Observations Of X-ray Bursts Research Articles

The Impact of Neutron Transfer Reactions on the Heating and Cooling of Accreted Neutron Star Crusts

Abstract Nuclear reactions heat and cool the crust of accreting neutron stars and need to be understood to interpret observations of X-ray bursts and long-term cooling in transiently accreting systems. It was recently suggested that previously ignored neutron transfer reactions may play a significant role in the nuclear processes. We present results from full nuclear network calculations that now include these reactions and determine their impact on crust composition, crust impurity, heating, and cooling. We find that a large number of neutron transfer reactions indeed occur and impact crust models. In particular, we identify a new type of reaction cycle that brings a pair of nuclei across the nuclear chart into equilibrium via alternating neutron capture and neutron release, interspersed with a neutron transfer. While neutron transfer reactions lead to changes in crust model predictions and need to be considered in future studies, previous conclusions concerning heating, cooling, and compositional evolution are remarkably robust.

Effects of the Nuclear Equation of State on Type I X-Ray Bursts: Interpretation of the X-Ray Bursts from GS 1826–24

Abstract Type I X-ray bursts are thermonuclear explosions on the neutron star (NS) surface caused by mass accretion from a companion star. Observations of X-ray bursts provide valuable information on X-ray binary systems, e.g., binary parameters, the chemical composition of accreted matter, and the nuclear equation of state (EOS). There have been several theoretical studies to constrain the physics of X-ray bursters. However, they have mainly focused on the burning layers above the solid crust of the NS, which brings up issues of the treatment of NS gravitational and internal energy. In this study, focusing on the microphysics inside NSs, we calculate a series of X-ray bursts using a general-relativistic stellar-evolution code with several NS EOSs. We compare the X-ray-burst models with the burst parameters of a clocked burster associated with GS 1826–24. We find a monotonic correlation between the NS radius and the light-curve profile. A larger radius shows a higher recurrence time and a large peak luminosity. In contrast, the dependence of light curves on the NS mass becomes more complicated, where neutrino cooling suppresses the efficiency of nuclear ignition. We also constrain the EOS and mass of GS 1826–24, i.e., stiffer EOSs, corresponding to larger NS radii, are not preferred due to a too-high peak luminosity. The EOS and the cooling and heating of NSs are important to discuss the theoretical and observational properties of X-ray bursts.

A Bayesian approach to matching thermonuclear X-ray burst observations with models

ABSTRACT We present a new method of matching observations of Type-I (thermonuclear) X-ray bursts with models, comparing the predictions of a semi-analytic ignition model with X-ray observations of the accretion-powered millisecond pulsar SAX J1808.4–3658 in outburst. We used a Bayesian analysis approach to marginalize over the parameters of interest and determine parameters such as fuel composition, distance/anisotropy factors, neutron star mass, and neutron star radius. Our study includes a treatment of the system inclination effects, inferring that the rotation axis of the system is inclined $\left(69^{+4}_{-2}\right)^\circ$ from the observers line of sight, assuming a flat disc model. This method can be applied to any accreting source that exhibits Type-I X-ray bursts. We find a hydrogen mass fraction of $0.57^{+0.13}_{-0.14}$ and CNO metallicity of $0.013^{+0.006}_{-0.004}$ for the accreted fuel is required by the model to match the observed burst energies, for a distance to the source of $3.3^{+0.3}_{-0.2}\, \mathrm{kpc}$. We infer a neutron star mass of $1.5^{+0.6}_{-0.3}\, \mathrm{M}_{\odot }$ and radius of $11.8^{+1.3}_{-0.9}\, \mathrm{km}$ for a surface gravity of $1.9^{+0.7}_{-0.4}\times 10^{14}\, \mathrm{cm}\, \mathrm{s}^{-2}$ for SAX J1808.4–3658.

NICER Observes the Effects of an X-Ray Burst on the Accretion Environment in Aql X-1

Abstract Accretion disks around neutron stars regularly undergo sudden strong irradiation by Type-I X-ray bursts powered by unstable thermonuclear burning on the stellar surface. We investigate the impact on the disk during one of the first X-ray burst observations with the Neutron Star Interior Composition Explorer (NICER) on the International Space Station. The burst is seen from Aql X-1 during the hard spectral state. In addition to thermal emission from the neutron star, the burst spectrum exhibits an excess of soft X-ray photons below 1 keV, where NICER’s sensitivity peaks. We interpret the excess as a combination of reprocessing by the strongly photoionized disk and enhancement of the pre-burst persistent flux, possibly due to Poynting–Robertson drag or coronal reprocessing. This is the first such detection for a short sub-Eddington burst. As these bursts are observed frequently, NICER will be able to study how X-ray bursts affect the disk and corona for a range of accreting neutron star systems and disk states.

