95 research outputs found
Supernova remnants and gamma-ray sources
A review of the possible relationship between gamma-ray sources and supernova
remnants (SNRs) is presented. Particular emphasis is given to the analysis of
the observational status of the problem of cosmic ray acceleration at SNR shock
fronts. All positional coincidences between SNRs and unidentified gamma-ray
sources listed in the Third EGRET Catalog at low Galactic latitudes are
discussed on a case by case basis. For several coincidences of particular
interest, new CO(J=1-0) and radio continuum maps are shown, and the mass
content of the SNR surroundings is determined. The contribution to the
gamma-ray flux observed that might come from cosmic ray particles (particularly
nuclei) locally accelerated at the SNR shock fronts is evaluated. We discuss
the prospects for future research in this field and remark on the possibilities
for observations with forthcoming gamma-ray instruments.Comment: Final version of a review article, to appear in the Physics Reports
(82 pages, 31 figures). Figures requiring high quality are just too large and
too many to be included here. Please download them from
http://www.angelfire.com/id/dtorres/down3.htm
Parallaxes for star forming regions in the inner Perseus spiral arm
We report trigonometric parallax and proper motion measurements of 6.7-GHz
CH3OH and 22-GHz H2O masers in eight high-mass star-forming regions (HMSFRs)
based on VLBA observations as part of the BeSSeL Survey. The distances of these
HMSFRs combined with their Galactic coordinates, radial velocities, and proper
motions, allow us to assign them to a segment of the Perseus arm with ~< 70
deg. These HMSFRs are clustered in Galactic longitude from ~30 deg to ~50,
neighboring a dirth of such sources between longitudes ~50 deg to ~90 deg.Comment: 18 pages, 4 figures, accepted for publication in The Astronomical
Journal. arXiv admin note: text overlap with arXiv:1312.385
Supernova-Remnant Origin of Cosmic Rays?
It is thought that Galactic cosmic ray (CR) nuclei are gradually accelerated
to high energies (up to ~300 TeV/nucleon, where 1TeV=10^12eV) in the expanding
shock-waves connected with the remnants of powerful supernova explosions.
However, this conjecture has eluded direct observational confirmation^1,2 since
it was first proposed in 1953 (ref. 3). Enomoto et al.^4 claim to have finally
found definitive evidence that corroborates this model, proposing that the
very-high-energy, TeV-range, gamma-rays from the supernova remnant (SNR) RX
J1713.7-3946 are due to the interactions of energetic nuclei in this region.
Here we argue that their claim is not supported by the existing multiwavelength
spectrum of this source. The search for the origin(s) of Galactic cosmic ray
nuclei may be closing in on the long-suspected supernova-remnant sources, but
it is not yet over.Comment: 4 pages, 1 Figur
Millimeter dust continuum emission unveiling the true mass of giant molecular clouds in the Small Magellanic Cloud
CO observations have been so far the best way to trace molecular gas in
external galaxies, but at low metallicity the gas mass deduced could be largely
underestimated. At present, the kinematic information of CO data cubes are used
to estimate virial masses and trace the total mass of the molecular clouds.
