Systematic search for VHE gamma-ray emission from X-ray-bright high-frequency BL Lac objects

Abstract

All but three (M87, BL Lac, and 3C 279) extragalactic sources detected so far at very high energy gamma-rays belong to the class of high-frequency-peaked BL Lac objects. This suggested to us a systematic scan of candidate sources with the MAGIC telescope, based on the Donato et al. compilation of X-ray blazars. The observations took place from 2004 December to 2006 March and cover northern sky sources visible under small zenith distances zd 30 degrees at culmination, constraining the declination to -2 degrees to +58 degrees. The sensitivity of the search was planned for detecting X-ray-bright [F(1 keV) > 2 mu Jy] sources emitting at least the same energy flux at 200 GeV as at 1 keV. To avoid strong gamma-ray attenuation close to the energy threshold, source redshift was constrained to z < 0.3. Of the 14 sources observed, 1ES 1218+304 (for the first time at VHE) and 1ES 2344+514 (strong detection in a low flux state) were detected in addition to the known bright TeV blazars Mrk 421 and Mrk 501. A marginal excess of 3.5 sigma from the position of 1ES 1011+496 was observed and then confirmed as a VHE gamma-ray source by a second MAGIC observation triggered by a high optical state. For the remaining sources, we present 99% c. l. upper limits on the integral flux greater than or similar to 200 GeV. We characterize the HBL sample (including all HBLs detected at VHE so far) by looking for correlations between their multifrequency spectral indices determined from simultaneous optical, archival X-ray, and radio luminosities, finding that VHE-emitting HBLs do not seem to constitute a unique subclass. The HBLs' absorption-corrected gamma-ray luminosities at 200 GeV are generally not higher than their X-ray luminosities at 1 keV.