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ATEL # 1601; E. M. Cackett (Michigan), R. Wijnands (Amsterdam), M. Reynolds (Michigan)
on 3 Jul 2008; 15:42 UT
Password Certification: Edward Cackett (firstname.lastname@example.org)
Subjects: Infra-Red, Request for Observations, Binaries, Neutron Stars, Transients
Following the recent announcement of an X-ray and optical outburst from a previously unknown Galactic transient possibly associated with 1RXH J173523.7-354013 (Israel et al. 2008, ATel #1528) we obtained near-infrared observations of the source using the PANIC camera on the 6.5-m Magellan Baade telescope to search for a possible near-infrared counterpart.
The observations were performed on 2008 May 25 09:12-10:02 UT, and we acquired J, H and Ks band images with a total of 600s, 300s, and 300s on-source time respectively. Seeing conditions were approximately 0.6". 2MASS sources within the field of view were used to determine the astrometric and photometric calibrations with a positional RMS of 0.1 arcsec and a photometric error <0.1 mag.
In all 3 near-IR bands we detect a source at a position of RA(J2000)=17:35:23.74 and Dec(J2000)=-35:40:16.6. This is only 0.5" from the location of the Swift UVOT counterpart of 1RXH J173523.7-354013 (ATel #1528), and therefore the positions are consistent within the errors. However, we note that the field is quite crowded and thus the chance probability of finding a counterpart coincident with the UVOT position is non-negligible. We determine magnitudes for this source of J = 15.4, H = 14.3 and Ks = 13.8.
Israel et al. (ATel #1528) suggested that the transient could either be an accreting neutron star in a binary (preferred) or a magnetar. If we have indeed detected the near-IR counterpart to the transient source, this would immediately rule out the magnetar hypothesis since the counterpart would be far too bright (compared to what is known from the confirmed magnetars). Therefore, we consider that it is most probable that the source is indeed an accreting neutron star. Moreover, the obtained J-H and H-K colours are consistent (after correcting for absorption) with a low-mass (K/M dwarf) star which would support the classification of the source as a low-mass X-ray binary, though the accretion disk could significantly contribute to the near-IR flux, which may change this tentative classification. However, high resolution and high sensitivity optical spectroscopic observations are highly desired to further shed light on the nature of this system.
This finding chart shows a PANIC J band image, with the Swift UVOT position marked with a green circle.