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 models 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 OH 18 cm lines against XTE J1810–197. We were unsuccessful in this, mainly because of its declining radio flux density. Analysis of H I 21 cm, OH 18 cm, and 12CO(2→ 1) emission toward XTE J1810–197 allows us to place a lower limit of NH 4.6 × 1021 cm−2 on the nonionized hydrogen column density to XTE J1810–197, consistent with estimates obtained from fits to its X-ray spectra.