arXiv subjects

A. R. King

Publications and source records attributed to A. R. King.

At least 91 records · Page 5Linked to original sources

arXiv2003

Black Widow Pulsars: the Price of Promiscuity

The incidence of evaporating 'black widow' pulsars (BWPs) among all millisecond pulsars (MSPs) is far higher in globular clusters than in the field. This implies a special formation mechanism for them in clusters. Cluster MSPs in wide binaries with WD companions exchange them for turnoff-mass stars. These new companions eventually overflow their Roche lobes because of encounters and tides. The millisecond pulsars eject the overflowing gas from the binary, giving mass loss on the binary evolution timescale. The systems are only observable as BWPs at epochs where this evolution is slow, making the mass loss transparent and the lifetime long. This explains why observed BWPs have low-mass companions. We suggest that at least some field BWPs were ejected from globular clusters or entered the field population when the cluster itself was disrupted.

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arXiv2003

A Variable Ultraluminous Supersoft X-ray Source in "The Antennae": Stellar-Mass Black Hole or White Dwarf?

The chandra monitoring observations of The Antennae (NGC 4038/39) have led to the discovery of a variable, luminous, supersoft source (SSS). This source is only detected at energies below 2 keV and, in 2002 May, reached count rates comparable to those of the nine ultraluminous X-ray sources (ULXs) detected in these galaxies. Spectral fits of the SSS data give acceptable results only for a $\sim$100--90 eV blackbody spectrum with an intrinsic absorption column of $N_{\rm H} \sim 2-3 \times 10^{21} \rm cm^{-2}$. For a distance of 19 Mpc, the best-fit observed luminosity increases from 1.7$\times 10^{38}$\ergs in 1999 December to 8.0$\times 10^{38}$\ergs in 2002 May. The intrinsic, absorption-corrected best-fit luminosity reaches 1.4$\times 10^{40}$ \ergs in 2002 May. The assumption of unbeamed emission would suggest a black hole of $\ga$100\msun. However, if the emission is blackbody at all times, as suggested by the steep soft spectrum, the radiating area would have to vary by a factor of $\sim10^3$, inconsistent with gravitational energy release from within a few Schwarzschild radii of a black hole. Viable explanations for the observed properties of the SSS are provided by anisotropic emission from either an accreting nuclear-burning white dwarf or an accreting stellar-mass black hole.

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arXiv2003

The Disk-Jet Connection in Microquasars and AGN

We propose a new model for the disk-jet connection in black hole x-ray transients and active galactic nuclei. In our model, the inner part of the accretion disk around the central black hole switches between two states. In one state, the accretion energy is dissipated locally (some within the disk, some within a corona) to produce the observed disk luminosity. In the second state, the accretion energy is deposited into the bulk flow of a relativistic jet. We associate the transition between the two states with the generation of a global, poloidal magnetic field. We show that this model can explain the observed behavior of black hole accretors.

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arXiv2003

A New Evolutionary Channel for Type Ia Supernovae

We show that long-period dwarf novae offer a promising route for making Type Ia supernovae. For typical dwarf nova duty cycles d ~ 0.1 - 0.01, mass is accreted by the white dwarf mainly during dwarf nova outbursts at rates allowing steady nuclear burning of most of the accreted matter. Mass gains up to ~ 0.4 Msun are possible in this way. Although these are too small to allow a 0.7 Msun WD to reach the Chandrasekhar mass, they are sufficient if the WD grew to ~ 1 Msun in a previous episode of thermal-timescale mass transfer, i.e. for those long-period dwarf novae which descend from supersoft binaries. A further advantage of this picture is that the supernova always occurs in a binary of small secondary/primary mass ratio, with the secondary having very little remaining hydrogen. Both features greatly reduce the possibility of hydrogen contamination of the supernova ejecta.

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arXiv2003

Resolved Jets and Long-Period Black Hole X-ray Novae

In this brief note we point out that the four spatially resolved relativistic jets among the 14 dynamically confirmed black hole X-ray novae are all in systems with long orbital periods. Many shorter period systems show transient radio outbursts which are attributed to jets, but these jets have not been spatially resolved. Super-Eddington accretion has been suggested as a requirement for jet formation and may be consistent with our compilation of luminosities, but some super-Eddington outbursts did not form spatially resolved jets. We speculate that some as yet unknown process (or combination of processes) favors formation of substantially larger jets in long period systems. Two short period systems show evidence for extent, but have not been resolved into multiple components as the long period systems have.

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arXiv2002

The Time-Variable Ultra-Luminous X-ray Sources of "The Antennae"

We report the first results of the Chandra temporal monitoring of the ultra-luminous X-ray sources (ULXs) in the Antennae galaxies (NGC 4038/39). Observations at four different epochs, covering time scales of 2 years to 2 months, show variability in seven out of nine ULXs, confirming that they are likely to be accreting compact X-ray binaries (XRBs). The seven variable ULXs exhibit a variety of temporal and spectral behaviors: one has harder X-ray colors with decreasing luminosity, similar to the black hole binary Cyg X-1, but four other ULXs show the opposite behavior. We suggest that the latter may be black-hole binaries accreting at very high rates.

