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

• Published in: The Astrophysical journal 2008-07, Vol.681 (2), p.944-953
• Main Authors: Aliu, E, Anderhub, H, Antoranz, P, Baixeras, C, Bartko, H, Bastieri, D, Bednarek, W, Berger, K, Bigongiari, C, Biland, A, Bosch-Ramon, V, Bretz, T, Britvitch, I, Camara, M, Chilingarian, A, Coarasa, J. A, Contreras, J. L, Cortina, J, Costado, M. T, Curtef, V, Danielyan, V, Dazzi, F, De Angelis, A, Delgado, C, de los Reyes, R, De Lotto, B, Dorner, D, Doro, M, Fagiolini, M, Ferenc, D, Firpo, R, Fonseca, M. V, Font, L, Fuchs, M, Galante, N, García-López, R. J, Goebel, F, Hakobyan, D, Hayashida, M, Hengstebeck, T, Höhne, D, Hose, J, Huber, S, Jacon, P, Jogler, T, Kritzer, R, Laille, A, Lindfors, E, Longo, F, Maneva, G, Mannheim, K, Mariotti, M, Martínez, M, Merck, C, Meyer, M, Moralejo, A, Nieto, D, Nilsson, K, Ninkovic, J, Otte, N, Oya, I, Panniello, M, Paoletti, R, Pascoli, D, Pauss, F, Pegna, R, Piccioli, A, Prandini, E, Puchades, N, Raymers, A, Rhode, W, Ribó, M, Rissi, M, Rügamer, S, Saggion, A, Saito, T. Y, Sánchez, A, Sartori, P, Scalzotto, V, Scapin, V, Schmitt, R, Schweizer, T, Shayduk, M, Shinozaki, K, Shore, S. N, Sillanpää, A, Sobczynska, D, Spanier, F, Stamerra, A, Stark, L. S, Takalo, L, Temnikov, P, Tescaro, D, Teshima, M, Torres, D. F, Turini, N, Wittek, W, Zandanel, F, Zanin, R, Zapatero, J
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/587499

All but three (M87, BL Lac, and 3C 279) extragalactic sources detected so far at very high energy -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 [image] at culmination, constraining the declination to -2 degree to +58 degree . The sensitivity of the search was planned for detecting X-ray-bright [[image] ] sources emitting at least the same energy flux at 200 GeV as at 1 keV. To avoid strong -ray attenuation close to the energy threshold, source redshift was constrained to [image]. 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 capital sigma from the position of 1ES 1011+496 was observed and then confirmed as a VHE -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 [image]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 -ray luminosities at 200 GeV are generally not higher than their X-ray luminosities at 1 keV.