24 research outputs found
Augmenting x-ray single particle imaging reconstruction with self-supervised machine learning
The development of X-ray Free Electron Lasers (XFELs) has opened numerous
opportunities to probe atomic structure and ultrafast dynamics of various
materials. Single Particle Imaging (SPI) with XFELs enables the investigation
of biological particles in their natural physiological states with unparalleled
temporal resolution, while circumventing the need for cryogenic conditions or
crystallization. However, reconstructing real-space structures from
reciprocal-space x-ray diffraction data is highly challenging due to the
absence of phase and orientation information, which is further complicated by
weak scattering signals and considerable fluctuations in the number of photons
per pulse. In this work, we present an end-to-end, self-supervised machine
learning approach to recover particle orientations and estimate reciprocal
space intensities from diffraction images only. Our method demonstrates great
robustness under demanding experimental conditions with significantly enhanced
reconstruction capabilities compared with conventional algorithms, and
signifies a paradigm shift in SPI as currently practiced at XFELs
Testing the data framework for an AI algorithm in preparation for high data rate X-ray facilities
The advent of next-generation X-ray free electron lasers will be capable of
delivering X-rays at a repetition rate approaching 1 MHz continuously. This
will require the development of data systems to handle experiments at these
type of facilities, especially for high throughput applications, such as
femtosecond X-ray crystallography and X-ray photon fluctuation spectroscopy.
Here, we demonstrate a framework which captures single shot X-ray data at the
LCLS and implements a machine-learning algorithm to automatically extract the
contrast parameter from the collected data. We measure the time required to
return the results and assess the feasibility of using this framework at high
data volume. We use this experiment to determine the feasibility of solutions
for `live' data analysis at the MHz repetition rate
A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope
Globular clusters with their large populations of millisecond pulsars (MSPs)
are believed to be potential emitters of high-energy gamma-ray emission. Our
goal is to constrain the millisecond pulsar populations in globular clusters
from analysis of gamma-ray observations. We use 546 days of continuous
sky-survey observations obtained with the Large Area Telescope aboard the Fermi
Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular
clusters. Steady point-like high-energy gamma-ray emission has been
significantly detected towards 8 globular clusters. Five of them (47 Tucanae,
Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices and clear evidence for an exponential cut-off in the range
1.0-2.6 GeV, which is the characteristic signature of magnetospheric emission
from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral
indices , however the presence of an exponential cut-off
can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC
6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral
properties. From the observed gamma-ray luminosities, we estimate the total
number of MSPs that is expected to be present in these globular clusters. We
show that our estimates of the MSP population correlate with the stellar
encounter rate and we estimate 2600-4700 MSPs in Galactic globular clusters,
commensurate with previous estimates. The observation of high-energy gamma-ray
emission from a globular cluster thus provides a reliable independent method to
assess their millisecond pulsar populations that can be used to make
constraints on the original neutron star X-ray binary population, essential for
understanding the importance of binary systems in slowing the inevitable core
collapse of globular clusters.Comment: Accepted for publication in A&A. Corresponding authors: J.
Kn\"odlseder, N. Webb, B. Pancraz
Fermi Large Area Telescope Constraints on the Gamma-ray Opacity of the Universe
The Extragalactic Background Light (EBL) includes photons with wavelengths
from ultraviolet to infrared, which are effective at attenuating gamma rays
with energy above ~10 GeV during propagation from sources at cosmological
distances. This results in a redshift- and energy-dependent attenuation of the
gamma-ray flux of extragalactic sources such as blazars and Gamma-Ray Bursts
(GRBs). The Large Area Telescope onboard Fermi detects a sample of gamma-ray
blazars with redshift up to z~3, and GRBs with redshift up to z~4.3. Using
photons above 10 GeV collected by Fermi over more than one year of observations
for these sources, we investigate the effect of gamma-ray flux attenuation by
the EBL. We place upper limits on the gamma-ray opacity of the Universe at
various energies and redshifts, and compare this with predictions from
well-known EBL models. We find that an EBL intensity in the optical-ultraviolet
wavelengths as great as predicted by the "baseline" model of Stecker et al.
(2006) can be ruled out with high confidence.Comment: 42 pages, 12 figures, accepted version (24 Aug.2010) for publication
in ApJ; Contact authors: A. Bouvier, A. Chen, S. Raino, S. Razzaque, A.
Reimer, L.C. Reye
Gamma-ray and radio properties of six pulsars detected by the fermi large area telescope
We report the detection of pulsed γ-rays for PSRs J0631+1036, J0659+1414, J0742-2822, J1420-6048, J1509-5850, and J1718-3825 using the Large Area Telescope on board the Fermi Gamma-ray Space Telescope (formerly known as GLAST). Although these six pulsars are diverse in terms of their spin parameters, they share an important feature: their γ-ray light curves are (at least given the current count statistics) single peaked. For two pulsars, there are hints for a double-peaked structure in the light curves. The shapes of the observed light curves of this group of pulsars are discussed in the light of models for which the emission originates from high up in the magnetosphere. The observed phases of the γ-ray light curves are, in general, consistent with those predicted by high-altitude models, although we speculate that the γ-ray emission of PSR J0659+1414, possibly featuring the softest spectrum of all Fermi pulsars coupled with a very low efficiency, arises from relatively low down in the magnetosphere. High-quality radio polarization data are available showing that all but one have a high degree of linear polarization. This allows us to place some constraints on the viewing geometry and aids the comparison of the γ-ray light curves with high-energy beam models
Rare
We have searched for baryon-containing radiative penguin decays in 9.7 × 106
events collected at the Υ(4S) with the CLEO detector. We find no evidence for such decays, and set a 90% confidence level upper limit of . Corrections to CLEO’s recent b → s
γ measurement due to decays are well within the errors quoted. A search for semileptonic decays of B mesons to inclusive final states in the same data sample found no evidence for such decays and set an upper limit of . These limits suggest that external W emission is not the dominant source of baryon production in B decay.
PACS: 13.20.HeDecays of Bottom Mesons – 13.40.HqElectromagnetic Decays – 13.60.RjBaryon Productio