Rudy Wijnands

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.

Chandra/high energy transmission grating spectrometer spectroscopy of the galactic black hole GX 339-4: A relativistic iron emission line and evidence for a Seyfert-like warm absorber

We observed the Galactic black hole GX 339-4 with the Chandra High Energy Transmission Grating Spectrometer (HETGS) for 75 ks during the decline of its 2002-2003 outburst. The sensitivity of this observation provides an unprecedented glimpse of a Galactic black hole at about a tenth of the luminosity of the outburst peak. The continuum spectrum is well described by a model consisting of multicolor disk blackbody (kT~=0.6 keV) and power-law (Gamma~=2.5) components. X-ray reflection models yield improved fits. A strong, relativistic Fe Kalpha emission line is revealed, indicating that the inner disk extends to the innermost stable circular orbit. The breadth of the line is sufficient to suggest that GX 339-4 may harbor a black hole with significant angular momentum. Absorption lines from H- and He-like O and He-like Ne and Mg are detected, as well as lines that are likely due to Ne II and Ne III. The measured line properties make it difficult to associate the absorption with the coronal phase of the interstellar medium. A scenario wherein the absorption lines are due to an intrinsic AGN-like warm-absorber geometry-perhaps produced by a disk wind in an extended disk-dominated state-may be more viable. We compare our results to Chandra observations of the Galactic black hole candidate XTE J1650-500 and discuss our findings in terms of prominent models for Galactic black hole accretion flows and connections to supermassive black holes.

The hard quiescent spectrum of the neutron star X-ray transient exo 1745-248 in the globular cluster terzan 5

We present a Chandra observation of the globular cluster Terzan 5 during times when the neutron star X-ray transient EXO 1745-248 located in this cluster was in its quiescent state. We detected the quiescent system with a (0.5-10 keV) luminosity of ~2 ? 1033 ergs s-1. This is similar to several other neutron-star transients observed in their quiescent states. However, the quiescent X-ray spectrum of EXO 1745-248 was dominated by a hard power-law component instead of the soft component that usually dominates the quiescent emission of other neutron star X-ray transients. This soft component could not conclusively be detected in EXO 1745-248, and we conclude that it contributed at most 10% of the quiescent flux in the energy range 0.5-10 keV. EXO 1745-248 is only the second known neutron-star transient whose quiescent spectrum is dominated by the hard component (SAX J1808.4-3658 is the other one). We discuss possible explanations for this unusual behavior of EXO 1745-248, its relationship to other quiescent neutron-star systems, and the impact of our results on understanding quiescent X-ray binaries. We also discuss the implications of our results on the way that the low-luminosity X-ray sources in globular clusters are classified.

High-Frequency Quasi-Periodic Oscillations in the Black Hole X-Ray Transient XTE J1650-500

We report the detection of high-frequency variability in the black hole X-ray transient XTE J1650-500. A quasi-periodic oscillation (QPO) was found at 250 Hz during a transition from the hard to the soft state. We also detected less coherent variability around 50 Hz that disappeared when the 250 Hz QPO showed up. There are indications that when the energy spectrum hardened the QPO frequency increased from ~110 to ~270 Hz, although the observed frequencies are also consistent with being 1 : 2 : 3 harmonics of each other. Interpreting the 250 Hz as the orbital frequency at the innermost stable orbit around a Schwarzschild black hole leads to a mass estimate of 8.2 M☉. The spectral results by Miller et al., which suggest considerable black hole spin, would imply a higher mass.

Monitoring Chandra Observations of the Quasi-persistent Neutron Star X-Ray Transient MXB 1659-29 in Quiescence: The Cooling Curve of the Heated Neutron Star Crust

We have observed the quasi-persistent neutron star X-ray transient and eclipsing binary MXB 1659-29 in quiescence on three occasions with Chandra. The purpose of our observations was to monitor the quiescent behavior of the source after its last prolonged (~2.5 yr) outburst that ended in 2001 September. The X-ray spectra of the source are consistent with thermal radiation from the neutron star surface. We found that the bolometric flux of the source decreased by a factor of 7-9 over the timespan of 1.5 yr between our first and last Chandra observations. The effective temperature also decreased, by a factor of 1.6-1.7. The decrease in time of the bolometric flux and effective temperature can be described using exponential decay functions, with e-folding times of ~0.7 and ~3 yr, respectively. Our results are consistent with the hypothesis that we observed a cooling neutron star crust that was heated considerably during the prolonged accretion event and that is still out of thermal equilibrium with the neutron star core. We could only determine upper limits for any luminosity contribution because of the thermal state of the neutron star core. The rapid cooling of the neutron star crust implies that it has a large thermal conductivity. Our results also suggest that enhanced cooling processes are present in the neutron star core.

Simultaneous radio and X-ray observations of the low-mass X-ray binary GX 13+1

We present the results of two simultaneous X-ray/radio observations of the low-mass X-ray binary GX 13+1, per- formed in July/ August 1999 with the Rossi X-ray Timing Explorer and the Very Large Array. In X-rays the source was observed in two distinct spectral states; a soft state, which had a corresponding 6 cm flux density of ∼0.25 mJy, and a hard state, which was much brighter at 1.3-7.2 mJy. For the radio bright observation we measured a delay between changes in the X-ray spectral hardness and the radio brightness of ∼40 min, similar to what has been found in the micro-quasar GRS 1915+105. We compare our results with those of GRS 1915+105 and the atoll/Z-type neutron star X-ray binaries. Although it has some properties that do not match with either atoll or Z sources, GX 13+1 seems more similar to the Z sources.

