98 research outputs found
Modal fields calculation using the finite difference beam propagation method
A method is described to construct modal fields for an arbitrary one- or two-dimensional refractive index structure. An arbitrary starting field is propagated along a complex axis using the slowly varying envelope approximation (SVEA). By choosing suitable values for the step-size, one mode is maximally increased in amplitude on propagating, until convergence has been obtained. For the calculation of the next mode, the mode just found is filtered out, and the procedure starts again. The method is tested for one-dimensional refractive index structures, both for nonabsorbing and for absorbing structures, and is shown to give fast convergenc
Multiwavelength spectral evolution during the 2011 outburst of the very faint X-ray transient Swift J1357.2-0933
We report our multiwavelength study of the 2011 outburst evolution of the
newly discovered black hole candidate X-ray binary Swift J1357.2-0933. We
analysed the Swift X-ray telescope and Ultraviolet/Optical telescope (UVOT)
data taken during the ~7 months duration of the outburst. It displayed a 2-10
keV X-ray peak luminosity of ~1E35(D/1.5 kpc)^2 erg s-1 which classifies the
source as a very faint X-ray transient. We found that the X-ray spectrum at the
peak was consistent with the source being in the hard state, but it softened
with decreasing luminosity, a common behaviour of black holes at low
luminosities or returning to quiescence from the hard state. The correlations
between the simultaneous X-ray and ultraviolet/optical data suggest a system
with a black hole accreting from a viscous disc that is not irradiated. The
UVOT filters provide the opportunity to study these correlations up to
ultraviolet wavelengths a regime so far unexplored. If the black hole nature is
confirmed, Swift J1357.2-0933 would be one of the very few established black
hole very-faint X-ray transients.Comment: 6 pages, 2 tables, 5 figures. Accepted by MNRA
Duurzame energie 2050 : verkenning rol van (agrarische) ondernemers in de energietransitie naar 2050
Rural entrepreneurs (farmers) are going to play a major role in the energy transitions towards 2050. In professional dialogues between farmers and pertinent stakeholders it was shown that farmers are already involved in all aspects and are in the right position. The transition needs to be facilitated in local initiatives
Discovery of mHz X-ray Oscillations in a Transient Ultraluminous X-ray Source in M82
We report the discovery of X-ray quasi-periodic oscillations (QPOs) at
frequencies of 3-4 mHz from a transient ultraluminous X-ray source (ULX)
X42.3+59 in M82. The QPOs are strong and broad and appear with weak or absent
red noise, and are detected only in Chandra observations when the source is
brighter than 10^40 ergs/s. The QPO behavior is similar to the type A-I QPOs
found in XTE J1550-564, which is a subclass of low frequency QPOs with
properties in between type A and B. Therefore, we identify the QPOs in X42.3+59
as of type A or B, and rule out the possibility of type C. With this
identification, the mass of the black hole in X42.3+59 can be inferred as in
the range of 12,000-43,000 solar masses by scaling the QPO frequency to that of
the type A/B QPOs in stellar mass black holes. Cool disk emission is detected
in one Chandra observation, and the disk inner radius suggests a similar black
hole mass range. Black holes of such a high mass are able to produce an energy
output in a manner similar to X42.3+59 by accreting from the interstellar
medium directly.Comment: 5 pages, 1 figure, accepted for publication in The Astrophysical
Journal Letter
Verbindend ondernemen : Betekenisvolle biologische sector, vandaag en (over)morgen
In dit âinspiratieboekjeâ Verbindend ondernemen ligt de nadruk op conceptueel-theoretische noties en redeneerlijnen die de bedoeling hebben de sociale dimensie van duurzaamheid â de P van people â perspectief en inhoud te geven
A transient I band excess in the optical spectrum of the accreting millisecond pulsar SAX J1808.4-3658
The optical counterpart of the transient, millisecond X-ray pulsar SAX
J1808.4-3658 was observed in four colours (BVRI) for five weeks during the 2005
June-July outburst. The optical fluxes declined by ~2 magnitudes during the
first 16 days and then commenced quasi-periodic secondary outbursts, with
time-scales of several days, similar to those seen in 2000 and 2002. The
broadband spectra derived from these measurements were generally consistent
with emission from an X-ray heated accretion disc. During the first 16 days
decline in intensity the spectrum became redder. We suggest that the primary
outburst was initiated by a viscosity change driven instability in the inner
disc and note the contrast with another accreting millisecond pulsar, XTE
J0929--314, for which the spectrum becomes bluer during the decline. On the
night of 2005 June 5 (HJD 2453527) the I band flux was ~0.45 magnitudes
brighter than on the preceding or following nights whereas the BVR bands showed
no obvious enhancement. A Type I X-ray burst was detected by the RXTE
spacecraft during this I band integration. It seems unlikely that reprocessed
radiation from the burst was sufficient to explain the observed increase. We
suggest that a major part of the I band excess was due to synchrotron emission
triggered by the X-ray burst. Several other significant short duration changes
in V-I were detected. One occurred at about HJD 2453546 in the early phase of
