Fermi-LAT detection of a hard-spectrum GeV flare from the BL Lac B2 1811+31
ATel #14060; R. Angioni (SSDC/INFN), E. Bissaldi (Politecnico & INFN Bari), S. Garrappa (DESY Zeuthen), F. Longo (University and INFN Trieste) and D. Kocevski (MSFC/NASA) on behalf of the Fermi Large Area Telescope Collaboration
on 2 Oct 2020; 15:59 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 a hard-spectrum gamma-ray flare from a source positionally consistent with the BL Lac object B2 1811+31, associated with the GeV gamma-ray source 4FGL J1813.5+3144 (The Fermi-LAT collaboration 2020, ApJS, 247, 33) at RA=273.396678 deg, Dec=31.738228 deg, (J2000, Petrov 2011, AJ, 142, 105). This source has a measured redshift of z=0.117 (Giommi et al. 1991, ApJ, 378, 77).
Preliminary analysis indicates that B2 1811+31 has been in an elevated GeV gamma-ray emission state in the past two days. On 1 October 2020, the LAT measured a daily averaged gamma-ray flux (E>100MeV) of (1.5+/-0.6) X 10^-7 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 11 relative to the average flux reported in the fourth Fermi-LAT catalog (4FGL). The corresponding photon index is 1.4+/-0.2, to be compared with the 4FGL value of 2.14+/-0.06, implying the source went through a very significant spectral hardening. This hard-spectrum state was accompanied by the detection of several E>10 GeV photons with probability >99% of having been emitted by B2 1811+31. The highest energy photon was observed on the same day at 23:14:24.561 UTC, with an energy of 61 GeV. The "Fermi All-sky Variability Analysis" (Ackermann et al. 2013, ApJ, 771, 57) weekly light curve shows a longer-timescale flare starting from April 2020 (https://fermi.gsfc.nasa.gov/ssc/data/access/lat/FAVA/LightCurve.php?ra=273.396678&dec=31.738228).
Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A Swift ToO request has been submitted. We encourage further 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.
