6,667 research outputs found

    FUSE Observations of a Full Orbit of Hercules X-1: Signatures of Disk, Star, and Wind

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    We observed an entire 1.7 day orbit of the X-ray binary Hercules X-1 with the Far Ultraviolet Spectroscopic Explorer (FUSE). Changes in the O VI 1032,1037 line profiles through eclipse ingress and egress indicate a Keplerian accretion disk spinning prograde with the orbit. These observations may show the first double-peaked accretion disk line profile to be seen in the Hercules X-1 system. Doppler tomograms of the emission lines show a bright spot offset from the Roche lobe of the companion star HZ Her, but no obvious signs of the accretion disk. Simulations show that the bright spot is too far offset from the Roche lobe to result from uneven X-ray heating of its surface. The absence of disk signatures in the tomogram can be reproduced in simulations which include absorption from a stellar wind. We attempt to diagnose the state of the emitting gas from the C III 977, C III 1175, and N III 991 emission lines. The latter may be enhanced through Bowen fluorescence.Comment: Accepted for publication in The Astrophysical Journa

    Stability and Evolution of Supernova Fallback Disks

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    We show that thin accretion disks made of Carbon or Oxygen are subject to the same thermal ionization instability as Hydrogen and Helium disks. We argue that the instability applies to disks of any metal content. The relevance of the instability to supernova fallback disks probably means that their power-law evolution breaks down when they first become neutral. We construct simple analytical models for the viscous evolution of fallback disks to show that it is possible for these disks to become neutral when they are still young (ages of a few 10^3 to 10^4 years), compact in size (a few 10^9 cm to 10^11 cm) and generally accreting at sub-Eddington rates (Mdot ~ a few 10^14 - 10^18 g/s). Based on recent results on the nature of viscosity in the disks of close binaries, we argue that this time may also correspond to the end of the disk activity period. Indeed, in the absence of a significant source of viscosity in the neutral phase, the entire disk will likely turn to dust and become passive. We discuss various applications of the evolutionary model, including anomalous X-ray pulsars and young radio pulsars. Our analysis indicates that metal-rich fallback disks around newly-born neutron stars and black holes become neutral generally inside the tidal truncation radius (Roche limit) for planets, at \~10^11 cm. Consequently, the efficiency of the planetary formation process in this context will mostly depend on the ability of the resulting disk of rocks to spread via collisions beyond the Roche limit. It appears easier for the merger product of a doubly degenerate binary, whether it is a massive white dwarf or a neutron star, to harbor planets because it can spread beyond the Roche limit before becoming neutral.[Abridged]Comment: 34 pages, 2 figures, accepted for publication in Ap

    Measurement of an AGN Central Mass on Centiparsec Scales: Results of Long-Term Optical Monitoring of Arp 102B

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    The optical spectrum of the broad-line radio galaxy Arp 102B has been monitored for more than thirteen years to investigate the nature of the source of its broad, double-peaked hydrogen Balmer emission lines. The shape of the lines varied subtly; there was an interval during which the variation in the ratio of the fluxes of the two peaks appeared to be sinusoidal, with a period of 2.16 years and an amplitude of about 16% of the average value. The variable part of the broad H-alpha line is well fit by a model in which a region of excess emission (a quiescent ``hot spot'') within an accretion disk (fitted to the non-varying portion of the double-peaked line) completes at least two circular orbits and eventually fades. Fits to spectra from epochs when the hot spot is not present allow determination of the disk inclination, while fits for epochs when it is present provide a measurement of the radius of the hot spot's orbit. From these data and the period of variation, we find that the mass within the hot spot's orbit is 2.2 +0.2/-0.7 times 10^8 solar masses, within the range of previous estimates of masses of active galactic nuclei. Because this mass is determined at a relatively small distance (~1000 AU) from the central body, it is extremely difficult to explain without assuming that a supermassive black hole lies within Arp 102B. The lack of any systematic change in the velocity of the blue peak over time yields a lower limit on the combined mass of the two bodies in a binary black hole model like that of Gaskell (1983) of 10^10 solar masses.Comment: 29 pages, including 6 figures; to appear in the Astrophysical Journal 199

