812 research outputs found
Can the unresolved X-ray background be explained by emission from the optically-detected faint galaxies of the GOODS project?
The emission from individual X-ray sources in the Chandra Deep Fields and
XMM-Newton Lockman Hole shows that almost half of the hard X-ray background
above 6 keV is unresolved and implies the existence of a missing population of
heavily obscured active galactic nuclei (AGN). We have stacked the 0.5-8 keV
X-ray emission from optical sources in the Great Observatories Origins Deep
Survey (GOODS; which covers the Chandra Deep Fields) to determine whether these
galaxies, which are individually undetected in X-rays, are hosting the
hypothesised missing AGN. In the 0.5-6 keV energy range the stacked-source
emission corresponds to the remaining 10-20 per cent of the total background --
the fraction that has not been resolved by Chandra. The spectrum of the stacked
emission is consistent with starburst activity or weak AGN emission. In the 6-8
keV band, we find that upper limits to the stacked X-ray intensity from the
GOODS galaxies are consistent with the ~40 per cent of the total background
that remains unresolved, but further selection refinement is required to
identify the X-ray sources and confirm their contribution.Comment: 7 pages, 1 figure, accepted for publication in MNRA
Bayesian Blocks, A New Method to Analyze Structure in Photon Counting Data
I describe a new time-domain algorithm for detecting localized structures
(bursts), revealing pulse shapes, and generally characterizing intensity
variations. The input is raw counting data, in any of three forms: time-tagged
photon events (TTE), binned counts, or time-to-spill (TTS) data. The output is
the most likely segmentation of the observation into time intervals during
which the photon arrival rate is perceptibly constant -- i.e. has a fixed
intensity without statistically significant variations. Since the analysis is
based on Bayesian statistics, I call the resulting structures Bayesian Blocks.
Unlike most, this method does not stipulate time bins -- instead the data
themselves determine a piecewise constant representation. Therefore the
analysis procedure itself does not impose a lower limit to the time scale on
which variability can be detected. Locations, amplitudes, and rise and decay
times of pulses within a time series can be estimated, independent of any
pulse-shape model -- but only if they do not overlap too much, as deconvolution
is not incorporated. The Bayesian Blocks method is demonstrated by analyzing
pulse structure in BATSE -ray data. The MatLab scripts and sample data
can be found on the WWW at: http://george.arc.nasa.gov/~scargle/papers.htmlComment: 42 pages, 2 figures; revision correcting mathematical errors;
clarifications; removed Cyg X-1 sectio
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SYNTHESIS AND CHARACTERIZATION OF CARBON AEROGEL NANOCOMPOSITES CONTAINING DOUBLE-WALLED CARBON NANOTUBES
Carbon aerogels (CAs) are novel mesoporous materials with applications such as electrode materials for super capacitors and rechargeable batteries, adsorbents and advanced catalyst supports. To expand the potential application for these unique materials, recent efforts have focused on the design of CA composites with the goal of modifying the structure, conductivity or catalytic activity of the aerogel. Carbon nanotubes (CNTs) possess a number of intrinsic properties that make them promising materials in the design of composite materials. In addition, the large aspect ratios (100-1000) of CNTs means that small additions (less than 1 vol%) of CNTs can produce a composite with novel properties. Therefore, the homogeneous incorporation of CNTs into a CA matrix provides a viable route to new carbon-based composites with enhanced thermal, electrical and mechanical properties. One of the main challenges in preparing CNT composites is achieving a good uniform dispersion of nanotubes throughout the matrix. CAs are typically prepared through the sol-gel polymerization of resorcinol with formaldehyde in aqueous solution to produce organic gels that are supercritically dried and subsequently pyrolyzed in an inert atmosphere. Therefore, a significant issue in fabricating CA-CNT composites is dispersing the CNTs in the aqueous reaction media. Previous work in the design of CACNT composites have addressed this issue by using organic solvents in the sol-gel reaction to facilitate dispersion of the CNTs. To our knowledge, no data has been published involving the preparation of CA composites containing CNTs dispersed in aqueous media. In this report, we describe a new method for the synthesis of monolithic CA-CNT composites that involves the sol-gel polymerization of resorcinol and formaldehyde in an aqueous solution containing a surfactant-stabilized dispersion of double-walled carbon nanotubes (DWNT). One of the advantages of this approach is that it allows one to uniformly distribute CNTs in the CA matrix without compromising the synthetic control that is afforded by traditional organic sol-gel chemistry over the CA structure. We will describe the physical characterization of these novel materials as well as the influence of DWNT loading on the electrical conductivity of the CA composite
The HELLAS2XMM survey: XI. Unveiling the nature of X-ray Bright Optically Normal Galaxies
X-ray Bright Optically Normal Galaxies (XBONGs) constitute a small but not
negligible fraction of hard X-ray selected sources in recent Chandra and
XMM-Newton surveys. Even though several possibilities were proposed to explain
why a relatively luminous hard X-ray source does not leave any significant
signature of its presence in terms of optical emission lines, the nature of
XBONGs is still subject of debate. We aim to a better understanding of their
nature by means of a multiwavelength and morphological analysis of a small
sample of these sources. Good-quality photometric near-infrared data
(ISAAC/VLT) of four low-redshift (z=0.1-0.3) XBONGs, selected from the
HELLAS2XMM survey, have been used to search for the presence of the putative
nucleus, applying the surface-brightness decomposition technique through the
