147 research outputs found
Biological and Economic Considerations in Establishing a Short-Rotation Bioenergy Plantation
The development of bio-fuel synthesis technologies has led to increased interest in woody crops grown specifically for energy production. These woody feedstocks typically involve fast-growing species (e.g., Salix spp., Populus spp.) planted at high densities using short rotations and intensive cultural practices like weed control and fertilization. Under ideal conditions, this type of system can produce 20 dry Mg/ha/yr, which is substantially higher than the 2.5-4 dry Mg/ha/yr produced by pine plantations in the southern U.S. Many of these plantings are projected to be established on lower quality agricultural lands. Recent attempts at establishing these plantations have highlighted some of the challenges that landowners will need to overcome to achieve levels of production that are financially attractive. This paper will address some of the pitfalls and hurdles that need to be overcome before woody bio-fuel plantations will become widespread
The accretion environment in Vela X-1 during a flaring period using XMM-Newton
We present analysis of 100 ks contiguous XMM-Newton data of the prototypical
wind accretor Vela X-1. The observation covered eclipse egress between orbital
phases 0.134 and 0.265, during which a giant flare took place, enabling us to
study the spectral properties both outside and during the flare. This giant
flare with a peak luminosity of erg
s allows estimates of the physical parameters of the accreted structure
with a mass of g.
We have been able to model several contributions to the observed spectrum
with a phenomenological model formed by three absorbed power laws plus three
emission lines. After analysing the variations with orbital phase of the column
density of each component, as well as those in the Fe and Ni fluorescence
lines, we provide a physical interpretation for each spectral component.
Meanwhile, the first two components are two aspects of the principal accretion
component from the surface of the neutron star, and the third component seems
to be the \textit{X-ray light echo} formed in the stellar wind of the
companion.Comment: Accepted. Astronomy and Astrophysics, 201
Rank-(n – 1) convexity and quasiconvexity for divergence free fields
The CAST experiment at CERN (European Organization of Nuclear Research)
searches for axions from the sun. The axion is a pseudoscalar particle that was
motivated by theory thirty years ago, with the intention to solve the strong CP
problem. Together with the neutralino, the axion is one of the most promising
dark matter candidates. The CAST experiment has been taking data during the
last two years, setting an upper limit on the coupling of axions to photons
more restrictive than from any other solar axion search in the mass range below
0.1 eV. In 2005 CAST will enter a new experimental phase extending the
sensitivity of the experiment to higher axion masses. The CAST experiment
strongly profits from technology developed for high energy physics and for
X-ray astronomy: A superconducting prototype LHC magnet is used to convert
potential axions to detectable X-rays in the 1-10 keV range via the inverse
Primakoff effect. The most sensitive detector system of CAST is a spin-off from
space technology, a Wolter I type X-ray optics in combination with a prototype
pn-CCD developed for ESA's XMM-Newton mission. As in other rare event searches,
background suppression and a thorough shielding concept is essential to improve
the sensitivity of the experiment to the best possible. In this context CAST
offers the opportunity to study the background of pn-CCDs and its long term
behavior in a terrestrial environment with possible implications for future
space applications. We will present a systematic study of the detector
background of the pn-CCD of CAST based on the data acquired since 2002
including preliminary results of our background simulations.Comment: 11 pages, 8 figures, to appear in Proc. SPIE 5898, UV, X-Ray, and
Gamma-Ray Space Instrumentation for Astronomy XI
Status of the XMM-Newton cross-calibration with SASv6.5.0
Further achievements of the XMM-Newton cross-calibration - XMM internal as
well as with other X-ray missions - are presented. We explain the major changes
in the new version SASv6.5 of the XMM-Newton science analysis system. The
current status of the cross-calibration of the three EPIC cameras is shown.
