3,162 research outputs found
Probing the effects of a thermonuclear X-ray burst on the neutron star accretion flow with NuSTAR
Observational evidence has been accumulating that thermonuclear X-ray bursts
ignited on the surface of neutron stars influence the surrounding accretion
flow. Here, we exploit the excellent sensitivity of NuSTAR up to 79 keV to
analyze the impact of an X-ray burst on the accretion emission of the neutron
star LMXB 4U 1608-52. The ~200 s long X-ray burst occurred during a hard X-ray
spectral state, and had a peak intensity of ~30-50 per cent of the Eddington
limit with no signs of photospheric radius expansion. Spectral analysis
suggests that the accretion emission was enhanced up to a factor of ~5 during
the X-ray burst. We also applied a linear unsupervised decomposition method,
namely non-negative matrix factorization (NMF), to study this X-ray burst. We
find that the NMF performs well in characterizing the evolution of the burst
emission and is a promising technique to study changes in the underlying
accretion emission in more detail than is possible through conventional
spectral fitting. For the burst of 4U 1608-52, the NMF suggests a possible
softening of the accretion spectrum during the X-ray burst, which could
potentially be ascribed to cooling of a corona. Finally, we report a small (~3
per cent) but significant rise in the accretion emission ~0.5 h before the
X-ray burst, although it is unclear whether this was related to the X-ray burst
ignition.Comment: 10 pages, 10 figures, 1 table, to appear in MNRA
Dynamics of Laterally Propagating Flames in X-ray Bursts. I. Burning Front Structure
We investigate the structure of laterally-propagating flames through the
highly-stratified burning layer in an X-ray burst. Two-dimensional
hydrodynamics simulations of flame propagation are performed through a rotating
plane-parallel atmosphere, exploring the structure of the flame. We discuss the
approximations needed to capture the length and time scales at play in an X-ray
burst and describe the flame acceleration observed. Our studies complement
other multidimensional studies of burning in X-ray bursts.Comment: Submitted to Ap
A Strong X-Ray Burst from the Low Mass X-Ray Binary EXO0748-676
We have observed an unusually strong X-ray burst as a part of our regular
eclipse timing observations of the low mass binary system EXO0748-676. The
burst peak flux was 5.2x10^-8 ergs cm^-2 s^-1, approximately five times the
normal peak X-ray burst flux observed from this source by RXTE. Spectral fits
to the data strongly suggest that photospheric radius expansion occurred during
the burst. In this Letter we examine the properties of this X-ray burst, which
is the first example of a radius expansion burst from EXO0748-676 observed by
RXTE. We find no evidence for coherent burst oscillations. Assuming that the
peak burst luminosity is the Eddington luminosity for a 1.4 solar mass neutron
star we derive a distance to EXO0748-676 of 7.7 kpc for a helium-dominated
burst photosphere and 5.9 kpc for a hydrogen-dominated burst photosphere.Comment: 15 pages including 2 figures and 1 table. Accepted for publication in
the Astrophysical Journa
Proton Drip-Line Calculations and the Rp-process
One-proton and two-proton separation energies are calculated for proton-rich
nuclei in the region . The method is based on Skyrme Hartree-Fock
calculations of Coulomb displacement energies of mirror nuclei in combination
with the experimental masses of the neutron-rich nuclei. The implications for
the proton drip line and the astrophysical rp-process are discussed. This is
done within the framework of a detailed analysis of the sensitivity of rp
process calculations in type I X-ray burst models on nuclear masses. We find
that the remaining mass uncertainties, in particular for some nuclei with
, still lead to large uncertainties in calculations of X-ray burst light
curves. Further experimental or theoretical improvements of nuclear mass data
are necessary before observed X-ray burst light curves can be used to obtain
quantitative constraints on ignition conditions and neutron star properties. We
identify a list of nuclei for which improved mass data would be most important.Comment: 20 pages, 9 figures, 2 table
Possible signatures for strange stars in stellar X-ray binaries
Kilohertz quasi-periodic brightness oscillations (kHz QPOs) observed in
certain X-ray burst sources may represent Keplerian frequencies in the inner
regions of the accretion disk in such systems. If this assumption is strictly
adhered to, we show here that if the central accretor in stellar X-ray burst
sources is a strange star (made up of u, d and s quarks in beta equilibrium,
referred to as strange matter) then the calculated QPO frequencies are
reconcilable with the observed QPO frequencies (corresponding to the highest
