545 research outputs found
Discovery of coherent millisecond X-ray pulsations in Aql X-1
We report the discovery of an episode of coherent millisecond X-ray pulsation
in the neutron star low-mass X-ray binary Aql X-1. The episode lasts for
slightly more than 150 seconds, during which the pulse frequency is consistent
with being constant. No X-ray burst or other evidence of thermonuclear burning
activity is seen in correspondence with the pulsation, which can thus be
identified as occurring in the persistent emission. The pulsation frequency is
550.27 Hz, very close (0.5 Hz higher) to the maximum reported frequency from
burst oscillations in this source. Hence we identify this frequency with the
neutron star spin frequency. The pulsed fraction is strongly energy dependent,
ranging from 10% (16-30 keV). We discuss possible physical
interpretations and their consequences for our understanding of the lack of
pulsation in most neutron star low-mass X-ray binaries. If interpreted as
accretion-powered pulsation, Aql X-1 might play a key role in understanding the
differences between pulsating and non-pulsating sources.Comment: 5 pages, 3 figures, accepted by ApJ Letters after minor revisions.
Slightly extended discussion. One author added. Uses emulateapj.cl
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.
Very hard states in neutron star low-mass X-ray binaries
We report on unusually very hard spectral states in three confirmed
neutron-star low-mass X-ray binaries (1RXS J180408.9-342058, EXO 1745-248, and
IGR J18245-2452) at a luminosity between ~ 10^{36-37} erg s^{-1}. When fitting
the Swift X-ray spectra (0.5 - 10 keV) in those states with an absorbed
power-law model, we found photon indices of \Gamma ~ 1, significantly lower
than the \Gamma = 1.5 - 2.0 typically seen when such systems are in their so
called hard state. For individual sources very hard spectra were already
previously identified but here we show for the first time that likely our
sources were in a distinct spectral state (i.e., different from the hard state)
when they exhibited such very hard spectra. It is unclear how such very hard
spectra can be formed; if the emission mechanism is similar to that operating
in their hard states (i.e., up-scattering of soft photons due to hot electrons)
then the electrons should have higher temperatures or a higher optical depth in
the very hard state compared to those observed in the hard state. By using our
obtained \Gamma as a tracer for the spectral evolution with luminosity, we have
compared our results with those obtained by Wijnands et al. (2015). We confirm
their general results in that also our sample of sources follow the same track
as the other neutron star systems, although we do not find that the accreting
millisecond pulsars are systematically harder than the non-pulsating systems.Comment: Accepted for publication in MNRA
Breaking the AMSP mould: the increasingly strange case of HETE J1900.1-2455
We present ongoing Rossi X-ray Timing Explorer (RXTE) monitoring observations
of the 377.3 Hz accretion-powered pulsar, HETE J1900.1-2455 Activity continues
in this system more than 3 years after discovery, at a mean luminosity of
4.4e36 erg/s (for d=5 kpc), although pulsations were present only within the
first 70 days. X-ray variability has increased each year, notably with a brief
interval of nondetection in 2007, during which the luminosity dropped to below
1e-3 of the mean level. A deep search of data from the intervals of
nondetection in 2005 revealed evidence for extremely weak pulsations at an
amplitude of 0.29% rms, a factor of ten less than the largest amplitude seen
early in the outburst.
X-ray burst activity continued through 2008, with bursts typically featuring
strong radius expansion. Spectral analysis of the most intense burst detected
by RXTE early in the outburst revealed unusual variations in the inferred
photospheric radius, as well as significant deviations from a blackbody. We
obtained much better fits instead with a comptonisation model.Comment: 8 pages, 3 figures, one table; 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.
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.
Type I X-ray bursts, burst oscillations and kHz quasi-periodic oscillations in the neutron star system IGR J17191-2821
We present a detailed study of the X-ray energy and power spectral properties
of the neutron star transient IGR J17191-2821. We discovered four instances of
pairs of simultaneous kilohertz quasi-periodic oscillations (kHz QPOs). The
frequency difference between these kHz QPOs is between 315 Hz and 362 Hz. We
also report on the detection of five thermonuclear type-I X-ray bursts and the
discovery of burst oscillations at ~294 Hz during three of them. Finally, we
report on a faint and short outburst precursor, which occurred about two months
before the main outburst. Our results on the broadband spectral and variability
properties allow us to firmly establish the atoll source nature of IGR
J17191-2821.Comment: 9 pages, 7 figures - accepted for publication in MNRA
Spectral evidence for jets from Accreting Millisecond X-ray Pulsars
Transient radio emission from X-ray binaries is associated with synchrotron
emission from collimated jets that escape the system, and accreting millisecond
X-ray pulsars (AMXPs) are no exception. Although jets from black hole X-ray
binaries are well-studied, those from neutron star systems appear much fainter,
for reasons yet uncertain. Jets are usually undetectable at higher frequencies
because of the relative brightness of other components such as the accretion
disc. AMXPs generally have small orbital separations compared with other X-ray
binaries and as such their discs are relatively faint. Here, I present data
that imply jets in fact dominate the radio-to-optical spectrum of outbursting
AMXPs. They therefore may provide the best opportunity to study the behaviour
of jets produced by accreting neutron stars, and compare them to those produced
by black hole systems.Comment: 4 pages, 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.
