461 research outputs found
The centrifugal force reversal and X-ray bursts
Heyl (2000) made an interesting suggestion that the observed shifts in QPO
frequency in type I X-ray bursts could be influenced by the same geometrical
effect of strong gravity as the one that causes centrifugal force reversal
discovered by Abramowicz and Lasota (1974). However, his main result contains a
sign error. Here we derive the correct formula and conclude that constraints on
the M(R) relation for neutron stars deduced from the rotational-modulation
model of QPO frequency shifts are of no practical interest because the correct
formula implies a weak condition R* > 1.3 Rs, where Rs is the Schwarzschild
radius. We also argue against the relevance of the rotational-modulation model
to the observed frequency modulations.Comment: 3 pages, Minor revisions, A&A Letters, in pres
Excitation of Trapped Waves in Simulations of Tilted Black Hole Accretion Disks with Magnetorotational Turbulence
We analyze the time dependence of fluid variables in general relativistic,
magnetohydrodynamic simulations of accretion flows onto a black hole with
dimensionless spin parameter a/M=0.9. We consider both the case where the
angular momentum of the accretion material is aligned with the black hole spin
axis (an untilted flow) and where it is misaligned by 15 degrees (a tilted
flow). In comparison to the untilted simulation, the tilted simulation exhibits
a clear excess of inertial variability, that is, variability at frequencies
below the local radial epicyclic frequency. We further study the radial
structure of this inertial-like power by focusing on a radially extended band
at 118 (M/10Msol)^-1 Hz found in each of the three analyzed fluid variables.
The three dimensional density structure at this frequency suggests that the
power is a composite oscillation whose dominant components are an over dense
clump corotating with the background flow, a low order inertial wave, and a low
order inertial-acoustic wave. Our results provide preliminary confirmation of
earlier suggestions that disk tilt can be an important excitation mechanism for
inertial waves.Comment: 8 Pages, 6 Figures, accepted for publication in Ap
XMM-Newton observations of AM CVn binaries : V396 Hya and SDSS J1240â01
We present the results of XMM-Newton observations of two AM CVn systems - V396 Hya and SDSS J1240-01. Both systems are detected in X-rays and in the UV: neither shows coherent variability in their light curves. We compare the rms variability of the X-ray and UV power spectra of these sources with other AM CVn systems. Apart from ES Cet, AM CVn sources are not strongly variable in X-rays, while in the UV the degree of variability is related to the systems apparent brightness. The X-ray spectra of V396 Hya and SDSS J1240-01 show highly non-solar abundances, requiring enhanced nitrogen to obtain good fits. We compare the UV and X-ray luminosities for 7 AM CVn systems using recent distances. We find that the X-ray luminosity is not strongly dependent upon orbital period. However, the UV luminosity is highly correlated with orbital period with the UV luminosity decreasing with increasing orbital period. We expect that this is due to the accretion disk making an increasingly strong contribution to the UV emission at shorter periods. The implied luminosities are in remarkably good agreement with predictions
Relativistic Astrophysics Explorer
The great success of the Rossi X-Ray Timing Explorer (RXTE) has shown that
X-ray timing is an excellent tool for the study of strong gravitational fields
and the measurement of fundamental physical properties of black holes and
neutron stars. Here, we describe a next-generation X-ray timing mission, the
Relativistic Astrophysics Explorer (RAE), designed to fit within the envelope
of a medium-sized mission. The instruments will be a narrow-field X-ray
detector array with an area of 6 m^2 equal to ten times that of RXTE and a
wide-field X-ray monitor. We describe the science made possible with this
mission, the design of the instruments, and results on prototype large-area
X-ray detectors.Comment: to appear in Advances in Space Research, 6 pages with one color
figure (low-res
Neutron specific heat in the crust of neutron stars from the nuclear band theory
The inner crust of neutron stars, formed of a crystal lattice of uclear
clusters immersed in a sea of unbound neutrons, may be the nique example of
periodic nuclear systems. We have calculated the neutron specific heat in the
shallow part of the crust using the band theory of solids with Skyrme
nucleon-nucleon interactions. We have also tested the validity of various
approximations. We have found that the neutron specific heat is well described
by that of a Fermi gas, while the motion of the unbound neutrons is strongly
affected by the nuclear lattice. These apparently contradictory results are
explained by the particular properties of the neutron Fermi surface
The Long Term Stability of Oscillations During Thermonuclear X-ray Bursts: Constraining the Binary X-ray Mass Function
We report on the long term stability of the millisecond oscillations observed
with the Rossi X-ray Timing Explorer (RXTE) during thermonuclear X-ray bursts
from the low mass X-ray binaries (LMXB) 4U 1728-34 and 4U 1636-53. We show that
bursts from 4U 1728-34 spanning more than 1.5 years have observed asymptotic
oscillation periods which are within 0.2 microsec. of each other, well within
the magnitude which could be produced by the orbital motion of the neutron star
in a typical LMXB. This stability implies a timescale to change the oscillation
period of > 23,000 years, suggesting a highly stable process such as stellar
rotation as the oscillation mechanism. We show that period offsets in three
distinct bursts from 4U 1636-53 can be plausibly interpreted as due to orbital
motion of the neutron star in this 3.8 hour binary system. We discuss the
constraints on the mass function which can in principle be derived using this
technique.Comment: 11 pages, 4 figures. AASTeX, to be published in the Astrophysical
Journal Letter
Microscopic sub-barrier fusion calculations for the neutron star crust
Fusion of very neutron rich nuclei may be important to determine the
composition and heating of the crust of accreting neutron stars. Fusion cross
sections are calculated using time-dependent Hartree-Fock theory coupled with
density-constrained Hartree-Fock calculations to deduce an effective potential.
