Statistics – Computation
Scientific paper
Jan 1982
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982aipc...77..349j&link_type=abstract
AIP Conference Proceedings, Volume 77, pp. 349-350 (1982).
Statistics
Computation
X-Ray Binaries, Neutron Stars
Scientific paper
Since the discovery of cosmic X-ray bursts in 1975, a wealth of observational information concerning this phenomenon has been obtained. X-ray burst have rise times of <~1 s, decay timescale of ~3-30 s, and intervals between bursts that are usually in the range of ~104-105 s. The burst spectra can often be well fitted by a blackbody spectrum from an emission region of maximum temperature ~3×107 K and roughly constant size. If X-ray burst sources are typically at distances of ~10 kpc, as indicated by their concentration in the direction of the galactic center, then they have maximum luminosities of ~1038 ergs s-1, total emitted energies of ~1039 ergs per burst, and effective blackbody radii of ~10 km. The observational properties of X-ray burst sources have recently been reviewed by Lewin and Joss.1
Thermonuclear flashes in the surface layers of accreting neutron stars are one of several mechanisms that have been proposed to account for X-ray bursts2,3. Computations of the evolution of such surface layers4 demonstrate that under many circumstances, the helium-burning shell is thermally unstable and should undergo flashes that result in the emission of X-ray bursts from the neutron-star photosphere. The calculated properties of these bursts are remarkably similar to those of bursts from most observed X-ray burst sources.
If the surface magnetic field of the neutron star is sufficiently strong to funnel the accretion flow onto the magnetic polar caps, then the rotation of the neutron star and its associated accretion pattern should result in the emission of periodic X-ray pulsations. However, the magnetic funneling shold also alter the structure of the surface layers so as to tend to suppress thermonuclear flashes5. Hence, the apparent dichotomy between X-ray burst sources and X-ray pulsars may be readily understood.
A number of theoretical and phenomenological problems remain to be resolved (see Lewin and Joss1 for a review). Among the theoretical issues currently under active investigation are the complex interactions between the helium-burning and hydrogen-burning shells6-10, the role of general relatistic effects11,12,10, and the conditions required to maintain the core of the neutron star in the thermal equilibrium13,10. Other considerations, such as violations of spherical symmetry and the role of dynamical effects in the outer surface layers, may also turn out to be important.
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