Fermi-LAT detection of a hard GeV flare from the TeV source VER J0521+211
ATel #13528; J. Sinapius (Univ. of Wuerzburg), R. Angioni (SSDC/INFN), and R. Ojha (NASA/GSFC/UMBC) on behalf of the Fermi-LAT Collaboration
on 27 Feb 2020; 14:05 UT
Credential Certification: Roberto Angioni (r.angioni90@gmail.com)
Subjects: >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 a hard gamma-ray flare from a source positionally consistent with the TeV source VER J0521+211, associated with the radio source TXS 0518+211 (a.k.a. RGB J0521+212) and the GeV gamma-ray source 4FGL J0521.7+2112 (The Fermi-LAT collaboration 2019, arXiv:1902.10045) at RA=80.441524 deg, Dec=21.214292 deg, (J2000, Beasley et al. 2002 ApJS, 141, 13). This source has a measured redshift of z=0.108 (Shaw et al. 2013, ApJ 764, 135) and a classification as a BL Lac object has been suggested (VERITAS Collaboration 2013, ApJ 776, 69).
Preliminary analysis indicates that VER J0521+211 has been in an elevated GeV gamma-ray emission state in the past month. On 25 February 2020, the LAT measured a daily averaged gamma-ray flux (E>100MeV) of (4.2+/-1.4) X 10^-7 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 4 relative to the average flux reported in the fourth Fermi-LAT catalog (4FGL). The corresponding photon index is 1.5+/-0.2, significantly smaller than the 4FGL value of 1.92+/-0.01. During this flare, the LAT detected three E>10 GeV photons associated with this source, with energies of 11, 59, 65 GeV. VER J0521+211 is also reported in the 2FHL catalog of LAT sources significantly detected in the 50 GeV - 2 TeV energy range during the first 80 months of the Fermi mission (2FHL J0521.7+2112, Ackermann et al. 2016, ApJS 222, 5). This source has been reported to be in an unprecedented high state at E>100 GeV by the VERITAS collaboration on the same day (ATel #13522). An analysis of LAT data in the same time interval reported by the VERITAS team also yields a >5 sigma detection, however the spectral parameters are not well constrained due to the low statistics. Overall, the LAT results appear consistent with the high state in the TeV band reported by VERITAS. No corresponding flare has been reported in the optical so far (ATel #13523).
Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. This source is part of the "LAT Monitored Sources" and consequently, a preliminary estimation of the daily gamma-ray flux observed by Fermi-LAT will be publicly available (https://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 Sara Buson (sara.buson@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.