Chandra observations of the neutron star soft X-ray transient RX J170930.2 − 263927 returning to quiescence

We present our analysis of Chandra observations obtained when the soft X-ray transient RX J170930.2 - 263927 (XTE J1709 - 267) returned to quiescence after an outburst. Using the type I burst peak luminosity found by Cocchi et al. and the value of NH we derived from our spectral fits, the distance to RX J170930.2 - 263927 can be constrained to 2.5-10 kpc. RX J170930.2 - 263927 is probably associated with the low-metallicity globular cluster NGC 6293, which has a tidal radius of 14.2 arcmin, as the projected distance to the centre of the cluster is approximately 25 pc (9-10 arcmin). If the association is correct, RX J170930.2 - 263927 would be at ~8.5 kpc. We determined that Loutburst/Lquiescence>~ 105 for this outburst. If the quiescent luminosity is caused by cooling of the neutron star core then enhanced core cooling processes were at work, implying a neutron star mass of ~1.7-1.8 Msolar. Combining our Chandra observations with archival ROSAT observations we show that the source very probably exhibits periods of sustained low-level accretion. Variable, low-level activity could provide an alternative explanation for some of the sources in the recently proposed category of faint soft X-ray transients. We found excess emission at ~0.6 keV. If such an excess is a unique feature of ultracompact systems, as was recently proposed, RX J170930.2 - 263927 must have a short orbital period as well. From the constraints on the distance and the non-detection of the optical counterpart with mV <20.5, we conclude that this system must have a low-mass companion.

The faint neutron star soft X-ray transient SAX J1810.8-2609 in quiescence

We present the analysis of a 35-ksec-long Chandra observation of the neutron star soft X-ray transient (SXT) SAX J1810.8-2609. We detect three sources in the field of view. The position of one of them is consistent with the location of the ROSAT error circle of SAX J1810.8-2609. The accurate Chandra position of that source coincides with the position of the proposed optical counterpart, strengthening the identification as the counterpart. We detected the neutron star SXT system in quiescence at an unabsorbed luminosity of similar to1 x 10(32) erg s(-1) (assuming a distance of 4.9 kpc). This luminosity is at the low end of quiescent luminosities found in other neutron star SXTs. This renders support to the existence of a group of faint soft X-ray transients of which the accreting millisecond X-ray pulsar SAX J1808.4-3658 is the most prominent member. The quiescent spectrum of SAX J1810.8-2609 is well-fitted with an absorbed power law with a photon index of 3.3 +/- 0.5. With a value of 3.3 x 10 21 cm 2, the galactic absorption is consistent with the value derived in outburst. Because the spectra of quiescent neutron star SXTs are often fitted with an absorbed blackbody or neutron star atmosphere plus power-law model, we also fitted the spectrum using those fitting functions. Both models provide a good fit to the data. If cooling of the neutron star core and/or crust is responsible for the soft part of the spectrum, the time-averaged mass accretion rate must have been very low (similar to5.7 x 10(13) M-circle dot yr(-1); assuming standard core cooling only) or the neutron star must be massive. We also discuss the possibility that the thermal spectral component in neutron stars in quiescence is produced by residual accretion.

Chandra Detections of Two Quiescent Black Hole X-Ray Transients

Using the Chandra X-Ray Observatory, we have detected the black hole transients V4641 Sgr and XTE J1859+226 in their low-luminosity quiescent states. The 0.3-8 keV luminosities are (4.0) × 1031 (d/7 kpc)2 ergs s-1 and (4.2) × 1031 (d/11 kpc)2 ergs s-1 for V4641 Sgr and XTE J1859+226, respectively. With the addition of these two systems, 14 out of the 15 transients with confirmed black holes (via compact object mass measurements) now have measured quiescent luminosities or sensitive upper limits. The only exception is GRS 1915+105, which has not been in quiescence since its discovery in 1992. The luminosities for V4641 Sgr and XTE J1859+226 are consistent with the median luminosity of 2 × 1031 ergs s-1 for the systems with previous detections. Our analysis suggests that the quiescent X-ray spectrum of V4641 Sgr is harder than for the other systems in this group, but because of the low statistical quality of the spectrum, it is not clear if V4641 Sgr is intrinsically hard or if the column density is higher than the interstellar value. Focusing on V4641 Sgr, we compare our results to theoretical models for X-ray emission from black holes in quiescence. Also, we obtain precise X-ray positions for V4641 Sgr and XTE J1859+226 via cross-correlation of the X-ray sources detected near our targets with IR sources in the Two Micron All Sky Survey catalog.

The aperiodic timing behaviour of the accretion-driven millisecond pulsar SAX J1808.4-3658

Abstract We studied the aperiodic X-ray timing behaviour of the accreting millisecond pulsar SAX J1808.4–3658. The source was recently found [Nature 424 (2003) 44] to be the first accreting millisecond pulsar that shows the kilohertz quasi-periodic oscillations (kilohertz QPOs) that are found in many other X-ray binaries with accreting neutron stars. The high frequency of these signals reflects the short dynamical time scales in the region near the compact object where they originate. We find that in addition to the kilohertz QPOs SAX J1808.4–3658 shows several low frequency timing features, based on which the source can be classified as a so–called atoll source. The frequencies of the variability components of the atoll sources follow a universal scheme of correlations. The correlations in SAX J1808.4–3658 are similar but show a shift in upper kilohertz QPO frequency. This discrepancy is perhaps related to a stronger or differently configured magnetic field.