180 research outputs found
Numerical simulations of flow and mass transport processes in the anterior chamber in the presence of an iris-fixated intraocular lens
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.
A Mathematical Model of Corneal Metabolism in the Presence of an Iris-Fixated Phakic Intraocular Lens
Purpose: Corneal endothelial cell loss is one of the possible complications associated with phakic iris-fixated intraocular lens (PIOL) implantation. We postulate that this might be connected to the alteration of corneal metabolism secondary to the lens implantation. Methods: A mathematical model of transport and consumption/production of metabolic species in the cornea is proposed, coupled with a model of aqueous flow and transport of metabolic species in the anterior chamber. Results: Results are presented both for open and closed eyelids. We showed that, in the presence of a PIOL, glucose availability at the corneal endothelium decreases significantly during sleeping. Conclusions: Implantation of a PIOL significantly affects nutrient transport processes to the corneal endothelium especially during sleep. It must still be verified whether this finding has a clinical relevance
A complex state transition from the black hole candidate Swift J1753.5-0127
We present our monitoring campaign of the outburst of the black-hole
candidate Swift J1753.5-0127, observed with the Rossi X-ray Timing Explorer and
the Swift satellites. After ~4.5 years since its discovery, the source had a
transition to the hard intermediate state. We performed spectral and timing
studies of the transition showing that, unlike the majority of the transient
black holes, the system did not go to the soft states but it returned to the
hard state after a few months. During this transition Swift J1753.5-0127
features properties which are similar to those displayed by the black hole
Cygnus X-1. We compared Swift J1753.5-0127 to one dynamically confirmed black
hole and two neutron stars showing that its power spectra are in agreement with
the binary hosting a black hole. We also suggest that the prolonged period at
low flux that followed the initial flare is reminiscent of that observed in
other X-ray binaries, as well as in cataclysmic variables.Comment: 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.
Parsec-Scale Bipolar X-ray Shocks Produced by Powerful Jets from the Neutron Star Circinus X-1
We report the discovery of multi-scale X-ray jets from the accreting neutron
star X-ray binary, Circinus X-1. The bipolar outflows show wide opening angles
and are spatially coincident with the radio jets seen in new high-resolution
radio images of the region. The morphology of the emission regions suggests
that the jets from Circinus X-1 are running into a terminal shock with the
interstellar medium, as is seen in powerful radio galaxies. This and other
observations indicate that the jets have a wide opening angle, suggesting that
the jets are either not very well collimated or precessing. We interpret the
spectra from the shocks as cooled synchrotron emission and derive a cooling age
of approximately 1600 yr. This allows us to constrain the jet power to be
between 3e35 erg/s and 2e37 erg/s, making this one of a few microquasars with a
direct measurement of its jet power and the only known microquasar that
exhibits stationary large-scale X-ray emission.Comment: 5 pages, 4 figures, to be published in the Astrophysical Journal
Letter
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
e-VLBI observations of Circinus X-1: monitoring of the quiescent and flaring radio emission on AU scales
A recent detection of the peculiar neutron star X-ray binary Circinus X-1
with electronic very long baseline interferometry (e-VLBI) prompted the
suggestion that compact, non-variable radio emission persists through the
entire 16.6-day orbit of the binary system. We present the results of a high
angular resolution monitoring campaign conducted with the Australian Long
Baseline Array in real-time e-VLBI mode. e-VLBI observations of Circinus X-1
were made on alternate days over a period of 20 days covering the full binary
orbit. A compact radio source associated with Circinus X-1 was clearly detected
at orbital phases following periastron passage but no compact radio emission
was detected at any other orbital phase, ruling out the presence of a
persistent, compact emitting region at our sensitivity levels. The jet was not
resolved at any epoch of our 1.4-GHz monitoring campaign, suggesting that the
ultrarelativistic flow previously inferred to exist in this source is likely to
be dark. We discuss these findings within the context of previous radio
monitoring of Circinus X-1.Comment: Accepted for publication in MNRAS. 7 pages, 5 figure
- …