Accurate positions for the ULXs NGC 7319-X4 and NGC 5474-X1 and limiting magnitudes for their optical counterparts

TL;DR

This study precisely locates two ultraluminous X-ray sources using Chandra and Hubble data, finds no optical counterparts down to specific magnitude limits, and discusses their possible natures based on spectral and variability analysis.

Contribution

The paper provides accurate positions for two ULXs, performs bore-sight corrections, and sets limiting magnitudes for their optical counterparts, enhancing understanding of their nature.

Findings

No optical counterparts detected within magnitude limits

Evidence of X-ray variability in NGC 5474-X1

Accurate positions improve ULX localization

Abstract

In this paper we report accurate Chandra positions for two ultraluminous X-ray sources: NGC 7319-X4 at Right Ascension (RA) = 339.02917(2) deg, Declination (Dec) = 33.97476(2) deg and NGC 5474-X1 at RA = 211.24859(3) deg, Dec = 53.63584(3) deg. We perform bore-sight corrections on the Chandra X-ray Satellite observations of these sources to get to these accurate positions of the X-ray sources and match these positions with archival optical data from the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. We do not find the optical counterparts: the limiting absolute magnitudes of the observations in the WFPC2 standard magnitude system are B = -7.9, V = -8.7 and I = -9.3 for NGC 7319-X4 and U = -6.4 for NGC 5474-X1. We report on the X-ray spectral properties and we find evidence for X-ray variability in NGC 5474-X1. Finally, we briefly discuss several options for the nature of these ULXs.