X‐Ray Observations and Infrared Identification of the Transient 7.8 s X‐Ray Binary Pulsar XTE J1829−098

Abstract

XMM-Newton and Chandra observations of the transient 7.8 s pulsar XTE J1829-098 are used to characterize its pulse shape and spectrum, and to facilitate a search for an optical or infrared counterpart. In outburst, the absorbed, hard X-ray spectrum with Γ = 0.76 ± 0.13 and NH = (6.0 ± 0.6) × 1022 cm-2 is typical of X-ray binary pulsars. The precise Chandra localization in a faint state leads to the identification of a probable infrared counterpart at R.A. = 18h29m43.98s, decl. = -09°51'23'' (J2000.0), with magnitudes K = 12.7, H = 13.9, I > 21.9, and R > 23.2. If this is a highly reddened O or B star, we estimate a distance of 10 kpc, at which the maximum observed X-ray luminosity is 2 × 1036 ergs s-1, typical of Be X-ray transients or wind-fed systems. The minimum observed luminosity is 3 × 1032(d/10 kpc)2 ergs s-1. We cannot rule out the possibility that the companion is a red giant. The two known X-ray outbursts of XTE J1829-098 are separated by ≈1.3 yr, which may be the orbital period or a multiple of it, with the neutron star in an eccentric orbit. We also studied a late M giant long-period variable that we found only 9'' from the X-ray position. It has a pulsation period of ≈1.5 yr, but is not the companion of the X-ray source.