Fermi-LAT and Swift observations of flaring activity from the FSRQ PKS 1954-388

ATel #15326; G. La Mura (LIP, Portugal), G. Principe (University of Trieste and INFN - Trieste, INAF-IRA Bologna), on behalf of the Fermi Large Area Telescope Collaboration
on 12 Apr 2022; 20:13 UT
Credential Certification: Giovanni La Mura (glamura@lip.pt)

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On March 29, 2022, the Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, observed increased gamma-ray activity from a source positionally consistent with the flat-spectrum radio quasar PKS 1954-388, also known as 4FGL J1958.0-3845 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 299.499247 deg, Decl. = -38.751766 deg (J2000; Ma et al. 1998, AJ, 116, 516), and redshift z=0.63 (Gattano et al. 2018, A&A, 618, 80). A preliminary analysis resulted in a daily average flux (E>100MeV) of (1.05 +/- 0.23) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only) with photon index 1.98+/-0.14. These values correspond to a flux increase of approximately 20 times and a harder spectral state with respect to the 12-year average values of the fourth Fermi-LAT catalog (4FGL-DR3), so a Swift ToO request was submitted.

Swift executed two observations of the source in photon counting mode, on April 6 and April 9, accumulating, respectively, 2180s and 1860s of XRT exposure time. An X-ray source positionally consistent with PKS 1954-388 was detected in both visits. Preliminary analysis suggests that the source could be modelled with an absorbed power-law and a black body thermal component, using a fixed Galactic Hydrogen column density of 8.39 X 10^20 cm^-2 (Willingale et al. 2013, MNRAS, 431, 394). On April 6, the X-ray data in the 0.3-10 keV band resulted in a flux estimate of (7.0+/-1.1) X 10^-12 erg cm^-2 s^-1, with a photon index of 1.7+/-0.3 (statistical uncertainty only). On April 9, the estimated flux had decreased to (5.9+/-0.4) X 10^-12 erg cm^-2 s^-1, with a photon index of 1.8+/-0.7. Both X-ray fluxes are significantly larger than the 2SXPS catalog value of 2.51 X 10^-12 erg cm^-2 s^-1 (Evans et al. 2020, ApJS, 247, 54).

The LAT monitoring provided simultaneous coverage of the gamma-ray evolution of this flare. On April 6, the source was detected with a daily gamma-ray flux (E>100MeV) of (0.6+/-0.2) X 10^-6 photons cm^-2 s^-1 and a spectral index of 2.4+/-0.3 (statistical uncertainty only). On April 9, it was detected with a daily gamma-ray flux (E>100MeV) of (0.7+/-0.2) X 10^-6 photons cm^-2 s^-1 and a spectral index of 2.2+/-0.2 (statistical uncertainty only). Both observed fluxes are more than 10 times brighter than the average flux reported in 4FGL. The average flux and spectral index for this source in 4FGL-DR3 are, respectively, (4.7+/-0.2) X 10^-8 photons cm^-2 s^-1 and 2.33+/-0.02.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. This source is included in the "LAT Monitored Sources" and, consequently, a preliminary estimation of the daily gamma-ray flux observed by Fermi-LAT will be publicly available ( http://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/ ). The light curve of this source can be accessed through the Fermi-LAT Light Curve Repository at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.html?source_name=4FGL_J1958.0-3845. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is Giacomo Principe (giacomo DOT principe AT inaf DOT 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. We further acknowledge commitment of the Swift Team, Observatory Duty Scientists, and PI, who made these observations possible.