Fermi LAT detection of a GeV gamma-ray flare from the BL Lac object 1RXS J054357.3-553206 (RBS 0679)

ATel #15432; Stefano Ciprini (1. INFN Roma Tor Vergata; 2. ASI Space Science Data Center; Italy), on behalf of the Fermi Large Area Telescope Collaboration
on 14 Jun 2022; 16:21 UT
Credential Certification: Stefano Ciprini (stefano.ciprini@ssdc.asi.it)

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The Large Area Telescope (LAT), one of two instruments on the Fermi Gamma-ray Space Telescope, has observed increasing gamma-ray emission from a source positionally consistent with the BL Lac object 1RXS J054357.3-553206 (also known as RBS 0679, SUMSS J054356-553203, BZB J0543-5532, 4FGL J0543.9-5531 and 3FHL J0543.9-5532, Abdollahi, et al. 2020, ApJS, 247, 33; Ajello et al. 2017, ApJS, 232, 18, and sometimes as RX J0543.9-5532), with radio coordinates (J2000) R.A.: 85.98837 deg, Dec.: -55.53536 deg (Mao 2011, NewAst, 16, 503). 1RXS J054357.3-553206 has a redshift of z=0.273 as measured by the X-shooter spectrograph at the ESO Very Large Telescope (Pita et al. 2014, A&A, 565, 12). As a note, this BL Lac object is also a member of the first NuSTAR catalog of blazars (Middei et al., 2022, MNRAS, accepted).

Preliminary analysis indicates that on 2022 June 12, 1RXS J054357.3-553206 was in a high state with a daily averaged gamma-ray flux (E>100 MeV) of (0.3+/-0.1) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), about 40 times greater than its twelve-year average flux reported in the fourth Fermi-LAT source catalog data release 3 (4FGL-DR3). The corresponding photon spectral index (E>100 MeV) of 1.8+/-0.2 (statistical uncertainty only) is consistent with the 4FGL-DR3 catalog value of 1.75+/-0.03.

The peak 6-hour integrated gamma-ray flux (E>100 MeV) of (0.7+/-0.2) X 10^-6 photons cm^-2 s^-1 was reached by 1RXS J054357.3-553206 on the interval 06:00-12:00 UT of the same day, with the corresponding photon spectral index of 2.1+/-0.2.

This is the first time that the Fermi-LAT Collaboration is announcing a gamma-ray flare on daily timescale from a gamma-ray point source spatially associated with this high-energy peaked BL Lac object. Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of 1RXS J054357.3-553206 will continue. In consideration of the ongoing activity of this source, we encourage multiwavelength observations. For this source the Fermi LAT contact person is S. Ciprini (stefano.ciprini[at]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.