Mapping the QCD Phase Transition with Accreting Compact Stars
We discuss an idea for how accreting millisecond pulsars could contribute to
the understanding of the QCD phase transition in the high-density nuclear
matter equation of state (EoS). It is based on two ingredients, the first one
being a ``phase diagram'' of rapidly rotating compact star configurations in
the plane of spin frequency and mass, determined with state-of-the-art hybrid
equations of state, allowing for a transition to color superconducting quark
matter. The second is the study of spin-up and accretion evolution in this
phase diagram. We show that the quark matter phase transition leads to a
characteristic line in the Omega-M plane, the phase border between neutron
stars and hybrid stars with a quark matter core. Along this line a change in
the pulsar's moment of inertia entails a waiting point phenomenon in the
accreting millisecond X-ray pulsar (AMXP) evolution: most of these objects
should therefore be found along the phase border in the Omega-M plane, which
may be viewed as the AMXP analog of the main sequence in the
Hertzsprung-Russell diagram for normal stars. In order to prove the existence
of a high-density phase transition in the cores of compact stars we need
population statistics for AMXP's with sufficiently accurate determination of
their masses and spin frequencies.Comment: 8 pages, 3 figures, to appear in Proceedings of the Conference on "A
Decade of Accreting Millisecond X-Ray Pulsars, Amsterdam, April 14-18, 200
The Advanced X-ray Timing Array (AXTAR)
AXTAR is an X-ray observatory mission concept, currently under study in the
U.S., that combines very large collecting area, broadband spectral coverage,
high time resolution, highly flexible scheduling, and an ability to respond
promptly to time-critical targets of opportunity. It is optimized for
submillisecond timing of bright Galactic X-ray sources in order to study
phenomena at the natural time scales of neutron star surfaces and black hole
event horizons, thus probing the physics of ultradense matter, strongly curved
spacetimes, and intense magnetic fields. AXTAR's main instrument is a
collimated, thick Si pixel detector with 2-50 keV coverage and 8 square meters
collecting area. For timing observations of accreting neutron stars and black
holes, AXTAR provides at least an order of magnitude improvement in sensitivity
over both RXTE and Constellation-X. AXTAR also carries a sensitive sky monitor
that acts as a trigger for pointed observations of X-ray transients and also
provides continuous monitoring of the X-ray sky with 20 times the sensitivity
of the RXTE ASM. AXTAR builds on detector and electronics technology previously
developed for other applications and thus combines high technical readiness and
well understood cost.Comment: 4 pages with 1 figure, 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.). Footnote and references adde
The very faint X-ray binary IGR J17062-6143: a truncated disc, no pulsations, and a possible outflow
We present a comprehensive X-ray study of the neutron star low-mass X-ray binary IGR J17062-6143, which has been accreting at low luminosities since its discovery in 2006. Analysing NuSTAR, XMM–Newton, and Swift observations, we investigate the very faint nature of this source through three approaches: modelling the relativistic reflection spectrum to constrain the accretion geometry, performing high-resolution X-ray spectroscopy to search for an outflow, and searching for the recently reported millisecond X-ray pulsations. We find a strongly truncated accretion disc at
77+22−18
gravitational radii (∼164 km) assuming a high inclination, although a low inclination and a disc extending to the neutron star cannot be excluded. The high-resolution spectroscopy reveals evidence for oxygen-rich circumbinary material, possibly resulting from a blueshifted, collisionally ionized outflow. Finally, we do not detect any pulsations. We discuss these results in the broader context of possible explanations for the persistent faint nature of weakly accreting neutron stars. The results are consistent with both an ultra-compact binary orbit and a magnetically truncated accretion flow, although both cannot be unambiguously inferred. We also discuss the nature of the donor star and conclude that it is likely a CO or O–Ne–Mg white dwarf, consistent with recent multiwavelength modelling
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