Detailed observations of an intermittent ultraluminous X-ray source indicate that its emission is unlikely to be powered by mass accretion onto an intermediate-mass black hole as previously thought. See Letter p.500
Ultraluminous X-ray sources (ULXs) are extragalactic sources of X-rays, powered by black holes, that are not coincident with galactic nuclei and have luminosities greater than 1039 erg per second — roughly the highest luminosity that stellar-mass black holes, which weigh less than about 30 solar masses, should be able to achieve. Many types of X-ray source could fit this definition, and there has been a recent multiplication of ULX types. On page 500 of this issue, Liu et al.1 use an intermittent ULX in the spiral galaxy M 101 — an object that purists might argue is not a 'real' ULX — to show that several aspects of our understanding of ULXs and, indeed, of black-hole formation, may need to be revised.
The luminosity and spectrum of this object, known as M 101 ULX-1 (Fig. 1), had suggested that it is an intermediate-mass black hole2. These black holes have masses in the range of 100 to 1,000 solar masses, and so are larger than black holes formed by the collapse of single massive stars, but smaller than the supermassive black holes that lurk in galactic nuclei. However, Liu and colleagues' radial-velocity measurements of M 101 ULX-1 show that it is likely to be a black hole with a mass of only 20–30 solar masses. It is not an intermediate-mass black hole, even though commonly used relations between mass, luminosity and temperature imply that it should be.
The Eddington luminosity of an accreting black hole occurs when the pressure of the infalling material that will produce radiation is balanced by the outward radiation pressure. This simple physical argument sets the maximum luminosity for a given black-hole mass, or the minimum mass an accretor must have to produce a given luminosity. High luminosities require high fuelling (accretion) rates, which require the black hole to have a stellar companion to provide the fuel. This, in turn, requires the black hole to have formed from the collapse of a massive star, and to have a mass less than about 30 solar masses.
A typical ULX, at the Eddington luminosity, must have a minimum mass of 100 to 1,000 solar masses, much larger than predicted by our understanding of how stars become black holes. Thus, either our knowledge of the stellar evolution leading to black holes is wrong, or our simple picture of accretion is wrong. Indeed, mechanisms for accretion beyond the Eddington limit have been proposed3, although their likelihood remains unclear.
Most of the time, M 101 ULX-1 has a luminosity 100-fold less than that of the ULX definition, but it occasionally flares into the ULX regime. Liu et al. show that the mass of M 101 ULX-1 is probably 20–30 solar masses, so its luminosity in outburst greatly exceeds the Eddington limit. But because the outbursts are relatively short (less than a week), this super-Eddington luminosity may be understandable. Thus, one might not think this source interesting. However, M 101 ULX-1 contradicts another common understanding about ULXs.
Under some highly simplifying assumptions, standard-accretion models predict the mass of a black hole to be proportional to T−4, where T, which is determined through spectral fitting4, is the characteristic temperature of a disk formed from the infalling material. More-massive accretors have lower disk temperatures. The disk temperatures of ULXs are lower than those of Galactic black-hole binary systems with measured masses, but at a given disk temperature, ULXs have luminosities that are 100-fold higher5. This difference has been taken as tentative evidence that ULXs are indeed much more massive than stellar-mass black holes. However, Liu et al. use archival X-ray data to show that M 101 ULX-1, in outburst, has the low temperature expected from a ULX, despite being a stellar-mass black hole. Thus, other ULXs previously thought to be intermediate-mass black holes on the basis of their luminosity and temperature may, in fact, be stellar-mass systems.
Liu and colleagues remind us of the need for multi-wavelength data to understand these objects; they used data from a deep year-long monitoring campaign with the Chandra X-ray Observatory, extensive optical imaging by the Hubble Space Telescope, and a major spectroscopic programme with the 8.1-metre Gemini telescope, although some of these data were obtained for other reasons. They have managed to resolve the controversial issue of the nature of the optical counterpart to M 101 ULX-1. This was once thought to be a massive 'B supergiant star', and then a lower-mass 'F star' whose emission was drowned in the optical light from the accretion disk. But the authors now confirm a previous suggestion6 that it is a Wolf–Rayet star, an evolved massive star undergoing strong mass loss. The spectroscopic observations also suggest that the system does not have the low metallicity (abundance of elements other than hydrogen and helium) that has come to be associated with ULXs7.
Thus, although these properties mean that M 101 ULX-1 is not a 'classic' ULX, it was formerly a particularly good intermediate-mass black-hole candidate. It is systems such as these that, when studied in this detail, will allow us to determine the conditions under which super-Eddington accretion occurs, whether ULX luminosities are super-Eddington, and whether intermediate-mass black holes are really needed to explain ULXs.