The spectra of x-ray transients

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Abstract
The properties of the persistent X‐ray sources (such as X‐ray bursts) are also seen from the transient X‐ray source suggesting that they are related binary systems containing accreting compact objects. The spectral properties of the X‐ray transients are considered and compared with those of the persistent sources. The spectral morphologies of the three classes of persistent source i.e. the X‐ray pulsars, the bulge sources and the accreting black holes also characterise the spectra of the X‐ray transients. A comparative study of the spectra of the transients with those of the persistent sources suggests that two thirds of the transients seen to date result from episodic accretion onto neutron stars with either low magnetic fields (the soft bulge‐like transients) or high magnetic fields (the hard pulsing transients). The remaining transients have unusually soft (ultra‐soft) spectra similar to those of the black hole candidates in their high state. For the systems containing late type lobe filling stars the recurrence interval of the outbursts coupled with the total outburst energy gives time averaged accretion rates of <1016.5 gm s−1, well below that required to power the bulge‐like sources. Two transients that after the ourburst became steady sources required accretion rates of ∼1016.5 gm s−1, both to power the outburst and the subsequent persistent emission. This suggests that for mass transfer rates below ∼1016.5 gm s−1 the mass transfer onto compact objects in semi‐detached late type systems becomes intermittent.

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