Fermi-LAT detection of enhanced gamma-ray activity from the flat-spectrum radio source SUMSS J065518-495205
ATel #14261; R. Angioni (SSDC/INFN) on behalf of the Fermi Large Area Telescope Collaboration
on 11 Dec 2020; 23:32 UT
Credential Certification: Roberto Angioni (r.angioni90@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 activity from a source positionally consistent with the flat-spectrum radio source SUMSS J065518-495205, also known as 4FGL J0654.6-4952 (The Fermi-LAT Collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 103.826958 deg, Decl. = -49.868444 deg (J2000; Murphy et al. 2010, MNRAS, 402, 2403), and no measured redshift.
Preliminary analysis indicates that SUMSS J065518-495205 was in an elevated gamma-ray emission state on 10 December 2020, with a daily averaged gamma-ray flux (E>100MeV) of (3+/-1) X 10^-7 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of about 70 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 1.8+/-0.2, and is significantly smaller than the 4FGL value of 2.36+/-0.14. The transient was significantly detected in the time interval 2020-12-10 18:00:00 - 2020-12-11 00:00:00 UTC as well, with flux (9+/-3) X 10^-7 photons cm^-2 s^-1, corresponding to a flux increase of a factor of about 200 relative to the 4FGL flux. During the same time interval, the Fermi-LAT detected a ~12 GeV photon with probability higher than 99% of being associated with this gamma-ray source, which was recorded at 2020-12-10 22:49:13.278 UTC.
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. Optical spectroscopic observation would be especially valuable since the source does not yet have a measured redshift. 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.
