2009ApJ...696..287S -
Astrophys. J., 696, 287-297 (2009/May-1)
Different types of ultraluminous X-ray sources in NGC 4631.
SORIA R. and GHOSH K.K.
Abstract (from CDS):
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 ∼ a few 1039 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 noncanonical 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 behavior of an accreting intermediate-mass black hole. We suggest that recurrent super-Eddington outbursts with photospheric expansion from a massive white dwarf (Mwd ≳ 1.3 M☉), 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 ~(5-10)x10–6 M☉/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 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 ≳50 M☉.
Abstract Copyright:
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Journal keyword(s):
black hole physics - X-rays: binaries - X-rays: individual: NGC 4631
Nomenclature:
Table 1: [SG2009] XN (Nos X1-X5).
Simbad objects:
16
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