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Fermi LAT detection of a GeV flare from blazar PKS 2155-83

ATEL # 2373; Eric Wallace (University of Washington), on behalf of the Fermi Large Area Telescope Collaboration
on 7 Jan 2010; 21:38 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Teddy Cheung (

Subjects: Gamma Ray, >GeV, AGN, Quasars

The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope has observed an increasing gamma-ray flux from a source positionally coincident with the blazar PKS 2155-83 (RA=22h02m19.9s, DEC= -83d38m13s, J2000; T. Mauch et al. 2003, MNRAS, 342, 1117). The radio source CRATES J2149-8315 (PMN J2149-8315, RA=21h49m49.9s, DEC=-83d15m26s, J2000; Mauch et al. 2003) is also within the error circle. Preliminary analysis indicates that on January 5, 2010, the source was in a high state, with a gamma-ray flux (E>100MeV) of (1.4 +/- 0.3)*10^-6 photons/cm^2/s (statistical uncertainty only), representing more than an order of magnitude increase over its average flux during the first 11 months of Fermi observations.

Since Fermi operates in an all-sky scanning mode, regular monitoring of this source will continue. Multiwavelength observations may help to distinguish which of the two blazars is flaring, and are strongly encouraged. The Fermi LAT contact person for this source is Eric Wallace (

The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan, and Sweden.

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