Different Types of Ultraluminous X-ray Sources in Ngc4631

We have re-examined the most luminous X-ray sources in the starburst galaxy NGC4631, using XMM-Newton, Chandra and ROSAT data. The most interesting source is a highly variable supersoft ULX. We suggest that its bolometric luminosity ∼ a few 10 erg s in the high/supersoft state: this is an order of magnitude lower than estimated in previous studies, thus reducing the need for extreme or exotic scenarios. Moreover, we find that this source was in a non-canonical low/soft (kT ∼ 0.1–0.3 keV) state during the Chandra observation. By comparing the high and low state, we argue that the spectral properties may not be consistent with the expected behaviour of an accreting intermediatemass black hole. We suggest that recurrent super-Eddington outbursts with photospheric expansion from a massive white dwarf (Mwd & 1.3M⊙), powered by non-steady nuclear burning, may be a viable possibility, in alternative to the previously proposed scenario of a super-Eddington outflow from an accreting stellar-mass black hole. The long-term average accretion rate required for nuclear burning to power such white-dwarf outbursts in this source and perhaps in other supersoft ULXs is ≈ 5–10 × 10M⊙ yr : this is comparable to the thermal-timescale mass transfer rate invoked to explain the most luminous hard-spectrum ULXs (powered by black hole accretion). The other four most luminous X-ray sources in NGC4631 (three of which can be classified as ULXs) appear to be typical accreting black holes, in four different spectral states: high/soft, convex-spectrum, power-law with soft excess, and simple power-law. None of them requires masses & 50M⊙. Subject headings: X-rays: binaries — X-rays: individual (NGC 4631) — black hole physics