306 research outputs found
Radius-expansion burst spectra from 4U 1728-34: an ultracompact binary?
Recent theoretical and observational studies have shown that ashes from
thermonuclear burning may be ejected during radius-expansion bursts, giving
rise to photoionisation edges in the X-ray spectra. We report a search for such
features in Chandra spectra observed from the low-mass X-ray binary 4U 1728-34.
We analysed the spectra from four radius-expansion bursts detected in 2006
July, and two in 2002 March, but found no evidence for discrete features. We
estimate upper limits for the equivalent widths of edges of a few hundred eV,
which for the moderate temperatures observed during the bursts, are comparable
with the predictions. During the 2006 July observation 4U 1728-34 exhibited
weak, unusually frequent bursts (separated by <2 hr in some cases), with
profiles and alpha-values characteristic of hydrogen-poor fuel. Recurrence
times as short as those measured are insufficient to exhaust the accreted
hydrogen at solar composition, suggesting that the source accretes hydrogen
deficient fuel, for example from an evolved donor. The detection for the first
time of a 10.77 min periodic signal in the persistent intensity, perhaps
arising from orbital modulation, supports this explanation, and suggests that
this system is an ultracompact binary similar to 4U 1820-30.Comment: 9 pages, 6 figures, accepted by Ap
Accreting neutron star spins and the equation of state
X-ray timing of neutron stars in low-mass X-ray binaries (LMXBs) with RXTE
has since 1996 revealed several distinct high-frequency phenomena. Among these
are oscillations during thermonuclear (type-I) bursts, which (in addition to
persistent X-ray pulsations) are thought to trace the neutron star spin. Recent
discoveries bring the total number of measured LMXB spin rates to 22. An open
question is why the majority of the ~100 known neutron stars in LMXBs show
neither pulsations nor burst oscillations.
Recent observations suggest that persistent pulsations may be more common
than previously thought, although detectable intermittently, and in some cases
at very low duty cycles. For example, the 377.3 Hz pulsations in HETE
J1900.1-2455 were only present in the first few months of it's outburst, and
have been absent since (although X-ray activity continues). Intermittent
(persistent) pulsations have since been detected in a further two sources. In
two of these three systems the pulsations appear to be related to the
thermonuclear burst activity, but in the third (Aql X-1) they are not. This
phenomenon offers new opportunities for spin measurements in known systems.
Such measurements can constrain the poorly-known neutron star equation of
state, and neutron stars in LMXBs offer observational advantages over
rotation-powered pulsars which make the detection of more rapidly-spinning
examples more likely. Even so, spin rates of at least 50% faster than the
present maximum appear necessary to give constraints stringent enough to
discriminate between the various models. Although the future prospects for such
rapidly-spinning objects do not appear optimistic, several additional
observational approaches are possible for LMXBs.Comment: 9 pages, 3 figures, includes tables of confirmed rapidly-rotating
accreting neutron stars (as of 2007 Nov. 7). Submitted to the proceedings of
the "40 Years of Pulsars: Millisecond Pulsars, Magnetars and More"
conference, Montreal, August 200
Non-axisymmetric low frequency oscillations of rotating and magnetized neutron stars
We investigate non-axisymmetric low frequency modes of a rotating and
magnetized neutron star, assuming that the star is threaded by a dipole
magnetic field whose strength at the stellar surface, , is less than G, and whose magnetic axis is aligned with the rotation axis. For modal
analysis, we use a neutron star model composed of a fluid ocean, a solid crust,
and a fluid core, where we treat the core as being non-magnetic assuming that
the magnetic pressure is much smaller than the gas pressure in the core. Here,
we are interested in low frequency modes of a rotating and magnetized neutron
star whose oscillation frequencies are similar to those of toroidal crust modes
of low spherical harmonic degree and low radial order. For a magnetic field of
G, we find Alfv\'en waves in the ocean have similar frequencies
to the toroidal crust modes, and we find no -modes confined in the ocean for
this strength of the field. We calculate the toroidal crustal modes, the
interfacial modes peaking at the crust/core interface, and the core inertial
modes and -modes, and all these modes are found to be insensitive to the
magnetic field of strength B_0\ltsim10^{12}G. We find the displacement vector
of the core -modes have large amplitudes around the rotation
axis at the stellar surface even in the presence of a surface magnetic field
G, where and are the spherical harmonic degree
and the azimuthal wave number of the -modes, respectively. We suggest that
millisecond X-ray variations of accretion powered X-ray millisecond pulsars can
be used as a probe into the core -modes destabilized by gravitational wave
radiation.Comment: 15pages, 7 figure
Ignition column depths of helium-rich thermonuclear bursts from 4U 1728-34
We analysed thermonuclear (type-I) X-ray bursts observed from the low-mass
X-ray binary 4U1728-34 by RXTE, Chandra and INTEGRAL. We compared the variation
in burst energy and recurrence times as a function of accretion rate with the
predictions of a numerical ignition model including a treatment of the heating
and cooling in the crust. We found that the measured burst ignition column
depths are significantly below the theoretically predicted values, regardless
of the assumed thermal structure of the neutron star interior. While it is
possible that the accretion rate measured by Chandra is underestimated, due to
