Lost and Found: A New Position and Infrared Counterpart for the X-Ray Binary Scutum X-1

Abstract

Using archival X-ray data, we find that the catalog location of the X-ray binary Scutum X-1 (Sct X-1) is incorrect and that the correct location is that of the X-ray source AX J183528-0737, which is 15' to the west. Our identification is made on the basis of the 112 s pulse period for this object detected in an XMM-Newton observation, as well as spatial coincidence between AX J183528-0737 and previous X-ray observations. Based on the XMM-Newton data and archival RXTE data, we confirm secular spin-down over 17 yr with period derivative P˙~3.9×10^-9 s s^-1, but do not detect a previously reported X-ray iron fluorescence line. We identify a bright (K_s=6.55) red (J-K_s=5.51) optical and infrared counterpart to AX J183528-0737 from 2MASS, a number of mid-IR surveys, and deep optical observations, which we use to constrain the extinction to and distance of Sct X-1. From these data, as well as limited near-IR spectroscopy, we conclude that Sct X-1 is most likely a binary system composed of a late-type giant or supergiant and a neutron star.

This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

Partially based on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA, and was made possible by the generous financial support of the W. M. Keck Foundation.