Simulating X-ray bursts during a transient accretion event

Abstract

Modelling of thermonuclear X-ray bursts on accreting neutron stars has to date focused on stable accretion rates. However, bursts are also observed during episodes of transient accretion. During such events, the accretion rate can evolve significantly between bursts, and this regime provides a unique test for burst models. The accretion-powered millisecond pulsar SAX J1808.4-3658 exhibits accretion outbursts every 2-3 years. During the well-sampled month-long outburst of 2002 October, four helium-rich X-ray bursts were observed. Using this event as a test case, we present the first multi-zone simulations of X-ray bursts under a time-dependent accretion rate. We investigate the effect of using a time-dependent accretion rate in comparison to constant, averaged rates. Initial results suggest that using a constant, average accretion rate between bursts may underestimate the recurrence time when the accretion rate is decreasing, and overestimate it when the accretion rate is increasing. Our model, with an accreted hydrogen fraction of $X=0.44$ and a CNO metallicity of $Z_{\mathrm{CNO}}=0.02$, reproduces the observed burst arrival times and fluences with root mean square (RMS) errors of $2.8\mathrm{h}$, and $0.11\times 10^{-6}\mathrm{erg}\mathrm{c}{\mathrm{m}}^{-2}$, respectively. Our results support previous modelling that predicted two unobserved bursts, and indicate that additional bursts were also missed by observations.

8 pages, 5 figures; minor corrections, added grid resolution paragraph in Sec 2.2, added Table 1 summarising model parameters; accepted for publication by MNRAS

References in corpus (4)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• Dependence of X-Ray Burst Models on Nuclear Reaction Rates
• Models of Type I X-ray Bursts from GS 1826-24: A Probe of rp-Process Hydrogen Burning
• Accretion-powered Millisecond Pulsar Outbursts

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• A cooling neutron star crust after recurrent outbursts: Modelling the accretion outburst history of Aql X-1
• Multi-epoch X-ray burst modelling: MCMC with large grids of 1D simulations
• Enhanced optical activity 12 days before X-ray activity, and a 4 day X-ray delay during outburst rise, in a low-mass X-ray binary
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• Hydrogen-triggered X-ray Bursts from SAX J1808.4-3658? The Onset of Nuclear Burning
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