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arXiv2002

AE Aquarii: how CVs descend from supersoft binaries

AE Aquarii is a propeller system. It has the shortest spin period among cataclysmic variables, and this is increasing on a 10^7 yr timescale. Its UV spectrum shows very strong carbon depletion vs nitrogen and its secondary mass indicates a star far from the zero-age main sequence. We show that these properties strongly suggest that AE Aqr has descended from a supersoft X-ray binary. We calculate the evolution of systems descending through this channel, and show that many of them end as AM CVn systems. The short spindown timescale of AE Aqr requires a high birthrate for such systems, implying that a substantial fraction of cataclysmic variables must have formed in this way. A simple estimate suggests that this fraction could be of order one-third of current CVs. We emphasize the importance of measurements of the C/N abundance ratio in CVs, particularly via the CIV1550 / NV1238 ratio, in determining how large the observed fraction is.

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arXiv2002

Black Hole Blackbodies

Many black-hole sources emit a substantial fraction of their luminosities in blackbody-like spectral components. It is usual to assume that these are produced in regions at least comparable in size to the hole's Schwarzschild radius, so that a measure of the emitting area provides an estimate of the black hole mass M. However there is then no guarantee that the source luminosity (if isotropic) obeys the Eddington limit corresponding to M. We show that the apparent blackbody luminosity L_sph and temperature T must obey the inequality L_sph < 2.3 10^44 (T/100 eV)^-4 erg s^-1 for this to hold. Sources violating this limit include ultrasoft AGN and some of the ultraluminous X-ray sources (ULXs) observed in nearby galaxies. We discuss the possible consequences of this result, which imply either super-Eddington or anisotropic emission in both cases. We suggest that the ultrasoft AGN are the AGN analogues of the ULXs.

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arXiv2002

The Brightest Black Holes

I suggest that there are two classes of ultraluminous X-ray sources (ULXs), corresponding to super-Eddington mass inflow in two situations: (a) thermal-timescale mass transfer in high-mass X-ray binaries, and (b) long-lasting transient outbursts in low-mass X-ray binaries. These two classes are exemplified by SS433 and microquasars like GRS 1915+105 respectively. The observed ULX population is a varying mixture of the two, depending on the star formation history of the host galaxy. ULXs in galaxies with vigorous star formation (such as the Antennae) are generally SS433--like, while ULXs in elliptical galaxies must be of the microquasar type. The latter probably have significantly anisotropic radiation patterns. They should also be variable, but demonstrating this may require observations over decades. The close analogy between models of X-ray binaries and active galactic nuclei (AGN) suggests that there should exist an apparently super-Eddington class of the latter, which may be the ultrasoft AGN, and a set of X-ray binaries with Doppler--boosted X-ray emission. These are presumably a subset of the ULXs, but remain as yet unidentified.

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arXiv2002

Blunting the Spike: the CV Minimum Period

The standard picture of CV secular evolution predicts a spike in the CV distribution near the observed short-period cutoff P_0 ~ 78 min, which is not observed. We show that an intrinsic spread in minimum (`bounce') periods P_b resulting from a genuine difference in some parameter controlling the evolution can remove the spike without smearing the sharpness of the cutoff. The most probable second parameter is different admixtures of magnetic stellar wind braking (at up to 5 times the GR rate) in a small tail of systems, perhaps implying that the donor magnetic field strength at formation is a second parameter specifying CV evolution. We suggest that magnetic braking resumes below the gap with a wide range, being well below the GR rate in most CVs, but significantly above it in a small tail.

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arXiv2002

The UV line spectrum of the soft X-ray transient XTE J1118+480: a CNO processed core exposed

We compare UV spectra of the recent soft X-ray transients XTE J1118+480 and XTE J1859+226. The emission line strengths in XTE J1118+480 strongly suggest that the accreting material has been CNO processed. We show that this system must have come into contact with a secondary star of about 1.5 solar masses, and an orbital period ~15hr, very close to the bifurcation value at which nuclear and angular momentum loss timescales are similar. Subsequent evolution to the current period of 4.1 hr was driven by angular momentum loss. In passing through a period of 7.75 hr the secondary star would have shown essentially normal surface abundances. XTE J1118+480 could thus represent a slightly later evolutionary stage of A0620-00. We briefly discuss the broad Ly alpha absorption wings in XTE J1118+480.

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arXiv2002

A source of high-velocity white dwarfs

We investigate whether the recently-observed population of high-velocity white dwarfs can be derived from a population of binaries residing initially within the thin disk of the Galaxy. In particular we consider binaries where the primary is sufficiently massive to explode as a type II supernova. A large fraction of such binaries are broken up when the primary then explodes as a supernova owing to the combined effects of the mass loss from the primary and the kick received by the neutron star on its formation. For binaries where the primary evolves to fill its Roche lobe, mass transfer from the primary leads to the onset of a common envelope phase during which the secondary and the core of the primary spiral together as the envelope is ejected. Such binaries are the progenitors of X-ray binaries if they are not broken up when the primary explodes. For those systems which are broken up, a large number of the secondaries receive kick velocities ~100 - 200 km/s and subsequently evolve into white dwarfs. We compute trajectories within the Galactic potential for this population of stars and relate the birthrate of these stars over the entire Galaxy to those seen locally with high velocities relative to the LSR. We show that for a reasonable set of assumptions concerning the Galactic supernova rate and the binary population, our model produces a local number density of high-velocity white dwarfs compatible with that inferred from observations. We therefore propose that a population of white dwarfs originating in the thin disk may make a significant contribution to the observed population of high-velocity white dwarfs.

