Superbursts and long bursts as surface phenomenon of compact objects

Abstract

X-ray bursts from compact stars is believed to be due to type I thermonuclear processes which are short lived, typically ~ 10 to 100 s. There are some low mass X-ray binaries (LMXB) like 4U 1820-30, 4U 1636-53, KS 1731-260 and Serpens X-1, known as Super Bursters (SB) which emit X-rays close to the Eddington luminosity limit for long periods of several hours. Recently there are reports of some long bursters (LB), which have bursts lasting 6-25 minutes, whereas the 4U 1735-44 has a burst period of 86 minutes. We suggest that these bursts from SB and LB may be due to 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. (1998) and Li et al. (1999a and 1999b) 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-s for each pair, on the average. Diquarks are expected to break up due to the explosion and shock of the TN process. The subsequent production of copious diquark pairing may produce sufficient energy to produce the very long bursts seen in SB or LB. The estimated total energy liberated, 10^{42} ergs, can be explained in our model with the calculated pair density ~ 0.275/fm^3 and a surface thickness of only half a micron, if the entire surface is involved. The depth of the surface involved in the process may be only few microns if the process is restricted to small part of the surface near the equator as suggested by Bildsten.