DIFFERENT TYPES OF ULTRALUMINOUS X-RAY SOURCES IN NGC 4631

We have re-examined the most luminous X-ray sources in the starburst galaxy NGC 4631, using XMM-Newton, Chandra, and ROSAT data. The most interesting source is a highly variable supersoft ultraluminous X-ray source (ULX). We suggest that its bolometric luminosity similar to a few 10(39) erg s(-1) 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 noncanonical low/soft (kT similar to 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 behavior of an accreting intermediate-mass black hole. We suggest that recurrent super-Eddington outbursts with photospheric expansion from a massive white dwarf (M(wd) greater than or similar to 1.3 M(circle dot)), powered by nonsteady 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 approximate to (5-10) x 10(-6) M(circle dot) yr(-1): 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 NGC 4631 ( 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 require masses greater than or similar to 50 M(circle dot).