Using a large sample of blazars, the pn energy redistribution at low energy
could be further calibrated, correcting the overestimation of fluxes in the
lowest energy regime. In the central CCDs of the MOSs, patches were identified
at the bore-sight positions, leading to an underestimation of the low energy
fluxes. The further improvement in the understanding of the cameras resulted in
a good agreement of the EPIC instruments down to lowest energies. The latest
release of the SAS software package already includes corrections for both
effects as shown in several examples of different types of sources. Finally the
XMM internal cross-calibration is completed by the presentation of the current
cross-calibration status between EPIC and RGS instruments. Major efforts have
been made in cross-calibrations with other X-ray missions, most importantly
with Chandra, of course, but also with currently observing satellites like
Swift.Comment: 6 pages, 23 figures. To appear in the proceedings of "The X-Ray
Universe 2005" conference, 2005 Sept 26-30, El Escorial, Madrid, Spai
2003--2005 INTEGRAL and XMM-Newton observations of 3C 273
The aim of this paper is to study the evolution of the broadband spectrum of
one of the brightest and nearest quasars 3C 273.
We analyze the data obtained during quasi-simultaneous INTEGRAL and XMM
monitoring of the blazar 3C 273 in 2003--2005 in the UV, X-ray and soft
gamma-ray bands and study the results in the context of the long-term evolution
of the source.
The 0.2-100 keV spectrum of the source is well fitted by a combination of a
soft cut-off power law and a hard power law. No improvement of the fit is
achieved if one replaces the soft cut-off power law by either a blackbody, or a
disk reflection model. During the observation period the source has reached the
historically softest state in the hard X-ray domain with a photon index
. Comparing our data with available archived X-ray data
from previous years, we find a secular evolution of the source toward softer
X-ray emission (the photon index has increased by
over the last thirty years). We argue that existing theoretical models have to
be significantly modified to account for the observed spectral evolution of the
source.Comment: 11 pages, accepted to A&
Simultaneous observations of the quasar 3C 273 with INTEGRAL, XMM-Newton and RXTE
INTEGRAL has observed the bright quasar 3C 273 on 3 epochs in January 2003 as
one of the first observations of the open programme. The observation on January
5 was simultaneous with RXTE and XMM-Newton observations. We present here a
first analysis of the continuum emission as observed by these 3 satellites in
the band from 3 keV to 500 keV. The continuum spectral energy distribution of
3C 273 was observed to be weak and steep in the high energies during this
campaign. We present the actual status of the cross calibrations between the
instruments on the three platforms using the calibrations available in June
2003.Comment: 4 figures, accepted for publication in A+A letter
The 2-79 keV X-ray Spectrum of the Circinus Galaxy with NuSTAR, XMM-Newton and Chandra: a Fully Compton-Thick AGN
The Circinus galaxy is one of the nearest obscured AGN, making it an ideal
target for detailed study. Combining archival Chandra and XMM-Newton data with
new NuSTAR observations, we model the 2-79 keV spectrum to constrain the
primary AGN continuum and to derive physical parameters for the obscuring
material. Chandra's high angular resolution allows a separation of nuclear and
off-nuclear galactic emission. In the off-nuclear diffuse emission we find
signatures of strong cold reflection, including high equivalent-width neutral
Fe lines. This Compton-scattered off-nuclear emission amounts to 18% of the
nuclear flux in the Fe line region, but becomes comparable to the nuclear
emission above 30 keV. The new analysis no longer supports a prominent
transmitted AGN component in the observed band. We find that the nuclear
spectrum is consistent with Compton-scattering by an optically-thick torus,
where the intrinsic spectrum is a powerlaw of photon index ,
the torus has an equatorial column density of cm and the intrinsic AGN keV luminosity is
erg/s. These values place Circinus along the same
relations as unobscured AGN in accretion rate-vs- and -vs-
phase space. NuSTAR's high sensitivity and low background allow us to study the
