Evidence of spreading layer emission in thermonuclear superbursts

Abstract: When a neutron star accretes matter from a companion star in a low-mass X-ray binary, the accreted gas settles onto the stellar surface through a boundary/spreading layer. On rare occasions the accumulated gas undergoes a powerful thermonuclear superburst powered by carbon burning deep below the neutron star atmosphere. In this paper, we apply the non-negative matrix factorization spectral decomposition technique to show that the spectral variations during a superburst from 4U 1636-536 can be explained by two distinct components: 1) the superburst emission characterized by a variable temperature black body radiation component, and 2) a quasi-Planckian component with a constant, $\sim$2.5 keV, temperature varying by a factor of $\sim$15 in flux. The spectrum of the quasi-Planckian component is identical in shape and characteristics to the frequency-resolved spectra observed in the accretion/persistent spectrum of neutron star low-mass X-ray binaries, and agrees well with the predictions of the spreading layer model by Inogamov & Sunyaev (1999). Our result is yet another observational evidence that superbursts - and possibly also normal X-ray bursts - induce changes in the disc-star boundary.