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Fermi LAT detection of a GeV flare from BL Lac PKS 0537-441

ATEL # 2124; D. Bastieri (INFN/University Padova) on behalf of the Fermi Large Area Telescope Collaboration
on 10 Jul 2009; 20:32 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Dario Gasparrini (

Subjects: Gamma Ray, >GeV, Request for Observations, AGN, Quasars

The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed an increasing gamma-ray flux from a source positionally consistent with PKS 0537-441, one of the most variable BL Lac objects (z=0.894, Peterson et al. 1976, ApJ, 207, L5).

LAT observations have shown an increasing flux for the past three weeks, reaching, above 100 MeV, 1.0 +/- 0.4 10^-6 ph cm^-2 s^-1 on July 8. A similar activity was previously reported by LAT in October 2008 (ATel #1759).

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the ongoing activity of these sources we strongly encourage multiwavelength observations. The Fermi LAT contact persons for this source is Gino Tosti (

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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