Fermi and Swift discovery of GeV gamma-ray and X-ray emission from the blazar S4 0444+63

ATel #10584; Stefano Ciprini (SSDC-ASI & INFN, Italy) on behalf of the Fermi Large Area Telescope Collaboration.
on 20 Jul 2017; 12:12 UT
Credential Certification: Stefano Ciprini (stefano.ciprini@asdc.asi.it)

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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 S4 0444+63 (also known as TXS 0444+634, GB6 J0449+6332 and 2FAV J0449+63.5, Abdollahi et al. 2017, ApJS, accepted), with radio coordinates, (J2000.0), R.A.: 72.34717 deg, Dec.: 63.53595 deg (Fey et al. 2004, AJ, 127, 3587). In particular S4 0444+63 is listed in the CGRaBS and CRATES catalogs and the 5BZ list. It has a redshift z=0.781 (Stickel & Kuhr 1993, A&AS, 101, 521).

Preliminary analysis indicates that on July 11, 12 and 13, 2017, S4 0444+63 was in an active state with daily averaged gamma-ray fluxes (E>100MeV) respectively of (0.3+/-0.1) X 10^-6, (0.4+/-0.1) X 10^-6, and (0.5+/-0.2) X 10^-6 photons cm^-2 s^-1. The corresponding photon spectral indexes (E>100MeV) are respectively 1.8+/-0.3, 1.9+/-0.2 and 2.0+/-0.2. This gamma-ray flaring source was also identified by the 'Fermi All-sky Variability Analysis' (FAVA, Ackermann et al. 2013, ApJ, 771, 57) on the weekly interval July 3-10, 2017. This source is not included in the published third Fermi-LAT source catalog (3FGL, Acero et al. 2015, ApJS 218, 23), in the Third EGRET catalog (3EG, Hartman et al, 1999 ApJS 123, 79) and has not been reported by AGILE so far. The closest cataloged gamma-ray source is 3FGL J0453.2+6321 with centroid position that is from 28 to 49 arcmin away from daily positions of July 11, 12 and 13 for this flaring gamma-ray source and outside the corresponding error circle radii.

Gamma-ray activity from S4 0444+63 was previously observed by the LAT in December 2012. S4 0444+63 was not previously observed by any X-ray instrument, and the ROSAT flux upper limit in the 0.1-2.4 keV band is 0.79X10^-12 erg cm^-2 s^-1.

High-urgency follow-up ToO Swift observations were performed on July 12 and 14, 2017 for a total exposure of about 6.1ks. Swift XRT data were taken in Photon Counting mode. A preliminary analysis shows that the X-ray spectrum (0.3-10 keV) can be fit by an absorbed power-law model with an HI column density set to the Galactic value of 1.98 X 10^21 cm^-2 (Kalberla et al. 2005, A&A, 440, 775). On July 12 the blazar S4 0444+63 was observed with a count rate of 0.044 ct s^-1 and an unabsorbed 0.3-10 keV flux of (2.6+/-0.8)X10^-12 erg cm^-2 s^-1, while on July 14 the count rate was 0.040 ct s^-1 and the unabsorbed 0.3-10 keV flux (1.7+/-0.9)X10^-12 erg cm^-2 s^-1. The corresponding X-ray photon spectral indexes of 1.0+/-0.6 and 1.9+/-0.8 might suggest a purely inverse Compton contribution to the X-ray spectrum, consistent with the indication of an infrared synchrotron peak frequency based on archival radio-to-optical data. These preliminary results and the known radio-to-optical source properties suggest S4 0444+63 could be a new gamma-ray and X-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 is S. Ciprini (stefano.ciprini@ssdc.asi.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 thank the Swift Team for making these observations possible, in particular B. Sbarufatti as the Swift Observatory Duty Scientist.