449 research outputs found
Accretion-powered Millisecond Pulsar Outbursts
The population of accretion-powered millisecond pulsars has grown rapidly
over the last four years, with the discovery of six new examples to bring the
total sample to seven. While the first six discovered are transients active for
a few weeks every two or more years, the most recently-discovered source HETE
J1900.1-2455, has been active for more than 8 months. We summarise the
transient behaviour of the population to estimate long-term time-averaged
fluxes, and equate these fluxes to the expected mass transfer rate driven by
gravitational radiation in order to constrain the distances. We also estimate
an upper limit of 6 kpc to the distance of IGR J00291+5934 based on the
non-detection of bursts from this source.Comment: 5 pages, 2 figures, to be published in: "The Transient Milky Way: a
perspective for MIRAX", eds. F. D'Amico, J. Braga & R. Rothschild, AIP Conf.
Pro
On the consistency of neutron-star radius measurements from thermonuclear bursts
The radius of neutron stars can in principle be measured via the
normalisation of a blackbody fitted to the X-ray spectrum during thermonuclear
(type-I) X-ray bursts, although few previous studies have addressed the
reliability of such measurements. Here we examine the apparent radius in a
homogeneous sample of long, mixed H/He bursts from the low-mass X-ray binaries
GS 1826-24 and KS 1731-26. The measured blackbody normalisation (proportional
to the emitting area) in these bursts is constant over a period of up to 60s in
the burst tail, even though the flux (blackbody temperature) decreased by a
factor of 60-75% (30-40%). The typical rms variation in the mean normalisation
from burst to burst was 3-5%, although a variation of 17% was found between
bursts observed from GS 1826-24 in two epochs. A comparison of the
time-resolved spectroscopic measurements during bursts from the two epochs
shows that the normalisation evolves consistently through the burst rise and
peak, but subsequently increases further in the earlier epoch bursts. The
elevated normalisation values may arise from a change in the anisotropy of the
burst emission, or alternatively variations in the spectral correction factor,
f_c, of order 10%. Since burst samples observed from systems other than GS
1826-24 are more heterogeneous, we expect that systematic uncertainties of at
least 10% are likely to apply generally to measurements of neutron-star radii,
unless the effects described here can be corrected for.Comment: 9 pages, 6 figures; accepted by Ap
Evidence for accretion rate change during type I X-ray bursts
The standard approach for time-resolved X-ray spectral analysis of
thermonuclear bursts involves subtraction of the pre-burst emission as
background. This approach implicitly assumes that the persistent flux remains
constant throughout the burst. We reanalyzed 332 photospheric radius expansion
bursts observed from 40 sources by the Rossi X-ray Timing Explorer, introducing
a multiplicative factor to the persistent emission contribution in our
spectral fits. We found that for the majority of spectra the best-fit value of
is significantly greater than 1, suggesting that the persistent emission
typically increases during a burst. Elevated values were not found solely
during the radius expansion interval of the burst, but were also measured in
the cooling tail. The modified model results in a lower average value of the
fit statistic, indicating superior spectral fits, but not yet to the
level of formal statistical consistency for all the spectra.
We interpret the elevated values as an increase of the mass accretion
rate onto the neutron star during the burst, likely arising from the effects of
Poynting-Robertson drag on the disk material. We measured an inverse
correlation of with the persistent flux, consistent with theoretical
models of the disc response. We suggest that this modified approach may provide
more accurate burst spectral parameters, as well as offering a probe of the
accretion disk structure.Comment: 15 pages, 12 figures, 4 table
Evidence for enhanced persistent emission during sub-Eddington thermonuclear bursts
The standard approach for time-resolved X-ray spectral analysis of
thermonuclear bursts involves subtraction of the pre-burst emission as
background. This approach implicitly assumes that the persistent flux remains
constant throughout the burst. We reanalyzed 332 photospheric radius expansion
bursts observed from 40 sources by the Rossi X-ray Timing Explorer, introducing
a multiplicative factor to the persistent emission contribution in our
spectral fits. We found that for the majority of spectra the best-fit value of
is significantly greater than 1, suggesting that the persistent emission
typically increases during a burst. Elevated values were not found solely
during the radius expansion interval of the burst, but were also measured in
the cooling tail. The modified model results in a lower average value of the
fit statistic, indicating superior spectral fits, but not yet to the
level of formal statistical consistency for all the spectra.
We interpret the elevated values as an increase of the mass accretion
rate onto the neutron star during the burst, likely arising from the effects of
Poynting-Robertson drag on the disk material. We measured an inverse
correlation of with the persistent flux, consistent with theoretical
models of the disc response. We suggest that this modified approach may provide
more accurate burst spectral parameters, as well as offering a probe of the
accretion disk structure.Comment: 15 pages, 9 figure
A Double Outburst from IGR J00291+5934: Implications for Accretion Disk Instability Theory
The accretion-powered millisecond pulsar IGR J00291+5934 underwent two ~10 d
long outbursts during 2008, separated by 30 d in quiescence. Such a short
quiescent period between outbursts has never been seen before from a neutron
star X-ray transient. X-ray pulsations at the 599 Hz spin frequency are
detected throughout both outbursts. For the first time, we derive a pulse phase
model that connects two outbursts, providing a long baseline for spin frequency
measurement. Comparison with the frequency measured during the 2004 outburst of
this source gives a spin-down during quiescence of -4(1)x10^-15 Hz/s,
approximately an order of magnitude larger than the long-term spin-down
observed in the 401 Hz accretion-powered pulsar SAX J1808.4-3658. If this
spin-down is due to magnetic dipole radiation, it requires a 2x10^8 G field
strength, and its high spin-down luminosity may be detectable with the Fermi
Large Area Telescope. Alternatively, this large spin-down could be produced by
gravitational wave emission from a fractional mass quadrupole moment of Q/I =
1x10^{-9}. The rapid succession of the outbursts also provides a unique test of
models for accretion in low-mass X-ray binaries. Disk instability models
generally predict that an outburst will leave the accretion disk too depleted
to fuel a second outburst after such a brief quiescence. We suggest a
modification in which the outburst is shut off by the onset of a propeller
effect before the disk is depleted. This model can explain the short quiescence
and the unusually slow rise of the light curve of the second 2008 outburst.Comment: 17 pages, 8 figures; accepted by Ap
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