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Swift/XRT non detection of MASTER OT J174534.82-643044.4

ATel #10308; A. Papitto (INAF-OAR Roma), E. Bozzo (ISDC Switzerland), F. Coti Zelati (ICE Barcelona), C. Ferrigno (ISDC Switzerland), N. Rea (ICE Barcelona)
on 22 Apr 2017; 08:50 UT
Credential Certification: Alessandro Papitto (papitto@ice.csic.es)

Subjects: Optical, Ultra-Violet, X-ray, Transient

A bright optical transient was discovered by MASTER Global Robotic Net (Gagarin et al. 2017, ATel #10258) on 2017-04-12.25627 UT, MASTER OT J174534.82-643044.4. Its position was determined with an accuracy of 0.7 arcsec, and was compatible with the X-ray source J174534.8-643044 reported in the 3XMM DR6 serendipitous source catalog (Rosen, Webb, Watson et al. 2016, A&A, 590, A1).

The transient was observed by Swift for 2 ks starting on 2017-04-16.262 (UTC). No X-ray source was detected by the Swift X-ray Telescope (XRT) at the position of the optical transient. An upper limit of 1.3E-13 erg/cm2/s was derived for the 0.5-10 keV flux (3 sigma confidence level), assuming the Galactic value for interstellar absorption (5.2E20 cm-2) and a spectrum described by a power law with photon index equal to 1.7. This upper limit is ~10 times larger than the 0.5-4.5 keV flux value reported by the 3XMM DR6 catalog for J174534.8-643044, (1.0+/-0.3)E-14 erg/cm2/s.

The Swift Ultraviolet/Optical Telescope (UVOT) observed the region of the optical transient with filters U and UVW1 for 380 and 888 s, respectively. A source with intensity 16.52+/-0.05 mag (U band) and 16.21+/-0.06 mag (UVW1 band) was detected at the position of the optical transient. These values are of the same order of the magnitude reported by Gagarin et al. (2017, ATel #10258, 16.7mag, unfiltered), suggesting that the optical transient was still active at the time of the Swift observation.

We also analysed archival Swift observations of the field performed on 2016-10-24.994 (id. 00084525002) and 2016-10-26.127 (id. 00084525003) for an exposure of ~ 300 and 200 s, respectively. No X-ray source was detected at the position of the optical transient, with an upper limit of ~5E-12 erg/cm2/s. The position of the transient was slightly outside the UVOT field of view, preventing a comparison with the detection obtained in the observation reported here.

We thank the Swift team for their support and the very rapid scheduling of the transient source.