Flux decay during thermonuclear X-ray bursts analysed with the dynamic power-law index method

Abstract

The cooling of type-I X-ray bursts can be used to probe the nuclear burning conditions in neutron star envelopes. The flux decay of the bursts has been traditionally modelled with an exponential, even if theoretical considerations predict power-law-like decays. We have analysed a total of 540 type-I X-ray bursts from five low-mass X-ray binaries observed with the Rossi X-ray Timing Explorer. We grouped the bursts according to the source spectral state during which they were observed (hard or soft), flagging those bursts that showed signs of photospheric radius expansion (PRE). The decay phase of all the bursts were then fitted with a dynamic power-law index method. This method provides a new way of probing the chemical composition of the accreted material. Our results show that in the hydrogen-rich sources the power-law decay index is variable during the burst tails and that simple cooling models qualitatively describe the cooling of presumably helium-rich sources 4U 1728-34 and 3A 1820-303. The cooling in the hydrogen-rich sources 4U 1608-52, 4U 1636-536, and GS 1826-24, instead, is clearly different and depends on the spectral states and whether PRE occurred or not. Especially the hard state bursts behave differently than the models predict, exhibiting a peculiar rise in the cooling index at low burst fluxes, which suggests that the cooling in the tail is much faster than expected. Our results indicate that the drivers of the bursting behaviour are not only the accretion rate and chemical composition of the accreted material, but also the cooling that is somehow linked to the spectral states. The latter suggests that the properties of the burning layers deep in the neutron star envelope might be impacted differently depending on the spectral state.

10 pages, 7 figures, accepted for publication in A&A

References in corpus (11)

• Modelling the behaviour of accretion flows in X-ray binaries
• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• Dependence of X-Ray Burst Models on Nuclear Reaction Rates
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• Models of Type I X-ray Bursts from GS 1826-24: A Probe of rp-Process Hydrogen Burning
• The effect of accretion on the measurement of neutron star mass and radius in the low-mass X-ray binary 4U 1608-52
• Spread of Matter over a Neutron-Star Surface During Disk Accretion: Deceleration of Rapid Rotation
• The influence of accretion geometry on the spectral evolution during thermonuclear (type-I) X-ray bursts
• First superburst from a classical low-mass X-ray binary transient
• Long tails on thermonuclear X-ray bursts from neutron stars: a signature of inward heating?
• 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

Cited by in corpus (4)

• Accretion disks and coronae in the X-ray flashlight
• Neutron star cooling and the rp process in thermonuclear X-ray bursts
• Broadband time-resolved spectroscopy of thermonuclear X-ray bursts from 4U 1636-536 using AstroSat
• Quasi-simultaneous Integral, Swift, And Nustar Observations Of The New X-ray Clocked Burster 1rxsj180408.9-342058