The observed properties of X-ray burst sources have recently been reviewed by Lewin and Clark (1980) and Lewin and Joss (1980). About thirty-five such sources are presently known, and they have a spatial distribution reminiscent of stellar Population II (see Figure 1). The salient features of these sources include burst rise times of ≲ls, decay time scales of ~3–100 s, peak luminosities of ~1039 ergs per burst, spectra that can generally be well fitted by blackbody emission from a surface with a constant effective radius of ~10 km and a peak temperature of ~3 × 107 K, and “tails” of softer X-ray emission that may persist for several minutes after the burst maximum. Profiles of bursts from some typical burst sources are shown in Figure 2. The intervals between bursts from a given source may be regular or erratic and are typically in the range of ~104−105 s; many sources undergo burst-inactive phases that can last for weeks or months. Most burst sources are also sources of persistent X-ray emission, and the ratio of average persistent luminosity to time-averaged burst luminosity is typically ~102 during burst-active phases. (The properties of the “Rapid Burster,” MXB1730-335, are different from those of all other known burst sources and will be discussed separately in §VI below.) There are few correlations among the burst flux, burst intervals, and persistent X-ray flux from any given source, and the detailed burst shapes vary from one source to another and often vary with time in a given source.
A census of young stellar population associated with the Herbig Be star HD 200775