Fermi-LAT detection of a hard-spectrum GeV flare from the FSRQ OV 591
ATel #14066; I. Mereu (INFN Perugia), C. C. Cheung (Naval Research Laboratory) on behalf of the Fermi Large Area Telescope Collaboration
on 6 Oct 2020; 19:41 UT
Credential Certification: Isabella Mereu (mereuisabella@gmail.com)
Subjects: Gamma Ray, >GeV, Request for Observations, AGN, Blazar
The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed enhanced gamma-ray flaring activity from a source positionally consistent with the flat spectrum radio quasar OV 591, also known as 4FGL J1955.4+5132 (The Fermi-LAT Collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 298.9280758 deg, Decl. = 51.5301517 deg (J2000; Johnston et al. 1995 AJ 110, 880), and redshift, z=1.223 (Lawrence et al. 1996 ApJS 107, 541).
Preliminary analysis indicates that OV 591 was in an elevated gamma-ray emission state on 5 October 2020 with a daily averaged gamma-ray flux (E>100MeV) of (4.5 +/- 1.2) X 10^-7 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 23 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 1.78 +/- 0.14 and is smaller than the 4FGL value of 2.53 +/- 0.06, indicating a spectral hardening accompanying the flux increase. This hard-spectrum state was accompanied by the detection of one E > 10 GeV photon (E = 11 GeV; time : 2020-10-05 12:49:03.636 UTC) with probability >99% of having been emitted by OV 591.
Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is Isabella Mereu (mereuisabella@gmail.com).
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.