additional persistent spectral components outside the sensitivity band, the
required correction factor is typically 3.6 and as high as 6, which is
implausible. Furthermore, such underestimation is even more unlikely for RXTE
and INTEGRAL, which have much broader bandpasses. Possible explanations for the
observed discrepancy include shear-triggered mixing of the accreted helium to
larger column depths, resulting in earlier ignition, or the fractional covering
of the accreted fuel on the neutron star surface.Comment: 12 pages, 5 figures, accepted for publication in Ap
Magnetic Field Evolution in Neutron Stars: One-Dimensional Multi-Fluid Model
This paper is the first in a series aimed at understanding the long-term
evolution of neutron star magnetic fields. We model the stellar matter as an
electrically neutral and lightly ionized plasma composed of three moving
particle species: neutrons, protons, and electrons, which can be converted into
each other by weak interactions (beta decays), suffer binary collisions, and be
affected by each other's macroscopic electromagnetic fields. Since the
evolution of the magnetic field occurs over thousands of years or more,
compared to dynamical time scales (sound and Alfv\'en) of milliseconds to
seconds, we use a slow-motion approximation in which we neglect the inertial
terms in the equations of motion for the particles. We restrict ourselves to a
one-dimensional geometry in which the magnetic field points in one Cartesian
direction but varies only along an orthogonal direction. We study the evolution
of the system in three different ways: (i) estimating time scales directly from
the equations, guided by physical intuition; (ii) a normal-mode analysis in the
limit of a nearly uniform system; and (iii) a finite-difference numerical
integration of the equations of motion. We find good agreement between our
analytical normal-mode solutions and the numerical simulations. We show that
the magnetic field and the particles evolve through successive
quasi-equilibrium states, on time scales that can be understood by physical
arguments. Depending of the parameter values the magnetic field can evolve by
ohmic diffusion or by ambipolar diffusion, the latter being limited either by
interparticle collisions or by relaxation to chemical equilibrium through beta
decays. The numerical simulations are further validated by verifying that they
satisfy the known conservation laws also in highly non-linear situations.Comment: Paper Accepted in Astronomy & Astrophysics: 24 April 2008, Paper
Reference Number: AA/2008/09466. Paper contains 8 Figures. In this version
the section: Summary and Conclusions has been expande
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 1997-98 were consistent with a stable recurrence time of 5.74 +/- 0.13
hr. The persistent intensity of GS 1826-24 increased by 36% between 1997-2000,
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 lightcurves over the full approx.
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 Mdot^(-1.05+/-0.02)
(where Mdot is assumed to be linearly proportional to the X-ray flux), so that
the ratio alpha between the integrated persistent and burst fluxes was
inversely correlated with Mdot. The average value of alpha was 41.7 +/- 1.6.
Both the alpha value, and the long burst durations indicate that the hydrogen
is burning during the burst via the rapid-proton (rp) process. The variation in
alpha with Mdot 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 alpha. We
discuss possible explanations, including extra heating between bursts, or that
the fraction of the neutron star covered by the accreted fuel increases with
Mdot.Comment: 9 pages, 6 figures, accepted by ApJ. Minor revisions following the
referee's repor
Models of Type I X-ray Bursts from GS 1826-24: A Probe of rp-Process Hydrogen Burning
The X-ray burster GS 1826-24 shows extremely regular Type I X-ray bursts
whose energetics and recurrence times agree well with thermonuclear ignition
models. We present calculations of sequences of burst lightcurves using
multizone models which follow the rp-process nucleosynthesis with an extensive
nuclear reaction network. The theoretical and observed burst lightcurves show
remarkable agreement. The models naturally explain the slow ~5s rise and long
~100s tails of these bursts, as well as their dependence on mass accretion
rate. This comparison provides further evidence for solar metallicity in the
accreted material in this source, and constrains the distance to the source.
The main difference is that the observed lightcurves do not show the distinct
two-stage rise of the models. This may reflect the time for burning to spread
over the stellar surface, or may indicate that our treatment of heat transport
or nuclear physics needs to be revised. The trends in burst properties with
accretion rate are well-reproduced by our spherically symmetric models which
include chemical and thermal inertia from the ashes of previous bursts. Changes
in the covering fraction of the accreted fuel are not required.Comment: 5 pages, 4 figures, to appear in ApJ letter
Helium-rich thermonuclear bursts and the distance to the accretion-powered millisecond pulsar SAX J1808.4-3658
We analysed 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
signficantly, 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 degrees.
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 approx. 0.1, and
require that the accreted hydrogen fraction X_0 and the CNO metallicity Z_CNO
are related by Z_CNO approx. 0.03(X_0/0.7)^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
which is in good agreement with our other estimates.Comment: 10 pages, 6 figures, accepted by Ap
Self-regulatory imagery and physical activity in older adults: A social-cognitive perspective
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