINSTEIN Medium Sensitivity Survey (EMSS), but the EMSS objects tend to be less X-ray dominated: They cover the region of 0.60 < $alpha$_OX < 1.70, and 0.25 < $alpha$_RO < 0.50 (Rector et al. 2000). The region covered by the HRX BL Lac sample does not overlap with that of the 1Jy BL Lacs, which show higher $alpha$_OX and/or $alpha$_RO values. The RGB sample (Laurent-Muehleisen et al. 1999), as an example for a sample with moderate radio (> 24 mJy at 6 cm) and X-ray flux limit, covers a wider range with 0.46 < $alpha$_OX < 1.72 and $alpha$_RO similar to the values of the HRX-BL Lacs. Only the three HRX-BL Lacs with $alpha$_OX > 1.2 are bona fide low frequency cut-off BL Lacs.
Therefore the overall spectral indices of this work can be used to test the different models for the spectral energy distribution of high frequency cut-off BL Lac objects, especially when combined with other RASS selected BL Lac objects (e.g. Nass et al. 1996).

5. References

6. Table and Figure