The cooling phase of Type-I X-ray bursts in 4U 1636-53

Abstract

Time-resolved spectra during the cooling phase of thermonuclear X-ray bursts in low-mass X-ray binaries (LMXBs) can be used to measure the radii and masses of neutron stars. We analyzed ~ 300 bursts of the LMXB 4U 1636-53 using data from the Rossi X-ray Timing Explorer. We divided the bursts in three groups, photospheric radius expansion (PRE), hard non-PRE and soft non-PRE bursts, based on the properties of the bursts and the state of the source at the time of the burst. For the three types of bursts, we found that the average relation between the bolometric flux and the temperature during the cooling phase of the bursts is significantly different from the canonical $F\propto T^4$ relation that is expected if the apparent emitting area on the surface of the neutron star remains constant as the flux decreases during the decay of the bursts. We also found that a single power law cannot fit the average flux-temperature relation for any of the three types of bursts, and that the flux-temperature relation for the three types of bursts is significantly different. Finally, for the three types of bursts, the temperature distribution at different flux levels during the decay of the bursts is significantly different. From the above we conclude that hard non-PRE bursts ignite in a hydrogen-rich atmosphere, whereas for soft non-PRE and PRE bursts the fuel is helium-rich. We further conclude that the metal abundance in the neutron star atmosphere decreases as the bursts decay, probably because the heavy elements sink faster in the atmosphere than H and He.

10 pages, 22 figures, submitted to MNRAS

References in corpus (4)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• The Mass and Radius of the Neutron Star in 4U 1820-30
• Ignition latitude and the shape of Type I X-ray bursts
• Long tails on thermonuclear X-ray bursts from neutron stars: a signature of inward heating?

Cited by in corpus (31)

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• The cooling phase of Type I X-ray bursts observed with RXTE in 4U 1820-30 does not follow the canonical F - T^4 relation
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• Variable spreading layer in 4U 1608-52 during thermonuclear X-ray bursts in the soft state
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• Broadband time-resolved spectroscopy of thermonuclear X-ray bursts from 4U 1636-536 using AstroSat
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• Kilohertz quasi-periodic oscillations in neutron-star X-ray binaries: Flattening of the lag spectrum with increasing luminosity
• The harmonic component of the millihertz quasi-periodic oscillations in 4U 1636-53
• New insight into the hard X-ray emission influenced by the type-\uppercase\expandafter{\romannumeral1} bursts observed by Insight-HXMT during outburst of 4U 1636--536
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• Detection of the thermonuclear X-ray bursts and dips from the X-ray binary 4U 1323-62 with AstroSat/LAXPC