frequency of 1.22 kHz, observed so far from the source 4U 1636-53) only for
particular values of the QCD-related parameters which describe the equation of
state of strange matter. We demonstrate that QPO frequencies in the very high
range (1.9-3.1) kHz can be understood in terms of a (non- magnetized) strange
star X-ray binary (SSXB) rather than a neutron star X-ray binary (NSXB). Future
discovery of such high frequency QPOs from X-ray burst sources will constitute
a new astrophysical di- agnostic for identifying solar mass range stable
strange stars in our galaxy.Comment: 4 pages, 2 figs., uses psbox.tex, submitted to A&
Rapid soft X-ray fluctuations in solar flares observed with the X-ray polychromator
Three flares observed by the Soft X-Ray Polychromator on the Solar Maximum Mission were studied. Flare light curves from the Flat Crystal Spectrometer and Bent Crystal Spectrometer were examined for rapid signal variations. Each flare was characterized by an initial fast (less than 1 min) burst, observed by the Hard X-Ray Burst Spectrometer (HXRBS), followed by softer gradual X-ray emission lasting several minutes. From an autocorrelation function analysis, evidence was found for quasi-periodic fluctuations with rise and decay times of 10 s in the Ca XIX and Fe XXV light curves. These variations were of small amplitude (less than 20%), often coincided with hard X-ray emissions, and were prominent during the onset of the gradual phase after the initial hard X-ray burst. It is speculated that these fluctuations were caused by repeated energy injections in a coronal loop that had already been heated and filled with dense plasma associated with the initial hard X-ray burst
Dependence of X-Ray Burst Models on Nuclear Reaction Rates
X-ray bursts are thermonuclear flashes on the surface of accreting neutron
stars and reliable burst models are needed to interpret observations in terms
of properties of the neutron star and the binary system. We investigate the
dependence of X-ray burst models on uncertainties in (p,),
(,), and (,p) nuclear reaction rates using fully
self-consistent burst models that account for the feedbacks between changes in
nuclear energy generation and changes in astrophysical conditions. A two-step
approach first identified sensitive nuclear reaction rates in a single-zone
model with ignition conditions chosen to match calculations with a
state-of-the-art 1D multi-zone model based on the {\Kepler} stellar evolution
code. All relevant reaction rates on neutron deficient isotopes up to mass 106
were individually varied by a factor of 100 up and down. Calculations of the 84
highest impact reaction rate changes were then repeated in the 1D multi-zone
model. We find a number of uncertain reaction rates that affect predictions of
light curves and burst ashes significantly. The results provide insights into
the nuclear processes that shape X-ray burst observables and guidance for
future nuclear physics work to reduce nuclear uncertainties in X-ray burst
models.Comment: 24 pages, 13 figures, 4 tables, submitte
X-ray burst induced spectral variability in 4U 1728-34
Aims. INTEGRAL has been monitoring the Galactic center region for more than a
decade. Over this time INTEGRAL has detected hundreds of type-I X-ray bursts
from the neutron star low-mass X-ray binary 4U 1728-34, a.k.a. "the slow
burster". Our aim is to study the connection between the persistent X-ray
spectra and the X-ray burst spectra in a broad spectral range. Methods. We
performed spectral modeling of the persistent emission and the X-ray burst
emission of 4U 1728-34 using data from the INTEGRAL JEM-X and IBIS/ISGRI
instruments. Results. We constructed a hardness intensity diagram to track
spectral state variations. In the soft state the energy spectra are
characterized by two thermal components - likely from the accretion disc and
the boundary/spreading layer - together with a weak hard X-ray tail that we
detect in 4U 1728-34 for the first time in the 40 to 80 keV range. In the hard
state the source is detected up to 200 keV and the spectrum can be described by
a thermal Comptonization model plus an additional component: either a powerlaw
tail or reflection. By stacking 123 X-ray bursts in the hard state, we detect
emission up to 80 keV during the X-ray bursts. We find that during the bursts
the emission above 40 keV decreases by a factor of about three with respect to
the persistent emission level. Conclusions. Our results suggest that the
enhanced X-ray burst emission changes the spectral properties of the accretion
disc in the hard state. The likely cause is an X-ray burst induced cooling of
the electrons in the inner hot flow near the neutron star.Comment: 7 pages, 5 figures, Accepted for publication in A&
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