S. Yow

MPhil 2006 (supervisor: G. Branduardi-Raymont)

This thesis is a study of the X-ray transient A0620-00. A0620-00 is a binary system consisting of a black hole candidate and a late-type main sequence star. Its last recorded outbursts were in 1917 and 1975. This black hole candidate binary system is chosen for this study as it is the brightest optical source in quiescence among black hole candidates. Furthermore its long quiescence period (∼60 years) allows ample opportunities to study the properties of the black hole candidate, the mass donor star and the accretion disk.

This investigation involves the analysis of a series of optical spectra of A0620-00 collected at the 2.3m ANU telescope at Siding Spring Observatory (SSO), over the span of 5 years (with observing campaigns in 2000, 2003, and 2004).

The most prominent spectral feature in these spectra of A0620-00 is a double peaked Hα emission line centred at 6562.8 Å. The double peaked line indicates the presence of an accretion disk. The physical conditions and dynamics of an X-ray transient that lead to the formation of this optical Balmer line can be inferred from the data. It is unclear if the mechanisms which produce the Balmer line during quiescence and during outbursts are similar.

The objective of this thesis is to look for variations in the optical spectra of A0620-00 during quiescence, and in particular, for changes in the profile and the strength of the normalised flux of the double peaked Hα emission line. Scrutiny of the Hα profile over time provides an insight into the possible changes in the physical conditions of the flow in the disk, hence the resulting radiation.

The analysis of the spectra found the Hα line profile to be varying. The asymmetry in the shape of the double peaked line profile and periodic variation of the equivalent widths support the presence of a hotspot, as well as a weak wind on the accretion disk. The line width of Hα varies over time which could be the result of an expanding of the inner edge of the accretion disc, resulting in the emission region being much closer to the compact object. There are also changes in the equivalent width that indicate a possible increase in the mass accretion rate, hence a higher luminosity. However this increase in accretion rate is not enough to induce a mini outburst in the system, nor to produce a strong wind in the disc. The diagnosis suggests that the accretion disc could be in a state of instability, where the inner edge is expanding and contracting.