THE INCLINATION OF THE SOFT X-RAY TRANSIENT A0620–00 AND THE MASS OF ITS BLACK HOLE

Abstract

We analyze photometry of the Soft X-ray Transient A0620-00 spanning nearly 30\nyears, including previously published and previously unpublished data. Previous\nattempts to determine the inclination of A0620 using subsets of these data have\nyielded a wide range of measured values of i. Differences in the measured value\nof i have been due to changes in the shape of the light curve and uncertainty\nregarding the contamination from the disk. We give a new technique for\nestimating the disk fraction and find that disk light is significant in all\nlight curves, even in the infrared. We also find that all changes in the shape\nand normalization of the light curve originate in a variable disk component.\nAfter accounting for this disk component, we find that all the data, including\nlight curves of significantly different shapes, point to a consistent value of\ni. Combining results from many separate data sets, we find i=51 plus or minus\n0.9 degrees, implying M=6.6 plus or minus 0.25 solar masses. Using our\ndynamical model and zero-disk stellar VIH magnitudes, we find d=1.06 plus or\nminus 0.12 kpc. Understanding the disk origin of non-ellipsoidal variability\nmay assist with making reliable determinations of i in other systems, and the\nfluctuations in disk light may provide a new observational tool for\nunderstanding the three-dimensional structure of the accretion disk.\n