Discovery of a BL Lacertae object (EXO 055625-3838.6) in the error box of H 0557-385.
Abstract
We present X-ray and optical data of a new BL Lacertae type object (EXO 055625-3838.6) which has been serendipitously discovered in an EXOSAT CMA image and is located within the error box of HO557-385 (4U0557-38, 3A0557-383), a hard X-ray source previously identified with a seyfert 1 galaxy. Although the soft X-ray flux of the BL Lac is 1O times higher than that of the seyfert and the EXOSAT data are fully consistent with the BL Lac being the only counterpart of H0557-355, a substantial contribution to the hard X-ray flux from the Seyfert galaxy cannot be excluded, The long-term hard X-ray light curve of H0557-385, obtained by combining data from different satellites, shows variability by a factor >= 10. Luminosity variability of a factor >= 22.8 in the 2-6 keV band, between two EXOSAT observations, has been found. In contrast, no significant variability was detected in any of the four sources present in the CMA image. If the hard X-ray flux of H0557-385 is mostly due to the BL Lac object, the number of sources belonging to this class in the HEAO1-A2 whole sky survey rises to five, The reclassification as BL Lacs of even a very small number of the HEAO1 survey sources would imply a substantial increase of a previous estimate of the space density of these objects at fluxes >10^-11^ erg cm^-2^ s^-1^. The implications for the BL Lac X-ray log N-log S relation are briefly discussed.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- January 1989
- DOI:
- 10.1093/mnras/236.2.375
- Bibcode:
- 1989MNRAS.236..375G
- Keywords:
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- Astronomical Spectroscopy;
- Bl Lacertae Objects;
- X Ray Sources;
- Error Analysis;
- Exosat Satellite;
- Light Curve;
- Seyfert Galaxies;
- Visible Spectrum;
- Astrophysics