Fermi-LAT detection of enhanced gamma-ray activity from BL Lac

ATel #14583; G. La Mura (LIP) on behalf of the Fermi Large Area Telescope Collaboration
on 29 Apr 2021; 02:08 UT
Credential Certification: Giovanni La Mura (glamura@lip.pt)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed enhanced gamma-ray activity from a source positionally consistent with BL Lacertae, also known as 4FGL J2207.1+4316 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A.=22:02:43.29137 Dec.=+42:16:39.9799 (J2000; Beasley et al. 2002, ApJS, 141, 13), and redshift z=0.0686 (Vermeulen et al., 1995, ApJ, 452, L5).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on 2021-04-27, with a daily averaged gamma-ray flux (E>100MeV) of (7.7+/-0.3) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 33 relative to the 10-year average flux reported in the 2nd data release of the fourth Fermi-LAT catalog (4FGL-DR2, Ballet et al., arXiv:2005.11208). The source was detected with a peak flux of (17.5+/-1.1) X 10^-6 photons cm^-2 s^-1 during 0:00-6:00 UTC on that day. This is the highest LAT flux ever observed for this source. The corresponding photon index is 1.89+/-0.03, and is smaller than the 4FGL value of 2.20+/-0.01, similar to what was observed in a previous event (ATel #14330). The highest energy photon associated with the source had an energy of 26 GeV, an association probability larger than 0.99, and was recorded at 19:21:10 UTC on 2021-04-27.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue and consequently, a preliminary estimation of the daily gamma-ray flux observed by Fermi-LAT will be publicly available ( http://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/ ). We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is Simone Garrappa (simone.garrappa at desy.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.