Neutral hydrogen absorption toward XTE J1810-197: The distance to a radio-emitting magnetar

We have used the Green Bank Telescope (GBT) to measure H I absorption against the anomalous X-ray pulsar XTE J1810-197. Using a flat rotation curve, we find that XTE J1810-197 is located at a distance of 3.4 ±0.6 kpc. The Galactic bar, however, does have an influence on Galactic rotation velocities that impacts the distance estimate of XTE J1810 -197. When rotation curve model s that include the Galactic bar are used, we find that XTE J1810 - 197 is located in the distance range of 3.1-4.3 kpc. From information gleaned from the literature, we find an upper limit on the distance to XTE J1810-197 of 4 kpc based on its infrared detection, its location just in front of an Infrared Dark Cloud (G10.74-0.13), and a measurement of the infrared absorption versus distance for this line of sight. Our best determination for the distance to XTE J1810-197 is thus 3.1-4 kpc. This distance, 3.1-4 kpc, is consistent with the distance to XTE J1810-197 of about 3.3 kpc derived from its dispersion measure, and estimates of 2-5 kpc obtained from fits to its X-ray spectra. We also used the GBT in an attempt to measure absorption in the four OH18 cm lines against XTE J1810-197. We were unsuccessful in this, mainly because of its declining radio flux density. Analysis of H121 cm, OH 18 cm, and 12CO(2 → 1) emission toward XTE J1810-197 allows us to placealower limit of NH ≳ 4.6 × 1021 cm_2on the nonionized hydrogen column density to XTE J1810-197, consistent with estimates obtained from fits to its X-ray spectra. © 2008. The American Astronomical Society. All rights reserved.