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Fermi LAT detection of a GeV flare from the BL Lac object Mrk 421

ATel #4261; F. D'Ammando (Dip. Fisica, Univ. Perugia and INFN), M. Orienti (Univ. Bologna, INAF-IRA Bologna) on behalf of the Fermi Large Area Telescope Collaboration
on 17 Jul 2012; 20:45 UT
Credential Certification: Filippo D'Ammando (filippo.dammando@iasf-roma.inaf.it)

Subjects: Gamma Ray, >GeV, Request for Observations, AGN, Black Hole, Blazar

Referred to by ATel #: 4272, 4451, 5107

The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed gamma-ray flaring activity from a source positionally consistent with the BL Lac object Mrk 421 (also known as 2FGL J1104.4+3812, Nolan et al. 2012, ApJS, 199, 31; R.A.= 11h04m27.3139s, Dec.= +38d12m31.799s, J2000.0, Fey et al. 2004, AJ, 127, 3587) at redshift z=0.03 (De Vaucouleurs et al. 1991, Third Reference Catalogue of bright galaxies v3.9).

Preliminary analysis indicates that the source brightened in gamma rays with a daily flux (E > 100 MeV) of (1.4+/-0.2) x10^-6 ph cm^-2 s^-1 (errors are statistical only) on 16 July 2012, a factor of 8 greater than the average flux reported in the second Fermi LAT catalog (2FGL, Nolan et al. 2012, ApJS, 199, 31). This is the highest flux observed for this source since the beginning of the Fermi mission. High level gamma-ray activity from this source was detected in June 2008 by AGILE (Donnarumma et al. 2009, ApJ, 691, L13).

Because Fermi operates in all-sky survey mode, gamma-ray monitoring of this source will continue. In consideration of the activity of this source we encourage multiwavelength observations. The Fermi LAT contact person for this source is D. Paneque (dpaneque@mppmu.mpg.de).

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.