Neutral Hydrogen Absorption Toward XTE J1810-197: the Distance to a Radio-Emitting Magnetar

TL;DR

This study measures the distance to the magnetar XTE J1810-197 using HI absorption data, confirming its location around 3.5 kpc, and provides insights into its environment and hydrogen column density.

Contribution

First precise distance measurement of XTE J1810-197 using HI absorption, considering different Galactic rotation models, and analysis of its surrounding interstellar medium.

Findings

Distance to XTE J1810-197 is approximately 3.5 kpc.

Hydrogen column density is estimated at ≥4.6×10^{21} cm^{-2}.

No OH absorption detected due to declining radio flux.

Abstract

We have used the Green Bank Telescope to measure HI absorption against the anomalous X-ray pulsar XTE J1810-197. Assuming a flat rotation curve, we find that XTE J1810-197 is located at a distance of 3.4(+0.5,-0.7) kpc. For a rotation curve that incorporates a model of the Galactic bar, we obtain a distance of 4.0(+0.3,-0.8) kpc. Using a rotation curve that incorporates a model of the Galactic bar and the spiral arms of the Galaxy, the distance is 3.7(+/-0.6) kpc. These values are 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. Overall, we determine that XTE J1810-197 is located at a distance of 3.5(+/-0.5) kpc, possibly not far in front of the infrared dark cloud G10.74-0.13. We also used the GBT in an attempt to measure absorption in the OH 1612, 1665, 1667, and 1720 MHz lines against XTE J1810-197. We were unsuccessful in this, mainly because of its declining radio flux density. Analysis of HI 21 cm, OH, and CO(2-1) emission toward XTE J1810-197 allows us to place a lower limit of $N_{\rm H} \ga 4.6 \times 10^{21}$ cm$^{-2}$ on the non-ionized hydrogen column density to \magnetar, consistent with estimates obtained from fits to its X-ray spectra.