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Astron. Astrophys. 354, 513-521 (2000)


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A large high-energy gamma-ray flare from the blazar 3C 273

W. Collmar 1, O. Reimer 1, K. Bennett 4, H. Bloemen 2, W. Hermsen 2, G.G. Lichti 1, J. Ryan 3, V. Schönfelder 1, H. Steinle 1, O.R. Williams 4 and M. Böttcher 5

1 Max-Planck-Institut für Extraterrestrische Physik, P.O. Box 1603, 85740 Garching, Germany
2 SRON-Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
3 University of New Hampshire, Institute for the Study of Earth, Oceans and Space, Durham, NH 03824, USA
4 Astrophysics Division, Space Science Department of ESA/ESTEC, 2200 AG Noordwijk, The Netherlands
5 Rice University, Space Physics and Astronomy Department, 6100 S. Main Street, Houston, TX 77005 - 1892, USA

Received 30 July 1999 / Accepted 23 November 1999

Abstract

The Compton Gamma-Ray Observatory (CGRO) experiments EGRET and COMPTEL observed the Virgo sky region continuously for 7 weeks between December 10, 1996 and January 28, 1997. The prominent quasar 3C 273 was found to be the brightest source in [FORMULA]-rays and was significantly detected by EGRET and COMPTEL. The EGRET experiment observed a time-variable flux at energies above 100 MeV, which reached in a 2-week flaring period (December 30, 1996 to January 14, 1997) its highest flux level observed during the CGRO-era. COMPTEL, however, does not observe obvious time variability at energies below [FORMULA]30 MeV contemporaneous to EGRET. In particular, no flare was observed, indicating that this outburst is solely a high-energy ([FORMULA]100 MeV) phenomenon. The energy spectrum between 3 MeV and 10 GeV is well represented by a simple power-law model. Below 3 MeV a spectral turnover is indicated. Performing spectral analysis for different time periods, we found evidence for a spectral hardening during the flaring period, which is consistent with the flare occurring mainly at the higher energies and with its absence at COMPTEL energies of a few MeV. This may be interpreted as an indication that the emission in the EGRET energy range is dominated by a different radiation mechanism than the MeV emission. We argue that the most likely mechanism for the high-energy flare is inverse-Compton scattering of reprocessed accretion-disk radiation.

Key words: gamma rays: observations – galaxies: active – galaxies: quasars: individual: 3C 273

Send offprint requests to: W. Collmar

Correspondence to: wec@mpe.mpg.de

© European Southern Observatory (ESO) 2000

Online publication: February 9, 2000

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