Identification of the new gamma-ray blazar NRAO 676 through Swift follow-up observations

ATel #4527; Stefano Ciprini, Dario Gasparrini, Sara Cutini (ASDC and INAF Rome, Italy) on behalf of the Fermi Large Area Telescope Collaboration
on 29 Oct 2012; 16:18 UT
Credential Certification: Stefano Ciprini (stefano.ciprini@asdc.asi.it)

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Following the recent gamma-ray flaring activity observed from a new gamma-ray source not included in any of the Fermi LAT catalogs and positionally associated to the flat spectrum radio quasar NRAO 676 (also known as TXS 2159+505, see Fermi LAT ATel#4486 and ATel #4182), we requested medium priority Swift Target of Opportunity (ToO) observations. This blazar is located at R.A.= 330.43140 deg, Dec.= 50.81566 deg (J2000, Beasley et al. 2002, ApJS, 141, 13) and has redshift z=1.899 (Sowards-Emmerd et al. 2005, ApJ, 626, 95).

Such follow-up observations by Swift were performed on 2012 October 21 and 24, when Fermi LAT detected a still high gamma-ray state with an averaged daily gamma-ray flux (E>100MeV) of (1.4+/-0.3) x 10^-6 photons cm^-2 s^-1 on October 21 and a daily flux of (2.5+/-0.3) and (1.3+/-0.3) x 10^-6 photons cm^-2 s^-1 on October 23 and October 24 respectively (statistical uncertainty only).

Swift XRT data are taken in photon counting mode for a net exposure of about 2.5ksec for both the observations of NRAO 676. For the October 21 observation the XRT X-ray spectrum can be fit by an absorbed power law with the HI column density fixed to the Galactic value of N_HI = 3.69 x 10^21 cm^-2 (Kalberla et al. 2005, A&A, 440, 775) and a hard photon spectral index of 1.33+/-0.08. The unabsorbed flux in the 0.3-10 keV energy band is (15+/-2) x10^-12 erg cm^-2 s^-1, which is a factor of about 3 higher than that previously detected by XRT on 2012 June 28 during a first Swift ToO dedicated to this source. The photon spectral index has a similar value.

For the October 24 observation the XRT X-ray spectrum can be fit by an absorbed power law with the HI column density fixed to the same Galactic value of N_HI and a hard photon spectral index of 1.30+/-0.09. The unabsorbed flux in the 0.3-10 keV energy band is (14+/-2) x10^-12 erg cm^-2 s^-1, again a factor of about 3 higher than that previously detected by XRT on 2012 June 28.

On 2012, June 27 and 28 NRAO 676 was not detected above 5 sigma by Fermi LAT (respectively with daily upper limits to the flux of 0.7x10^-6 and 1.0x10^-6 photons cm^-2 s^-1) suggesting a lower gamma-ray emission state that appears correlated with the fainter X-ray flux observed on June 28 (around 1AM, UT) by Swift XRT.

Because of the positional association and the synergetic Fermi-Swift observations of variable and correlated flux levels at GeV and X-ray bands on 2012 June 28, October 21 and October 24 we claim the identification of this new gamma-ray source with the flat spectrum radio quasar NRAO 676.

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the identification of this new gamma-ray source, and the ongoing flaring activity in X-ray and GeV gamma-ray band, we encourage multiwavelength observations. This blazar is now one of the LAT Monitored Sources at FSSC (GSFC) and consequently, a preliminary, uncalibrated estimation of the daily gamma-ray flux observed by Fermi LAT is publicly available (http://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/NRAO_676). For this source the Fermi LAT contact persons are S. Ciprini (stefano.ciprini@asdc.asi.it) and E. A. Hays (elizabeth.a.hays@nasa.gov).

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 and the Observation Duty Scientists (ODS) for the scheduling of these two observations of October 2012.