Infrared Observations of the Neutron Star X-ray Transient KS 1731-260
ATel #75; Jerome A. Orosz (Utrecht University), Charles D. Bailyn and Katie Whitman (Yale University)
on 6 Sep 2001; 14:54 UT
Credential Certification: Jerome A. Orosz (J.A.Orosz@astro.uu.nl)
Subjects: Infra-Red, Binary, Neutron Star, Transient
We have obtained J-band images of the field of the neutron star X-ray
transient KS 1731-260 on July 13, 2001 using the YALO 1m telescope at
Cerro Tololo Interamerican Observatory and the ANDICAM optical/IR
camera. We compared our image with the J-band image obtained June 1,
1996 with the Canada-France-Hawaii Telescope (Barret, Motch, &
Predehl, 1998, A&A, 329, 965). The counterpart suggested by Wijnands et
al. (ATEL #72) is the only star in the ROSAT error circle that is
significantly variable between the two images.
Using the programs DAOPHOT and ALLSTAR (Stetson, 1987,
PASP, 99, 191) to compute instrumental magnitudes and the J magnitudes
of several field stars given in Barret et al. (1998) to calibrate the
zero point, we find J = 17.18 +/- 0.14 for the counterpart in the CFHT
image and J = 18.62 +/- 0.21 in the YALO image. Using a distance of
8.3 +/- 0.3 kpc (Smith, Morgan, & Bradt, 1997, ApJ, L137) and a J-band
extinction of A_J = 2.0 +/- 0.3 (Barret et al. 1998), we find an
absolute J-magnitude of 2.0 +/- 0.4. This is significantly
brighter than the absolute J-magnitude of a K or M dwarf (e.g. a K0V
star has M_J = 4.5, Allen's Astrophysical Quantities, 4th ed.). We
therefore conclude that the companion star is either significantly
evolved or that its spectral type is much earlier than K, assuming the
accretion disk is faint in the infrared and that there are no other
sources of light such as an unresolved field star (which is the case
for Aql X-1, Callanan, Filippenko, & Garcia, 1999, IAU Circular #7086;
Chevalier et al., 1999, A&A, 347, L51).
We thank Christian Motch for providing the archival CFHT data.
