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[en] A complex three-ring shaped structure has been reported extending some arc seconds around the SN1987A central spot by several authors. This structure is believed to arise when a thin shell of matter surrounding the supernova was illuminated by the initial extreme-ultra-violet flash from the explosion of the progenitor. In this view the two ring-shaped outer loops, ∼ 3'' in size, are interpreted as limb brightening of an hourglass-shaped nebula surrounding the supernova, while the smaller central ring is located at the waist of the hourglass. Their explanation of the two external loops is different: keeping the hourglass-shaped nebula scenario, they believe that these loops result from the interaction of this nebula with a double beam of relativistic particles emitted by a young pulsar formed by the supernova
[en] The emphasis on the reduction of gaseous radioactive effluent associated with PET radiochemistry laboratories has increased. Various radioactive gas capture strategies have been employed historically including expensive automated compression systems. We have implemented a new cost-effective strategy employing gas capture bags with electronic feedback that are integrated with the cyclotron safety system. Our strategy is suitable for multiple automated 18F radiosynthesis modules and individual automated 11C radiosynthesis modules. We describe novel gas capture systems that minimize the risk of human error and are routinely used in our facility. (paper)
[en] The recent proposal that most AM Her systems have essentially identical white dwarf masses (∼ 0.6-0.7 Msolarmasses) implies a direct relation between their surface field-strengths, B1 and magnetic moments. We show that for B1 > Bmax ≅ 4 x 107 G, magnetic breaking via the white dwarf magnetic field may drive the binary evolution so rapidly as to cause the effective disappearance of these systems. They end as detached systems with low mass degenerate companions and orbital periods P≤ 7 hr. This may explain the existence of apparently isolated, rapidly rotating and highly magnetic white dwarfs. Very old systems may appear as non-magnetic white dwarfs with detached brown dwarf companions. We stress that the apparent lack of unpolarized but synchronously rotating CVs at short orbital periods is a serious problem unless most magnetic CVs have B1 ≥ 107 G. (author)
[en] We show that current ideas for forming the period gap in cataclysmic variables (CVs) give a simple explanation for the spike in the observed distribution of magnetic CVs, provided that they have a bimodal distribution of white dwarf masses M1 at the start of their secular evolution. The majority should have M1 very close to a value of ∼ 0.6-0.7 Msolarmasses and form at periods of ∼ 4-6 hr, with a much smaller group having larger M1 and forming at periods up to 10 hr. Nova explosions remove almost exactly the accreted mass for the low-mass group, so that M1 remains at the initial value of ∼ 0.6-0.7 Msolarmasses while the high-mass group may lose small amounts of mass. These conclusions are supported by nova theory and offer the possibility of an observational test of gap forming hypotheses. (author)
[en] We report the discovery of a new ultraluminous X-ray source associated with a globular cluster in the elliptical galaxy NGC 4649. The X-ray source was initially detected with a luminosity below 5 × 1038 erg s–1, but in subsequent observations 7 and 11 years later it had brightened substantially to 2-3 × 1039 erg s–1. Over the course of six separate observations it displayed significant spectral variability, in both continuum slope and absorption column. Short-term variability in the X-ray flux was also present in at least one observation. The properties of this object appear consistent with a stellar-mass black hole accreting at super-Eddington rates (i.e., in the ultraluminous accretion state), although a highly super-Eddington neutron star cannot be excluded. The coincidence of an increase in absorption column with a possible enhancement in short-term variability in at least one observation is suggestive of a clumpy, radiatively driven wind crossing our line of sight to the object.
