A systematic search for near-infrared counterparts to ultraluminous X-ray sources

Abstract

The nature of ultraluminous X-ray sources (ULXs) remains unclear as it is still not understood whether they contain stellar mass black holes or intermediate-mass black holes (IMBHs). As a ?first step to answer this question we report a systematic search for nearinfrared counterparts to ULXs. We observed 23 ULX X-ray positions, typically accurate to within 1 arcsecond, in host galaxies <10 Mpc in the Ks-band with LIRIS at the WHT on La Palma. The data reduction, astrometry and the coaddition of the images was performed with data reduction package Theli. Relative photometry with respect to 2MASS stars produced apparent magnitudes of the detections and limits on the depth of the images. We found 6 detections within the error circle of the ULX X-ray positions. Based on chance alignment calculations we conclude that all detections are very likely associated with the ULX detections in X-ray. We measured apparent magnitudes in Ks of the detections of 15.8 to 20.0. The corresponding absolute magnitudes ranged from -8.1 to -14.2, which are suggestive of red supergiants with the exception of the brightest detection, at X-ray position X-2 in NGC 2500. This extended source is most likely a foreground source or star cluster. These detections represent the fi?rst identi?cations of near-IR counterparts to ULXs to date. The brightest counterparts found are primary candidates for spectroscopy to unambiguously identify the donor star, and ultimately time-resolved photometric and spectroscopic observations to obtain a mass-limit to the black hole in the ULX.