DIFFERENT TYPES OF ULTRALUMINOUS X-RAY SOURCES IN NGC 4631
- Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT (United Kingdom)
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 {approx} a few 10{sup 39} erg s{sup -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 {approx} 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 {sub wd} {approx}> 1.3 M {sub sun}), 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 {approx}(5-10) x 10{sup -6} M {sub sun} yr{sup -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 {approx}>50 M {sub sun}.
- OSTI ID:
- 21296219
- Journal Information:
- Astrophysical Journal, Vol. 696, Issue 1; Other Information: DOI: 10.1088/0004-637X/696/1/287; Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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