Simultaneous Constraints on the Mass and Radius of Aql X–1 from Quiescence and X-Ray Burst Observations

Abstract The measurement of neutron star mass and radius is one of the most direct ways to distinguish between various dense matter equations of state. The mass and radius of accreting neutron stars hosted in low-mass X-ray binaries can be constrained by several methods, including photospheric radius expansion from type I X-ray bursts and from quiescent spectra. In this paper, we apply for the first time these two methods simultaneously to constrain the mass and radius of Aql X–1. The quiescent spectra from Chandra and XMM-Newton, and photospheric radius expansion bursts from RXTE are used. The determination of the mass and radius of Aql X–1 is also used to verify the consistency between the two methods and to narrow down the uncertainties of the neutron star mass and radius. It is found that the distance to Aql X–1 should be in the range of 4.0–5.75 kpc, based on the overlapping confidence regions between photospheric radius expansion burst and quiescent spectra methods. In addition, we show that the mass and radius determined for the compact star in Aql X–1 are compatible with strange star equations of state and conventional neutron star models.

CONSTRAINING MODELS OF TWIN-PEAK QUASI-PERIODIC OSCILLATIONS WITH REALISTIC NEUTRON STAR EQUATIONS OF STATE

ABSTRACT Twin-peak quasi-periodic oscillations (QPOs) are observed in the X-ray power-density spectra of several accreting low-mass neutron star (NS) binaries. In our previous work we have considered several QPO models. We have identified and explored mass–angular-momentum relations implied by individual QPO models for the atoll source 4U 1636-53. In this paper we extend our study and confront QPO models with various NS equations of state (EoS). We start with simplified calculations assuming Kerr background geometry and then present results of detailed calculations considering the influence of NS quadrupole moment (related to rotationally induced NS oblateness) assuming Hartle–Thorne spacetimes. We show that the application of concrete EoS together with a particular QPO model yields a specific mass–angular-momentum relation. However, we demonstrate that the degeneracy in mass and angular momentum can be removed when the NS spin frequency inferred from the X-ray burst observations is considered. We inspect a large set of EoS and discuss their compatibility with the considered QPO models. We conclude that when the NS spin frequency in 4U 1636-53 is close to 580 Hz, we can exclude 51 of the 90 considered combinations of EoS and QPO models. We also discuss additional restrictions that may exclude even more combinations. Namely, 13 EOS are compatible with the observed twin-peak QPOs and the relativistic precession model. However, when considering the low-frequency QPOs and Lense–Thirring precession, only 5 EOS are compatible with the model.

Detection of burning ashes from thermonuclear X-ray bursts

Abstract When neutron stars (NS) accrete gas from low-mass binary companions, explosive nuclear burning reactions in the NS envelope fuse hydrogen and helium into heavier elements. The resulting thermonuclear (type-I) X-ray bursts produce energy spectra that are fit well with black bodies, but a significant number of burst observations show deviations from Planck spectra. Here we present our analysis of RXTE/PCA observations of X-ray bursts from the NS low-mass X-ray binary HETE J1900.1-2455. We have discovered that the non-Planckian spectra are caused by photoionization edges. The anticorrelation between the strength of the edges and the colour temperature suggests that the edges are produced by the nuclear burning ashes that have been transported upwards by convection and become exposed at the photosphere. The atmosphere model fits show that occasionally the photosphere can consist entirely of metals, and that the peculiar changes in blackbody temperature and radius can be attributed to the emergence and disappearance of metals in the photosphere. As the metals are detected already in the Eddington-limited phase, it is possible that a radiatively driven wind ejects some of the burning ashes into the interstellar space.

