3,221 research outputs found
Designing strategies to control grey mould in strawberry cultivation using decision support systems
Grey mould is one of the major diseases in strawberry cultivation. Fungicides to control Botrytis cinerea are applied frequently during flowering and sometimes at harvest. Reduction of pesticide use is one of the major aims of the Dutch government. Implementation of a Decision Support System (DSS) helps to achieve this goal. Pin point timing of fungicide application can possibly improve the efficacy of the treatment and reduce the number of spray applications. Predicted weather data to forecast infection risks are used by most DSS’s. However in strawberry cultivation irrigation is a daily practice. The effect of overhead irrigation on the Botrytis infection risk is unknown. This is one of the reasons that strawberry growers infrequently use DSS’s. Therefore adaptation of the model to agricultural management is necessary. Under low disease pressure DSS BoWaS controlled Botrytis fruit rot 62% better then routine applications of fungicides, with a 50% reduction of fungicide input. Adding an irrigation or a disease pressure sub-routine did not improve the model under low disease pressure. BoWaS based on disease pressure and weather resulted in better control of grey mould then the weather based BoWaS, under high disease pressure. Adding an irrigation rule did not improve the model further. Using the modified BoWaS reduced fungicide input with 36% compared to routine applications with the same efficacy
Coronal--Temporal Correlations in GX339-4: Hysteresis, Possible Reflection Changes, and Implications for ADAFs
We present spectral fits and timing analysis of Rossi X-ray Timing Explorer
observations of GX339-4. These observations were carried out over a span of
more than two years and encompassed both the soft/high and hard/low states.
Hysteresis in the soft state/hard state transition is observed. The hard state
exhibits a possible anti-correlation between coronal compactness (i.e.,
spectral hardness) and the covering fraction of cold, reflecting material. The
correlation between `reflection fraction' and soft X-ray flux, however, appears
to be more universal. Furthermore, low flux, hard state observations - taken
over a decline into quiescence- show that the Fe line, independent of
`reflection fraction', remains broad and at a roughly constant equivalent
width, counter to expectations from ADAF models. All power spectral densities
(PSD) of the hard state X-ray lightcurves are describable as the sum of just a
few broad, quasi-periodic features with frequencies that roughly scale as
coronal compactness to the -3/2 power. Similar to observations of Cyg X-1, time
lags between soft and hard variability anti-correlate with coronal compactness.
A stronger correlation is seen between the time lags and the `reflection
fraction'.Comment: 29 Pages, 17 Figures, 6 Tables. Accepted for Publication in MNRAS.
(Abstract Abridged
XMM-Newton Observations of the Be/X-ray transient A0538-66 in quiescence
We present XMM-Newton observations of the recurrent Be/X-ray transient
A0538-66, situated in the Large Magellanic Cloud, in the quiescent state.
Despite a very low luminosity state of (5-8)E33 ergs/s in the range 0.3-10 keV,
the source is clearly detected up to ~8 keV. and can be fitted using either a
power law with photon index alpha=1.9+-0.3 or a bremsstrahlung spectrum with
kT=3.9+3.9-1.7 keV. The spectral analysis confirms that the off-state spectrum
is hard without requiring any soft component, contrary to the majority of
neutron stars observed in quiescence up to now.Comment: Accepted for proceedings of 5th INTEGRAL Worksho
RXTE Observation of Cygnus X-1: II. Timing Analysis
We present timing analysis for a Rossi X-ray Timing Explorer observation of
Cygnus X-1 in its hard/low state. This was the first RXTE observation of Cyg
X-1 taken after it transited back to this state from its soft/high state.
