Transient X-ray rings around dwarf novae

Abstract

The possibility that point X-ray sources possess extended haloes arising from scattering of X rays by interstellar grains has been discussed by several authors (see, for example, refs 1, 2) and recent observations using the Einstein X-ray satellite have suggested that observational evidence for such haloes is now becoming available. For example, Rolf³ has shown that the X-ray source GX 339–4 has a discernible halo out to ∼20 arc min, while Catura⁴ has argued that several low latitude X-ray sources may have haloes that extend to ≳10 arc min. Much of the theoretical work on such haloes to date has involved steady (time-independent) X-ray sources. While haloes around such sources can provide useful information about the scattering medium, variable X-ray sources are potentially much more powerful probes of the interstellar dust distribution^5,6. We discuss here the scattering of soft X rays emitted by dwarf novae in outburst, during which these objects undergo short (∼1 day) eruption at soft X-ray wavelengths^7,8. We also consider the nature of the apparently transient soft X-ray ‘ring’ around the dwarf nova SU UMa (ref. 9) and we show that the properties of such a ring are consistent with those expected from the effect under consideration.