Swift Follow-up of the New Gamma-ray Source PKS B2258-022
ATel #7479; S. Buson (INFN/Univ. of Padova), M. Ajello (Clemson Univ.), F. D'Ammando (Univ./INAF-IRA Bologna), M. Orienti (INAF-IRA Bologna), D. Gasparrini (ASDC/INFN Roma) on behalf of the Fermi Large Area Telescope Collaboration and A. Segreto (INAF-IASF Palermo)
on 6 May 2015; 14:49 UT
Credential Certification: Sara Buson (sara.buson@unipd.it)
Subjects: Gamma Ray, >GeV, Request for Observations, AGN, Blazar, Quasar
Referred to by ATel #: 7480
Following the detection of a new gamma-ray source by Fermi-LAT on 2015 April 25 (ATel #7445), two Swift target of opportunity observations were performed on 2015 April 25 and 26. Swift X-ray telescope (XRT) data were taken in Photon Counting mode for a total exposure of about 3.7 ks and 3.9 ks, respectively. During both observations, only one bright source is detected by the Swift/XRT at RA =23h 01m 7.8s, Dec = -01d 58â 3.3'' (J2000) with a 90 % error radius of 4.0 arcsec. The XRT source, well described by an absorbed power law with absorption consistent with the Galactic one, displays a spectrum with a photon index of 1.2+/-0.2 and a 0.3-10 keV flux of (2.7+/-0.8)x10^-12 erg cm^-2 s^-1.
No previous X-ray observations of this source exist in the literature. However, the hard photon index argues in favour of a flat-spectrum radio quasar (FSRQ). The FSRQ PKS B2258-022 is spatially coincident with the bright XRT source and is the most probable associated object. The source was very likely detected in an X-ray high state since the extrapolated XRT flux to the Swift-BAT band (15-150keV) of ~2x10^-11 erg cm^-2 s^-1 is in tension with the 5 sigma upper limit of 1.2x10^-11 erg cm^-2 s^-1 from the analysis of 100 months BAT survey data (Segreto et al 2010, A&A, 510, 47). This is consistent with the FSRQ being the counterpart of the Fermi-LAT source.
Further multiwavelength observations are encouraged. For this source the Fermi-LAT contact person is M. Orienti (orienti@ira.inaf.it).
We thank the Swift team and the Swift PI for promptly accepting and scheduling the ToO observation, in particular K. L. Page as the Swift Observatory Duty Scientist.
