Detection of two X-ray sources in the error circle of the Fermi/LAT source 2FGL J1551.9+0855

ATel #4551; S. de Jong, V. Beckmann (APC/Univ. Paris Diderot, France), J. Kennea (PSU, USA), S. Soldi (CEA/Saclay, France), C. R. Shrader, N. Gehrels (NASA/GSFC, USA)
on 6 Nov 2012; 19:17 UT
Credential Certification: Volker Beckmann (beckmann@apc.univ-paris7.fr)

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We have observed the field of the Fermi/LAT detected source 2FGL J1551.9+0855. The observations were made with Swift/XRT and started on 2012-08-01T08:22 for an exposure time of 2.5 ks. We report the first detection of two X-ray sources within the error circle of 2FGL J1551.9+0855. One of the sources has a position consistent with the Seyfert 1 galaxy LSBC F727-V01. The X-ray source, with a measured count rate of 0.010 +/-0.002 c/s in the XRT energy band, is located at RA=237.9181 and DEC=8.8744 with 2.7 arcsec uncertainty (90% confidence radius), consistent with the optical position of LSBC F727-V01 at RA=237.918824 and DEC=8.873930 (J2000.0). The XRT data can be represented by a power law model with a photon index 1.9 +/- 0.4 and fixed Galactic hydrogen column density NH=3.4e20cm^-2. The 2-10 keV flux is 3e-13 ergs/cm^2/s, which corresponds to a luminosity of 3.4e+42 ergs/s (z=0.0705). The low intrinsic absorption, the X-ray luminosity and spectral slope are consistent with average properties of Seyfert 1 galaxies. The 0.1-2.4 keV model flux is 4e-13 ergs/cm^2/s, which is consistent with the RASS upper limit of ~5e-13 ergs/cm^2/s in this energy band. There are no ROSAT sources reported within 5 arcmin of the XRT detection. The UVOT data yield a U-band brightness of 18.0 mag. Earlier observations of the source by Swift in 2011 gave a measurement of >18.2 mag. In the error circle of 2FGL J1551.9+0855 we also observed the blazar BZB J1552+0850 at RA=238.0153 and DEC=8.8456 with an error circle of 4.1 arcsec (90% confidence radius). This is 7.6 arcsec from the optical AGN core, but the XRT error radius might be underestimated by the methods we applied. The source has a flux of 2E-13 ergs/cm^2/s between 0.1-2.4 keV, assuming a power law with index=2, consistent with the RASS upper limit of ~5e-13 ergs/cm^2/s. BZB J1552+0850 has been associated with 2FGL J1551.9+0855 in the second Fermi/LAT catalogue. Even though both, BZB J1552+0850 and LSBC F727-V01, are positioned within the ~7 arcmin error circle of the Fermi source, we can assume that the former is the counterpart of 2FGL J1551.9+0855, since blazars are well established gamma-ray emitter, unlike Seyfert 1 type objects.