Session 104 - Accretion-Powered X-Ray Sources.
Display session, Thursday, January 18
North Banquet Hall, Convention Center

## [104.02] Preliminary Mass Determination of the Soft X-ray Transient A0620-00

B. Lane (Caltech), J. E. McClintock (SAO), R. A. Remillard (MIT)

Presented here are photometric data for the soft X-ray transient A0620-00, from observations made regularly since 1981 in the I and B+V bands. The orbital light curves obtained have been fitted to an ellipsoidal binary model containing a K4-K5 dwarf and a compact object. The best-fitting value for the orbital inclination angle, i, is strongly dependent on light contributions from an accretion disk; for this reason the fitting was done for a range of possible disk contributions. The resulting range of inclination is 42^\circ \leq i \leq 63^\circ. When combined with previously published results for the mass function, f(M, i) = 2.91 \pm 0.08 M_ødot, this inclination range implies a mass range of 4.9 - 11.6 M_ødot for the compact primary. These results strengthen the case made earlier that A0620-00 contains a black hole.

The secular variability of the photometric light curves has also been studied. Variations on the order of 10% are apparent, and a phase variation of 180^\circ on a two-year timescale was observed. No straightforward explanation of this behaviour has yet been proposed.