24 research outputs found
Broad Band X-ray Spectra of M31 Sources with BeppoSAX
We present the first spectral study of the X-ray emitting stellar sources in
M31 in the energy band from 0.1 to 10 keV. We find that the globular cluster
sources have spectral characteristics consistent with those of the Milky Way
object, namely that the spectrum can be described by a thermal model with 6-20
keV from 2 to 10 keV. Evidence of high absorption in some of these sources is
most likely an indication that they lie in or behind the HI ring in the disk of
the galaxy.
We also find one peculiar globular cluster source, with spectral
characteristics more typically associated with either High Mass X-ray Binaries
or black hole candidates. We therefore suggest that either the source has been
wrongly identified with a globular cluster or that the system contains a Black
Hole. We confirm earlier report that the spectrum of the bulge of M31 is
consistent with the superposition of many LMXB spectra. It is likely that a
large fraction of the 15-30 keV detection obtained from the PDS instrument is
associated with the bulge, thus extending the spectral data for this complex of
sources up to 30 keV. The high energy part of the spectrum can be parameterized
with typical LMXB spectra, while at low energies an additional component is
required.Comment: Accepted for publication in A&A, main journa
How to guess a gradient
How much can you say about the gradient of a neural network without computing
a loss or knowing the label? This may sound like a strange question: surely the
answer is "very little." However, in this paper, we show that gradients are
more structured than previously thought. Gradients lie in a predictable
low-dimensional subspace which depends on the network architecture and incoming
features. Exploiting this structure can significantly improve gradient-free
optimization schemes based on directional derivatives, which have struggled to
scale beyond small networks trained on toy datasets. We study how to narrow the
gap in optimization performance between methods that calculate exact gradients
and those that use directional derivatives. Furthermore, we highlight new
challenges in overcoming the large gap between optimizing with exact gradients
and guessing the gradients
Testing general relativity with accretion onto compact objects
The X-ray emission of neutron stars and black holes presents a rich
phenomenology that can lead us to a better understanding of their nature and to
address more general physics questions: Does general relativity apply in the
strong gravity regime? Is spacetime around black holes described by the Kerr
metric? This white paper considers how we can investigate these questions by
studying reverberation mapping and quasi-periodic oscillations in accreting
systems with a combination of high-spectral and high-timing resolution. In the
near future, we will be able to study compact objects in the X-rays in a new
way: advancements in transition-edge sensors (TES) technology will allow for
electron-volt-resolution spectroscopy combined with nanoseconds-precision
timing.Comment: White paper submitted for Astro2020 Decadal Survey. 8 pages, 2
figure
The time derivative of the kilohertz quasi-periodic oscillations in 4U 1636-53
We analysed all archival RXTE observations of the neutron-star low-mass X-ray
binary 4U 1636-53 up to May 2010. In 528 out of 1280 observations we detected
kilohertz quasi-periodic oscillations (kHz QPOs), with ~ 65% of these
detections corresponding to the so-called lower kHz QPO. Using this QPO we
measured, for the first time, the rate at which the QPO frequency changes as a
function of QPO frequency. For this we used the spread of the QPO frequency
over groups of 10 consecutive measurements, sampling timescales between 320 and
1600 s, and the time derivative of the QPO frequency over timescales of 32 to
160 s. We found that: (i) Both the QPO-frequency spread and the QPO time
derivative decrease by a factor ~ 3 as the QPO frequency increases. (ii) The
average value of the QPO time derivative decreases by a factor of ~ 2 as the
timescale over which the derivative is measured increases from less than 64 s
to 160 s. (iii) The relation between the absolute value of the QPO time
derivative and the QPO frequency is consistent with being the same both for the
positive and negative QPO-frequency derivative. We show that, if either the
lower or the upper kHz QPO reflects the Keplerian frequency at the inner edge
of the accretion disc, these results support a scenario in which the inner part
of the accretion disc is truncated at a radius that is set by the combined
effect of viscosity and radiation drag.Comment: 8 pages, 6 figures, Accepted for publication in MNRA
The kilohertz quasi-periodic oscillations during the Z and atoll phases of the unique transient XTE J1701--462
We analysed 866 observations of the neutron-star low-mass X-ray binary XTE
J1701-462 during its 2006-2007 outburst. XTE J1701-462 is the only example so
far of a source that during an outburst showed, beyond any doubt, spectral and
timing characteristics both of the Z and atoll type. We found that the lower
kHz QPO in the atoll phase has a significantly higher coherence and fractional
rms amplitude than any of the kHz QPOs seen during the Z phase, and that in the
