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 \Gamma = 0.76 \pm 0.13 and N _ { H } = ( 6.0 \pm 0.6 ) \times 10 ^ { 22 } 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. = 18 ^ { h } 29 ^ { m } 43. ^ { s } 98 , decl .
= -09 ^ { \circ } 51 ^ { \prime } 23 ^ { \prime \prime } .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 \times 10 ^ { 36 } ergs s ^ { -1 } , typical of Be X-ray transients or wind-fed systems .
The minimum observed luminosity is 3 \times 10 ^ { 32 } ( d / 10 { kpc } ) ^ { 2 } ergs s ^ { -1 } .
We can not rule out the possibility that the companion is a red giant .
The two known X-ray outbursts of XTE J1829 - 098 are separated by \approx 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 ^ { \prime \prime } from the X-ray position .
It has a pulsation period of \approx 1.5 yr , but is not the companion of the X-ray source .