Near-IR observations of the anomalous X-ray pulsar CXO J164710.2-455216

ATel #910; Z. Wang, V. M. Kaspi (McGill U.), D. Osip, N. Morrell (LCO), D. L. Kaplan, and D. Chakrabarty (MIT)
on 4 Oct 2006; 14:02 UT
Credential Certification: Z. Wang (wangzx@physics.mcgill.ca)

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We observed the field of the AXP CXO J164710.2-455216 on 2006 September 29 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 (LCO) in Chile. The total exposure was 27 min, reaching a limiting magnitude of 20.3 mag (3-sigma). We astrometrically calibrated our image using 23 2MASS stars, and the resulting nominal uncertainty for locating the X-ray source (Muno et al. 2006 ApJ 636 L41) on the image is 0.4 arcsec (90% confidence). No near-IR objects were detected within the error circle.

Previously on 2006 May 17, we obtained a Ks image with the same exposure time and using the same telescope/instrument. The 3-sigma limiting magnitude of the image was 21 mag, as the condition was excellent with the seeing around 0.35 arcsec during the observation. Two very faint objects were found near the X-ray source position (within 0.6 arcsec). However, neither of them was detected in September's image. Since the X-ray source has brightened by approximately 70 times comparing its X-ray fluxes in the outburst (ATEL #901) and quiescence (Muno et al. 2006), an IR counterpart would be about 4.6 mag brighter if the IR and X-ray fluxes are correlated (Tam et al. 2004 ApJ 617 L53; Rea et al. 2004 A&A 425 L5). This suggests that neither of the faint sources is the IR counterpart.

The unabsorbed flux from this source is uncertain. Using the results of ATEL #896 and ATEL #901, we estimate an unabsorbed flux of 6.2E-11 ergs/cm^2/s between 0.5-10 keV on September 29, comparable to the unabsorbed flux of AXP XTE J1810-197 (Rea et al. 2004). Assuming XTE J1810-197's dereddened Ks= 20.7 mag for CXO J164710.2-455216 and adding the reddening to the Westerlund 1 cluster (Av=12.9; Piatti et al. A&AS 1998 127 423), we expect Ks= 22.2 mag, much fainter than our limiting magnitude.

Alternatively, our observations rule out a main sequence star down to spectral type M2 (an M2 dwarf has 20.7 mag in Ks at a distance of 5 kpc), if considering the X-ray source as an X-ray binary. This strongly supports that CXO J164710.2-455216 is an AXP.