Duncan K. Galloway

Thermonuclear (Type I) X-Ray Bursts Observed by the Rossi X-Ray Timing Explorer

We have assembled a sample of 1187 thermonuclear (type I) X-ray bursts from observations of 48 accreting neutron stars by the Rossi X-ray Timing Explorer, spanning more than 10 years. The sample contains examples of two of the three theoretical ignition regimes (confirmed via comparisons with numerical models) and likely examples of the third. We present a detailed analysis of the variation of the burst profiles, energetics, recurrence times, presence of photospheric radius expansion, and presence of burst oscillations, as a function of accretion rate. We estimated the distance for 35 sources exhibiting radius-expansion bursts, and found that the peak flux of such bursts varies typically by 13%. We classified sources into two main groups based on the burst properties: (1) both long and short bursts (indicating mixed H/He accretion), and (2) consistently short bursts (primarily He accretion), and we calculated the mean burst rate as a function of accretion rate for the two groups. The decrease in burst rate observed at > 0.06dot MEdd (~2 × 10^37 ergs s^−1) is associated with a transition in the persistent spectral state and (as has been suggested previously) may be related to the increasing role of steady He burning. We found many examples of bursts with recurrence times <30 minutes, including burst triplets and even quadruplets. We describe the oscillation amplitudes for 13 of the 16 burst oscillation sources, as well as the stages and properties of the bursts in which the oscillations are detected. The burst properties are correlated with the burst oscillation frequency; sources spinning at <400 Hz generally have consistently short bursts, while the more rapidly spinning systems have both long and short bursts. This correlation suggests either that shear-mediated mixing dominates the burst properties, or alternatively that the nature of the mass donor (and hence the evolutionary history) has an influence on the long-term spin evolution.

Nuclear-powered millisecond pulsars and the maximum spin frequency of neutron stars

Millisecond pulsars are neutron stars that are thought to have been spun-up by mass accretion from a stellar companion. It is not known whether there is a natural brake for this process, or if it continues until the centrifugal breakup limit is reached at submillisecond periods. Many neutron stars that are accreting mass from a companion star exhibit thermonuclear X-ray bursts that last tens of seconds, caused by unstable nuclear burning on their surfaces. Millisecond-period brightness oscillations during bursts from ten neutron stars (as distinct from other rapid X-ray variability that is also observed) are thought to measure the stellar spin, but direct proof of a rotational origin has been lacking. Here we report the detection of burst oscillations at the known spin frequency of an accreting millisecond pulsar, and we show that these oscillations always have the same rotational phase. This firmly establishes burst oscillations as nuclear-powered pulsations tracing the spin of accreting neutron stars, corroborating earlier evidence. The distribution of spin frequencies of the 11 nuclear-powered pulsars cuts off well below the breakup frequency for most neutron-star models, supporting theoretical predictions that gravitational radiation losses can limit accretion torques in spinning up millisecond pulsars.

DYNAMICS OF THE TURBIDITY MAXIMUM IN THE FLY RIVER ESTUARY, PAPUA-NEW-GUINEA

Field studies in the Fly River estuary, Papua New Guinea, show that the turbidity maximum exists only at spring tides. The wind is important in wave-driven fluidization of the bed. The erosion rate varies with the sixth power of the water velocity. The suspended sediment settling velocity varies nonlinearly with the concentration. At least three-quarters of the river sediment inflow appears to be trapped in the estuary. A numerical hydrodynamics-sediment transport model is able to reproduce a number of the key features of the turbidity maximu, and suggests that the turbidity maximum is due to the simultaneous influence of the baroclinic circulation and the tidal pumping.

Discovery of a High-Latitude Accreting Millisecond Pulsar in an Ultracompact Binary

We have identified the third known accretion-powered millisecond pulsar, XTE J0929-314, with the Rossi X-Ray Timing Explorer. The source is a faint, high Galactic latitude X-ray transient (d 5 kpc) that was in outburst during 2002 April-June. The 185 Hz (5.4 ms) pulsation had a fractional rms amplitude of 3%-7% and was generally broad and sinusoidal, although occasionally double-peaked. The hard X-ray pulses arrived up to 770 ?s earlier than the soft X-ray pulses. The pulsar was spinning down at an average rate of = (-9.2 ? 0.4) ? 10-14 Hz s-1; the spin-down torque may arise from magnetic coupling to the accretion disk, a magnetohydrodynamic wind, or gravitational radiation from the rapidly spinning pulsar. The pulsations were modulated by a 43.6 minute ultracompact binary orbit, yielding the smallest measured mass function (2.7 ? 10-7 M?) of any stellar binary. The binary parameters imply an 0.01 M? white dwarf donor and a moderately high inclination. We note that all three known accreting millisecond pulsars are X-ray transients in very close binaries with extremely low mass transfer rates. This is an important clue to the physics governing whether or not persistent millisecond pulsations are detected in low-mass X-ray binaries.