A bright thermonuclear X-ray burst simultaneously observed withChandraand RXTE

The prototypical accretion-powered millisecond pulsar SAX J1808.4-3658 was observed simultaneously with Chandra-LETGS and RXTE-PCA near the peak of a transient outburst in November 2011. A single thermonuclear (type-I) burst was detected, the brightest yet observed by Chandra from any source, and the second-brightest observed by RXTE. We found no evidence for discrete spectral features during the burst; absorption edges have been predicted to be present in such bursts, but may require a greater degree of photospheric expansion than the rather moderate expansion seen in this event (a factor of a few). These observations provide a unique data set to study an X-ray burst over a broad bandpass and at high spectral resolution (lambda/delta-lambda=200-400). We find a significant excess of photons at high and low energies compared to the standard black body spectrum. This excess is well described by a 20-fold increase of the persistent flux during the burst. We speculate that this results from burst photons being scattered in the accretion disk corona. These and other recent observations of X-ray bursts point out the need for detailed theoretical modeling of the radiative and hydrodynamical interaction between thermonuclear X-ray bursts and accretion disks.

Multiwavelength Observations of EXO 0748–676. I. Reprocessing of X‐Ray Bursts

We present high time-resolution multiwavelength observations of X-ray bursts in the low-mass X-ray binary UY Vol. Strong reprocessed signals are present in the ultraviolet and optical, lagged and smeared with respect to the X-rays. The addition of far-ultraviolet coverage for one burst allows much tighter constraints on the temperature and geometry of the reprocessing region than previously possible. A blackbody reprocessing model for this burst suggests a rise in temperatures during the burst from 18,000 to 35,000 K and an emitting area comparable to that expected for the disk and/or irradiated companion star. The lags are consistent with those expected. The single-zone blackbody model cannot reproduce the ratio of optical to ultraviolet flux during the burst, however. The discrepancy seems too large to explain with deviations from a local blackbody spectrum and more likely indicates that a range of reprocessing temperatures are required. Comparable results are derived from other bursts, and in particular the lag and smearing both appear shorter when the companion star is on the near side of the disk as predicted. The burst observed by HST also yielded a spectrum of the reprocessed light. It is dominated by continuum, with a spectral shape consistent with the temperatures derived from lightcurve modeling. Taken as a whole, our observations confirm the standard paradigm of prompt reprocessing distributed across the disk and companion star, with the response dominated by a thermalized continuum rather than by emission lines.

Rotational Evolution during Type I X‐Ray Bursts

The rotation rates of six weakly-magnetic neutron stars accreting in low-mass X-ray binaries have most likely been measured by Type I X-ray burst observations with RXTE. The nearly coherent oscillations detected during the few seconds of thermonuclear burning are most simply understood as rotational modulation of brightness asymmetries on the neutron star surface. We show that, as suggested by Strohmayer and colleagues, the frequency changes of 1-2 Hz observed during bursts are consistent with angular momentum conservation as the burning shell hydrostatically expands and contracts. We calculate how vertical heat propagation through the radiative outer layers of the atmosphere and convection affect the coherence of the oscillation. We show that the evolution of the rotational profile depends strongly on whether the burning layers are composed of pure helium or mixed hydrogen/helium. Our results help explain the absence (presence) of oscillations from hydrogen-burning (helium-rich) bursts that was found by Muno and collaborators. We investigate angular momentum transport within the burning layers and the recoupling of the burning layers with the star. We show that the Kelvin-Helmholtz instability is quenched by the strong stratification, and that mixing between the burning fuel and underlying ashes by the baroclinic instability does not occur. However, the baroclinic instability may have time to operate within the differentially rotating burning layer, potentially bringing it into rigid rotation.

Constraints on the mass-radius relation of neutron stars from observations of X-ray emission from their surfaces

The author discusses available methods to use observations of X-ray bursts to constrain the mass-radius relation of neutron stars, with the aim of obtaining information on the equation of state of the high-density matter inside neutron stars.

The numbers of fast electrons in solar flares as deduced from hard X-ray and microwave spectral data

The previously reported discrepancy between the number of fast solar flare electrons deduced from hard X-ray burst observations and microwave observations of the same events is reexamined. The thin and thick-target models for hard X-ray production and their consequences for gyrosynchrotron emission of microwaves are discussed. Previous work that led to the perceived discrepancy is reviewed, and it is suggested that the discrepancy can be reduced when thick-target hard X-ray emission is assumed. The number of electrons in microwaves and hard X-rays are calculated assuming a homogeneous source for a sample of flares given by Wiehl et al. (1983). The numbers are given separately under the two assumptions of thin and thick-target hard X-ray emission. It is found that when the latter emission is assumed, the number of electrons deduced from the microwaves is very similar to the number deduced from the hard X-rays. 36 references.

