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Register a new userThe Amplitude Evolution and Harmonic Content of Millisecond Oscillations in Thermonuclear X-ray Bursts
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We present a comprehensive observational and theoretical analysis of the amplitudes and profiles of oscillations that occur during thermonuclear X-ray bursts from weakly-magnetized neutron stars in low mass X-ray binaries. Our sample contains 59 oscillations from six sources observed with the Rossi X-ray Timing Explorer. The oscillations that we examined occurred primarily during the decaying portions of bursts, and lasted for several seconds each. We find that the oscillations are as large as 15% during the declines of the bursts, and they appear and disappear due to intrinsic variations in their fractional amplitudes. However, the maxima in the amplitudes are not related to the underlying flux in the burst. We derive folded profiles for each oscillation train to study the pulse morphologies. The mean rms amplitudes of the oscillations are 5%, although the eclipsing source MXB 1659-298 routinely produces 10% oscillations in weak bursts. We also produce combined profiles from all of the oscillations from each source. Using these pulse profiles, we place upper limits on the fractional amplitudes of harmonic and half-frequency signals of 0.3% and 0.6%, respectively (95% confidence). We then compare the pulse morphologies to theoretical profiles from models with one or two antipodal bright regions on the surface of a rotating neutron star. We find that if one bright region is present on the star, it must either lie near the rotational pole or cover nearly half the neutron star in order to be consistent with the observed lack of harmonic signals. If an antipodal pattern is present, the hot regions must form very near the rotational equator. We discuss how these geometric constraints challenge current models for the production of brightness variations on the surface of a neutron star. (abridged)
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We use archival data from the Rossi X-Ray Timing Explorer to examine 125 type I X-ray bursts from the 9 weakly magnetic accreting neutron stars where millisecond oscillations have been detected during some bursts. We find that oscillations from the 6 fast (approximately 600 Hz) sources are almost always observed during radius expansion bursts, whereas oscillations from the 3 slow (about 300 Hz) sources are about equally likely to be found in bursts both with and without photospheric radius expansion. This strongly suggests that the distinction between these two source groups cannot be an observational selection effect, but must instead arise from some physical mechanism.
Type-I X-ray bursts arise from unstable thermonuclear burning of accreted fuel on the surface of neutron stars. In this chapter we review the fundamental physics of the burning processes, and summarise the observational, numerical, and nuclear experimental progress over the preceding decade. We describe the current understanding of the conditions that lead to burst ignition, and the influence of the burst fuel on the observational characteristics. We provide an overview of the processes which shape the burst X-ray spectrum, including the observationally elusive discrete spectral features. We report on the studies of timing behaviour related to nuclear burning, including burst oscillations and mHz quasi-periodic oscillations. We describe the increasing role of nuclear experimental physics in the interpretation of astrophysical data and models. We survey the simulation projects that have taken place to date, and chart the increasing dialogue between modellers, observers, and nuclear experimentalists. Finally, we identify some open problems with prospects of a resolution within the timescale of the next such review.
We report the discovery of burst oscillations at the spin frequency in ten thermonuclear bursts from the accreting millisecond X-ray pulsar (AMXP) IGR J17511-3057. The burst oscillation properties are, like those from the persistent AMXPs SAX J1808.4-3658 and XTE J1814-338, anomalous compared to burst oscillations from intermittent pulsars or non-pulsing LMXBs. Like SAX J1808.4-3658 they show frequency drifts in the rising phase rather than the tail. There is also evidence for harmonic content. Where IGR J17511-3057 is unusual compared to the other two persistent pulsars is that oscillations are not detected throughout all bursts. As accretion rate drops the bursts get brighter and their rise/decay time scales become shorter, while the oscillation amplitude falls below the detection threshold: first in the burst peak and then also in the rise. None of the bursts from IGR J17511-3057 show evidence for photospheric radius expansion (which might be expected to suppress oscillation amplitude) which allow us to set an upper limit to the distance of 6.9 kpc. We discuss the implications of our results for models of the burst oscillation mechanism.
It has been known for nearly three decades that the energy spectra of thermonuclear X-ray bursts are often well-fit by Planck functions with temperatures so high that they imply a super-Eddington radiative flux at the emitting surface, even during portions of bursts when there is no evidence of photospheric radius expansion. This apparent inconsistency is usually set aside by assuming that the flux is actually sub-Eddington and that the fitted temperature is so high because the spectrum has been distorted by the energy-dependent opacity of the atmosphere. Here we show that the spectra predicted by currently available conventional atmosphere models appear incompatible with the highest-precision measurements of burst spectra made using the Rossi X-ray Timing Explorer, such as during the 4U 1820-30 superburst and a long burst from GX 17+2. In contrast, these measurements are well-fit by Bose-Einstein spectra with high temperatures and modest chemical potentials. Such spectra are very similar to Planck spectra. They imply surface radiative fluxes more than a factor of three larger than the Eddington flux. We find that segments of many other bursts from many sources are well-fit by similar Bose-Einstein spectra, suggesting that the radiative flux at the emitting surface also exceeds the Eddington flux during these segments. We suggest that burst spectra can closely approximate Bose-Einstein spectra and have fluxes that exceed the Eddington flux because they are formed by Comptonization in an extended, low-density radiating gas supported by the outward radiation force and confined by a tangled magnetic field.
When the upper layer of an accreting neutron star experiences a thermonuclear runaway of helium and hydrogen, it exhibits an X-ray burst of a few keV with a cool-down phase of typically 1~minute. When there is a surplus of hydrogen, hydrogen fusion is expected to simmer during that same minute due to the rp process, which consists of rapid proton captures and slow beta-decays of proton-rich isotopes. We have analyzed the high-quality light curves of 1254 X-ray bursts, obtained with the Proportional Counter Array on the Rossi X-ray Timing Explorer between 1996 and 2012, to systematically study the cooling and rp process. This is a follow-up of a study on a selection of 37 bursts from systems that lack hydrogen and show only cooling during the bursts. We find that the bolometric light curves are well described by the combination of a power law and a one-sided Gaussian. The power-law decay index is between 1.3 and 2.1 and similar to that for the 37-bursts sample. There are individual bursters with a narrower range. The Gaussian is detected in half of all bursts, with a typical standard deviation of 50~s and a fluence ranging up to 60% of the total fluence. The Gaussian appears consistent with being due to the rp process. The Gaussian fluence fraction suggests that the layer where the rp process is active is underabundant in H by a factor of at least five with respect to cosmic abundances. Ninety-four percent of all bursts from ultracompact X-ray binaries lack the Gaussian component, and the remaining 6% are marginal detections. This is consistent with a hydrogen deficiency in these binaries. We find no clear correlation between the power law and Gaussian light-curve components.
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