In this paper we report upper limits for a sample of eight X-ray selected BL Lac objects and the object "BL Lacertae" observed in 1997/1998 with the HEGRA CT-system. Seven sources were studied with three telescopes during one to ten hours. The TeV upper limits range from 15% to 65% of the Crab flux. The conversion in units of cm-2 s-1 is done assuming the slope of the spectrum and a model of CIB absorption.
The sources BL Lacertae and 1ES 2344+51.4 were observed with the 4-CT-system for approximately 15 h each. BL Lacertae was flaring in July, 1997 in optical and soft gamma-ray. The source 1ES 2344+51.4 is a very good VHE candidate due to its similarity to the well established strong TeV sources Mkn 421 and Mkn 501. Our upper limits are at the level of 10% of the Crab flux.
The HEGRA IACT system could detect TeV sources out to a redshift of 0.15. Beyond this, intergalactic absorption is expected to reduce the 500 GeV flux considerably. Since BL Lac sources are known to be very variable, a search for TeV emission is much more promising when guided by observations in other wavelengths, especially by observations in the X-ray energy band.
© European Southern Observatory (ESO) 2000
Online publication: January 18, 2000