short time-scale variability of Circinus at X-ray energies above 10 keV for the
first time. The lack of detected variability favors a Compton-thick absorber,
in line with the the spectral fitting results.Comment: Accepted for publication in Ap
Timing variability of Vela X-1 during a bright flare
The X-ray Universe 2014, edited by Jan-Uwe Ness. Online at http://www.cosmos.esa.int/web/xmm-newton/2014-symposium/, id.130We present an in-depth analysis of the temporal behaviour of the HMXB Vela X-1 which exhibited strongly varying flux levels during an observation by XMM-Newton. During the 100 ks observation the source went from being in a highly absorbed initial state, to one of increased activity which was followed by a giant X-ray flare, before finally reaching a settled state with low absorption. The lower absorption state allows the normally absorbed pulse profile below 1 keV to be studied. Vela X-1 also showed multiple changes in the structure of the pulse profile including a phase lag with the harder photons leading the softer ones. A change in the pulse period, significant at approximately the 2σ level, was observed at the beginning of the powerful flare and is likely related to the large accreted mass. For comparison purposes, an earlier XMM-Newton observation in a lower active state is used to put the variations in pulse profile and phase lag in context
XMM-Newton observations of 3C 273
A series of nine XMM-Newton observations of the radio-loud quasar 3C 273 are
presented, concentrating mainly on the soft excess. Although most of the
individual observations do not show evidence for iron emission, co-adding them
reveals a weak, broad line (EW ~ 56 eV). The soft excess component is found to
vary, confirming previous work, and can be well fitted with multiple blackbody
components, with temperatures ranging between ~40 and ~330 eV, together with a
power-law. Alternatively, a Comptonisation model also provides a good fit, with
a mean electron temperature of ~350 eV, although this value is higher when the
soft excess is more luminous over the 0.5-10 keV energy band. In the RGS
spectrum of 3C 273, a strong detection of the OVII He-alpha absorption line at
zero redshift is made; this may originate in warm gas in the local
intergalactic medium, consistent with the findings of both Fang et al. (2003)
and Rasmussen et al. (2003).Comment: 12 pages, 19 figures, accepted for publication in MNRA
The relative and absolute timing accuracy of the EPIC-pn camera on XMM-Newton, from X-ray pulsations of the Crab and other pulsars
Reliable timing calibration is essential for the accurate comparison of
XMM-Newton light curves with those from other observatories, to ultimately use
them to derive precise physical quantities. The XMM-Newton timing calibration
is based on pulsar analysis. However, as pulsars show both timing noise and
glitches, it is essential to monitor these calibration sources regularly. To
this end, the XMM-Newton observatory performs observations twice a year of the
Crab pulsar to monitor the absolute timing accuracy of the EPIC-pn camera in
the fast Timing and Burst modes. We present the results of this monitoring
campaign, comparing XMM-Newton data from the Crab pulsar (PSR B0531+21) with
radio measurements. In addition, we use five pulsars (PSR J0537-69, PSR
B0540-69, PSR B0833-45, PSR B1509-58 and PSR B1055-52) with periods ranging
from 16 ms to 197 ms to verify the relative timing accuracy. We analysed 38
XMM-Newton observations (0.2-12.0 keV) of the Crab taken over the first ten
years of the mission and 13 observations from the five complementary pulsars.
All the data were processed with the SAS, the XMM-Newton Scientific Analysis
Software, version 9.0. Epoch folding techniques coupled with \chi^{2} tests
were used to derive relative timing accuracies. The absolute timing accuracy
was determined using the Crab data and comparing the time shift between the
main X-ray and radio peaks in the phase folded light curves. The relative
timing accuracy of XMM-Newton is found to be better than 10^{-8}. The strongest
X-ray pulse peak precedes the corresponding radio peak by 306\pm9 \mus, which
is in agreement with other high energy observatories such as Chandra, INTEGRAL
and RXTE. The derived absolute timing accuracy from our analysis is \pm48 \mus.Comment: 16 pages, 9 figures. Accepted for publication on A&
- …