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 H-alpha monitoring campaign of the Be X-ray binarysystems HDE 245770 = A 0535+26 and X Per. We use the H-alpha equivalent widthstogether with adopted values of the Be star effective temperature, diskinclination, and disk outer boundary to determine the half-maximum emissionradius of the disk as a function of time. The observations of HDE 245770document the rapid spectral variability that apparently accompanied theregeneration of a new circumstellar disk. This disk grew rapidly during theyears 1998 - 2000, but then slowed in growth in subsequent years. The outerdisk radius is probably truncated by resonances between the disk gas andneutron star orbital periods. Two recent X-ray outbursts appear to coincidewith the largest disk half-maximum emission radius attained over the lastdecade. Our observations of X Per indicate that its circumstellar disk hasrecently grown to near record proportions, and concurrently the system hasdramatically increased in X-ray flux, presumably the result of enhanced massaccretion from the disk. We find that the H-alpha half-maximum emission radiusof the disk surrounding X Per reached a size about six times larger than thestellar radius, a value, however, that is well below the minimum separationbetween the Be star and neutron star. We suggest that spiral arms excited bytidal interaction at periastron may help lift disk gas out to radii whereaccretion by the neutron star companion becomes more effective.Comment: accepted to ApJ, 24 pages including 3 tables & 10 figure