Fermi-LAT detection of gamma-ray flaring activity from the peculiar radio source PKS 1413+135
ATel #13049; R. Angioni (MPIfR-Bonn), C. C. Cheung (NRL), S. Buson (Uni. Wuerzburg/UMBC) on behalf of the Fermi Large Area Telescope Collaboration
on 29 Aug 2019; 14:23 UT
Credential Certification: Roberto Angioni (r.angioni90@gmail.com)
Subjects: Gamma Ray, >GeV, Request for Observations, AGN
The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed gamma-ray flaring activity from a source positionally consistent with the peculiar radio source PKS 1413+135, also known as 4FGL J1416.1+1320 (The Fermi-LAT collaboration 2019, arXiv:1902.10045), with coordinates R.A. = 213.99507 deg, Decl. = +13.33992 deg (J2000; Fey et al. 2015 AJ, 150, 58), and redshift z=0.247 (Stocke et al. 1992 ApJ, 400, 17). While PKS 1413+135 was initially classified as a BL Lac (McHardy et al. 1991 MNRAS, 249, 742), its parsec-scale jet suggests it might be a young radio source (Perlman et al. 1996 AJ, 111, 1839; Gugliucci et al. 2005 ApJ, 622, 136).
Preliminary analysis indicates that this source was in an elevated gamma-ray emission state during the past ten days, reaching a daily averaged gamma-ray flux (E>100MeV) of (4.8+/-1.5) X 10^-7 photons cm^-2 s^-1 (statistical uncertainty only) on 28 August 2019. The latter corresponds to a flux increase of a factor of 38 with respect to the average flux reported in the fourth Fermi-LAT catalog (4FGL). This is the highest LAT daily flux ever observed for this source. The corresponding photon index is 1.8+/-0.2, and is significantly smaller than the 4FGL value of 2.41+/-0.07. The source showed an even harder spectrum on 19 August and 25 August, with a measured power-law photon index of 1.4+/-0.3 and 1.4+/-0.2, respectively. This hard-spectrum state was accompanied by the detection of several E>10 GeV photons with probability >99% of having been emitted by the target source. The highest energy photon was observed on 19 August 2019 at 06:55:11.961 UTC, with an energy of ~64 GeV.
Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. This source is being added to the "LAT Monitored Sources" and consequently, a preliminary estimation of the daily gamma-ray flux observed by Fermi-LAT will be publicly available at http://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/. Target of opportunity observations with the Neil Gehrels Swift Observatory have been requested. Given that the source's flux has been increasing over the past three days, and due to the observed remarkable spectral hardening, we strongly encourage further multifrequency observations of this source. For this source, the Fermi-LAT contact persons are R. Angioni (angioni@mpifr-bonn.mpg.de) and 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.
