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

We present results from an Hα monitoring campaign of the Be X-ray binary systems HDE 245770 = A0535+26 and X Per. We use the Hα equivalent widths together with adopted values of the Be star effective temperature, disk inclination, and disk outer boundary to determine the half-maximum emission radius of the disk as a function of time. The observations of HDE 245770 document the rapid spectral variability that apparently accompanied the regeneration of a new circumstellar disk. This disk grew rapidly during the years 1998-2000, but then slowed in growth in subsequent years. The outer disk radius is probably truncated by resonances between the disk gas and neutron star orbital periods. Two recent X-ray outbursts appear to coincide with the largest disk half-maximum emission radius attained over the last decade. Our observations of X Per indicate that its circumstellar disk has recently grown to near-record proportions, and concurrently the system has dramatically increased in X-ray flux, presumably the result of enhanced mass accretion from the disk. We find that the Hα half-maximum emission radius of the disk surrounding X Per reached a size about 6 times larger than the stellar radius, a value, however, that is well below the minimum separation between the Be star and neutron star. We suggest that spiral arms excited by tidal interaction at periastron may help lift disk gas out to radii where accretion by the neutron star companion becomes more effective.