  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.
