SIMBAD references

X-ray bursts from compact stars are believed to be a result of Type I thermonuclear processes which are short-lived, typically ∼10 to 100 s. There are some low mass X-ray binaries, such as 4U 1820-30, 4U 1636-53, KS1731-260 and Serpens X-1, known as super bursters (SBs) which emit X-rays close to the Eddington luminosity limit for long periods of several hours. Recently, there have been reports of some long bursters (LBs), which have bursts lasting 6-25 min, whereas 4U 1735-44 has a burst period of 86 min.

We suggest that these bursts from SBs and LBs may be a result of breaking and re-formation of diquark pairs, on the surface of realistic strange quark stars. We use the beta equilibrated u, d and s quark model of Dey et al. and Li et al. and allow for spin-dependent hyperfine interaction between quarks. The interaction produces pairing of specific colour-spin diquarks, leading to further lowering of energy by several MeV for each pair, on average.

Diquarks are expected to break up because of the explosion and shock of the thermonuclear process. The subsequent production of copious diquark pairing may produce sufficient energy to produce the very long bursts seen in SBs or LBs. We do not claim to be able to model the complicated process fully. However, the estimated total energy liberated, 10⁴² erg, can be explained in our model with the calculated pair density ∼0.275 fm^–3 and a surface thickness of only half a µm, if the entire surface is involved. The depth of the surface involved in the process may be only few µm if the process is restricted to a small part of the surface near the equator, as suggested by Bildsten.

If SBs and LBs are surface phenomena, then recurrent super bursts, found near 4U 1636-53 by Wijnands at an interval of 4.7 yr, and the quick cooling of KS 1731-260 could be natural in this model.