X-ray burst observations of Serpens X-1

Fifty-seven X-ray bursts observed with SAS 3 in the period 1976 July to 1979 March are reported. Their general features (e.g., maximum burst flux, integrated burst flux, spectral hardness, and rise time) and the relations between them are discussed. Also a comparison is made between these burst features and the associated persistent X-ray flux. The latter appears to be correlated with the maximum burst flux; the temperature of the plasma that produces the persistent flux is roughly proportional to this flux.

Hakucho observations of X-ray bursts from 4U 1702-42

Hakucho observations of X-ray bursts from 4U 1702-42

The effect of reverse currents on the dynamics of nonthermal electron beams in solar flares and on their emitted X-ray bremsstrahlung

view Abstract Citations (133) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The effect of reverse currents on the dynamics of nonthermal electron beams in solar flares and on their emitted X-ray bremsstrahlung Emslie, A. G. Abstract Recognizing the possible importance of reverse current ohmic energy losses from beams of electrons injected into the solar atmosphere during flares, we solve the equations governing the dynamics of such beams, taking into account energy and momentum losses of the beam particles by both collisional and ohmic heating processes. It is found that for injected fluxes compatible with hard X-ray burst observations of fairly large flares, the inclusion of ohmic terms results in a substantial modification of the variation of electron energy and pitch angle with depth in the target. The chromospheric energy deposition due to the beam, and X-ray bremsstrahlung emitted by the beam, are also evaluated for a variety of empirical flare model atmospheres, and it is found that, although both these quantities decrease as the injected electron flux increases, their ratio is insensitive to the parameters of the problem. Further, the emitted nonthermal X-ray flux is found to be bounded by a limiting value only marginally consistent with observations, thus suggesting that a significant portion of the X-ray flux is thermal in origin. The dependence of the results obtained on the model atmosphere used in the calculations is discussed. Publication: The Astrophysical Journal Pub Date: February 1980 DOI: 10.1086/157709 Bibcode: 1980ApJ...235.1055E Keywords: Bremsstrahlung; Electron Beams; Solar Atmosphere; Solar Flares; Solar Physics; Solar X-Rays; Atmospheric Models; Energy Spectra; X Ray Spectra; Solar Physics full text sources ADS |

Optical observations of X-ray bursts from MXB 1837 + 05

Optical observations of the X-ray burst source MXB 1837 + 05 (Ser X-1) made at different observatories are discussed. Six X-ray bursts were detected with SAS-3 during these observations, but no simultaneous optical signals were seen during or after any of these bursts. For an assumed burst duration of 10 s, the 3σ upper limits of the ratio of the detected energy in an optical burst to that in an X-ray burst range from 3 × 10−5 to 6 × 10−5.

HEAO Observations of X-ray bursts from MXB1730–335

BURST activity was observed1 on 18 March 1978, from MXB1730–335, the rapid burster, from which no activity had been observed a few days earlier2. HEAO 1 was pointed ( + y axis) at the rapid burster from ∼ 18.00–21.00 UT on 31 March 1978. The source was actively bursting in the ‘Mode I’ pattern3, in which very large type II bursts4 are first followed by a long (several minutes) quiescent period and then by a train of closely spaced, small bursts. Because type I and type II bursts probably have different underlying physical mechanisms4, it is important to study both types over a large energy range. We report here on a search for hard X-ray emission from the rapid burster and on evidence for spectral evolution in the type II bursts.

Average properties of X-ray burst sources

Observations of X-ray bursts from 10 sources reveal a remarkable uniformity in the properties of X-ray burst sources: if the peak luminosity in a burst is a standard candle the effective black body radius observed in bursts, is the same for all burst sources.

Observations of X-ray bursts from the vicinity of 3U1727–33

ARIEL V experiment C observed the region of sky near 3U1727–33 between March 8 and 10, 1976 and again between March 24 and 25. The experiment1 consists of a multiwire collimated proportional counter with a field of view of 3.5° FWHM. The experiment is, however, offset from the spin axis of the satellite by 1.75°, so the effective field of view per spin cycle (4–5 s) is a Gaussian-like curve of 5.0° FWHM centred on the spin axis. The field of view during the present observations included the positions of 3U1727 33 (ref. 2), MX1716–30 (ref. 3) and two recently discovered burst sources, MXB1730–335 and MXB1728–34 (ref. 4).

Evidence from Hard X-Rays for Two-Stage Particle Acceleration in a Solar Flare

Solar flare two stage particle acceleration from X-ray burst observations, discussing induced electric field and Fermi mechanism