My primary area of research interest is the nature and properties of Galactic neutron star binaries. A neutron star is the extreme product of a supernova explosion; the surface density, temperature, and magnetic field strength are all many orders of magnitude in excess of anything achievable on Earth. Accreting binaries, in which gas from a normal stellar companion falls under gravity onto a neutron star, offer a unique window on some rich physics via satellite-based X-ray observations. X-ray studies of accreting neutron stars have only been possible for a few decades, and there remain many outstanding questions about the neutron star's magnetic field geometry, surface thermonuclear processes, internal structure, and the angular momentum balance. Accreting binaries can be studied using a range of techniques:
Persistently pulsing neutron stars in X-ray binaries are found with both low-mass and high-mass mass donors, although the growing class of millisecond accreting pulsars have only low-mass companions. These rapidly-rotating (190-600 Hz) sources are the immediate progenitors of "recycled" millisecond radio pulsars, although the details of the evolutionary path remain unclear. The observation that 3 of the five known examples are in ``ultracompact'' binaries (Porb~40 min) with highly evolved companions is a significant clue. Our followup RXTE observations of two of the five sources discovered in the last 3 years, XTE J0929-314 (P orb=43.6 min), and IGR J00291+5934 (P orb=2.46 hr) allowed us to precisely determine the orbital parameters and place limits on the distances and companion masses. Optical and high-resolution X-ray spectroscopic followup observations of these sources can also help to determine the system properties.
These sources are of added interest due to the possibility that they emit detectable levels of gravitational radiation. Low-mass X-ray binaries are thought to be old systems, with a long history of spin-up via accretion. The majority of the 16 presently known accreting neutron star spins come from the coherent oscillations (observed only during thermonuclear X-ray bursts) in about 12 of the 80 known burst sources. A Bayesian analysis of the combined distribution of neutron star spin frequencies suggests that some process (possibly gravitational radiation) serves to limit the maximum spin frequency of neutron stars in low-mass binaries to a frequency well below the likely breakup speed. It is expected that gravitational waves (GW) will be detected for the first time by long-baseline interferometers (e.g. LIGO) within the next 5-10 years; neutron stars in accreting binaries are the only persistent GW source candidates with precisely known kHz frequencies. An ongoing program exists to detect and characterize new examples of such rapidly-rotating neutron stars, in addition to completing the catalogue of known sources by determining their orbital periods (essential if optimal GW sensitivity is to be achieved).
Long-period (P=0.1-10000 s) pulsars represent a much larger known population with more than 100 examples, but are nevertheless a fertile area for study. These sources are typically transient, and are observeable generally only in outburst. X-ray observations of new transient outbursts can determine the spin and orbital properties, as well as (through broadband X-ray spectroscopy) the magnetic field strength. Such observations can provide surprises, also. In an X-ray study of a recent outburst of the 18.7 s pulsar KS 1947+30, we detected evidence for a "glitch" analogous to those events observed in young radio pulsars, with fractional change in frequency of +4x10-5. Glitches have also been detected in so-called ``anomalous'' X-ray pulsars, leading to the unifying indications that they are a general phenomenon in neutron stars subjected to external torques.
Thermonuclear (type-I) bursts are observed in the majority of accreting neutron-star binaries with low magnetic fields, including two of the six accreting millisecond pulsars known at present. These bursts are caused by unstable thermonuclear ignition of accumulated H and He fuel, and manifest as a rapid (~1 s) rise in the observed X-ray intensity by a factor of 5-20, followed by an approximately exponential decay as the heated atmosphere cools. Studies of these bursts can provide details of the nuclear reactions taking place under conditions (g~1014 cm s-2, B~1012 G) which cannot be duplicated on Earth. Furthermore, they can also in principle allow measurements of fundamental properties of the neutron stars, such as the mass and radius, and thus provide constraints on the (theoretically rather uncertain) equation of state.
An alternative avenue for such constraints is the detection of discrete emission features from the surface of the star, presently a priority for neutron-star studies. Such detections can allow precise measurement of the gravitational redshift and hence the compactness of the neutron star. Unfortunately, most accreting neutron stars have intrinsically featureless spectra, although the studies to date have not exhausted the possibility of detections near the peak of thermonuclear bursts. Any discrete features that are present are generally attributable instead to neutral absorption edges from circumstellar or interstellar material. Such features can still provide detailed information about the source environment, for example the O emission lines and neutral absorption edge observed by XMM-Newton in the long-period symbiotic binary 4U 17009+24. Emission lines can also arise from the accretion disk or related outflows, as in the case of Cir X-1. This source exhibits a complex behaviour with line profiles and neutral absorption which vary significantly on the supposed 16.6 d orbital period. High-resolution X-ray spectroscopy provides a powerful probe of the environments of these sources, as does X-ray imaging, which can characterise the outflows present in some systems including Cir X-1. Jets are also present in accreting white dwarfs; a Chandra observation of the white dwarf binary CH Cygni revealed extended X-ray emission arising from the ~1000 km/s outflow seen previously only in the radio and optical wavebands. This is only the second X-ray jet detected in a white dwarf binary.
Finally, observations of optical counterparts of Galactic X-ray sources provide important information which is often not available from X-ray observations alone. The majority of these sources, unfortunately, lie towards the Galactic center, where absorption frequently makes the counterparts extremely faint in the optical band. Infra-red observations are in many cases the only way to detect and measure the properties of the counterparts, and so plays an important role for such studies. Spectroscopy also offers a wealth of information regarding the environment of X-ray binaries, through the observation of emission and absorption lines arising from the neutron star environment.
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