Models for Type I X‐Ray Bursts with Improved Nuclear Physics

Abstract

Multizone models of Type I X-ray bursts are presented that use an adaptive nuclear reaction network of unprecedented size, up to 1300 isotopes, for energy generation and include the most recent measurements and estimates of critical nuclear physics. Convection and radiation transport are included in calculations that carefully follow the changing composition in the accreted layer, both during the bursts themselves and in their ashes. Sequences of bursts, up to 15 in one case, are followed for two choices of accretion rate and metallicity, up to the point at which a limit cycle equilibrium is established. ForṀ ¼ 1:75 Â 10 À9 M yr À1 (andṀ ¼ 3:5 Â 10 À10 M yr À1 , for low metallicity), combined hydrogen-helium flashes occur. These bursts have light curves with slow rise times (seconds) and long tails. The rise times, shapes, and tails of these light curves are sensitive to the efficiency of nuclear burning at various waiting points along the rp-process path, and these sensitivities are explored. Each displays ''compositional inertia'' in that its properties are sensitive to the fact that accretion occurs onto the ashes of previous bursts that contain leftover hydrogen, helium, and CNO nuclei. This acts to reduce the sensitivity of burst properties to metallicity. Only the first anomalous burst in one model produces nuclei as heavy as A ¼ 100. For the present choice of nuclear physics and accretion rates, other bursts and models make chiefly nuclei with A % 64. The amount of carbon remaining after hydrogen-helium bursts is typically P1% by mass and decreases further as the ashes are periodically heated by subsequent bursts. Foṙ M ¼ 3:5 Â 10 À10 M yr À1 and solar metallicity, bursts are ignited in a hydrogen-free helium layer. At the base of this layer, up to 90% of the helium has already burned to carbon prior to the unstable ignition of the helium shell. These helium-ignited bursts have (1) briefer, brighter light curves with shorter tails, (2) very rapid rise times (<0.1 s), and (3) ashes lighter than the iron group.

XMM-Newtonand INTEGRAL view of the hard state of EXO 1745−248 during its 2015 outburst

“…Mass accretion rate should have then been high enough to allow stable hydrogen burning between bursts, but part of the accreted hydrogen was left unburnt at the burst onset and contributed to produce a longer event with respect to pure helium bursts. Combined hydrogenhelium flashes are expected to occur for mass accretion rates larger than ≃ 0.1ṁ Edd (for solar metallicity, lower values are expected for low metallicity, Woosley et al 2004), whereṁ Edd is the Eddington accretion rate per unit area on the NS surface (8.8 × 10 4 g cm −2 s, or 1.3 × 10 −8 M ⊙ yr −1 averaged over the surface of a NS with a radius of 10 km). The persistent broadband X-ray luminosity of EXO 1745-248 during the observations considered here indicates a mass accretion rate of 8.5 × 10 −10 d 2 5.5 ≃ 0.05Ṁ Edd M ⊙ yr −1 for a 1.4 M ⊙ NS with a 10 km radius, lower than the above threshold not to exhaust hydrogen before the burst onset.…”

What ignites on the neutron star of 4U 0614+091?

“…However, there are problems with this scenario: (1) producing enough C during H/He burning (e.g., Schatz et al 2003;Woosley et al 2004), (2) heating the neutron star ocean strongly enough to reach ignition temperature (e.g., Cumming et al 2006;Keek et al 2008), and (3) accreting rapidly enough for the C to survive to the ignition depth (Cumming & Bildsten 2001;Cumming et al 2006).…”

Restless quiescence: thermonuclear flashes between transient X-ray outbursts

“…In general, He ignites completely in a fraction of a second, while unstable hydrogen burning is prolonged to about 100 s through slow beta decays in the rp process (e.g., Cumming 2003;Woosley et al 2004;Heger et al 2007;Fisker et al 2008). The duration of a flash is a convolution of the burning process time and the cooling time.…”