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arXiv2001

The Short Period Supersoft Source in M31

We show that the recently discovered short period supersoft source in M31 is probably a progenitor of a magnetic CV. The white dwarf spins asynchronously because of the current high accretion rate. However its fieldstrength is typical of an AM Herculis system, which is what it will ultimately become. We discuss the relevance of this system to CV evolution, and its relation to some particular CVs with special characteristics.

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arXiv2001

Disc Instabilities in Soft X-ray Transients

I briefly review the theory of soft X-ray transient systems. Irradiation of the accretion disc faces by the central X-ray source determines both the occurrence and the nature of the outbursts, in particular forcing these to be long viscous events and producing exponential decays at short orbital periods. Soft X-ray transients constitute the majority of LMXBs, persistent systems being largely confined to a subset of neutron-star LMXBs with periods P < 2 d. It appears that LMXBs very frequently contain nuclear-evolved companions, even at short orbital periods. In long-period transients (P > 20 d) the outburst recurrence times must become extremely long (> 1500 yr). The outbursts are highly super-Eddington, markedly reducing the accretion efficiency. Spinup of a neutron-star primary to millisecond periods probably cannot occur for orbital periods > 200 d, in agreement with observations of binary pulsars.

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arXiv2001

Ultraluminous X-Ray Sources in External Galaxies

We investigate models for the class of ultraluminous non-nuclear X-ray sources (ULXs) seen in a number of galaxies and probably associated with star-forming regions. Models where the X-ray emission is assumed to be isotropic run into several difficulties. In particular formation of sufficient numbers of the required ultramassive black-hole X-ray binaries is problematic, and the likely transient behaviour of the resulting systems is not in good accord with observation. The assumption of mild X-ray beaming suggests instead that ULXs may represent a shortlived but extremely common stage in the evolution of a wide class of X-ray binaries. The best candidate for this is the phase of thermal-timescale mass transfer inevitable in many intermediate and high-mass X-ray binaries. This in turn suggests a link with the Galactic microquasars. The short lifetimes of high-mass X-ray binaries would explain the association of ULXs with episodes of star formation. These considerations still allow the possibility that individual ULXs may contain extremely massive black holes.

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arXiv2001

Multiwavelength observations revealing the outbursts of the two soft X-ray transients XTE J1859+226 and XTE J1118+480

We report here on multiwavelength observations of the two new soft X-ray transients (SXTs) XTE J1859+226 and XTE J1118+480, which we observed with HST/RXTE/UKIRT. For XTE J1118+480 we also used EUVE since it is located at a very high galactic latitude and suffers from very low extinction. The two sources exhibited very different behaviour. XTE J1859+226 seems quite normal and therefore a good object for testing the accretion mechanisms in place during the outbursts, XTE J1118+480 is much more unusual because it exhibits i) a low X-ray to optical ratio and ii) a strong non-thermal contribution in the radio to optical domain, which is likely to be due to synchrotron emission. We concentrate here on the near-infrared (NIR) and optical observations of these two systems.

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arXiv2000

Assisted stellar suicide: the wind-driven evolution of the recurrent nova T Pyxidis

We show that the extremely high luminosity of the short-period recurrent nova T Pyx in quiescence can be understood if this system is a wind-driven supersoft x-ray source (SSS). In this scenario, a strong, radiation-induced wind is excited from the secondary star and accelerates the binary evolution. The accretion rate is therefore much higher than in an ordinary cataclysmic binary at the same orbital period, as is the luminosity of the white dwarf primary. In the steady state, the enhanced luminosity is just sufficient to maintain the wind from the secondary. The accretion rate and luminosity predicted by the wind-driven model for T Pyx are in good agreement with the observational evidence. X-ray observations with Chandra or XMM may be able to confirm T Pyx's status as a SSS. T Pyx's lifetime in the wind-driven state is on the order of a million years. Its ultimate fate is not certain, but the system may very well end up destroying itself, either via the complete evaporation of the secondary star, or in a Type Ia supernova if the white dwarf reaches the Chandrasekhar limit. Thus either the primary, the secondary, or both may currently be committing assisted stellar suicide.

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arXiv2000

Type Ia Supernovae and Remnant Neutron Stars

On the basis of the current observational evidence, we put forward the case that the merger of two CO white dwarfs produces both a Type Ia supernova explosion and a stellar remnant, the latter in the form of a magnetar. The estimated occurrence rates raise the possibility that many, if not most, SNe Ia might result from white dwarf mergers.

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