Swift and NOT follow-up of the proposed X-ray counterpart of Fermi J0641-0317: evidence for association with the extra-galactic radio source PMN J0641-0320

ATel #6139; A. Papitto (ICE, CSIC-IEEC); P. D'Avanzo (INAF-Brera)
on 15 May 2014; 09:49 UT
Credential Certification: Alessandro Papitto (papitto@ice.csic.es)

Tweet

We report on X-ray and optical observations of the transient gamma-ray source Fermi J0641-0317, which has been discovered between April 14 and April 21, 2014 (Kocevski et al. 2014, ATel #6098; Ajello et al. 2014, ATel #6100, #6101).

Swift observed the field for 1.9 ks starting on May 13, 2014 at 08:44 (UT). We detected only one source in the X-ray Telescope (XRT) image, at position of RA,DEC (J2000)=06:41:51.11, -03:20:49.2 (+/- 2.0"; 90% confidence level). This source is compatible with the Swift X-ray counterpart of Fermi J0641-0317 proposed in ATel #6100. It is observed at an average count rate of 0.09(1) c/s, and the spectrum is described by a power law with a photon index of 0.7 (+0.6,-0.5), absorbed by a column density with NH=8(+10,-6)x10²¹ cm^-2. This gives a 0.3-10 keV flux of 7.0(+2.2,-1.7)x10^-12 erg cm^-2 s^-1, compatible with the flux level reported in ATel #6101. No optical counterpart is detected by UVOT within the error circle of the X-ray source, with 3 sigma upper limits of V=18.6, B=19.6, U=19.2, UW1=19.5, UM2=20.7, UW2=20.1 (Vega system), evaluated considering also other Swift observations performed between April 24 and May 13.

To search for the optical counterpart we observed the field around the X-ray source with the Nordic Optical Telescope (NOT) equipped with ALFOSC. We obtained three imaging exposures with the R filter, each one lasting 300 s. The observations were carried out at high airmass on 2014 May 13 between 20:49 and 21:07 (UT). In the co-added image we detect an object at RA, Dec (J2000) = 06:41:51.1, -03:20:48.7 (+/- 0.4"; 1 sigma c. l.). The object has a magnitude R=20.1 +/- 0.2 (calibrated against the USNOB1 catalogue).

The position of the optical source is compatible with the error circle of the source detected by XRT, and both are consistent with the position of the flat-spectrum, very bright (~680 mJy at 8.2-21.7 GHz, Trushkin et al. ATel #6122) radio source PMN J0641-0320 already proposed as a counterpart of Fermi J0641-0317 by Ajello et al. (ATel #6100). This source has been previously catalogued as a compact extragalactic radio source (Lanyi et al. 2010, AJ, 139, 1695), and the precise X-ray and optical position we have reported here reinforce its association with the proposed X-ray counterpart of Fermi J0641-0317.

Future observations will be used to study the source variability.

We thank the Swift and NOT team for the prompt scheduling of the observations.