X-ray Bursts

Abstract

An X-ray burst is a sudden increase (rise time of order seconds) of the X-ray brightness of an X-ray source, which after reaching its peak decays, generally within a minute. The sky distribution of X-ray burst sources indicates that they are galactic objects (see Fig. 1); their concentration to the direction of the galactic center tells us that they lie at typical distances of ∼ 8 kpc, with corresponding peak luminosities of order 1038 erg s−1. The X-ray and optical properties of the persistent emission of X-ray burst sources show that they are low-mass X-ray binaries, in which mass is transferred from a rather normal low-mass (< 1 M⊙) star to a neutron star. The persistent emission is caused by the conversion of kinetic energy of the transferred matter into heat, at a rate of ∼ GM/R (∼ 0.1c2) per gram of accreted matter. The bursts are caused by unstable thermonuclear burning of material that has accumulated on the neutron star (‘thermonuclear flash’).The global properties of X-ray bursts, in particular their dependence on the mass accretion rate, are fairly well understood. Different from the case of γ-ray bursts (see the contributions by Fishman, Hartmann and Kouveliotou to this Colloquium) the relevant question about X-ray bursts is not ‘What are they?’, but rather ‘What use are they?’. As we will argue here, X-ray bursts may provide us information on the mass and radius of a neutron star. This usefulness of X-ray bursts derives from the fact that the burst emission originates from the surface of the neutron star, unlike the persistent emission caused by mass accretion, of which we only know that it comes from the neutron star’s near vicinity.