Discovery of a red supergiant counterpart to RX J004722.4–252051, a ULX in NGC 253

Abstract

We present two epochs of near-infrared spectroscopy of the candidate red supergiant counterpart to RX J004722.4-252051, a ULX in NGC 253. We measure radial velocities of the object and its approximate spectral type by cross-correlating our spectra with those of known red supergiants. Our VLT/X-shooter spectrum is best matched by that of early M-type supergiants, confirming the red supergiant nature of the candidate counterpart. The radial velocity of the spectrum, taken on 2014, August 23, is 417 ± 4 km s −1 . This is consistent with the radial velocity measured in our spectrum taken with Magellan/MMIRS on 2013, June 28, of 410 ± 70 km s −1 , although the large error on the latter implies that a radial velocity shift expected for a black hole of tens of M can easily be hidden. Using nebular emission lines we find that the radial velocity due to the rotation of NGC 253 is 351 ± 4 km s −1 at the position of the ULX. Thus the radial velocity of the counterpart confirms that the source is located in NGC 253, but also shows an offset with respect to the local bulk motion of the galaxy of 66 ± 6 km s −1 . We argue that the most likely origin for this displacement lies either in a SN kick, requiring a system containing a 50 M black hole, and/or in orbital radial velocity variations in the ULX binary system, requiring a 100 M black hole. We therefore conclude that RX J004722.4-252051 is a strong candidate for a ULX containing a massive stellar black hole.

NIR counterparts to ULXs (III): completing the photometric survey and selected spectroscopic results★

“…Based only on photometry, of the 38 detected NIR counterparts, 20 have NIR colours and/or magnitudes consistent with an RSG, 11 are preliminarily classified as stellar clusters, 6 are consistent with being an AGN, and 1 is too faint to be either an RSG or an SC/AGN (see Table B1). After a spectroscopic follow-up of 12 out of these 38 counterparts, we confirmed 5 RSGs (Heida et al 2015a(Heida et al , 2016, and this work), while 4 sources photometrically classified as RSG turned out to be nebulae (Heida et al 2016;López et al 2019, and this work). For the remaining three followed-up sources, one RSG candidate is a background AGN (this work), one stellar cluster candidate turned out to be a nebula (this work; see Table B1), and the last RSG candidate is too faint to be classified.…”

“…Of these 33 NIR sources, 19 had the absolute magnitude consistent with an RSG. So far, we performed spectroscopic follow-up observations for 7 out of these 19 candidate RSGs and confirmed the RSG nature for 3 counterparts: ULX RX J004722.4−252051 (in NGC 253; Heida et al 2015a), ULX J022721+333500 (in NGC 925), and ULX J120922+295559 (in NGC 4136; Heida et al 2016). Additionally, we discovered that four counterparts, initially classified as RSG based on photometry, are actually nebulae partially powered by the X-ray emission of the ULX: ULX J022727+333443 (in NGC 925), ULX J120922+295551 (in NGC 4136), ULX Ho II X-1 (in Holmberg II; Heida et al 2016), and [SST2011] J110545.…”

Discovery and analysis of a ULX nebula in NGC 3521

“…The effects of the bright ULX on its environment has been seen in other Hii regions surrounding (or close to) a ULX (e.g. Mucciarelli et al 2005;Heida et al 2015). Since the presence of the ULX seems to affect the line flux ratios of J1105, and its radial velocity is consistent with the radial velocity of NGC 3521 at that position, we deem it likely that the ULX and the nebular source are physically related.…”

A 78 Day X-Ray Period Detected From NGC 5907 Ulx1 by Swift

“…Currently, we have no independent observational constraints on the nature of the stellar companion in NGC 5907 ULX1 owing to both its distance and the obscuring column toward this source (Ñ 10 H 22 cm −2 ; the host galaxy NGC 5907 is seen edge-on). However, Heida et al (2014Heida et al ( , 2015 have recently reported a number of ULXs with candidate red supergiant companions, demonstrating that some of the ULX population likely do have evolved counterparts. Indeed, if we assume Roche-lobe overflow and that the period is orbital, we can estimate a density for the stellar counterpart of r~´-3 10 5 g cm −3 (Faulkner et al 1972), implying the counterpart may be either an M giant or an F supergiant (Drilling & Landolt 2000).…”