253 research outputs found
Future Probes of the Neutron Star Equation of State Using X-ray Bursts
Observations with NASA's Rossi X-ray Timing Explorer (RXTE) have resulted in
the discovery of fast (200 - 600 Hz), coherent X-ray intensity oscillations
(hereafter, "burst oscillations") during thermonuclear X-ray bursts from 12 low
mass X-ray binaries (LMXBs). It is now beyond doubt that these oscillations
result from spin modulation of the thermonuclear burst flux from the neutron
star surface. Among the new timing phenomena revealed by RXTE the burst
oscillations are perhaps the best understood, in the sense that many of their
properties can be explained in the framework of this relatively simple model.
Because of this, detailed modelling of burst oscillations can be an extremely
powerful probe of neutron star structure, and thus the equation of state (EOS)
of supra-nuclear density matter. The new discoveries have spurred much new
theoretical work on thermonuclear burning and propagation on neutron stars, so
that in the near future it is not unreasonable to think that detailed physical
models of the time dependent flux from burning neutron stars will be available
for comparison with the observed pulse profiles from a future, large collecting
area X-ray timing observatory. In addition, recent high resolution burst
spectroscopy with XMM/Newton suggests the presence of redshifted absorption
lines from the neutron star surface during bursts. This leads to the
possibility of using large area, high spectral resolution measurements of X-ray
bursts as a precise probe of neutron star structure. In this work I will
explore the precision with which constraints on neutron star structure, and
hence the dense matter EOS, can be made with the implementation of such
programs.Comment: 8 pages, 7 figures, AIP conference proceedings format. Contribution
to "X-ray Timing 2003: Rossi and Beyond." meeting held in Cambridge, MA,
November, 200
Precision X-ray Timing of RX J0806.3+1527 with CHANDRA: Evidence for Gravitational Radiation from an Ultracompact Binary
RX J0806.3+1527 is a candidate double degenerate binary with possibly the
shortest known orbital period. The source shows an 100% X-ray intensity
modulation at the putative orbital frequency of 3.11 mHz (321.5 s). If the
system is a detached, ultracompact binary gravitational radiation should drive
spin-up with a magnitude of ~10-16 Hz/s. Here we describe the results of the
first phase coherent X-ray monitoring campaign on RX J0806.3+1527 with Chandra.
We obtained a total of 70 ksec of exposure in 6 epochs logarithmically spaced
over 320 days. These data conclusively show that the X-ray frequency is
increasing at a rate of 3.77 +- 0.8 x 10-16 Hz/s. Using our new ephemeris we
are able to phase up all the earlier Chandra and ROSAT data and show they are
consistent with a rate of 3.63 +- 0.06 x 10-16 Hz/s over the past decade. This
value appears consistent with that recently derived by Israel et al. largely
from monitoring of the optical modulation, and is in rough agreement with the
solutions reported initially by Hakala et al., based on ground-based optical
observations. The large spin-up is consistent with gravitational radiation
losses driving the evolution. An intermediate polar (IP) scenario where the
observed X-ray period is the spin period of an accreting white dwarf appears
less tenable. If the ultracompact scenario is correct, then the X-ray flux
cannot be powered by stable accretion which would drive the components apart,
suggesting a new type of energy source (perhaps electromagnetic) may power the
X-ray flux.Comment: 23 pages, 9 figures, AASTeX, accepted for publication in the
Astrophysical Journa
Discovery of a 115 Day Orbital Period in the Ultraluminous X-ray Source NGC 5408 X-1
We report the detection of a 115 day periodicity in SWIFT/XRT monitoring data
from the ultraluminous X-ray source (ULX) NGC 5408 X-1. Our ongoing campaign
samples its X-ray flux approximately twice weekly and has now achieved a
temporal baseline of ~485 days. Periodogram analysis reveals a significant
periodicity with a period of 115.5 +- 4 days. The modulation is detected with a
significance of 3.2 e-4. The fractional modulation amplitude decreases with
increasing energy, ranging from 0.13 above 1 keV to 0.24 below 1 keV. The shape
of the profile evolves as well, becoming less sharply peaked at higher
energies. The periodogram analysis is consistent with a periodic process,
however, continued monitoring is required to confirm the coherent nature of the
modulation. Spectral analysis indicates that NGC 5408 X-1 can reach 0.3 - 10
keV luminosities of ~2 e40 ergs/s. We suggest that, like the 62 day period of
the ULX in M82 (X41.4+60), the periodicity detected in NGC 5408 X-1 represents
the orbital period of the black hole binary containing the ULX. If this is true
then the secondary can only be a giant or supergiant star.Comment: Accepted for Publication in the Astrophysical Journal Letter
Where Are the r-modes? Chandra Observations of Millisecond Pulsars
We present the results of {\it Chandra} observations of two non-accreting
millisecond pulsars, PSRs J16402224 (J1640) and J17092313 (J1709), with
low inferred magnetic fields and spin-down rates in order to constrain their
surface temperatures, obtain limits on the amplitude of unstable -modes in
them, and make comparisons with similar limits obtained for a sample of
accreting low-mass X-ray binary (LMXB) neutron stars. We detect both pulsars in
the X-ray band for the first time. They are faint, with inferred soft X-ray
fluxes ( keV) of and erg
cm s for J1640 and J1709, respectively. Spectral analysis
assuming hydrogen atmosphere emission gives global effective temperature upper
limits ( confidence) of K for J1640 and K for J1709, where the low end of the range corresponds to
canonical neutron stars (), and the upper end corresponds to
higher-mass stars (). Under the assumption that -mode
heating provides the thermal support, we obtain dimensionless -mode
amplitude upper limits of and for J1640 and J1709, respectively, where again the low end of the
range corresponds to lower-mass, canonical neutron stars ().
These limits are about an order of magnitude lower than those we derived
previously for a sample of LMXBs, except for the accreting millisecond X-ray
pulsar (AMXP) SAX J1808.43658, which has a comparable amplitude limit to
J1640 and J1709.Comment: 9 pages, 4 figures, published in Ap
X-ray Burst Oscillations: From Flame Spreading to the Cooling Wake
Type I X-ray bursts are thermonuclear flashes observed from the surfaces of
accreting neutron stars (NSs) in Low Mass X-ray Binaries. Oscillations have
been observed during the rise and/or decay of some of these X-ray bursts. Those
seen during the rise can be well explained by a spreading hot spot model, but
large amplitude oscillations in the decay phase remain mysterious because of
the absence of a clear-cut source of asymmetry. To date there have not been any
quantitative studies that consistently track the oscillation amplitude both
during the rise and decay (cooling tail) of bursts. Here we compute the light
curves and amplitudes of oscillations in X-ray burst models that realistically
account for both flame spreading and subsequent cooling. We present results for
several such "cooling wake" models, a "canonical" cooling model where each
patch on the NS surface heats and cools identically, or with a
latitude-dependent cooling timescale set by the local effective gravity, and an
"asymmetric" model where parts of the star cool at significantly different
rates. We show that while the canonical cooling models can generate
oscillations in the tails of bursts, they cannot easily produce the highest
observed modulation amplitudes. Alternatively, a simple phenomenological model
with asymmetric cooling can achieve higher amplitudes consistent with the
observations.Comment: 8 pages, 7 figures, Accepted for publication in ApJ, Additional
calculations and discussion compared to v
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