DIFFERENT TYPES OF ULTRALUMINOUS X-RAY SOURCES IN NGC 4631
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Abstract:
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 ULX. We suggest that its bolometric luminosity ~ a few 10^{39} 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 intermediate-mass black hole. We suggest that recurrent super-Eddington outbursts with photospheric expansion from a massive white dwarf (M_{wd} >~ 1.3 M_{sun}), 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 x 10^{-6} M_{sun}/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 requires masses >~ 50 M_{sun}.Keywords:
ROSAT
Black hole (networking)
X-ray transient
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Intermediate polar
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The ROSAT WFC survey has provided us with evidence for the existence of a previously unidentified sample of hot white dwarfs (WD) in non-interacting binary systems, through the detection of EUV and soft X-ray emission. These stars are hidden at optical wavelengths due to their close proximity to much more luminous main sequence (MS) companions (spectral type K or earlier). However, for companions of spectral type A5 or later the white dwarfs are easily visible at far-UV wavelengths, and can be identified in spectra taken by IUE. Eleven white dwarf binary systems have previously been found in this way from ROSAT, EUVE and IUE observations (e.g. Barstow et al. 1994). In this paper we report the discovery of three more such systems through our programmes in recent episodes of IUE. The new binaries are HD2133, RE J0357+283 (whose existence was predicted by Jeffries, Burleigh and Robb 1996), and BD+27 1888. In addition, we have independently identified a fourth new WD+MS binary, RE J1027+322, which has also been reported in the literature by Genova et al. (1995), bringing the total number of such systems discovered as a result of the EUV surveys to fifteen. We also discuss here six stars which were observed as part of the programme, but where no white dwarf companion was found. Four of these are coronally active. Finally, we present an analysis of the WD+K0IV binary HD18131 (Vennes et al. 1995), which includes the ROSAT PSPC X-ray data.
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Chandrasekhar limit
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The BeppoSAX Wide Field Cameras detected a transient in NGC6440 on 1998 Aug 22. ROSAT did not detect this source on 1998 Sep 8, indicating that the X-ray flux had decreased by a factor 400 at least, and/or that the X-ray spectrum had become appreciably softer. Analysis of archival ROSAT HRI data reveals two sources associated with NGC6440; one of these may be the transient in quiescence. We have also obtained B and R images of NGC6440 on 1998 Aug 26 and again on 1999 July 15, when the transient had returned to quiescence. Subtraction of these images reveals one object in the core which was brighter in B, but not in R, during the X-ray outburst. We estimate B=22.7 and (B-R)o<0 on 1998 Aug 26, which makes it a viable candidate counterpart.
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The BeppoSAX Wide Field Cameras detected a transient in NGC6440 on 1998 Aug 22. ROSAT did not detect this source on 1998 Sep 8, indicating that the X-ray flux had decreased by a factor 400 at least, and/or that the X-ray spectrum had become appreciably softer. Analysis of archival ROSAT HRI data reveals two sources associated with NGC6440; one of these may be the transient in quiescence. We have also obtained B and R images of NGC6440 on 1998 Aug 26 and again on 1999 July 15, when the transient had returned to quiescence. Subtraction of these images reveals one object in the core which was brighter in B, but not in R, during the X-ray outburst. We estimate B=22.7 and (B-R)o<0 on 1998 Aug 26, which makes it a viable candidate counterpart.
ROSAT
X-ray transient
Transient (computer programming)
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We report observations which identify the optical/IR counterpart to the ROSAT X-ray transient RX J0117.6−7330. The counterpart is suggested to be a B1–B2 star (luminosity class III–V) showing an IR excess and strong Balmer emission lines. The distance derived from reddening and systemic velocity measurements is consistent with the distance derived from X-ray measurements and puts the source in the Small Magellanic Cloud (SMC).
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Balmer series
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One of the challenges to increasing the mass of a white dwarf through accretion is the tendency for the accumulating hydrogen to ignite unstably and potentially trigger mass loss. It has been known for many years that there is a narrow range of accretion rates for which the hydrogen can burn stably, allowing for the white dwarf mass to increase as a pure helium layer accumulates. We first review the physics of stable burning, providing a clear explanation for why radiation pressure stabilization leads to a narrow range of accretion rates for stable burning near the Eddington limit, confirming the recent work of Nomoto and collaborators. We also explore the possibility of stabilization due to a high luminosity from beneath the burning layer. We then examine the impact of the β-decay-limited "hot" CNO cycle on the stability of burning. Although this plays a significant role for accreting neutron stars, we find that for accreting white dwarfs, it can only increase the range of stably burning accretion rates for metallicities <0.01 Z☉.
Eddington luminosity
Black dwarf
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Carbon-oxygen white dwarf (CO WD) and helium star binary is one of the ways that can lead to accretion-induced collapse (AIC). The continuous accretion may increase the mass of the white dwarf until at a certain condition, carbon burning off-center might be initiated and alter CO into ONe WD. This paper is intended to analyze the long-term evolution of the CO WD accreting helium material. The stellar evolution code used in this research is MESA (Modules for Experiments in Stellar Astrophysics). MESA creates CO WD by evolving ZAMS star with an initial mass of 6 M ⊙ to produce 0.9 M ⊙ CO WD. The accretion rates are 4×10 −6 and 4×10 −7 M ⊙ /year which consist of mostly helium. It shows that for the high accretion rate, the helium burning on the surface of the WD is stable. It is predicted that it will continue to be stable for a very long time because of the high supply of matter. For the lower one, the burning experiences a fluctuation from the beginning of the accretion. For both cases, if carbon burning off-center has occurred, it may lead to the creation of ONe WD and eventually AIC.
Intermediate polar
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