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 $\log N-\log P$ 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, $90\%$ 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