Helium-rich Thermonuclear Bursts and the Distance to the Accretion-powered Millisecond Pulsar SAX J1808.4–3658

We analyzed Rossi X-ray Timing Explorer observations of the accretion-powered 401 Hz pulsar SAX J1808.4-3658, in order to precisely determine the source distance. While the fluences for the five transient outbursts observed from 1996 were constant to within the uncertainties, the outburst interval varied significantly, so that the time-averaged flux (and accretion rate) decreased by around 40%. By equating the time-averaged X-ray flux with the expected mass transfer rate from gravitational radiation, we derived a lower limit on the distance of 3.4 kpc. Combined with an upper limit from assuming that the four radius-expansion thermonuclear bursts observed during the 2002 October outburst reached at most the Eddington limit for a pure He atmosphere, we found that the probable distance range for the source is 3.4-3.6 kpc. The implied inclination, based on the optical/IR properties of the counterpart, is i 30°. We compared the properties of the bursts with an ignition model. The time between bursts was long enough for hot CNO burning to significantly deplete the accreted hydrogen, so that ignition occurred in a pure helium layer underlying a stable hydrogen-burning shell. This is the first time that this burning regime has been securely observationally identified. The observed energetics of the bursts give a mean hydrogen fraction at ignition of X ≈ 0.1, and require that the accreted hydrogen fraction X0 and the CNO metallicity ZCNO are related by ZCNO ≈ 0.03(X0/0.7)2. We show that in this burning regime, a measurement of the burst recurrence time and energetics allows the local accretion rate onto the star to be determined independently of the accreted composition, giving a new method for estimating the source distance that is in good agreement with our other estimates.

Periodic Thermonuclear X-Ray Bursts from GS 1826–24 and the Fuel Composition as a Function of Accretion Rate

We analyze 24 type I X-ray bursts from GS 1826-24 observed by the Rossi X-Ray Timing Explorer between 1997 November and 2002 July. The bursts observed between 1997 and 1998 were consistent with a stable recurrence time of 5.74 ? 0.13 hr. The persistent intensity of GS 1826-24 increased by 36% between 1997 and 2000, by which time the burst interval had decreased to 4.10 ? 0.08 hr. In 2002 July the recurrence time was shorter again, at 3.56 ? 0.03 hr. The bursts within each epoch had remarkably identical light curves over the full ?150 s burst duration; both the initial decay timescale from the peak and the burst fluence increased slightly with the rise in persistent flux. The decrease in the burst recurrence time was proportional to -1.05?0.02 (assuming that is linearly proportional to the X-ray flux), so that the ratio ? between the integrated persistent and burst fluxes was inversely correlated with . The average value of ? was 41.7 ? 1.6. Both the ?-value and the long burst durations indicate that the hydrogen is burning during the burst via the rapid-proton (rp) process. The variation in ? with implies that hydrogen is burning stably between bursts, requiring solar metallicity (Z ~ 0.02) in the accreted layer. We show that solar metallicity ignition models naturally reproduce the observed burst energies but do not match the observed variations in recurrence time and burst fluence. Low-metallicity models (Z ~ 0.001) reproduce the observed trends in recurrence time and fluence but are ruled out by the variation in ?. We discuss possible explanations, including extra heating between bursts or that the fraction of the neutron star covered by the accreted fuel increases with .

Discovery of the Accretion-powered Millisecond X-Ray Pulsar IGR J00291+5934

We report on observations of the sixth accretion-powered millisecond pulsar, IGR J00291+5934, with the Rossi X-Ray Timing Explorer. The source is a faint recurrent X-ray transient initially identified by the International Gamma-Ray Astrophysics Laboratory. The 599 Hz (1.67 ms) pulsation had a fractional rms amplitude of 8% in the 2-20 keV range, and its shape was approximately sinusoidal. The pulses show an energy-dependent phase delay, with the 6-9 keV pulses arriving up to 85 μs earlier than those at lower energies. No X-ray bursts, dips, or eclipses were detected. The neutron star is in a circular 2.46 hr orbit with a very low-mass donor, most likely a brown dwarf. The binary parameters of the system are similar to those of the first known accreting millisecond pulsar, SAX J1808.4-3658. Assuming that the mass transfer is driven by gravitational radiation and that the 2004 outburst fluence is typical, the 3 yr recurrence time implies a distance of at least 4 kpc.

