10,085 research outputs found
Diffuse X-ray emission from Abell clusters
Abell clusters of galaxies, specifically Abell 401 and Abell 399 regions, are discussed. Two sets of satellite observations were made. One was a lunar occultation pointed observation where there were two orbits in which to look at the regions while the moon scanned over it. The other was a scanning observation through the area. Two diffuse models of the X-ray emission were developed. Also, during the investigation three binary cluster systems were found where at least one member was an X-ray source. They were Abell 508, 509, 2177, 2178, 2204, and 2210
X-ray imaging above 3 keV
Energy dependent X-ray maps of clusters of galaxies are discussed. A design for a stacked, etched grid collimator is discussed
What can gamma rays tell us about binary x ray sources and SNR's
Black holes, neutron stars in binary x ray source systems, and supernova remnants (SNRs) are briefly discussed
Search for cold gas in clusters with and without cooling flows
The dominant galaxy in each of approx. 40 clusters was studied using co-added Infrared Astronomy Satellite (IRAS) survey data, and 11 of these galaxies were observed for CO (J=1 to 0) emission with the 12 m telescope at Kitt Peak. Half of the galaxies in the sample are in clusters reported to have cooling flows while the other half are not. Six of the galaxies appear to have been detected by IRAS at fairly low flux levels, in addition to one previously known strong detection; all seven have reported cooling flows. No detectable CO emission (to 2 to 3 mK) was found in any of the 11 galaxies observed. Assuming accretion rates of approx. 100 Solar Mass yr(-1), the star formation rates and efficiencies in these galaxies must be quite high in order to render the CO undetectable. At the same time, the infrared luminosities of these galaxies is unremarkable, suggesting that the correlation between star formation efficiency and infrared luminosity found for spirals may not hold for cooling flows
Constraints on the Gamma-ray Burst Luminosity Function from PVO and BATSE
We examine the width of the gamma-ray burst luminosity function through the
distribution of GRB peak fluxes as detected by the Pioneer Venus Orbiter (PVO)
and the Burst and Transient Source Experiment (BATSE). The strength of the
analysis is greatly enhanced by using a merged catalog of peak fluxes from both
instruments with good cross-calibration of their sensitivities. The range of
peak fluxes is increased by approximately a factor of 20 relative to the BATSE
catalog. Thus, more sensitive investigations of the
distribution are possible. We place constraints on the width of the luminosity
function of gamma-ray bursts brighter than the BATSE completeness limit by
comparing the intensity distribution in the merged catalog with those produced
by a variety of spatial density and luminosity functions. For the models
examined, of the {\em detectable\/} bursts have peak luminosities within
a range of 10, indicating that the peak luminosities of gamma-ray bursts span a
markedly less wide range of values than many other of their measurable
properties. We also discuss for which slopes of a power-law luminosity function
the observed width is at the upper end of the constrained range. This is
important in determining the power-law slopes for which luminosity-duration
correlations could be important.Comment: 10 pages latex + 2 uuencoded figures; APJL accepte
Dosimetric verification of the anisotropic analytical algorithm for radiotherapy treatment planning
BACKGROUND AND PURPOSE:
To investigate the accuracy of photon dose calculations performed by the Anisotropic Analytical Algorithm, in homogeneous and inhomogeneous media and in simulated
treatment plans.
MATERIALS AND METHODS:
Predicted dose distributions were compared with ionisation chamber and film
measurements for a series of increasingly complex situations. Initially, simple and complex fields in a
homogeneous medium were studied. The effect of inhomogeneities was investigated using a range of
phantoms constructed of water, bone and lung substitute materials. Simulated treatment plans were
then produced using a semi-anthropomorphic phantom and the delivered doses compared to the doses predicted by the Anisotropic Analytical Algorithm.
RESULTS:
In a homogeneous medium, agreement was found to be within 2% dose or 2mm dta in most
instances. In the presence of heterogeneities, agreement was generally to within 2.5%. The simulated
treatment plan measurements agreed to within 2.5% or 2mm.
Conclusions: The accuracy of the algorithm was found to be satisfactory at 6MV and 10MV both in homogeneous and inhomogeneous situations and in the simulated treatment plans. The algorithm was more accurate than the Pencil Beam Convolution model, particularly in the presence of low density heterogeneities
Rate zonal density gradient ultracentrifugation analysis of repair of radiation damage to the folded chromosome of Escherichia coli
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution April 1978The structure of the membrane-free nucleoid of Escherichia coli and
of unfolded chromosomal DNA was investigated by sedimentation on neutral
sucrose gradients after irradiation with 60Co gamma-rays and ultraviolet
light (2S4nm). Irradiation both in vivo and in vitro was used as a
molecular probe of the constraints on DNA~packaging in the bacterial
chromosome. The extremely gentle lysis and unfolding procedures which
were developed yielded undamaged, replicating genomes, thus permitting
direct measurement of the formation and repair of DNA double-strand breaks
at biologically-significant doses of ionizing radiation.