    Applications of BGP-reflection functors: isomorphisms of cluster algebras

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    Given a symmetrizable generalized Cartan matrix AA, for any index kk, one can define an automorphism associated with A,A, of the field Q(u1,>...,un)\mathbf{Q}(u_1, >..., u_n) of rational functions of nn independent indeterminates u1,...,un.u_1,..., u_n. It is an isomorphism between two cluster algebras associated to the matrix AA (see section 4 for precise meaning). When AA is of finite type, these isomorphisms behave nicely, they are compatible with the BGP-reflection functors of cluster categories defined in [Z1, Z2] if we identify the indecomposable objects in the categories with cluster variables of the corresponding cluster algebras, and they are also compatible with the "truncated simple reflections" defined in [FZ2, FZ3]. Using the construction of preprojective or preinjective modules of hereditary algebras by Dlab-Ringel [DR] and the Coxeter automorphisms (i.e., a product of these isomorphisms), we construct infinitely many cluster variables for cluster algebras of infinite type and all cluster variables for finite types.Comment: revised versio

    Boundaries of Semantic Distraction: Dominance and Lexicality Act at Retrieval

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    Three experiments investigated memory for semantic information with the goal of determining boundary conditions for the manifestation of semantic auditory distraction. Irrelevant speech disrupted the free recall of semantic category-exemplars to an equal degree regardless of whether the speech coincided with presentation or test phases of the task (Experiment 1) and occurred regardless of whether it comprised random words or coherent sentences (Experiment 2). The effects of background speech were greater when the irrelevant speech was semantically related to the to-be-remembered material, but only when the irrelevant words were high in output dominance (Experiment 3). The implications of these findings in relation to the processing of task material and the processing of background speech is discussed

    Low-energy Coulomb excitation of 62^{62}Fe and 62^{62}Mn following in-beam decay of 62^{62}Mn

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    Sub-barrier Coulomb-excitation was performed on a mixed beam of 62^{62}Mn and 62^{62}Fe, following in-trap β\beta^{-} decay of 62^{62}Mn at REX-ISOLDE, CERN. The trapping and charge breeding times were varied in order to alter the composition of the beam, which was measured by means of an ionisation chamber at the zero-angle position of the Miniball array. A new transition was observed at 418~keV, which has been tentatively associated to a (2+,3+)1g.s.+(2^{+},3^{+})\rightarrow1^{+}_{g.s.} transition. This fixes the relative positions of the β\beta-decaying 4+4^{+} and 1+1^{+} states in 62^{62}Mn for the first time. Population of the 21+2^{+}_{1} state was observed in 62^{62}Fe and the cross-section determined by normalisation to the 109^{109}Ag target excitation, confirming the B(E2)B(E2) value measured in recoil-distance lifetime experiments.Comment: 9 pages, 10 figure

    A pilot cluster randomised trial to assess the effect of a structured communication approach on quality of life in secure mental health settings: the Comquol study

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    Background There is a lack of research in forensic settings examining therapeutic relationships. A structured communication approach, placing patients’ perspectives at the heart of discussions about their care, was used to improve patients’ quality of life in secure settings. The objectives were to: • Establish the feasibility of the trial design • Determine the variability of the outcomes of interest • Estimate the costs of the intervention • If necessary, refine the intervention Methods A pilot cluster randomised controlled trial was conducted. Data was collected from July 2012 to January 2015 from participants in 6 medium secure in–patient services in London and Southern England. 55 patients and 47 nurses were in the intervention group with 57 patients and 45 nurses in the control group. The intervention comprised 6 nurse-patient meetings over a 6 month period. Patients rated their satisfaction with a range of domains followed by discussions on improving patient identified problems. Assessments took place at baseline, 6 months, and 12 months. Participants were not blind to their allocated group. The primary outcome was self-reported quality of life collected by a researcher blind to participants’ allocation status. Results The randomisation procedures and intervention approach functioned well. The measures used were understood by the participants and gave relevant outcome information. The response rates were good with low patient withdrawal rates. The quality of life estimated treatment effect was 0.2 (95% CI: -0.4 to 0.8) at 6 months and 0.4 (95% CI: -0.3 to 1.1) indicating the likely extreme boundaries of effect in the main trial. The estimated treatment effect of the primary outcome is clinically important, and a positive effect of the intervention is not ruled out. The estimate of the ICC for the primary outcome at 6 and 12 months was 0.04 (0.00 to 0.17) and 0.05 (0.00 to 0.18). The cost of the intervention was £529 per patient. Conclusions The trial design was viable as the basis for a full-scale trial. A full trial is justified to estimate the effect of the intervention with greater certainty. The variability of the outcomes could be used to calculate numbers needed for a full-scale trial. Ratings of need for therapeutic security may be useful in any future study
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