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.

8 pages, 8 figures. Accepted for publication in ApJ

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Cited by in corpus (12)

• Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra
• Accretion disks and coronae in the X-ray flashlight
• The direct cooling tail method for X-ray burst analysis to constrain neutron star masses and radii
• X-ray burst induced spectral variability in 4U 1728-34
• NICER observations of the evidence of Poynting-Robertson drag and disk reflection during type I X-ray bursts from 4U 1636$-$536
• NICER views moderate, strong, and extreme photospheric expansion bursts from the ultracompact X-ray binary 4U 1820$-$30
• Variable spreading layer in 4U 1608-52 during thermonuclear X-ray bursts in the soft state
• Broadband time-resolved spectroscopy of thermonuclear X-ray bursts from 4U 1636-536 using AstroSat
• The thermonuclear X-ray bursts of 4U 1730-22
• Flux decay during thermonuclear X-ray bursts analysed with the dynamic power-law index method
• Impact of Accretion Assumptions on Pulse Profile Modelling of Superburst Oscillations in 4U 1636-536
• NICER observations of type-I X-ray bursts from the ultra-compact X-ray binary M15 X-2