First X-ray detection of TXS 1520+344 by Swift

ATel #15398; G. La Mura (LIP, Portugal), on behalf of the Fermi Large Area Telescope Collaboration
on 24 May 2022; 14:45 UT
Credential Certification: Giovanni La Mura (glamura@lip.pt)

Tweet

On May 5, 2022, the Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, observed gamma-ray flaring activity from a source positionally consistent with the flat-spectrum radio quasar TXS 1520+344 (see ATel #15367), with coordinates R.A. = 230.72866 deg, Decl. = +34.23586 deg (J2000; Petrov & Taylor 2011, AJ, 142, 89), and redshift z=1.285 (Richards et al. 2009, ApJS, 180, 67). This source is not present in any LAT published catalog and no previous detections were reported by AGILE or EGRET. Following this first gamma-ray detection, a Swift ToO request was submitted.

Swift executed two observations of the source in photon counting mode, on May 7 and May 9, accumulating, respectively, 1496.86s and 1489.33s of XRT exposure times. An X-ray source with decaying flux, positionally consistent with TXS 1520+344, was detected in the two visits. Preliminary analysis suggests that the source could be modeled with an absorbed power-law, using a fixed Galactic Hydrogen column density of 1.90 X 10^20 cm^-2 (Willingale et al. 2013, MNRAS, 431, 394). On May 7, the X-ray data in the 0.3-10keV band resulted in a flux estimate of (0.50+/-0.26) X 10^-12 erg cm^-2 s^-1, with a photon index of 1.8+/-0.4 (statistical uncertainty only). On May 9, the estimated flux had decreased to (0.23+/-0.12) X 10^-12 erg cm^-2 s^-1, with a photon index of 2.7+/-0.7. No previous X-ray detection is reported for this source.

Simultaneous observations were carried out by the UVOT instrument. UVOT detected TXS 1520+344 with the following results (magnitudes not corrected for Galactic extinction):

Obs. Date	Filter	Exp. Time	Magnitude (AB)
May 7	UVM2	1483.15s	20.64 +/- 0.09
May 9	U	1489.01s	19.19 +/- 0.04

For comparison, the source is present in the revised GALEX catalog with an AB magnitude of 21.92 +/- 0.44 in the FUV band and an AB magnitude of 21.28 +/- 0.22 mag in the NUV band (Bianchi et al. 2017, ApJS, 230, 24), therefore the enhanced emission state was likely affecting the UV band as well. An optical increase of brightness with respect to historical data was also observed by MASTER-OAGH on May 7 (see ATel #15369).

The source was no longer detected by Fermi-LAT at the time of the Swift observations. On May 7, the Fermi-LAT 95% confidence upper limit for a power-law source with index 2.0 at the position of TXS 1520+344 is 0.10 x 10^-6 ph cm^-2 s^-1, while the exposure time for the region on May 9 is too short to place any meaningful constraint.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is Giovanni La Mura (glamura@lip.pt).

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. We further acknowledge commitment of the Swift Team, Observatory Duty Scientists, and PI, who made these observations possible.