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

ATel #14777; S. Buson (Univ. of Wuerzburg) and R. de Menezes (Univ. of Sao Paulo, Univ. of Wuerzburg) on behalf of the Fermi-LAT collaboration
on 12 Jul 2021; 20:09 UT
Credential Certification: Sara Buson (sara.buson@gmail.com)

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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 is undergoing an elevated gamma-ray emission state. On 2021-07-11, the source displayed a daily averaged gamma-ray flux (E>100MeV) of (4.9+/-0.3) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of about 30 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 (6.3+/-0.8) X 10^-6 photons cm^-2 s^-1 during 6:00-12:00 UTC on that day. The corresponding photon index is 1.95+/-0.05, and is smaller than the 4FGL value of 2.20+/-0.01, similar to what was observed in previous events (ATel #14330, ATel #14583). The highest-energy photon associated with the source had an energy of ~120 GeV, and was recorded at 14:24:01.000 UTC on 2021-07-10.

An automatic GCN/Fermi-LAT Notice has been issued reporting a comparable daily gamma-ray flux (see https://gcn.gsfc.nasa.gov/other/1626101153_fermi.txt). Continued bright activity from BL Lacerate has been reported also at other wavelengths, see e.g. ATel #14751, ATel #14773 and ATel #14774.

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.