least-squares fitting program GALFIT. The surface brightness decomposition
allows us to reveal a nuclear point-like source, likely to be responsible of
the X-ray emission, in two out of the four sources. The results indicate that
moderate amounts of gas and dust, covering a large solid angle (possibly 4pi)
at the nuclear source, combined with the low nuclear activity, may explain the
lack of optical emission lines. The third XBONG is associated with an X-ray
extended source and no nuclear excess is detected in the near infrared at the
limits of our observations. The last source is associated to a close (d< 1
arcsec) double system and the fitting procedure cannot achieve a firm
conclusion.Comment: 20 pages, 12 figures, A&A in pres
Limits on the high redshift growth of massive black holes
We place firm upper limits on the global accretion history of massive black
holes at z>5 from the recently measured unresolved fraction of the cosmic X-ray
background. The maximum allowed unresolved intensity observed at 1.5 keV
implies a maximum accreted-mass density onto massive black holes of rho_acc <
1.4E4 M_sun Mpc^{-3} for z>5. Considering the contribution of lower-z AGNs, the
value reduces to rho_acc < 0.66E4 M_sun Mpc^{-3}. The tension between the need
for the efficient and rapid accretion required by the observation of massive
black holes already in place at z>7 and the strict upper limit on the accreted
mass derived from the X-ray background may indicate that black holes are rare
in high redshift galaxies, or that accretion is efficient only for black holes
hosted by rare galaxies.Comment: 5 pages, 1 figure, published in A&A Letter
Graphene oxide nanosheets modulate spinal glutamatergic transmission and modify locomotor behaviour in an in vivo zebrafish model
Graphene oxide (GO), an oxidised form of graphene, is widely used for biomedical applications, due to its dispersibility in water and simple surface chemistry tunability. In particular, small (less than 500 nm in lateral dimension) and thin (1-3 carbon monolayers) graphene oxide nanosheets (s-GO) have been shown to selectively inhibit glutamatergic transmission in neuronal cultures in vitro and in brain explants obtained from animals injected with the nanomaterial. This raises the exciting prospect that s-GO can be developed as a platform for novel nervous system therapeutics. It has not yet been investigated whether the interference of the nanomaterial with neurotransmission may have a downstream outcome in modulation of behaviour depending specifically on the activation of those synapses. To address this problem we use early stage zebrafish as an in vivo model to study the impact of s-GO on nervous system function. Microinjection of s-GO into the embryonic zebrafish spinal cord selectively reduces the excitatory synaptic transmission of the spinal network, monitored in vivo through patch clamp recordings, without affecting spinal cell survival. This effect is accompanied by a perturbation in the swimming activity of larvae, which is the locomotor behaviour generated by the neuronal network of the spinal cord. Such results indicate that the impact of s-GO on glutamate based neuronal transmission is preserved in vivo and can induce changes in animal behaviour. These findings pave the way for use of s-GO as a modulator of nervous system function
JASPAR 2016: a major expansion and update of the open-access database of transcription factor binding profiles.
JASPAR (http://jaspar.genereg.net) is an open-access database storing curated, non-redundant transcription factor (TF) binding profiles representing transcription factor binding preferences as position frequency matrices for multiple species in six taxonomic groups. For this 2016 release, we expanded the JASPAR CORE collection with 494 new TF binding profiles (315 in vertebrates, 11 in nematodes, 3 in insects, 1 in fungi and 164 in plants) and updated 59 profiles (58 in vertebrates and 1 in fungi). The introduced profiles represent an 83% expansion and 10% update when compared to the previous release. We updated the structural annotation of the TF DNA binding domains (DBDs) following a published hierarchical structural classification. In addition, we introduced 130 transcription factor flexible models trained on ChIP-seq data for vertebrates, which capture dinucleotide dependencies within TF binding sites. This new JASPAR release is accompanied by a new web tool to infer JASPAR TF binding profiles recognized by a given TF protein sequence. Moreover, we provide the users with a Ruby module complementing the JASPAR API to ease programmatic access and use of the JASPAR collection of profiles. Finally, we provide the JASPAR2016 R/Bioconductor data package with the data of this release
Prospects in space-based Gamma-Ray Astronomy
With the unequalled INTEGRAL observatory, ESA has provided a unique tool to
the astronomical community that has made Europe the world leader in the field
of gamma-ray astronomy. INTEGRAL provides an unprecedented survey of the soft
gamma-ray sky, revealing hundreds of sources of different kinds, new classes of
objects, extraordinary views of antimatter annihilation in our Galaxy, and
fingerprints of recent nucleosynthesis processes. While INTEGRAL provides the
longly awaited global overview over the soft gamma-ray sky, there is a growing
need to perform deeper, more focused investigations of gamma-ray sources,
comparable to the step that has been taken in X-rays by going from the ROSAT
survey satellite to the more focused XMM-Newton observatory. Technological
advances in the past years in the domain of gamma-ray focusing using Laue
diffraction techniques have paved the way towards a future European gamma-ray
mission, that will outreach past missions by large factors in sensitivity and
angular resolution. Such a future Gamma-Ray Imager will allow to study particle
acceleration processes and explosion physics in unprecedented depth, providing
essential clues on the intimate nature of the most violent and most energetic
processes in the Universe.Comment: 8 pages, 7 figures, to be published in the Proceedings of the 39th
ESLAB Symposiu
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