Millimeter dust emission can also be used as a dense gas tracer and could
unveil H2 envelopes lacking CO. These different tracers must be compared in
different environments. This study compares virial masses to masses deduced
from millimeter emission, in two GMC samples: the local molecular clouds in our
Galaxy and their equivalents in the Small Magellanic Cloud (SMC), one of the
nearest low metallicity dwarf galaxy. In our Galaxy, mass estimates deduced
from millimeter emission are consistent with masses deduced from gamma ray
analysis and trace the total mass of the clouds. Virial masses are
systematically larger (twice on average) than mass estimates from millimeter
dust emission. This difference decreases toward high masses and has already
been reported in previous studies. In the SMC however, molecular cloud masses
deduced from SIMBA millimeter observations are systematically higher (twice on
average for conservative values of the dust to gas ratio and dust emissivity)
than the virial masses from SEST CO observations. The observed excess can not
be accounted for by any plausible change of dust properties. Taking a general
form for the virial theorem, we show that a magnetic field strength of ~15
micro Gauss in SMC clouds could provide additional support to the clouds and
explain the difference observed. Masses of SMC molecular clouds have therefore
been underestimated so far. Magnetic pressure may contribute significantly to
their support.Comment: 10 pages, 2 figures, Astronomy & Astrophysics accepte
The circadian clock regulates rhythmic erythropoietin expression in the murine kidney
Generation of circadian rhythms is cell-autonomous and relies on a transcription/translation feedback loop controlled by a family of circadian clock transcription factor activators including CLOCK, BMAL1 and repressors such as CRY1 and CRY2. The aim of the present study was to examine both the molecular mechanism and the hemopoietic implication of circadian erythropoietin expression. Mutant mice with homozygous deletion of the core circadian clock genes cryptochromes 1 and 2 (Cry-null) were used to elucidate circadian erythropoietin regulation. Wild-type control mice exhibited a significant difference in kidney erythropoietin mRNA expression between circadian times 06 and 18. In parallel, a significantly higher number of erythropoietin-producing cells in the kidney (by RNAscope®) and significantly higher levels of circulating erythropoietin protein (by ELISA) were detected at circadian time 18. Such changes were abolished in Cry-null mice and were independent from oxygen tension, oxygen saturation, or expression of hypoxia-inducible factor 2 alpha, indicating that circadian erythropoietin expression is transcriptionally regulated by CRY1 and CRY2. Reporter gene assays showed that the CLOCK/BMAL1 heterodimer activated an E-box element in the 5' erythropoietin promoter. RNAscope® in situ hybridization confirmed the presence of Bmal1 in erythropoietin-producing cells of the kidney. In Cry-null mice, a significantly reduced number of reticulocytes was found while erythrocyte numbers and hematocrit were unchanged. Thus, circadian erythropoietin regulation in the normoxic adult murine kidney is transcriptionally controlled by master circadian activators CLOCK/BMAL1, and repressors CRY1/CRY2. These findings may have implications for kidney physiology and disease, laboratory diagnostics, and anemia therapy
DNA-bridging by an archaeal histone variant via a unique tetramerisation interface
In eukaryotes, histone paralogues form obligate heterodimers such as H3/H4 and H2A/H2B that assemble into octameric nucleosome particles. Archaeal histones are dimeric and assemble on DNA into 'hypernucleosome' particles of varying sizes with each dimer wrapping 30 bp of DNA. These are composed of canonical and variant histone paralogues, but the function of these variants is poorly understood. Here, we characterise the structure and function of the histone paralogue MJ1647 from Methanocaldococcus jannaschii that has a unique C-terminal extension enabling homotetramerisation. The 1.9 Å X-ray structure of a dimeric MJ1647 species, structural modelling of the tetramer, and site-directed mutagenesis reveal that the C-terminal tetramerization module consists of two alpha helices in a handshake arrangement. Unlike canonical histones, MJ1647 tetramers can bridge two DNA molecules in vitro. Using single-molecule tethered particle motion and DNA binding assays, we show that MJ1647 tetramers bind ~60 bp DNA and compact DNA in a highly cooperative manner. We furthermore show that MJ1647 effectively competes with the transcription machinery to block access to the promoter in vitro. To the best of our knowledge, MJ1647 is the first histone shown to have DNA bridging properties, which has important implications for genome structure and gene expression in archaea
Status of the connection between unidentified EGRET sources and supernova remnants: The case of CTA 1
In this paper we briefly comment on the observational status of the possible
physical association between unidentified EGRET sources and supernova remnants
(SNRs) in our Galaxy. We draw upon recent results presented in the review by
Torres et al. (Physics Reports, 2003), concerning molecular gas in the vicinity
of all 19 SNRs found to be positionally coincident with EGRET sources at low
Galactic latitudes. In addition, we present new results regarding the supernova
remnant CTA~1. Our findings disfavor the possibility of a physical connection
with the nearby (in projection) EGRET source. There remains possible, however,
that the compact object produced in the supernova explosion be related with the
observed -ray flux.Comment: Presented for the proceedings of the II Workshop on Unidentified
Gamma-Ray Sources, Hong Kong, June 1-4, 2004. To appear in Astrophysics and
Space Science. Some changes to address referee's and readers' remarks.
References added. Results unchange
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