1,721,028 research outputs found
THE 3-RING STRUCTURE OF SUPERNOVA-1987A
No full textThe current scenario for the complex less than or equal to 3-arcsec structures surrounding SN 1987A is that they result from the emission of a thin shell of matter surrounding the supernova, illuminated by the initial extreme-ultraviolet flash from the explosion of the progenitor. Before the Hubble Space Telescope (HST) the two ring-shaped loops, similar to 3 arcsec in size, clearly visible in all images taken more than two years after the explosion in the UV-to-optical continuum, were interpreted as limb brightening of the outer parts of this nebula. However, the 1994 HST images show that these structures are actually ring-like. Accordingly, we propose a different explanation for these two rings: allowing for the presence of a shell, we suggest that the rings are part of this shell brightened by the interaction of the double beam of relativistic particles emitted from a young pulsar formed after the explosion
THE MASS OF THE COMPANION OF PSR-1718-19
No full textThe recently-discovered binary pulsar 1718 - 19 displays periodic obscuration. We show that free-free absorption in a spherical outflow from the companion star reproduces the observed light curves for a system inclination of order 35-degrees. This implies a companion of mass congruent-to 0.2 M.; if it is close to the main sequence this star is well inside its Roche lobe. The spindown torque in the wind could account for the large pulsar period derivative without the need to invoke a magnetic field which is unusually strong for a globular cluster pulsar. The stellar wind of a detached companion provides a natural source for the obscuring matter. This and the derived companion mass tend to support the capture formation scenario favored by Wijers & Paczynski (1993). However, it is also possible that the system descended from a low-mass X-ray binary which ceased mass transfer at an orbital period of about 3 hr, and has since been widened by adiabatic expansion of the companion through pulsar ablation
A firm upper limit to the radius of the neutron star in SAX J1808.4-3658
No full textWe show that observations of X-ray pulsing from SAX J1808.4-3658 place a firm upper limit of 13.8m(1/3) km on the radius of the neutron star, where m is its mass in solar units. The Limit is independent of distance, or assumptions about the magnetospheric geometry, and could be significantly tightened by observations of the pulsations in the near future. We discuss the implications for the equation of state and the possible neutron star mass
The age of PSR J1012+5307
No full textLorimer et al. have recently reported that the spin-down age (similar to 7 x 10(9) yr) of the millisecond pulsar PSR J1012 + 5307 is much longer than the cooling age (3 x 10(8) yr) of its white dwarf companion. A scenario where the neutron star magnetic field decays spontaneously, and conservative accretion of all the mass lost by the secondary then 'recycles' the neutron star as a millisecond pulsar, cannot explain this discrepancy: the pulsar spins too rapidly at the end of the accretion phase to reach the current value within the white dwarf cooling time. However, if the decay of the neutron star magnetic field is induced by the accretion process itself, spin-up and field decay occur on similar time-scales, and the neutron star spin period cannot in general reach the usual short equilibrium value. The pulsar is thus able to accrete all the mass lost by the companion, reaching its current period and period derivative within the white dwarf cooling time
Spin and orbital evolution in low-mass binary pulsars
No full textWe consider the spin and orbital evolution of low-mass binary radio pulsars which are not members of globular clusters. We show that their current spin periods can be understood in terms of their likely mass-transfer history. Systems with orbital periods greater than or similar to 50 days are the endpoints of low-mass X-ray binaries whose orbital evolution is driven by the nuclear expansion of the secondary. For shorter periods, orbital angular momentum losses are significant, and the mass-transferring progenitor undergoes a phase of Eddington or super-Eddington mass transfer which spins up the neutron star to an Eddington-limited equilibrium period. These systems subsequently enter a propeller phase and are probably undetectable as low-mass X-ray binaries, largely resolving the inferred discrepancy between the current number of binary pulsars and their likely progenitors. Spin-down of the neutron stars in this propeller phase is evidently ineffective. The observed relation between spin period, magnetic field, and orbital period is in apparent agreement with simple ideas of the equilibrium spin period but requires much lower fastness parameters than predicted by current theory
Black hole binaries and X-ray transients
No full textWe consider transient behavior in low-mass X-ray binaries (LMXBs). We show that if this results from a disk instability, the secondary star must be significantly evolved when mass transfer starts, particularly if the primary is a neutron star. For P less than or similar to 2 days, most neutron star systems will be persistent X-ray sources, whereas the slower orbital evolution of black hole systems means that most of them are likely to be transient. Both types of transient system must have extreme mass ratios (<0.1). For orbital periods P greater than or similar to 2 days, most LMXBs will be transient regardless of whether the primary is a neutron star or a Mack hole
PSR J1012+5307: younger than it looks?
No full textLorimer et al. have recently reported that the spin-down age (similar to 7 x 10(9) yr) of the low-mass binary pulsar PSR J1012+5307 is much higher than the cooling age (3 x 10(8) yr) of its white dwarf companion. The proposed solutions for this discrepancy are outlined and discussed. In particular, the revised cooling age estimate proposed by Alberts ct al. agrees with data from other low-mass binary pulsar systems if a transition to the 'classical' cooling regime occurs between similar to 0.14 and similar to 0.28 M.. If this transition is excluded, PSR J1012+5307 seems to have finished its accretion phase far from the spin-up line
Does the thermal disk instability operate in active galactic nuclei?
No full textWe examine all possible stationary, optically thick, geometrically thin accretion disk models relevant for active galactic nuclei (AGN) and identify the physical regimes in which they are stable against the thermal-viscous hydrogen ionization instability. Self-gravity and irradiation effects are included. We find that most if not all AGN disks are unstable. Observed AGN therefore represent the outburst state, although some or all quasars could constitute a steady population having markedly higher fueling rates than other AGN. This finding has important implications for the AGN mass supply and for the presence of supermassive black holes in nearby spirals
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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