Keck/MOSFIRE spectroscopy of five ULX counterparts
We present H-band spectra of the candidate counterparts of five ultraluminous X-ray sources (ULXs; two in NGC 925, two in NGC 4136 and Holmberg II X-1) obtained with Keck/MOSFIRE (Multi-Object Spectrometer for Infra-Red Exploration). The candidate counterparts of two ULXs (J022721+333500 in NGC 925 and J120922+295559 in NGC 4136) have spectra consistent with (M-type) red supergiants (RSGs). We obtained two epochs of spectroscopy of the candidate counterpart to J022721+333500, separated by 10 months, but discovered no radial velocity variations with a 2σ upper limit of 40 km s^(−1). If the RSG is the donor star of the ULX, the most likely options are that either the system is seen at low inclination (<40°) or the black hole mass is less than 100 M_⊙, unless the orbital period is longer than 6 years, in which case the obtained limit is not constraining. The spectrum of the counterpart to J120922+295559 shows emission lines on top of its stellar spectrum, and the remaining three counterparts do not show absorption lines that can be associated with the atmosphere of a star; their spectra are instead dominated by emission lines. Those counterparts with RSG spectra may be used in the future to search for radial velocity variations, and, if those are present, determine dynamical constraints on the mass of the accretor.
Additional Information© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2016 March 22. Received 2016 February 25; in original form 2015 November 5. First published online March 28, 2016. MH would like to thank Nick Konidaris for his help with the MOSFIRE DRP. We thank Tom Marsh for developing MOLLY. TPR acknowledges support from STFC as part of the consolidated grant award ST/L00075X/1. The work of DJW and DS was carried out at Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. PGJ acknowledges support from ERC consolidator grant number 647208. The data presented herein were 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 the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.
Published - MNRAS-2016-Heida-771-8.pdf
Submitted - 1603.07024v1.pdf