3,104 research outputs found
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.
Evolution of the bursting-layer wave during a Type 1 X-ray burst
In a popular scenario due to Heyl, quasi periodic oscillations (QPOs) which
are seen during type 1 X-ray bursts are produced by giant travelling waves in
neutron-star oceans. Piro and Bildsten have proposed that during the burst
cooling the wave in the bursting layer may convert into a deep crustal
interface wave, which would cut off the visible QPOs. This cut-off would help
explain the magnitude of the QPO frequency drift, which is otherwise
overpredicted by a factor of several in Heyl's scenario. In this paper, we
study the coupling between the bursting layer and the deep ocean. The coupling
turns out to be weak and only a small fraction of the surface-wave energy gets
transferred to that of the crustal-interface wave during the burst. Thus the
crustal-interface wave plays no dynamical role during the burst, and no early
QPO cut-off should occur.Comment: 8 pages, submitted to MNRA
CREATION OF SOIL PERMEABILITY MAPS TROUGH OBIA CLASSIFICATION OF VERY HIGH-RESOLUTION SATELLITE IMAGES
Abstract. In the last few months, we have been working on images acquired by the WorldView3 satellite over the city of Pavia with the final intent to create a soil permeability map. These maps can be particularly useful in various fields, such as water management and public green, for evaluate the correlation between overbuilt areas and pollution, the influence of vegetation on the temperature in within the different areas of the city, for the planning and monitoring of a sustainable transition of cities. To create such maps, it is essential to be able to identify various objects lying in the images, in our case we have done a classification of the image using the software Trimble eCognition™, applying Object-based Image Analysis (OBIA) approach and various classification methods, by applying fuzzy logic and supervised classification. The objects generated through various segmentations have been classified into 7 classes, water, fields, cultivated fields / low vegetation, high vegetation, roads, red roofs, and white roofs. And from the comparison with the manually defined ground truth, an overall accuracy degree of 80% was achieved. Furthermore, by applying various aggregation strategies, by combining the cultivated fields / low vegetation and high vegetation classes, we achieved a better overall accuracy of 91%
The appearance of a compact jet in the soft-intermediate state of 4U 1543-47
Recent advancements in the understanding of jet-disc coupling in black hole
candidate X-ray binaries (BHXBs) have provided close links between radio jet
emission and X-ray spectral and variability behaviour. In 'soft' X-ray states
the jets are suppressed, but the current picture lacks an understanding of the
X-ray features associated with the quenching or recovering of these jets. Here
we show that a brief, ~4 day infrared (IR) brightening during a predominantly
soft X-ray state of the BHXB 4U 1543-47 is contemporaneous with a strong X-ray
Type B quasi-periodic oscillation (QPO), a slight spectral hardening and an
increase in the rms variability, indicating an excursion to the
soft-intermediate state (SIMS). This IR 'flare' has a spectral index consistent
with optically thin synchrotron emission and most likely originates from the
steady, compact jet. This core jet emitting in the IR is usually only
associated with the hard state, and its appearance during the SIMS places the
'jet line' between the SIMS and the soft state in the hardness-intensity
diagram for this source. IR emission is produced in a small region of the jets
close to where they are launched (~ 0.1 light-seconds), and the timescale of
the IR flare in 4U 1543-47 is far too long to be caused by a single, discrete
ejection. We also present a summary of the evolution of the jet and X-ray
spectral/variability properties throughout the whole outburst, constraining the
jet contribution to the X-ray flux during the decay.Comment: Accepted to MNRAS. 11 pages, 6 figure
The identification of MAXI J1659-152 as a black hole candidate
We report on the analysis of all 65 pointed Rossi X-ray Timing Explorer
observations of the recently discovered soft X-ray transient MAXI J1659-152
(initially referred to as GRB 100925A). The source was studied in terms of its
evolution through the hardness-intensity diagram (HID) as well as its X-ray
variability properties. MAXI J1659-152 traced out an anti-clockwise loop in the
HID, which is commonly seen in transient low-mass X-ray binaries. The
variability properties of the source, in particular the detection of type-B and
type-C low-frequency quasi-periodic oscillations, and the way they evolve along
the HID track, indicate that MAXI J1659-152 is a black hole candidate. The
spectral and variability properties of MAXI J1659-152 imply that the source was
observed in the hard and soft intermediate states during the RXTE observations,
with several transitions between these two states.Comment: 6 pages, 4 figures, accepted for publication in ApJ
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.
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