290 research outputs found
Modeling pulse profiles of accreting millisecond pulsars
I review the basic observational properties of accreting millisecond pulsars
that are important for understanding the physics involved in formation of their
pulse profiles. I then discuss main effects responsible for shaping these
profiles. Some analytical results that help to understand the results of
simulations are presented. Constraints on the pulsar geometry and the neutron
star equation of state obtained from the analysis of the pulse profiles are
discussed.Comment: Invited review, to appear in the proceedings of the workshop 'A
Decade of Accreting Millisecond X-ray Pulsars' (Amsterdam, April 2008
Modelling the outburst profile of X-ray powered millisecond pulsars
The outbursts of low mass X-ray binaries are prolonged relative to those of
dwarf nova cataclysmic variables as a consequence of X-ray irradiation of the
disc. We show that the time-scale of the decay light curve and its luminosity
at a characteristic time are linked to the radius of the accretion disc. Hence
a good X-ray light curve permits two independent estimates of the disc radius.
In the case of the millisecond pulsars SAX J1808.4-3658 and XTE J0929-314 the
agreement between these estimates is very strong. Our analysis allows new
determinations of distances and accretion disc radii. Our analysis will allow
determination of accretion disc radii for sources in external galaxies, and
hence constrain system parameters where other observational techniques are not
possible. We also use the X-ray light curves to estimate the mass transfer
rate. The broken exponential decay observed in the 2002 outburst of SAX
J1808.4-3658 may be caused by the changing self-shadowing of the disc.Comment: To be published in the proceedings of the workshop 'A Decade of
Accreting Millisecond X-ray pulsars' (Amsterdam 14-18 April 2008; Eds.
Wijnands et al.
Lighthouses with two lights: burst oscillations from the accretion-powered millisecond pulsars
The key contribution of the discovery of nuclear-powered pulsations from the
accretion-powered millisecond pulsars (AMPs) has been the establishment of
burst oscillation frequency as a reliable proxy for stellar spin rate. This has
doubled the sample of rapidly-rotating accreting neutron stars and revealed the
unexpected absence of any stars rotating near the break-up limit. The resulting
`braking problem' is now a major concern for theorists, particularly given the
possible role of gravitational wave emission in limiting spin. This, however,
is not the only area where burst oscillations from the AMPs are having an
impact. Burst oscillation timing is developing into a promising technique for
verifying the level of spin variability in the AMPs (a topic of considerable
debate). These sources also provide unique input to our efforts to understand
the still-elusive burst oscillation mechanism. This is because they are the
only stars where we can reliably gauge the role of uneven fuel deposition and,
of course, the magnetic field.Comment: Invited review, to appear in the proceedings of the workshop 'A
decade of accreting millisecond x-ray pulsars' (Amsterdam, April 2008
Timing the accretion flow around accreting millisecond pulsars
At present, ten years after they were first discovered, ten accreting
millisecond pulsars are known. I present a study of the aperiodic X-ray
variability in three of these systems, which led to the discovery of
simultaneous kHz quasi periodic oscillations in XTE J1807-294 and extremely
strong broadband noise at unusually low variability frequencies in IGR
J00291+5934. Furthermore, we classified SWIFT J1756.9-2508 as an atoll source
and measured in its 2007 outburst spectral and variability properties typical
of the extreme island state. I also give detailed estimates of the total
fluence during the studied outbursts.Comment: To be published in the proceedings of the workshop 'A Decade of
Accreting Millisecond X-ray pulsars' (Amsterdam 14-18 April 2008; Eds.
Wijnands et al.
Motion of the hot spot and spin torque in accreting millisecond pulsars
The primary concern of this contribution is that accreting millisecond
pulsars (AMXPs) show a much larger amount of information than is commonly
believed. The three questions to be addressed are: 1. Is the apparent spin
torque observed in AMXPs real ? 2. Why do we see correlations and
anti-correlations between fractional amplitudes and timing residuals in some
AMXPs ? 3. Why the timing residuals, the lightcurve and the 1Hz QPO in SAX
J1808.43658 are related ?Comment: To be published in the proceedings of the workshop 'A Decade of
Accreting Millisecond X-ray pulsars' (Amsterdam 14-18 April 2008; Eds.
Wijnands et al.
The Spin Distribution of Millisecond X-ray Pulsars
The spin frequency distribution of accreting millisecond X-ray pulsars cuts
off sharply above 730 Hz, well below the breakup spin rate for most neutron
star equations of state. I review several different ideas for explaining this
cutoff. There is currently considerable interest in the idea that gravitational
radiation from rapidly rotating pulsars might act to limit spin up by
accretion, possibly allowing eventual direct detection with gravitational wave
interferometers. I describe how long-term X-ray timing of fast accreting
millisecond pulsars like the 599 Hz source IGR J00291+5934 can test the
gravitational wave model for the spin frequency limit.Comment: 8 pages with 2 figures, to appear in the proceedings of "A Decade of
Accreting Millisecond X-ray Pulsars", Amsterdam, April 2008, eds. R. Wijnands
et al. (AIP Conf. Proc.
Intermittent accreting millisecond pulsars: light houses with broken lamps?
Intermittent accreting millisecond X-ray pulsars are an exciting new type of
sources. Their pulsations appear and disappear either on timescales of hundreds
of seconds or on timescales of days. The study of these sources add new
observational constraints to present models that explain the presence or not of
pulsations in neutron star LMXBs. In this paper we present preliminary results
on spectral and aperiodic variability studies of all intermittent AMSPs, with a
particular focus on the comparison between pulsating and non pulsating periods.Comment: 4 pages, 2 figures; to appear in the proceedings of the workshop "A
Decade of Accreting Millisecond X-ray Pulsars", Amsterdam, April 2008, eds.
R. Wijnands et al. (AIP Conf. Proc.
- …