Fermi-LAT detection of flaring gamma-ray activity from the FSRQ PKS 0403-13, and continued activity in GPS radio source OS 300 (4C +33.38, B2 1600+33)

ATel #15146; Stefano Ciprini (INFN Roma Tor Vergata, & ASI Space Science Data Center, Rome, Italy), on behalf of the the Fermi Large Area Telescope Collaboration
on 27 Dec 2021; 20:59 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 emission from a source positionally consistent with the flat spectrum radio quasar (FSRQ) PKS 0403-13 (also known as TXS 0403-132, OF -105, RX J0405.5-1308, and 4FGL J0405.6-1308), with radio counterpart position R.A.: 61.391680 deg, Dec.: -13.137136 deg (J2000.0, Fey et al. 2004, AJ, 127, 3587) and with redshift z=0.5706+/-0.0001 (Marziani et al. 1996, ApJS, 104, 37).

Preliminary analysis indicates that on December 25, 2021, PKS 0403-13 was in a high state with a daily averaged gamma-ray flux (E>100 MeV) of (1.0+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), about 80 times greater than the average flux reported in the 4FGL catalog. The corresponding daily averaged spectral photon index (E>100 MeV) of 2.7+/-0.2 is greater than the fourth Fermi-LAT catalog (4FGL) value of 2.55+/-0.05. This is second time that the Fermi-LAT Collaboration has announced flaring gamma-ray activity from PKS 0403-13 (ATel#9242).

The LAT has also observed continuing and increasing gamma-ray activity from a source positionally consistent with the extragalactic radio source OS 300 (also known as 4C +33.38, B2 1600+33 and 4FGL J1602.1+3324, Abdollahi, et al. 2020, ApJS, 247, 33), with radio counterpart position R.A. = 240.530 deg, Dec. = +33.448 deg (J2000; Johnston et al. 1995, AJ, 110, 880). OS 300 is a Giga-Hertz Peaked Spectrum (GPS) radio source (Snellen et al. 2000 MNRAS 319, 445) with photometric redshift estimation of z = 1.1 (Snellen et al. 2000 MNRAS 319, 445).

Preliminary analysis indicates that OS 300 was in a flaring state on 2021 December 26, with a daily averaged gamma-ray flux (E>100MeV) of (1.5+/-0.1) X 10^-6 photons cm^-2 s^-1 (uncertainties are statistical only), and a peak 6-hr flux of (2.4+/-0.4) X 10^-6 photons cm^-2 s^-1 from 12:00-18:00 UTC. The former corresponds to a flux increase of a factor of about 330 relative to the average flux reported in the 4FGL catalog. This is the fourth, and largest, flaring episode associated with this source ever observed by the LAT (see ATels #13931, #14422, #15126). The daily photon index is 2.0+/-0.1, smaller than the 4FGL average value of 2.2+/-0.1.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of PKS 0403-13 and OS 300 will continue. These sources are part of the "FSSC LAT Monitored Sources" and, consequently, a preliminary estimation of the daily gamma-ray flux observed by Fermi-LAT will be publicly available at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/PKS_0403-13 , and at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/OS_300 , respectively. PKS 0403-13 has also an entry in the FSSC light curve repository (https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.php?source_name=4FGL_J0405.6-1308). We encourage multifrequency observations of both the gamma-ray sources. For PKS 0403-13 the Fermi LAT contact person is S. Ciprini (stefano.ciprini@ssdc.asi.it). For OS 300, the Fermi-LAT contact person is C.C. Cheung (teddy.cheung@nrl.navy.mil).

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.