the first secondary outburst and may be due to a mass transfer instability or
to another synchrotron emission event.Comment: 7 pages, 5 figures, accepted by MNRAS, 10 May 2006. Ver. 2 - larger
Table 2 + various modest text change
The outburst duration and duty-cycle of GRS 1915+105
The extraordinarily long outburst of GRS 1915+105 makes it one of the most
remarkable low-mass X-ray binaries (LMXBs). It has been in a state of constant
outburst since its discovery in 1992, an eruption which has persisted ~100
times longer than those of more typical LXMBs. The long orbital period of GRS
1915+105 implies that it contains large and massive accretion disc which is
able to fuel its extreme outburst. In this paper, we address the longevity of
the outburst and quiescence phases of GRS 1915+105 using Smooth Particle
Hydrodynamics (SPH) simulations of its accretion disc through many outburst
cycles. Our model is set in the two-alpha framework and includes the effects of
the thermo-viscous instability, tidal torques, irradiation by central X-rays
and wind mass loss. We explore the model parameter space and the examine the
impact of the various ingredients. We predict that the outburst of GRS 1915+105
should last a minimum of 20 years and possibly up to ~100 years if X-ray
irradiation is very significant. The predicted recurrence times are of the
order of 10^4 years, making the X-ray duty cycle a few 0.1%. Such a low duty
cycle may mean that GRS 1915+105 is not an anomaly among the more standard
LMXBs and that many similar, but quiescent, systems could be present in the
Galaxy.Comment: 10 pages, 9 figures, accepted for publication by MNRA
Discovery of a ~1 Hz quasi-periodic oscillation in the low-mass X-ray binary 4U 1746-37
We have discovered a ~1 Hz quasi-periodic oscillation (QPO) in the persistent
X-ray emission and during type I X-ray bursts of the globular cluster source,
dipper and low-mass X-ray binary (LMXB) 4U 1746-37. The QPO properties resemble
those of QPOs found recently in the LMXB dippers 4U 1323-62, and EXO 0748-676,
which makes 4U 1746-37 the third source known to exhibit this type of QPOs. We
present evidence for X-ray spectral changes in this source similar to those
observed in LMXBs referred to as atoll sources. We detect two states, a low
intensity and spectrally hard state, and a higher intensity and spectrally soft
state. This may explain the different spectral characteristics reported for 4U
1746-37 earlier. The high intensity state resembles the banana branch state of
atoll sources. The QPOs are only seen in the low intensity state, and are
absent when the source is in the banana branch. This strongly suggests that
either the accretion disk or an extended central source change shape between
the low intensity state and the banana branch. Twelve bursts were detected, of
which 5 took place while the source was on the banana branch and 7 when the
source was in the low intensity state. The bursts occurring on the banana
branch had an e-folding time ~3 times longer than those which occurred in the
low intensity state. Whereas previously detected dips showed only a decrease in
count rate of ~15%, we found in one observation a dip in which the count rate
dropped from ~200 counts per second to ~20 counts per second. This dip lasted
only ~250 seconds, during which clear spectral hardening occured. This is the
first time strong evidence for spectral changes during a dip are reported for
this source.Comment: 17 pages, accepted for publication in Ap
X-ray and Optical Study of Low Core Density Globular Clusters NGC6144 and E3
We report on the Chandra X-ray Observatory and Hubble Space Telescope
observation of two low core density globular clusters, NGC6144 and E3. By
comparing the number of X-ray sources inside the half-mass radius to those
outside, we found 6 X-ray sources within the half-mass radius of NGC6144, among
which 4 are expected to be background sources; 3 X-ray sources are also found
within the half-mass radius of E3, of which 3 is expected to be background
source. Therefore, we cannot exclude that all our sources are background
sources. However, combining the results from X-ray and optical observations, we
found that 1-2 sources in NGC6144 and 1 source in E3 are likely to be
cataclysmic variables and that 1 source in NGC6144 is an active binary, based
on the X-ray and optical properties. The number of faint X-ray sources in
NGC6144 and E3 found with Chandra and HST is higher than a prediction based on
collision frequency, but is closer to that based on mass. Our observations
strongly suggest that the compact binary systems in NGC6144 and E3 are
primordial in origin.Comment: 28 pages, 9 figures, 6 tables, Accepted for publication in Ap
1 Hz flaring in SAX J1808.4--3658: flow instabilities near the propeller stage
We present a simultaneous periodic and aperiodic timing study of the
accreting millisecond X-ray pulsar SAX J1808.4-3658. We analyze five outbursts
of the source and for the first time provide a full and systematic
investigation of the enigmatic phenomenon of the 1 Hz flares observed during
the final stages of some of the outbursts. We show that links between
pulsations and 1 Hz flares might exist, and suggest they are related with
hydrodynamic disk instabilities that are triggered close to the
disk-magnetosphere boundary layer when the system is entering the propeller
regime.Comment: Accepted by The Astrophysical Journal Updates and referee comments
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