A dipping black-hole X-ray binary candidate in NGC 55

Abstract

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.

9 pages, 7 figures, accepted for publication in MNRAS