55 research outputs found
Diffractive X-ray Telescopes
Diffractive X-ray telescopes using zone plates, phase Fresnel lenses, or
related optical elements have the potential to provide astronomers with true
imaging capability with resolution several orders of magnitude better than
available in any other waveband. Lenses that would be relatively easy to
fabricate could have an angular resolution of the order of micro-arc-seconds or
even better, that would allow, for example, imaging of the distorted space-
time in the immediate vicinity of the super-massive black holes in the center
of active galaxies What then is precluding their immediate adoption? Extremely
long focal lengths, very limited bandwidth, and difficulty stabilizing the
image are the main problems. The history, and status of the development of such
lenses is reviewed here and the prospects for managing the challenges that they
present are discussed.Comment: 46 pages, 15 figures, invited review paper to be published in a
special issue on "X-Ray Focusing: Techniques and Applications" of the online
journal "X-Ray Optics & Instrumentation
Positrons in the Galaxy: Their Births, Marriages and Deaths
High energy (approximately GeV) positrons are seen within cosmic rays and observation of a narrow line at 511 keV shows that positrons are annihilating in the galaxy after slowing down to approximately keV energies or less. Our state of knowledge of the origin of these positrons, of the formation of positronium 'atoms', and of the circumstances of their annihilation or escape from the galaxy are reviewed and the question of whether the two phenomena are linked is discussed
The doubling of the superorbital period of Cyg X-1
We study properties of the superorbital modulation of the X-ray emission of
Cyg X-1. We find that it has had a stable period of about 300 d in soft and
hard X-rays and in radio since 2005 until at least 2010, which is about double
the previously seen period. This new period, seen in the hard spectral state
only, is detected not only in the light curves but also in soft X-ray hardness
ratios and in the amplitude of the orbital modulation. On the other hand, the
spectral slope in hard X-rays, >20 keV, averaged over superorbital bins is
constant, and the soft and hard X-rays and the radio emission change in phase.
This shows that the superorbital variability consists of changing the
normalization of an intrinsic spectrum of a constant shape and of changes of
the absorbing column density with the phase. The maximum column density is
achieved at the superorbital minimum. The amplitude changes are likely to be
caused by a changing viewing angle of an anisotropic emitter, most likely a
precessing accretion disc. The constant shape of the intrinsic spectrum shows
that this modulation is not caused by a changing accretion rate. The modulated
absorbing column density shows the presence of a bulge around the disc centre,
as proposed previously. We also find the change of the superorbital period from
about 150 d to about 300 d to be associated with almost unchanged average X-ray
fluxes, making the period change difficult to explain in the framework of
disc-irradiation models. Finally, we find no correlation of the X-ray and radio
properties with the reported detections in the GeV and TeV gamma-ray range.Comment: MNRAS, in press, 8 page
When a Standard Candle Flickers
The Crab is the only bright steady source in the X-ray sky. The Crab consists of a pulsar wind nebula, a synchrotron nebula, and a cloud of expanding ejecta. On small scales, the Crab is extremely complex and turbulent. X-ray astronomers have often used the Crab as a standard candle to calibrate instruments, assuming its spectrum and overall flux remains constant over time. Four instruments (Fermi/GBM, RXTE/PCA, Swift/BAT, INTEGRAL/ISGRI) show a approx.5% (50 m Crab) decline in the Crab from 2008-2010. This decline appears to be larger with increasing energy and is not present in the pulsed flux, implying changes in the shock acceleration, electron population or magnetic field in the nebula. The Crab is known to be dynamic on small scales, so it is not too surprising that its total flux varies as well. Caution should be taken when using the Crab for in-orbit calibrations
All-Sky Monitoring of Variable Sources with Fermi GBM
This slide presentation reviews the monitoring of variable sources with the Fermi Gamma Ray Burst Monitor (GBM). It reviews the use of the Earth Occultation technique, the observations of the Crab Nebula with the GBM, and the comparison with other satellite's observations. The instruments on board the four satellites indicate a decline in the Crab from 2008-2010
Large expert-curated database for benchmarking document similarity detection in biomedical literature search
Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe
26th Annual Computational Neuroscience Meeting (CNS*2017): Part 3 - Meeting Abstracts - Antwerp, Belgium. 15–20 July 2017
This work was produced as part of the activities of FAPESP Research,\ud
Disseminations and Innovation Center for Neuromathematics (grant\ud
2013/07699-0, S. Paulo Research Foundation). NLK is supported by a\ud
FAPESP postdoctoral fellowship (grant 2016/03855-5). ACR is partially\ud
supported by a CNPq fellowship (grant 306251/2014-0)
Sensitivity of coded mask telescopes
Simple formulae are often used to estimate the sensitivity of coded mask X-ray or gamma-ray telescopes, but these are strictly only applicable if a number of basic assumptions are met. Complications arise, for example, if a grid structure is used to support the mask elements, if the detector spatial resolution is not good enoug
- …