Optical Counterparts of the Nearest Ultraluminous X-Ray Sources

Abstract

We present a photometric survey of the optical counterparts of ultraluminous X-ray sources (ULXs) observed with the Hubble Space Telescope in nearby ( 5 Mpc) galaxies. Of the 33 ULXs with Hubble & Chandra data, 9 have no visible counterpart, placing limits on their M V of ∼ -4 to -9, enabling us to rule out O-type companions in 4 cases. The refined positions of two ULXs place them in the nucleus of their host galaxy. They are removed from our sample. Of the 22 remaining ULXs, 13 have one possible optical counterpart, while multiple are visible within the error regions of other ULXs. By calculating the number of chance coincidences, we estimate that 13±5 are the true counterparts. We attempt to constrain the nature of the companions by fitting the SED and M V to obtain candidate spectral types. We can rule out O-type companions in 20 cases, while we find that one ULX (NGC 253 ULX2) excludes all OB-type companions. Fitting with X-ray irradiated models provides constraints on the donor star mass and radius. For 7 ULXs, we are able to impose inclinationdependent upper and/or lower limits on the black holes mass, if the extinction to the assumed companion star is not larger than the Galactic column. These are NGC 55 ULX1, NGC 253 ULX1, NGC 253 ULX2, NGC 253 XMM6, Ho IX X-1, IC342 X-1 & NGC 5204 X-1. This suggests that 10 ULXs do not have O companions, while none of the 18 fitted rule out B-type companions.

Irradiated, colour-temperature-corrected accretion discs in ultraluminous X-ray sources

“…This resulted in a sample of five ULXs, details of which are shown in Table 1. For four of the five sources in the sample, Gladstone et al (2013) identified only a single potential optical counterpart in the HLA images; the exception to this was NGC 1313 X-2, which had two potential counterparts. Here, we use only the counterpart to NGC 1313 X-2 previously identified by Liu et al (2007) and confirmed by Grisé et al (2008).…”

“…To carry out broad-band spectral modelling, we require both X-ray and optical data. Thus, we selected all of the ULXs with broadened disc spectra in Sutton et al (2013), which also had optical counterparts identified by Gladstone et al (2013) in Hubble Space Telescope (HST) Hubble Legacy Archive (HLA 4 ) data. This resulted in a sample of five ULXs, details of which are shown in Table 1.…”

“…HST magnitudes for the sample ULXs were taken from Gladstone et al (2013). These were derived from an assortment of the advanced camera for surveys high resolution channel (ACS-HRC), advanced camera for surveys wide field channel (ACS-WFC) and Wide Field Planetary Camera 2 (WFPC2) observations over a range of dates (see Table 3 for details).…”

“…These were derived from an assortment of the advanced camera for surveys high resolution channel (ACS-HRC), advanced camera for surveys wide field channel (ACS-WFC) and Wide Field Planetary Camera 2 (WFPC2) observations over a range of dates (see Table 3 for details). The Galactic-extinction-corrected Vega magnitudes (table 4 of Gladstone et al 2013) were converted into units of photons cm −2 s −1 . When converting the WFPC2 magnitudes, we used values of zero-points and photflam from the WFPC2 data handbook, along with filter wavelengths and bandwidths from the WFPC2 filter archive.…”

“…Indeed, point-like optical counterparts have been identified for a number of ULXs (Goad et al 2002;Liu, Bregman & Seitzer 2002;Kaaret et al 2004;Liu, Bregman & Seitzer 2004;Kuntz et al 2005;Mucciarelli et al 2005;Soria et al 2005;Liu et al 2007;Grisé et al 2008Grisé et al , 2011Roberts, Levan & Goad 2008;Rosado, Ghosh & Fuentes-Carrera 2008;Gladstone et al 2013), many of which are blue in colour, and appear to be consistent with large early-type companions, although a minority of the potential counterparts in the sample of Gladstone et al (2013) have colours which rule out O-type stars. However, it may not be possible to trivially identify stellar counterparts to ULXs.…”