Burst events with timescales of seconds were discovered by the Vela satellites and reported in 1973.' These "soft y-ray" bursts have energies of -150 keV and occur at a rate of -10 yr-' at the level of sensitivity of the Vela detectors.* T w o bursts detected by the Kosmos 428 satellite were reported in 1975.3 These "hard x-ray" bursts had temporal structures that resembled those of the soft y-ray bursts but softer spectra and much smaller fluxes.$ Independently in 1975, the ANS satellite discovered4 x-ray bursts that originated from the previously known x-ray source 3U 1820-30 in the globular cluster NGC6624. Analysis of earlier SAS-3 observations quickly confirmed this result and led to the remarkable additional discovery that the bursts recurred quasiperiodically on a timescale of 4h4 with a phase jitter of a few percent.5Since then, there have been many additional reports of x-ray bursts, and more than 20 burst sources are now known. Observations of x-ray bursts have recently been reviewed by Grindlay6 and Lewin' and are also discussed elsewhere in this volume in the articles by Gursky' and Lewin.' F I G U R E 1 shows schematically the light curvc of a burst source. Typically, the bursts have rise times 6 t b <, Is, durations Atb z 3'-IOs, and repetition timescales f b --I h -I O h . Peak burst luminosities are L b -103'erg sec-', and burst energies are Eb 1 0~~-1 0~ erg. Many, and perhaps all, burst sources have "steady" background luminosities Lo 510"erg scc-', and some have "active" and "inactive" states that last from weeks to months. TABLE I gives repetition timescales. burst energies, and the ratio A of the background luminosity Lo to the time-averaged burst luminosity = Eb/t,, while the source is in an "active" state for the sources for which this information is available at the time of writing.Identification of 3111820-30 as a source of bursts and the subsequent discovery that these bursts recur quasiperiodically represented a breakthrough in the field of fast timescale x-ray and y-ray phenomena: one now knew where and, often, approximately when to look for a burst. Data from successive bursts could be accumulated for a given source, and astrophysics with bursts could begin.I n this review, we shall survey compact star (either a neutron star or a degenerate dwarf) models of the bursts. These models fall into two distinct categories, which we shall discuss separately. Under ACCRETION MODELS, we discuss bursts Annals N e w Y o r k Academy of Sciences "b ' tb4 FIGURE I . Characteristic luminosities and timescales associated with bursting behavior. L b is the peak luminosity of the burst, and Lo is the steady background luminosity (if any) of the source. Shown are the burst risetime, d i b , duration, A l b , and repetition timescale, i b . (From Lamb ei a/. '' By permission of The Asirophysical Journal.)that are produced by instabilities that involve the magnetosphere of an accreting neutron star or degenerate dwarf. Under NUCLEAR BURNING MODELS, we discuss bursts that are produced by thermonuclea...
