Astronomy and Astrophysics – Astrophysics
Scientific paper
2004-03-18
Mon.Not.Roy.Astron.Soc. 351 (2004) 1063
Astronomy and Astrophysics
Astrophysics
9 pages, 7 figures, accepted for publication in MNRAS
Scientific paper
10.1111/j.1365-2966.2004.07848.x
XMM-Newton EPIC observations have revealed a bright point-like X-ray source in the nearby Magellanic-type galaxy NGC 55. At the distance of NGC 55, the maximum observed X-ray luminosity of the source, designated as XMMU J001528.9-391319, is L(x) ~ 1.6 x 10^{39} erg/s, placing the object in the ultraluminous X-ray source (ULX) regime. The X-ray lightcurve exhibits a variety of features including a significant upward drift over the 60 ks observation. Most notably a series of X-ray dips are apparent with individual dips lasting for typically 100-300 s. Some of these dips reach almost 100 percent diminution of the source flux in the 2.0-4.5 keV band. The EPIC CCD spectra can be modelled with two spectral components, a very soft powerlaw continuum (Gamma ~ 4) dominant below 2 keV, plus a multi-colour disc (MCD) component with an inner-disc temperature kT ~ 0.8 keV. The observed upperward drift in the X-ray flux can be attributed to an increase in the level of the MCD component, whilst the normalisation of the powerlaw continuum remains unchanged. The dipping episodes correspond to a loss of signal from both spectral components, although the blocking factor is at least a factor two higher for the MCD component. XMMU J001528.9-391319 can be considered as a candidate black-hole binary (BHB) system. A plausible explanation of the observed temporal and spectral behaviour is that we view the accretion disc close to edge-on and that, during dips, orbiting clumps of obscuring material enter our line of sight and cause significant blocking or scattering of the hard thermal X-rays emitted from the inner disc. In contrast, the more extended source of the soft powerlaw flux is only partially covered by the obscuring matter during the dips.
Roberts Timothy P.
Stobbart A-M.
Warwick Robert S.
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