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ATEL # 935; Z. Wang (McGill), F. K. Baganoff (MIT), M. Muno (Caltech), M. Morris (UCLA), N. Morrell (LCO), A. Muzzin, D. Gilbank, P. Hsieh (Toronto), D. Kaplan (MIT)
on 7 Nov 2006; 13:46 UT
Password Certification: Z. Wang (email@example.com)
Subjects: Infra-Red, Binaries, Black Holes, Neutron Stars
We observed the field of the Galactic center (GC) transient Swift J174540.2-290005 on 2006 October 30 and 31 at Ks band, using the near-IR imaging camera PANIC (Persson's Auxilliary Nasmyth Infrared Camera) on the Magellan/Baade Telescope at Las Campanas Observatory in Chile. The exposures were 3.3 and 6.7 min in the first and second night, reaching the limiting magnitudes of 17.3 and 17.7 mag (3-sigma), respectively. We did not detect any sources within the improved 0.4 arcsec Chandra error circle (90% confidence; Muno et al. 2005 ApJ 622 L113), while we note that there were two faint sources within a typical 0.7 arcsec Chandra error circle (90% confidence) and a dozen of sources within the 3.6 arcsec Swift error circle (90% containment; ATel #921).
Comparing the two images to a Ks image of the same field that we obtained on 2005 May 25 with an exposure of 0.83 min and using the same telescope/instrument, we did not detect any significant brightening from any of the sources. This supports that none of them was the counterpart to CXOGC J174540.0-290005 (Muno et al. 2005), since we expect a correlated IR flux brightening, either by approximately 0.5 mag for a high-mass X-ray binary (e.g., Clark et al. 2000 A&A 356 50) or several magnitudes for a low-mass X-ray binary (LMXB; e.g., Russell et al. 2006 MNRAS 371 1334).
Our IR observations would detect a main sequence star down to spectral type B5 (assuming a source distance of 8.0 kpc and a reddening Av=31 to the GC), suggesting that, if this transient is an X-ray binary with an accreting neutron star or black hole (Muno et al. 2005), it would likely be a LMXB.