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Fermi LAT detection of a GeV flare from the vicinity of Fermi bright source 0FGLJ1641.4+3939

ATEL # 2226; Luis C. Reyes (U. of Chicago - KICP) and C. C. Cheung (NRL/NRC) on behalf of the Fermi-LAT Collaboration
on 4 Oct 2009; 21:04 UT
Password Certification: Luis C. Reyes (

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

The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed increased gamma-ray activity from the vicinity of Fermi bright source 0FGL J1641.4+3939 (Abdo, A. et al. 2009 ApJ, 700, 597). Preliminary analysis of all-sky survey data over the 36 hour period from Sep 29 (00 UT) - Sep 30 (12 UT) resulted in the detection of a source at RA = 250.661 deg, Dec = 40.1502 deg (J2000) with 95% confidence error circle= 14.0 arcmin (statistical only). Including a systematic error in the way described in A. Abdo et al. (2009, ApJS, 183, 46), the well-known blazar 3C345 is just outside the edge of the 95% confidence error circle. Indeed, a confident identification of the gamma-ray source in this region remains challenging given the abundance of possible counterparts: 3C345, NRAO 512 and CLASS J1641+3935. Consequently, follow-up multiwavelength observations are encouraged for identification via correlated variability.

Preliminary analysis indicates that the during the 36 hour period, the average gamma-ray flux (E>100MeV) was (0.6 +/- 0.2) x 10-6 ph cm-2 s-1, with a peak flux of (1.3 +/- 0.5) x 10-6 ph cm-2 s-1 found during the 6 hr period Sep 30 (00-06 UT); errors are statistical only. The peak flux represents a relative increase up to a factor of ~8 with respect to the average flux of 0FGL J1641.4+3939 during the first year of Fermi operations. The increased activity in gamma-rays, if originating from 3C345, would be consistent with the recently reported outbursting optical activity (ATEL #2222).

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. For this source the Fermi LAT contact person is Luis C. Reyes (

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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