Fermi-LAT detection of enhanced gamma-ray activity from the FSRQ TXS 2013+370
ATel #14252; R. Angioni (SSDC/INFN) on behalf of the Fermi Large Area Telescope Collaboration
on 8 Dec 2020; 19:59 UT
Credential Certification: Roberto Angioni (r.angioni90@gmail.com)
Subjects: Gamma Ray, >GeV, Request for Observations, AGN, Blazar, Quasar
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 the flat-spectrum radio quasar TXS 2013+370, also known as MG2 J201534+3710 and 4FGL J2015.5+3710 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 303.869708 deg, Decl. = 37.183198 deg (J2000; Fomalont et al. 2003, AJ, 126, 2562), and redshift z=0.859 (Shaw et al. 2013, ApJ, 764, 135).
Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on 6 December 2020, with a daily averaged gamma-ray flux (E>100MeV) of (1.2+/-0.4) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 10 relative to the average flux reported in the fourth Fermi-LAT catalog (4FGL). This is the highest LAT daily flux ever observed for this source. The corresponding photon index is 2.1+/-0.2, and is significantly smaller than the 4FGL value of 2.45+/-0.03. TXS 2013+370 continued to be detected on daily time scales on 7 December 2020, with flux (9+/-3) X 10^-7 photons cm^-2 s^-1 and a photon index 1.9+/-0.2. The transient was significantly detected in the time interval 2020-12-07 00:00:00 - 2020-12-07 06:00:00 UTC as well, with flux (1.8+/-0.8) X 10^-6 photons cm^-2 s^-1. During the same time interval, the Fermi-LAT detected a ~16 GeV photon with probability higher than 99% of being associated with this gamma-ray source, which was recorded at 2020-12-07 03:23:05.327 UTC. TXS 2013+370 has undergone several periods of enhanced gamma-ray activity during the Fermi-LAT mission, with a major flare being observed in early 2009, which was accompanied by changes in the parsec-scale jet structure (Traianou et al. 2020, A&A, 634, 112).
Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. This source is being added to the "LAT Monitored Sources" 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 Roberto Angioni (roberto.angioni@ssdc.asi.it).
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.
