The Nature of Ultraluminous Compact X‐Ray Sources in Nearby Spiral Galaxies

Abstract

Studies were made of ASCA spectra of seven ultra-luminous compact X-ray sources (ULXs) in nearby spiral galaxies; M33 X-8 (Takano et al. two sources in NGC 4565 (Mizuno et al. 1999). With the 0.5-10 keV luminosities in the range 10 39−40 ergs s −1 , they are thought to represent a class of enigmatic X-ray sources often found in spiral galaxies. For some of them, the ASCA data are newly processed, or the published spectra are reanalyzed. For others, the published results are quoted. The ASCA spectra of all these seven sources have been described successfully with so called multi-color disk blackbody (MCD) emission arising from optically-thick standard accretion disks around black holes. Except the case of M33 X-8, the spectra do not exhibit hard tails. For the source luminosities not to exceed the Eddington limits, the black holes are inferred to have rather high masses, up to ∼ 100 solar masses. However, the observed innermost disk temperatures of these objects, T in = 1.1 − 1.8 keV, are too high to be compatible with the required high black-hole masses, as long as the standard accretion disks around Schwarzschild black holes are assumed. Similarly high disk temperatures are also observed from two Galactic transients with superluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually high disk temperature may be explained by the black hole rotation, which makes the disk get closer to the black hole, and hence hotter.

X-ray observations of ultraluminous X-ray sources

“…where M is the mass of the accreting object in solar masses (Makishima et al 2000). Hence, for an object obeying the Eddington limit, at a luminosity of 10 39 erg s −1 its mass must be > ∼ 7.7M ⊙ .…”

“…Most notably, the novel wide bandpass CCD spectroscopy afforded by ASCA allowed the spectra of many ULXs to be measured over the 0.5 − 10 keV range for the first time. This revealed some of them to be well-fitted by the multi-colour disc blackbody model, that describes the optically-thick thermal X-ray emission of an accretion disc around a black hole (Makishima et al 2000). Additionally, multi-epoch observations of some ULXs saw them apparently transiting between spectral states described by either the multi-colour disc blackbody model or a power-law, similar to the transition between low-and high-states seen in Galactic black hole X-ray binaries (Kubota et al 2001).…”

“…This gave the first strong corroborating evidence -on top of their extraordinary luminosities -that most ULXs are accreting black holes. However, these results were not without problems, notably that the disc temperatures measured by Makishima et al (2000) were far too hot to be reconciled with the masses suggested by the high luminosities of the sources (see below). It was suggested that this could be explained by rapidly spinning (Kerr metric) black holes, in which the inner edge of the accretion discs are far closer to the black hole -and therefore hotter -than in the non-rotating (Schwarzschild) case.…”

Spectral state transitions of the Ultraluminous X-ray Source IC 342 X-1

“…Their high luminosities suggest they are either intermediate mass black holes (IMBH) (Colbert & Mushotzky 1999;Makishima et al 2000;Kaaret et al 2001), that their emission is beamed (e.g. Poutanen et al 2007;King 2009), or that they are stellar-mass BHBs that are emitting at super-Eddington rates (e.g.…”

Unlocking the nature of ultraluminous X‐ray sources using their X‐ray spectra

“…Simple phenomenological models revealed warm disclike spectra in ASCA data, suggesting possible rapidly spinning (Kerr) StMBHs (Makishima et al 2000). Increases in spectral quality allowed fitting of two component models (multi-coloured disc + power-law), indicating the presence of a soft excess at lower energies, well fit by a cool disc.…”