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Fermi and Swift discovery of GeV gamma-ray and X-ray emission from the blazar PKS 2246+208

ATel #12403; S. Ciprini (INFN Roma Tor Vergata & SSDC ASI, Roma, Italy), Roberto Angioni (MPIfR, Bonn, Germany), on behalf of the Fermi Large Area Telescope Collaboration
on 18 Jan 2019; 10:23 UT
Credential Certification: Stefano Ciprini (stefano.ciprini@ssdc.asi.it)

Subjects: Ultra-Violet, X-ray, Gamma Ray, >GeV, Request for Observations, AGN, Blazar, Quasar

Referred to by ATel #: 12693

The Large Area Telescope (LAT), one of two instruments on the Fermi Gamma-ray Space Telescope, has observed a flare in gamma rays from a source positionally consistent with the flat spectrum radio quasar PKS 2246+208 (also known as TXS 2246+208 and MG3 J224902+2106), with radio coordinates (J2000.0) R.A.: 342.252361 deg, Dec.: 21.11745 deg (Beasley et al. 2002, ApJS, 141, 13) and also listed in the CGRaBS, CRATES and 5BZ catalogs. PKS 2246+208 has a redshift z=1.274 (Sowards-Emmerd et al. 2005, ApJ, 626, 95), and it has never been reported in X-ray source catalogs.

Preliminary analysis indicates that PKS 2246+208 was in an active gamma-ray state in the first part of this January 2019. In particular it was detected at least on January 3, 6, 7 and 12, 2019, with daily averaged gamma-ray fluxes (E>100MeV) respectively of (0.4+/-0.1) X 10^-6, (0.4+/-0.1) X 10^-6, (0.2+/-0.1) X 10^-6 and (0.6+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical error only). The corresponding photon spectral indexes (E>100MeV) are respectively 2.2+/-0.2, 2.1+/-0.2, 1.7+/-0.2 and 2.3+/-0.3.

PKS 2246+208 is not included in any published Fermi-LAT gamma-ray source catalog, nor in the third EGRET catalog and has not been reported by AGILE or INTEGRAL so far. It is located at about five degrees distance from 3C 454.3. In addition PKS 2246+208 is associated with a source in the forthcoming fourth Fermi-LAT source catalog (4FGL), which includes sources detected based on their average flux over the first 8 years of the Fermi mission.

High-urgency follow-up ToO Swift observations were performed on January 8, 11 and 14, 2019 for a total exposure of about 5.1 ks. Swift XRT preliminary analysis shows an X-ray excess from PKS 2246+208 for the first time, with a count rate for the three pointings of 0.017+/-0.003, 0.019+/-0.004 and 0.011+/-0.003 counts/s respectively.

The de-reddened Swift UVOT flux density values, obtained using the official calibration (Breeveld et al.2011) and a Galactic value E(B-V)=0.052 in the direction of the source (Schlafly et al.2011) on January 8, 11 and 14, 2019, indicate optical and UV band fluxes up to a factor of 10 greater than the archival flux densities. For two optical filters de-reddened magnitudes are in the following ranges: U(mag)=[17.21+/-0.10 - 16.61+/-0.08]; B(mag)=[18.31+/-0.12 - 17.84+/-0.10]; and for the 3 UV filters are in the following ranges: W1(mag)=[17.91+/-0.13 - 16.31+/-0.07]; M2(mag)=[18.57+/-0.02 - 17.30+/-0.01]; W2(mag)=[ 18.07+-0.10 - 16.85+/-0.07] .

These preliminary results and the previously known radio-to-optical source properties suggest PKS 2246+208 is now an identified new X-ray and new gamma-ray blazar. Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the ongoing activity of this source we encourage multiwavelength observations. For this source the contact persons are S. Ciprini (stefano.ciprini at ssdc.asi.it) and R. Angioni (angioni at mpifr-bonn.mpg.de).

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 thank the Swift Team for making these observations possible, in particular C. Gronwall as the Swift Observatory Duty Scientist.