In vitro UV-irradiation of nucleoids resulted in an increase in the
observed rate of sedimentation due to the formation of an unknown photo-product.
In contrast, UV-irradiation of wild-type cells in vivo showed
evidence of the formation of incision breaks which resulted in the relaxation
of supercoiling in the nucleoid. Strand breakage was also observed
following in vivo UV-irradiation of a uvrB-5 strain, but at a lower rate
and also accompanied by considerable unfolding of the chromosome. Such
lesions may have been the result of direct photochemical reactions in the
nucleoid, or enzyme activity associated with a uvr-independent mode of
repair.
The number of domains of supercoiling was estimated at 170 per
genome equivalent of DNA based on measurements of relaxation caused by
single-strand break formation in in vivo- and in vitro-gamma-irradiated
folded chromosomes. Similar estimates based on the target size of RNA
molecules responsible for maintaining the compact packaging of the
nucleoid predicted negligible unfolding due to the formation of RNA single-strand
breaks at doses up-to 10 Krad, and were born out by experimental
measurements.
Unfolding of the nucleoid in vitro by limit-digestion with RNase or
by heating at 70° resulted in DNA complexes with sedimentation coefficients
of 1030±59S and 625±15S respectively. The difference in these
rates was apparently due to more complete deproteinization and thus less
mass in the heated material. These structures are believed to represent
intact, replicating genomes in the form of complex-theta structures
containing 2-3 genome equivalents of DNA.
The rate of formation of double-strand breaks was determined from
molecular weight measurements of thermally unfolded chromosomal DNA gamma-irradiated
in vitro. Break formation was linear with dose up to 10 Krad,
resulting in 0.27 double-strand breaks per kilorad per genome equivalent
of DNA and requiring 1080 eV/double-strand break. The influence of
possible non-linear DNA conformations of these calculations is discussed.
Repair of ionizing radiation damage to folded chromosomes was
observed within 2-3 hours of post-irradiation incubation in growth medium.
A model based on recombinational repair is proposed to explain the formation
of 2200-2300S material during early stages of incubation and subsequent
changes in the gradient profiles. Such behavior is not observed
for post-irradiation incubation of wild-type cells in buffer or for a
recA-13 strain incubated in growth medium. Association of unrepaired DNA
with plasma membrane is proposed to explain the formation of a peak of
rapidly sedimenting material (>>3100S) during the later stages of repair.
Direct evidence of repair of double-strand breaks during post-irradiation
incubation in growth medium was obtained from gradient
profiles of DNA from RNAse-digested chromosomes. The sedimentation
coefficient of broken molecules was restored to the value of unirradiated
DNA after 2-3 hours of incubation, and the fraction of the DNA repaired in
this fashion was equal to the fraction of cells which survived at the
same dose. An average of 2.7 double-strand breaks per genome per lethal
event was observed, suggesting that 1-2 double-strand breaks per genome
are repairable in this strain of E. coli
A Tidal Flare Candidate in Abell 1795
As part of our ongoing archival X-ray survey of galaxy clusters for tidal
flares, we present evidence of an X-ray transient source within 1 arcmin of the
core of Abell 1795. The extreme variability (a factor of nearly 50), luminosity
(> 2 x 10^42 erg s^{-1}), long duration (> 5 years) and supersoft X-ray
spectrum (< 0.1 keV) are characteristic signatures of a stellar tidal
disruption event according to theoretical predictions and to existing X-ray
observations, implying a massive >~10^5 M_sun black hole at the centre of that
galaxy. The large number of X-ray source counts (~700) and long temporal
baseline (~12 years with Chandra and XMM-Newton) make this one of the
best-sampled examples of any tidal flare candidate to date. The transient may
be the same EUV source originally found contaminating the diffuse ICM
observations of Bowyer et al. (1999), which would make it the only tidal flare
candidate with reported EUV observations and implies an early source luminosity
1-2 orders of magnitude greater. If the host galaxy is a cluster member then it
must be a dwarf galaxy, an order of magnitude less massive than the quiescent
galaxy Henize 2-10 which hosts a massive black hole that is difficult to
reconcile with its low mass. The unusual faintness of the host galaxy may be
explained by tidal stripping in the cluster core.Comment: Accepted by MNRAS 2013 July 23. 27 pages, 10 figure
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