[en] From a deep multi-epoch Chandra observation of the elliptical galaxy NGC 3379 we report the spectral properties of eight luminous LMXBs (LX >1.2 x 1038 erg s-1). We also present a set of spectral simulations, produced to aid the interpretation of low-count single-component spectral modeling. These simulations demonstrate that it is possible to infer the spectral states of X-ray binaries from these simple models and thereby constrain the properties of the source. Of the eight LMXBs studied, three reside within globular clusters (GCs) and one is a confirmed field source. Due to the nature of the luminosity cut, all sources are either neutron star (NS) binaries emitting at or above the Eddington luminosity or black hole (BH) binaries. The spectra from these sources are well described by single-component models, with parameters consistent with Galactic LMXB observations, where hard-state sources have a range in photon index of 1.5-1.9 and thermally dominant (TD) sources have inner-disk temperatures between ∼0.7and1.55 keV. The large variability observed in the brightest GC source (LX >4 x 1038 erg s-1) suggests the presence of a BH binary. At its most luminous this source is observed in a TD state with kTin = 1.5 keV, consistent with a BH mass of ∼4 Msun. This observation provides further evidence that GCs are able to retain such massive binaries. We also observed a source transitioning from a bright state (LX ∼1 x 1039 erg s-1), with prominent thermal and non-thermal components, to a less luminous hard state (LX = 3.8 x 1038 erg s-1, Γ= 1.85). In its high flux emission, this source exhibits a cool-disk component of ∼0.14 keV, similar to spectra observed in some ultraluminous X-ray sources (ULXs). Such a similarity indicates a possible link between 'normal' stellar-mass BHs in a high accretion state and ULXs.
[en] We present the properties of the discrete X-ray sources detected in our monitoring program of the globular cluster (GC) rich elliptical galaxy, NGC 4278, observed with Chandra ACIS-S in six separate pointings, resulting in a co-added exposure of 458 ks. From this deep observation, 236 sources have been detected within the region overlapped by all observations, 180 of which lie within the D 25 ellipse of the galaxy. These 236 sources range in L X from 3.5 x 1036 erg s-1 (with 3σ upper limit ≤1 x 1037 erg s-1) to ∼2 x 1040 erg s-1, including the central nuclear source which has been classified as a LINER. From optical data, 39 X-ray sources have been determined to be coincident with a GC, these sources tend to have high X-ray luminosity, with 10 of these sources exhibiting L X > 1 x 1038 erg s-1. From X-ray source photometry, it has been determined that the majority of the 236 point sources that have well-constrained colors have values that are consistent with typical low-mass X-ray binary spectra, with 29 of these sources expected to be background objects from the log N-log S relation. There are 103 sources in this population that exhibit long-term variability, indicating that they are accreting compact objects. Three of these sources have been identified as transient candidates, with a further three possible transients. Spectral variations have also been identified in the majority of the source population, where a diverse range of variability has been identified, indicating that there are many different source classes located within this galaxy.
[en] We report a detailed spectral analysis of the population of low-mass X-ray binaries (LMXBs) detected in the elliptical galaxy NGC 4278 with Chandra. Seven luminous sources were studied individually, four in globular clusters (GCs) and three in the stellar field. The range of (0.3-8 keV) LX for these sources is ∼(3-8) x 1038 erg s-1, suggesting that they may be black hole binaries (BHBs). Fitting the data with either single thermal accretion disk or power-law (PO) models results in best-fit temperatures of ∼0.7-1.7 keV and Γ ∼ 1.2-2.0, consistent with those measured in Galactic BHBs. Comparison of our results with simulations allows us to discriminate between disk and power-law-dominated emission, pointing to spectral/luminosity variability, reminiscent of Galactic BHBs. The BH masses derived from a comparison of our spectral results with the LX ∼ T 4in relation of Galactic BHBs are in the 5-15 Msun range, as observed in the Milky Way. The analysis of joint spectra of sources selected in three luminosity ranges (LX ≥ 1.5 x 1038 erg s-1, 6 x 1037ergs-1 ≤ LX < 1.5 x 1038 erg s-1, and LX < 6 x 1037 erg s-1) suggests that while the high-luminosity sources have prominent thermal disk emission components, power-law components are likely to be important in the mid- and low-luminosity spectra. Comparing low-luminosity average spectra, we find a relatively larger NH in the GC spectrum; we speculate that this may point to either a metallicity effect or to intrinsic physical differences between field and GC accreting binaries. Analysis of average sample properties uncovers a previously unreported LX-RG correlation (where RG is the galactocentric radius) in the GC-LMXB sample, implying richer LMXB populations in more central GCs. No such trend is seen in the field LMXB sample. We can exclude that the GC LX-RG correlation is the by-product of a luminosity effect and suggest that it may be related to the presence of more compact GCs at smaller galactocentric radii, fostering more efficient binary formation.