Optical and X-ray observations of the neutron star soft X-ray transient XTE J1709-267

In this paper we report on the discovery of the optical counterpart to the neutron star soft X-ray transient (SXT) XTE J1709-267 at an R-band magnitude of R = 20.5 ± 0.1 and 22.24 ± 0.03, in outburst and quiescence, respectively. We further report the detection of type I X-ray bursts in RXTE data obtained during an outburst of the source in 2002. These bursts show a precursor before the onset of the main burst event, reminiscent of photospheric radius expansion bursts. Sifting through the archival RXTE data for the burster 4U 1636-53, we found a nearly identical burst with precursor in 4U 1636-53. A comparison of this burst to true photospheric radius expansion bursts in 4U 1636-53 leads us to conclude that these bursts-with-precursor do not reach the Eddington limit. Nevertheless, from the burst properties we can derive that the distance to XTE J1709-267 is consistent with the distance of the Globular Cluster NGC 6293. We further report on the analysis of a 22.7 ks observation of XTE J1709-267 obtained with the Chandra satellite when the source was in quiescence. We found that the source has a soft quiescent spectrum which can be fit well by an absorbed black body or neutron star atmosphere model. A power law contributes less than ∼20 per cent to the 0.5-10 keV unabsorbed flux of (1.0 ± 0.3) × 10 -13 erg cm -2 s -1 . This flux is only slightly lower than the flux measured right after the outburst in 2002. This is in contrast to the recent findings for MXB 1659-29, where the quiescent source flux decreased gradually by a factor of ∼7-9 over a period of 18 months. Finally, we compared the fractional power-law contribution to the unabsorbed 0.5-10 keV luminosity for neutron star SXTs in quiescence for which the distance is well-known. We find that the power-law contribution is low only when the source quiescent luminosity is close to ∼1-2 x 10 33 erg s -1 . Both at higher and lower values the power-law contribution to the 0.5-10 keV luminosity increases. We discuss how models for the quiescent X-ray emission can explain these trends.

The Frequency Stability of Millisecond Oscillations in Thermonuclear X-Ray Bursts

We analyze the frequency evolution of millisecond oscillations observed during type I X-ray bursts with the Rossi X-Ray Timing Explorer, in order to establish the stability of the mechanism underlying the oscillations. Our sample contains 68 pulse trains detected in a search of 159 bursts from eight accreting neutron stars. As a first step, we confirm that the oscillations usually drift upward in frequency by about 1% toward an apparent saturation frequency. Previously noted anomalies, such as drifts toward lower frequencies as the oscillations disappear (spin-down episodes) and instances of two signals present simultaneously at frequencies separated by a few Hz, occur in 5% of oscillations. Having verified the generally accepted description of burst oscillations, we proceed to study the coherence of the oscillations during individual bursts and the dispersion in the asymptotic frequencies in bursts observed over five years. On short timescales, we find that 30% of the oscillation trains do not appear to evolve smoothly in phase. This suggests either that two signals are present simultaneously with a frequency difference too small to resolve (1 Hz), that the frequency evolution is discontinuous, or that discrete phase jumps occur. On timescales of years, the maximum frequencies of the oscillations exhibit fractional dispersions of Δνmax/ 4 × 10-3. In the case of 4U 1636-536, this dispersion is uncorrelated with the known orbital phase, which indicates that a mechanism besides orbital Doppler shifts prevents the oscillations from appearing perfectly stable. In the course of this analysis, we also search for connections between the properties of the oscillations and the underlying bursts. We find that the magnitudes of the observed frequency drifts are largest when the oscillations are first observed at the start of the burst, which suggests that their evolution begins when the burst is ignited. We also find that radius expansion appears to temporarily interrupt the oscillation trains. We interpret these results under the assumption that the oscillations originate from anisotropies in the emission from the surfaces of these rotating neutron stars.

The effect of accretion on the measurement of neutron star mass and radius in the low-mass X-ray binary 4U 1608−52

Spectral measurements of thermonuclear (type-I) X-ray bursts from low mass X-ray binaries have been used to measure neutron star (NS) masses and radii. A number of systematic issues affect such measurements and have raised concerns as to the robustness of the methods. We present analysis of the X-ray emission from bursts observed from 4U 1608–52 at various persistent fluxes. We find a strong dependence of the burst properties on the flux and spectral hardness of the persistent emission before burst. Bursts occurring during the low-accretion rate (hard) state exhibit evolution of the black body normalisation consistent with the theoretical predictions of NS atmosphere models. However, bursts occurring during the high-accretion rate (soft) state show roughly constant normalisation, which is inconsistent with the NS atmosphere models and therefore these bursts cannot be easily used to determine NS parameters. We analyse the hard-state burst to put the lower limit on the neutron star radius in 4U 1608–52 of 13 km (for masses 1.2–2.4 M⊙). The best agreement with the theoretical NS mass-radius relations is achieved for source distances in the range 3.1–3.7 kpc. We expect that the radius limit will be 10 per cent lower if spectral models including rapid rotation are used instead.