Joint Hα and X‐Ray Observations of Massive X‐Ray Binaries. III. The Be X‐Ray Binaries HDE 245770 = A0535+26 and X Persei

Abstract

We present results from an H-alpha monitoring campaign of the Be X-ray binary\nsystems HDE 245770 = A 0535+26 and X Per. We use the H-alpha equivalent widths\ntogether with adopted values of the Be star effective temperature, disk\ninclination, and disk outer boundary to determine the half-maximum emission\nradius of the disk as a function of time. The observations of HDE 245770\ndocument the rapid spectral variability that apparently accompanied the\nregeneration of a new circumstellar disk. This disk grew rapidly during the\nyears 1998 - 2000, but then slowed in growth in subsequent years. The outer\ndisk radius is probably truncated by resonances between the disk gas and\nneutron star orbital periods. Two recent X-ray outbursts appear to coincide\nwith the largest disk half-maximum emission radius attained over the last\ndecade. Our observations of X Per indicate that its circumstellar disk has\nrecently grown to near record proportions, and concurrently the system has\ndramatically increased in X-ray flux, presumably the result of enhanced mass\naccretion from the disk. We find that the H-alpha half-maximum emission radius\nof the disk surrounding X Per reached a size about six times larger than the\nstellar radius, a value, however, that is well below the minimum separation\nbetween the Be star and neutron star. We suggest that spiral arms excited by\ntidal interaction at periastron may help lift disk gas out to radii where\naccretion by the neutron star companion becomes more effective.\n