The Dynamics and Outcomes of Rapid Infall onto Neutron Stars

Abstract

We present an extensive study of accretion onto neutron stars in which the velocity of the neutron star and structure of the surrounding medium is such that the Bondi-Hoyle accretion exceeds 10 −3 M ⊙ y −1 . Two types of initial conditions are considered for a range of entropies and chemical compositions: an atmosphere in pressure equilibrium above the neutron star, and a freely falling inflow of matter from infinity (also parametrized by the infall rate). We then evolve the system with one-and two-dimensional hydrodynamic codes to determine the outcome. For most cases, hypercritical (also termed "super Eddington") accretion due to rapid neutrino cooling allows the neutron star to accrete above the Bondi-Hoyle rate as previously pointed out by Chevalier. However, for a subset of simulations which corresponds to evolutionarily common events, convection driven by neutrino heating can lead to explosions by a mechanism similar to that found in core-collapse supernovae.Armed with the results from our calculations, we are in a position to predict the fate of a range of rapid-infall neutron star accretors present in certain low-mass X-ray binaries, common envelope systems, supernova fallbacks and Thorne-Zytkow objects (TZOs). A majority of the common envelope systems that we considered led to explosions expelling the envelope, halting the neutron star's inward spiral, and allowing the formation of close binary systems. As a result, the smothered neutron stars produced in the collisions studied by Davies & Benz may also explode, probably preventing them from forming millisecond pulsars. For the most massive supernovae, in which the fallback of material towards the neutron star after a successful explosion is large, we find that a black hole is formed in a few seconds. Finally, we argue that the current set of TZO formation scenarios is inadequate and leads instead to hypercritical accretion and black hole formation. Moreover, it appears that many of the current TZ models have structures ill-suited for modeling by mixing length convection. This has prompted us to develop a simple test to determine the viability of this approximation for a variety of convective systems.

Magnetar Spin‐Down, Hyperenergetic Supernovae, and Gamma‐Ray Bursts

“…Otherwise, we expect this very energetic shock to alter the nucleosynthetic signature of the hypernova with respect to normal supernovae, producing excess nickel as inferred from, e.g., SN 1998bw and SN 2003dh (Nakamura et al 2001a(Nakamura et al , 2001bMaeda et al 2003;Woosley & Heger 2003). If magnetars are born in progenitors with extended envelopes, these strong magnetocentrifugal outflows might also prevent late-time fallback accretion (Chevalier 1989;Woosley & Weaver 1995;Fryer et al 1996). Although the simple analysis presented here assumes sphericity, because the outflow is magnetocentrifugally driven, we expect asymmetries in the ejecta due to collimation (e.g., Sakurai 1985;Begelman & Li 1994).…”

Asymmetric Accretion Flows Within a Common Envelope

“…The hydrodynamics of this scenario have been considered by (Taam et al 1978;Bodenheimer & Taam 1984). Accretion onto the NS during this phase is relatively efficient as neutrinos provide a cooling channel (Houck & Chevalier 1991;Chevalier 1993Chevalier , 1996Fryer et al 1996), leading to the suggestion that the NS might grow to collapse to a BH in some, but not all, cases (Chevalier 1993;Brown 1995;Bethe & Brown 1998;Fryer & Woosley 1998;Belczynski et al 2002;Kalogera et al 2007;Chevalier 2012).…”

A Three‐Stage Model for the Inner Engine of Gamma‐Ray Bursts: Prompt Emission and Early Afterglow

“…Hence, most of the ejected material will be ejected from the polar regions, and it will be collimated by the magnetic field; i.e., it is a jet (ORV05). We assume an accretion rate ofṁ $ 10 À4 M s À1 onto the quark star (similar to what is expected for neutron stars; Fryer et al 1996). Explaining the physical process that limits the accretion rate is beyond the scope of this work and is left for future work.…”

“…As typical values we use the same accretion rate onto quark stars as for neutron stars,ṁ QS % 10 À4 M s À1 (Fryer et al 1996 in fact the accretion rate depends on the maximum temperature the star is heated to as in ORV05) andṁ BH % 1 M s À1 (De Villiers et al 2005), giving ṁ % 10 À4 , and find that the duration of the black hole era is much shorter than the quark star era. The ratio between the energy produced by the inner engine in the two stages can be found by…”