Spectroscopy of optical counterparts of ultraluminous X-ray sources

Abstract

Abstract. Here we present the results of panoramic and long-slit observations of eight ULX nebular counterparts held with the 6m SAO telescope. In two ULXNe we detected for the first time signatures of high excitation ([O iii]λ5007 / Hβ > 5). Two of the ULXs were identified with young (T ∼ 5 − 10 M yr) massive star clusters. Four of the eight ULX Nebulae (ULXNe) show bright high-excitation lines. This requires existence of luminous (∼ 10 38 ÷ 10 40 ergs s −1 ) UV/EUV sources coinciding with the X-ray sources. Other 4 ULXNe require shock excitation of the gas with shock velocities of 20-100km s −1 . However, all the studied ULXN spectra show signatures of shock excitation, but even those ULXNe where the shocks are prevailing show presence of a hard ionizing source with the luminosity at least ∼ 10 38 ergs s −1 . Most likely shock waves, X-ray and EUV ionization act simultaneously in all the ULXNe, but they may be roughly separated in two groups, shock-dominated and photoionization-dominated ULXNe. The ULXs have to produce strong winds and/or jets powering their nebulae with ∼ 10 39 ergs s −1 . Both the wind/jet activity and the EUV source needed are consistent with the suggestion that ULXs are high-mass X-ray binaries with the supercritical accretion disks of the SS433 type.

CHANDRAX-RAY ANDHUBBLE SPACE TELESCOPEIMAGING OF OPTICALLY SELECTED KILOPARSEC-SCALE BINARY ACTIVE GALACTIC NUCLEI. I. NATURE OF THE NUCLEAR IONIZING SOURCES

“…Since the nuclei in our targets have X-ray luminosities much higher than typical ULXs, they are most likely of a different origin. Further evidence against the ULX scenario includes: (1) the BHs are expected to be supermassive given their host bulge properties (Liu et al 2010a); (2) the observed [O III] luminosities are significantly higher than those for ULXs (Abolmasov et al 2007); and (3) the X-ray point sources in our targets are nuclear given astrometric uncertainties, although ULXs could also live close to nuclear regions.…”

Spectral variability of ultraluminous X-ray sources

“…Strong beaming, however, can be ruled out in some cases by observations of photo-ionised nebulae surrounding the ULXs because the observed line properties require luminosities comparable to the observed X-ray luminosities (Pakull & Mirioni 2003;Kaaret et al 2004;Abolmasov et al 2007;Kaaret & Corbel 2009). …”

Ultraluminous X-ray Sources and Their Nebulae

“…It was realised that an enhanced He IIλ 4686 line is common for many ULXNe [9,10] including even ULX bubbles [11,12]. The He II emission is often proposed to be a wind or atmospheric emission of the central object.…”

“…This result may appear to be the first detection of orbital motion for a ULX. Usually (see [11] and [7]) the He IIλ 4686 emission is narrow (FW HM 200 km s −1 ) but is emitted in the vicinity of the X-ray source therefore the best explanation is a compact HeIII-region.…”

“…According to Larsen [16], the cluster is about 12 Myr old. Optical spectrophotometry [11] reveals rather complicated stellar population spectrum with some excess in nebular lines.…”