An Optical Study of Stellar and Interstellar Environments of Seven Luminous and Ultraluminous X‐Ray Sources

Abstract

We have studied the stellar and interstellar environments of two luminous X-ray sources and five ultraluminous X-ray sources (ULXs) in order to gain insight into their nature. Archival Hubble Space Telescope images were used to identify the optical counterparts of the ULXs Ho IX X-1 and NGC 1313 X-2, and to make photometric measurements of the local stellar populations of these and the luminous source IC 10 X-1. We obtained high-dispersion spectroscopic observations of the nebulae around these seven sources to search for He ii k4686 emission and to estimate the expansion velocities and kinetic energies of these nebulae. Our observations did not detect nebular He ii emission from any source, with the exception of LMC X-1; this is either because we missed the He iii regions or because the nebulae are too diffuse to produce He ii surface brightnesses that lie within our detection limit. We compare the observed ionization and kinematics of the supershells around the ULXs Ho IX X-1 and NGC 1313 X-2 with the energy feedback expected from the underlying stellar population to assess whether additional energy contributions from the ULXs are needed. In both cases, we find insufficient UV fluxes or mechanical energies from the stellar population; thus these ULXs may be partially responsible for the ionization and energetics of their supershells. All seven sources that we studied are in young stellar environments, and six of them have optical counterparts with masses k7 M ; thus, these sources are most likely high-mass X-ray binaries.

THE ULTRALUMINOUS X-RAY SOURCES NGC 1313 X-1 AND X-2: A BROADBAND STUDY WITHNuSTARANDXMM-Newton

“…This is in agreement with the lack of signatures of strong outflows that should be associated with highly super-Eddington accretion (see, e.g., Poutanen et al 2007), such as photoionized bubbles (that are seen instead for other sources, e.g., in Pakull & Mirioni 2002;Ramsey et al 2006) or discrete atomic features in their high-energy spectra that could be associated with either iron emission or absorption from a wind (Walton et al 2012). An alternative explanation is that these winds are not pointing toward the observer and the source is observed almost face-on (this would also agree with the lack of variability; see, e.g., Middleton et al 2011a;Sutton et al 2013).…”

Spectral State Transitions of the Ultraluminous X-Ray Sources X-1 and X-2 in NGC 1313

“…In the luminosity versus temperature (L-kT) diagram of bright ULXs in nearby galaxies (Feng & Kaaret 2005), they appear in the low-temperature, high-luminosity class, which contains sources most likely to be IMBH candidates. Ramsey et al (2006) find a unique optical counterpart to X-2 in Hubble Space Telescope images and suggest that the companion star is a B1-B2 giant. NGC 1313 X-2 resides in an optical nebular supershell (Pakull & Mirioni 2003), whose kinetic energy is much larger than a typical supernova explosion.…”

“…Merloni (2003) proposed a coupled magnetic disk-corona solution, in which the disk is stable when it is corona-dominated at high accretion rates. Most bright ULXs have spectra dominated by a soft power-law component (Feng & Kaaret 2005) and are thought to be accreting from high-mass companion stars via Roche lobe overflow (Liu et al 2004;Kaaret et al 2004a;Ramsey et al 2006;Kaaret et al 2006aKaaret et al , 2006b). Detailed modeling of accretion flows that produce powerful coronae at high accretion rates should be important for understanding ULXs and also for determining if super-Eddington accretion occurs (Merloni 2003).…”

Ultraluminous X-ray Sources and Their Nebulae

“…[18] obtained echelle spectra of MF16 and detected high-velocity wings at about 200 km s −1 . In the recent work by Ramsey et al [13] echelle-spectroscopy of MH9 is reported. Lines are found to be broadened by ∼ 100 km s −1 .…”

“…The youngest ages detected are about 4 Myr [13,15] but it is difficult to judge about the higher age limit due to detectability problems. ULX nebulae residing in large star-forming regions are more difficult to detect.…”