X-ray Observations and Infrared Identification of the Transient

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) × 10 22 cm 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. = 182943.98, decl. = −095123.0 (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× 10 ergs s, typical of Be X-ray transients or wind-fed systems. The minimum observed luminosity is 3× 10(d/10 kpc) ergs s. 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. Subject headings: pulsars: individual (XTE J1829−098) — stars: neutron — stars: AGB and postAGB — stars: individual (2MASS J182944.55−095120.3) — stars: variable: other — X-rays: binaries