Fermi LAT detection of hard-state GeV gamma-ray activity from BL Lac objects MG2 J083121+2629 and PKS 0139-09

ATel #16266; Stefano Ciprini (INFN Roma Tor Vergata, & ASI Space Science Data Center, Roma, Italy); Denis Bernard (LLR, Ecole Polytechnique & CNRS / IN2P3, Paris, France), on behalf of the the Fermi Large Area Telescope Collaboration.
on 3 Oct 2023; 23:07 UT
Credential Certification: Stefano Ciprini (stefano.ciprini@ssdc.asi.it)

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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 candidate BL Lac object MG2 J083121+2629, also known as 87GB 082818.7+264044 and 4FGL J0831.4+2631 (Abdollahi et al. 2022, ApJS, 260, 53), with radio coordinates R.A. = 127.82636 deg, Decl. = 26.50698 deg (J2000; Petrov & Taylor 2011, AJ, 142, 89), and unknown redshift.

Preliminary analysis indicates that MG2 J083121+2629 was in an elevated and hard gamma-ray emission state during the past three days, reaching daily averaged gamma-ray fluxes (E>100MeV) of (1.5+/-0.7) X 10^-7 photons cm^-2 s^-1 and (4.1+/-1.3) X 10^-7 photons cm^-2 s^-1 (statistical uncertainties only) respectively on September 30 and October 2, 2023. These correspond to a flux increase of a factor of about 30 and 80 with respect to the average flux reported in the fourth Fermi-LAT catalog (4FGL), the latter being the greatest flux ever observed by the LAT for that source. The corresponding photon indices are 1.6+/-0.2 and 1.8+/-0.2 respectively, where the 4FGL value is 2.498+/-0.115.

The Fermi LAT also has observed gamma-ray enhanced activity from a source positionally consistent with the BL Lac object PKS 0139-09, also known as OC -065 and 4FGL J0141.4-0928 (Abdollahi et al. 2022, ApJS, 260, 53), with radio coordinates R.A. = 25.357634 deg, Decl. = -9.478798 deg (J2000; Beasley et al. 2002, ApJS, 141, 13), and redshift (debated) z=0.733 (Stocke & Rector 1997, ApJ, 489, 17).

Preliminary analysis indicates that PKS 0139-09 was in an elevated and hard gamma-ray emission state during the past two days, reaching daily averaged gamma-ray fluxes (E>100MeV) of (4.7+/-1.8) X 10^-7 photons cm^-2 s^-1 and (2.0+/-1.1) X 10^-7 photons cm^-2 s^-1 (statistical uncertainties only) respectively on September 30 and October 1, 2023. These correspond to flux increases of factors of about 15 and 10 with respect to the average flux reported in the fourth Fermi-LAT catalog (4FGL), the former being the greatest flux ever observed by the LAT for that source. The corresponding photon indices are 2.1+/-0.3 and 1.7+/-0.3 respectively, where the 4FGL value is 2.135+/-0.019.

For both blazars, spatially consistent ~30 GeV photons were detected during these periods. This is the first time that the Fermi LAT Collaboration has reported enhanced gamma-ray activity from either of these sources. Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of both MG2 J083121+2629 and PKS 0139-09 will continue. We encourage multifrequency observations of these gamma-ray sources. For these sources, the Fermi-LAT contact persons are D. Bernard (denis.bernard[at]in2p3.fr) and 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.