RXTE's large effective area, superior timing capabilities, and ability to
obtain long, uninterrupted observations have allowed us to obtain measurements
of the power spectral density (PSD), coherence function, and Fourier time lags
to a decade lower in frequency and half a decade higher in frequency than
typically was achieved with previous instruments. Notable aspects of our
observations include a weak 0.005 Hz feature in the PSD coincident with a
coherence recovery; a `hardening' of the high-frequency PSD with increasing
energy; a broad frequency range measurement of the coherence function,
revealing rollovers from unity coherence at both low and high frequency; and an
accurate determination of the Fourier time lags over two and a half decades in
frequency. As has been noted in previous similar observations, the time delay
is approximately proportional to f^(-0.7), and at a fixed Fourier frequency the
time delay of the hard X-rays compared to the softest energy channel tends to
increase logarithmically with energy. Curiously, the 0.01-0.2 Hz coherence
between the highest and lowest energy bands is actually slightly greater than
the coherence between the second highest and lowest energy bands. We carefully
describe all of the analysis techniques used in this paper, and we make
comparisons of the data to general theoretical expectations. In a companion
paper, we make specific comparisons to a Compton corona model that we have
successfully used to describe the energy spectral data from this observation.Comment: To Be Published in the Astrophysical Journal. 18 pages. Uses
emulatepaj.st
RXTE Observation of Cygnus X-1: Spectral Analysis
We present the results of the analysis of the broad-band spectrum of Cygnus
X-1 from 3.0 to 200 keV, using data from a 10 ksec observation by the Rossi
X-ray Timing Explorer. The spectrum can be well described phenomenologically by
an exponentially cut-off power law with a photon index Gamma = 1.45 +/- 0.02 (a
value considerably harder than typically found), e-folding energy E_fold = 162
+/- 9 keV, plus a deviation from a power law that formally can be modeled as a
thermal blackbody with temperature kT_bb = 1.2 +/1 0.2 keV. Although the 3 - 30
keV portion of the spectrum can be fit with a reflected power law with Gamma =
1.81 +/- 0.01 and covering fraction f = 0.35 +/- 0.02, the quality of the fit
is significantly reduced when the HEXTE data in the 30 - 100 keV range is
included, as there is no observed hardening in the power law within this energy
range. As a physical description of this system, we apply the accretion disc
corona models of Dove, Wilms & Begelman (1997) --- where the temperature of the
corona is determined self-consistently. A spherical corona with a total optical
depth tau = 1.6 +/- 0.1 and an average temperature kT_c = 87 +/- 5 keV,
surrounded by an exterior cold disc, does provide a good description of the
data (reduced chi-squared = 1.55). These models deviate from the data by up to
7% in the 5 - 10 keV range, and we discuss possible reasons for these
discrepancies. However, considering how successfully the spherical corona
reproduces the 10 - 200 keV data, such ``photon-starved'' coronal geometries
seem very promising for explaining the accretion processes of Cygnus X-1.Comment: Revised version (added content). 8 pages, 6 figures, 1 table.tex
file, latex, uses mn.sty. Accepted for publication in MNRA
Chandra and RXTE spectroscopy of the accreting msec pulsar IGR J00291+5934
We report on an observation of the recently discovered accreting millisecond X-ray pulsar IGR J00291+5934 performed with the RXTE-Proportional Counter Array (PCA) and Chandra-High Energy Transmission Grating Spectrometer (HETGS). The RXTE data are from a twoweek follow-up of the source, while the Chandra observation took place around the end of the follow-up, about 12 days after the discovery of
the source, when the source flux had decreased already by a factor of ten. The analysis of the Chandra data allowed us to extract the most precise X-ray position of IGR J00291+5934, RA = 00h 29m 03.08s, and Dec =+59â—¦ 34 19.2 (0.6 error), compatible with the optical and radio ones.
We find that the spectra of IGR J00291+5934 can be described by a combination of a thermal component and a power-law. Along the outburst detected by PCA, the power-law photon index showed no particular trend, while the thermal component (∼1 keV, interpreted as a hot spot on
the neutron star surface) became weaker until non-detection. In the simultaneous observation of the weak Chandra /RXTE spectrum, there was no longer any indication of the ∼1 keV thermal component, while we detected a colder thermal component (∼0.4 keV) that we interpret as the
emission from the cold disc. A hint of a 6.4 keV iron line was detected, together with an excess around 6.8 keV and absorption feature around 7.1 keV. The last two features have never been detected in the spectra of accretion-driven millisecond pulsars before and, if confirmed, would
suggest the presence of an expanding hot corona with high outflow velocities
The column density towards LMC X-1
We measure the neutral absorption towards the black hole X-ray binary system
LMC X-1 from six archival soft X-ray spectra obtained with the gratings and/or
CCD detectors on Chandra, XMM-Newton, and Swift. Four spectral models for the
soft continuum have been investigated. While the powerlaw model may
overestimate NH considerably, the others give consistent results. Taking the
lower metalicity of the Large Magellanic Cloud into account, we find equivalent
hydrogen column densities of N_H = (1.0-1.3)*10^22 cm^-2, with a systematic
dependence on the orbital phase. This variation of the neutral absorption can
nearly explain the orbital modulation of the soft X-ray flux recently detected
with the All Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE).Comment: 4 pages, accepted for publication as a Letter in Astronomy and
Astrophysic
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