arXiv subjects

Andrew King

Publications and source records attributed to Andrew King.

95 records · Page 6Linked to original sources

arXiv2002

Gamma-ray bursts, supernova kicks, and gravitational radiation

We suggest that the collapsing core of a massive rotating star may fragment to produce two or more compact objects. Their coalescence under gravitational radiation gives the resulting black hole or neutron star a significant kick velocity, which may explain those observed in pulsars. A gamma-ray burst can result only when this kick is small. Thus only a small fraction of core-collapse supernovae produce gamma-ray bursts. The burst may be delayed significantly (hours - days) after the supernova, as suggested by recent observations. If this picture is correct, core-collapse supernovae should be significant sources of gravitational radiation with a chirp signal similar to a coalescing binary.

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arXiv2000

The violent past of Cygnus X-2

Cygnus X-2 appears to be the descendant of an intermediate-mass X-ray binary (IMXB). Using Mazzitelli's (1989) stellar code we compute detailed evolutionary sequences for the system and find that its prehistory is sensitive to stellar input parameters, in particular the amount of core overshooting during the main-sequence phase. With standard assumptions for convective overshooting a case B mass transfer starting with a 3.5 M_sun donor star is the most likely evolutionary solution for Cygnus X-2. This makes the currently observed state rather short-lived, of order 3 Myr, and requires a formation rate > 1e-7 - 1e-6 per yr of such systems in the Galaxy. Our calculations show that neutron star IMXBs with initially more massive donors (> 4 M_sun) encounter a delayed dynamical instability; they are unlikely to survive this rapid mass transfer phase. We determine limits for the age and initial parameters of Cygnus X-2 and calculate possible dynamical orbits of the system in a realistic Galactic potential, given its observed radial velocity. We find trajectories which are consistent with a progenitor binary on a circular orbit in the Galactic plane inside the solar circle that received a kick velocity < 200 km/s at the birth of the neutron star. The simulations suggests that about 7% of IMXBs receiving an arbitrary kick velocity from a standard kick velocity spectrum would end up in an orbit similar to Cygnus X-2, while about 10% of them reach yet larger Galactocentric distances.

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arXiv1998

Supernova Kicks, Magnetic Braking, and Neutron-Star Binaries

We consider the formation of low-mass X-ray binaries containing accreting neutron stars via the helium-star supernova channel. The predicted relative number of short-period transients provides a sensitive test of the input physics in this process. We investigate the effect of varying mean kick velocities, orbital angular momentum loss efficiencies, and common envelope ejection efficiencies on the subpopulation of short-period systems, both transient and persistent. Guided by the thermal-viscous disk instability model in irradiation-dominated disks, we posit that short-period transients have donors close to the end of core-hydrogen burning. We find that with increasing mean kick velocity the overall short-period fraction, s, grows, while the fraction, r, of systems with evolved donors among short-period systems drops. This effect, acting in opposite directions on these two fractions, allows us to constrain models of LMXB formation through comparison with observational estimates of s and r. Without fine tuning or extreme assumptions about evolutionary parameters, consistency between models and current observations is achieved for a regime of intermediate average kick magnitudes of about 100-200 km/s, provided that (i) orbital braking for systems with donor masses in the range 1-1.5 solar masses is weak, i.e., much less effective than a simple extrapolation of standard magnetic braking beyond 1.0 solar mass would suggest, and (ii) the efficiency of common envelope ejection is low.

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arXiv1997

The Transient Nature of GRO J1655-40 and its Evolutionary State

We consider the evolutionary state of the black-hole X-ray source GRO J1655-40 in the context of its transient nature. Recent optical observations show that the donor in GRO J1655-40 is an intermediate-mass star (~ 2.3 solar masses) crossing the Hertzsprung gap. Usually in such systems the donor's radius expansion drives a near-Eddington or super-Eddington mass transfer rate which would sustain a persistently bright accretion disk. We show that GRO J1655-40 is close to a narrow parameter range where disk instabilities can occur. This range corresponds to a short-lived evolutionary stage where the secondary's radius expansion stalls (or reverses), with a correspondingly lower mass transfer rate. If GRO J1655-40 belongs to this class of transients the predicted accretion rates imply large populations of luminous persistent and transient sources, which are not seen in X-rays. The transient nature of the related system GRS 1915+105 may reflect spectral variations in a bolometrically persistent source rather than a genuine luminosity increase.

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arXiv1997

Why Low-Mass Black-Hole Binaries Are Transient

We consider transient behavior in low-mass X-ray binaries. In short-period neutron-star systems (orbital period less than ~ 1d) irradiation of the accretion disk by the central source suppresses this except at very low mass transfer rates. Formation constraints however imply that a significant fraction of these neutron star systems have nuclear-evolved main-sequence secondaries and thus mass transfer rates low enough to be transient. But most short-period low-mass black-hole systems will form with unevolved main-sequence companions and have much higher mass transfer rates. The fact that essentially all of them are nevertheless transient shows that irradiation is weaker, as a direct consequence of the fundamental black-hole property - the lack of a hard stellar surface.

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