same frequency range, atoll lower kHz QPOs show coherence and fractional rms
amplitude, respectively, 2 and 3 times larger than the Z kHz QPOs. Out of the
707 observations in the Z phase, there is no single observation in which the
kHz QPOs have a coherence or rms amplitude similar to those seen when XTE
J1701-462 was in the atoll phase, even though the total exposure time was about
5 times longer in the Z than in the atoll phase. Since it is observed in the
same source, the difference in QPO coherence and rms amplitude between the Z
and atoll phase cannot be due to neutron-star mass, magnetic field, spin,
inclination of the accretion disk, etc. If the QPO frequency is a function of
the radius in the accretion disk in which it is produced, our results suggest
that in XTE J1701-462 the coherence and rms amplitude are not uniquely related
to this radius. Here we argue that this difference is instead due to a change
in the properties of the accretion flow around the neutron star. Regardless of
the precise mechanism, our result shows that effects other than the geometry of
space time around the neutron star have a strong influence on the coherence and
rms amplitude of the kHz QPOs, and therefore the coherence and rms amplitude of
the kHz QPOs cannot be simply used to deduce the existence of the innermost
stable circular orbit around a neutron star.Comment: 11 pages, 5 figures, 2 tables, submitted to MNRA
Intrinsic and dust-induced polarization in gamma-ray burst afterglows: the case of GRB 021004
Polarization measurements for the optical counterpart to GRB 021004 are
presented and discussed. Our observations were performed with the TNG and the
VLT-UT3 (Melipal) during the first and fourth night after the gamma-ray burst
discovery. We find robust evidence of temporal evolution of the polarization,
which is therefore, at least partially, intrinsic to the optical transient. We
do not find convincing evidence of wavelength dependence for the intrinsic
polarization of the transient, in agreement with current polarization models
for optical afterglows. We discuss the role of dust, both in our galaxy and in
the host, in modifying the transmitted polarization vector, showing how a
sizable fraction of the observed polarized flux is due to Galactic selective
extinction, while it is not possible to single out any clear contribution from
dust in the host galaxy. We discuss how our data compare to those obtained by
different groups showing that a two-component model is required to describe the
complete dataset. This is not surprising given the complex lightcurve of GRB
021004.Comment: 9 pages, 6 postscript figures, A&A in pres
Modelling long term effects of cropping and managements systems on soil organic matter, C/N dynamics and crop growth
While simulation of cropping systems over a few years might reflect well the short term effects of management and cultivation, long term effects on soil properties and their consequences for crop growth and matter fluxes are not captured. Especially the effect on soil carbon sequestration/depletion is addressed by this task. Simulations of an ensemble of crop models are performed as transient runs over a period of 120 year using observed weather from three stations in Czech Republic (1961-2010) and transient long time climate change scenarios (2011-2080) from five GCM of the CMIP5 ensemble to assess the effect of different cropping and management systems on carbon sequestration, matter fluxes and crop production in an integrative way. Two cropping systems are regarded comprising two times winter wheat, silage maize, spring barley and oilseed rape. Crop rotations differ regarding their organic input from crop residues, nitrogen fertilization and implementation of catch crops. Models are applied for two soil types with different water holding capacity. Cultivation and nutrient management is adapted using management rules related to weather and soil conditions. Data of phenology and crop yield from the region of the regarded crops were provided to calibrate the models for crops of the rotations. Twelve models were calibrated in this first step. For the transient long term runs results of four models were submitted so far. Outputs are crop yields, nitrogen uptake, soil water and mineral nitrogen contents, as well as water and nitrogen fluxes to the atmosphere and groundwater. Changes in the carbon stocks and the consequences for nitrogen mineralisation, N fertilization and emissions also considered.
The Large Observatory For X-ray Timing: LOFT
LOFT, the Large Observatory for X-ray Timing, is a new space mission concept devoted to observations of Galactic and extra-Galactic sources in the X-ray domain with the main goals of probing gravity theory in the very strong field environment of black holes and other compact objects, and investigating the state of matter at supra-nuclear densities in neutron stars. The instruments on-board LOFT, the Large area detector and the Wide Field Monitor combine for the first time an unprecedented large effective area (~10 m2 at 8 keV) sensitive to X-ray photons mainly in the 2-30 keV energy range and a spectral resolution approaching that of CCD-based telescopes (down to 200 eV at 6 keV). LOFT is currently competing for a launch of opportunity in 2022 together with the other M3 mission candidates of the ESA Cosmic Vision Progra