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Systematic Search for VHE Gamma-Ray Emission from X-Ray-bright High-Frequency BL Lac Objects

Systematic Search for VHE Gamma-Ray Emission from X-Ray-bright High-Frequency BL Lac Objects

Authors :
Angelis, De
los, de
Lotto, De
Source :
The Astrophysical Journal; July 2008, Vol. 681 Issue: 2 p944-953, 10p
Publication Year :
2008

Abstract

All but three (M87, BL Lac, and 3C 279) extragalactic sources detected so far at very high energy g-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 <IMG SRC="eq-00001.gif" ALT="\mathrm{zd}\, < 30^{\circ }"/> zd < 30deg at culmination, constraining the declination to -2deg to +58deg. The sensitivity of the search was planned for detecting X-ray-bright [ <IMG SRC="eq-00002.gif" ALT="F(1\ \mathrm{keV}\,) > 2\ \mu \mathrm{Jy}\,"/> F(1 keV) > 2 m Jy ] sources emitting at least the same energy flux at 200 GeV as at 1 keV. To avoid strong g-ray attenuation close to the energy threshold, source redshift was constrained to <IMG SRC="eq-00003.gif" ALT="z < 0.3"/> 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 s from the position of 1ES 1011+496 was observed and then confirmed as a VHE g-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 [?]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 g-ray luminosities at 200 GeV are generally not higher than their X-ray luminosities at 1 keV.

Details

Language :
English
ISSN :
0004637X and 15384357
Volume :
681
Issue :
2
Database :
Supplemental Index
Journal :
The Astrophysical Journal
Publication Type :
Periodical
Accession number :
ejs18480685