Systems studied include 16O+16O, 16O+24O, 24O+24O, 12C+16O, and 12C+24O. We
find remarkable agreement with experimental cross sections for the fusion of
stable nuclei. Our simulations use the SLy4 Skyrme force that has been
previously fit to the properties of stable nuclei, and no parameters have been
fit to fusion data. We compare our results to the simple S\~{a}o Paulo static
barrier penetration model. For the asymmetric systems 12C+24O or 16O+24O we
predict an order of magnitude larger cross section than those predicted by the
S\~{a}o Paulo model. This is likely due to the transfer of neutrons from the
very neutron rich nucleus to the stable nucleus and dynamical rearrangements of
the nuclear densities during the collision process. These effects are not
included in potential models. This enhancement of fusion cross sections, for
very neutron rich nuclei, can be tested in the laboratory with radioactive
beams.Comment: 9 pages, 11 figures, corrected small errors in Figs 10, 11, Phys.
Rev. C in pres
High-frequency Quasi-Periodic Oscillations from GRS 1915+105 in its C state
We report the results of a systematic timing analysis of RXTE observations of
GRS 1915+105 when the source was in its variability class theta, characterized
by alternating soft and hard states on a time scale of a few hundred seconds.
The aim was to examine the high-frequency part of the power spectrum in order
to confirm the hecto-Hertz Quasi-Periodic Oscillations (QPO) previously
reported from observations from mixed variability behaviours. During the hard
intervals (corresponding to state C in the classification of Belloni et al.,
2000, A&A, 35, 271), we find a significant QPO at a frequency of ~170 Hz,
although much broader (Q~2) than previously reported. No other significant peak
is observed at frequencies >30 Hz. A time-resolved spectral analysis of
selected observations shows that the hard intervals from class theta show a
stronger and steeper (Gamma=2.8-3.0) power-law component than hard intervals
from other classes. We discuss these results in the framework of hecto-Hertz
QPOs reported from GRS 1915+105 and other black-hole binaries.Comment: 7 pages, 8 figures, accepted for publication on MNRA
Carbon Flashes in the Heavy Element Ocean on Accreting Neutron Stars
We show that burning of a small mass fraction of carbon in a neutron star
ocean is thermally unstable at low accumulated masses when the ocean contains
heavy ashes from the hydrogen burning rapid proton (rp) process. The key to
early unstable ignition is the low thermal conductivity of a heavy element
ocean. The instability requires accretion rates in excess of one-tenth the
Eddington limit when the carbon mass fraction is 0.1 or less. The unstable
flashes release 10^{42} to 10^{43} ergs over hours to days, and are likely the
cause of the recently discovered large Type I X-ray bursts (so-called
``superbursts'') from six Galactic low mass X-ray binaries. In addition to
explaining the energetics, recurrence times, and durations of the superbursts,
these mixed carbon/heavy element flashes have an accretion rate dependence of
unstable burning similar to that observed. Though the instability is present at
accretion rates near Eddington, there is less contrast with the accretion
luminosity there, explaining why most detections are made at accretion rates
between 0.1 and 0.3 Eddington. Future comparisons of time dependent
calculations with observations will provide new insights into the rp process.Comment: Submitted to Astrophysical Journal Letters (6 pages, 3 figures
An Fe XXVI Absorption Line in the Persistent Spectrum of the Dipping Low Mass X-ray Binary 1A 1744-361
We report on Chandra X-ray Observatory (CXO) High-Energy Transmission Grating
(HETG) spectra of the dipping Low Mass X-ray Binary (LMXB) 1A 1744-361 during
its July 2008 outburst. We find that its persistent emission is well modeled by
a blackbody (kT ~ 1.0 keV) plus power-law ( ~ 1.7) with an absorption
edge at 7.6 keV. In the residuals of the combined spectrum we find a
significant absorption line at 6.961+/-0.002 keV, consistent with the Fe XXVI
(hydrogen-like Fe) 2 - 1 transition. We place an upper limit on the velocity of
a redshifted flow of v < 221 km/s. We find an equivalent width for the line of
27^+2_-3 eV, from which we determine a column density of 7+/-1x10^17 cm^-2 via
a curve-of-growth analysis. Using XSTAR simulations, we place a lower limit on
the ionization parameter of > 10^3.6 erg cm/s. The properties of this line are
consistent with those observed in other dipping LMXBs. Using Rossi X-ray Timing
Explorer (RXTE) data accumulated during this latest outburst we present an
updated color-color diagram which clearly shows that 1A 1744-361 is an "atoll"
source. Finally, using additional dips found in the RXTE and CXO data we
provide an updated orbital period estimate of 52+/-5 minutes.Comment: 8 pages, 7 figures, submitted to Ap
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