Volume 5, Issue 4
December 1984, pages 331-506
pp 331-348 December 1984
The evidence for unseen mass (which is briefly reviewed) suggests that the cosmological density parameter Ω is at least 0.1–0.2. An Einstein-de-Sitter ‘flat’ universe with Ω = 1 — which is appealing for theoretical reasons — can only be reconciled with the data if the galaxies are more ‘clumped’ than the overall mass distribution, and are poor tracers of the unseen mass even on scales of several Mpc. Possible forms for the unseen mass are discussed; and feedback processes are outlined whereby galaxy formation can be suppressed in underdense regions.
pp 349-368 December 1984
pp 369-388 December 1984
Arrival times from a fast, quiet pulsar can be used to obtain accurate determinations of pulsar parameters. In the case of the millisecond pulsar, PSR 1937 + 214, the remarkably small rms residual to the timing fit indicates that precise measurements of position, proper motion and perhaps even trigonometric parallax will be possible (Backer 1984). The variances in these parameters, however, will depend strongly on the nature of the underlying noise spectrum. We demonstrate that for very red spectrai.e. those dominated by low-frequency noise, the uncertainties can be larger than the present estimates (based on a white-noise model) and can even grow with the observation period. The possibility of improved parameter estimation through prewhitening’ the data and the application of these results to other pulsar observations are briefly discussed. The post-fit rms residual of PSR 1937 + 214 may be used to limit the energy density of a gravitational radiation background at periods of a few months to years. However, fitting the pulsar position and pulse-emission times filters out significant amounts of residual power, especially for observation periods of less than three years. Consequently the present upper bound on the energy density of gravitational waves Ωg <3 × 10-4RΜs2, though already more stringent than any other available, is not as restrictive as had been previously estimated. The present limit is insufficient to exclude scenarios which use primordial cosmic strings for galaxy formation, but should improve rapidly with time.
pp 389-402 December 1984
Supernovae of both Type I (hydrogen-poor) and Type II (hydrogen-rich) can be expected to occur among binary stars. Among massive stars (>10 M•), the companion makes it more difficult for the primary to develop an unstable core of >1.4.M• while still retaining the extended, hydrogen-rich envelope needed to make a typical Type II light curve. Among 1–10 M• stars, on the other hand, a companion plays a vital role in currently popular models for Type I events, by transferring material to the primary after it has become a stable white dwarf, and so driving it to conditions where either core collapse or explosive nuclear burning will occur. Several difficulties (involving nucleosynthesis, numbers and lifetimes of progenitors, the mass-transfer mechanism,etc.) still exist in these models. Some of them are overcome by a recent, promising scenario in which the secondary also evolves to a degenerate configuration, and the two white dwarfs spiral together to produce a hydrogen-free explosion, long after single stars of the same initial masses have ceased to be capable of fireworks.
pp 403-423 December 1984
We conclude that pulsar-driven supernova remnants (SNRs) are extremely rare objects. Indeed an analysis of the known sample of plerions suggests a very low birthrate ∼ 1 in 240 years. Long-lived and bright plerions like the Crab nebula are likely to be produced only when the pulsar has an initial period ∼ 10–20 milliseconds and a field ∼ 1012 G. Such pulsars inside rapidly expanding shell remnants should also produce detectable plerions. The extreme rarity of SNRs with such hybrid morphology leads us to conclude that these pulsars must have been born with an initial period larger than ∼ 35–70 milliseconds.
pp 425-428 December 1984
The compact source 0125 + 628 in the centre of the galactic supernova remnant G 127.1 + 0.5 has been re-observed in HI absorption using the Westerbork Synthesis Radio Telescope (WSRT). The outer arm HI absorption atV = -95km s-1 has been confirmed. The absorption spectrum is similar to that of the nearby extragalactic source 0123 + 633. We discuss the arguments concerning an extragalactic origin of 0125 + 628 and conclude that it is most likely extragalactic and not an SS 433 type object.
pp 429-473 December 1984
We present total-intensity and linear-polarization observations with the Very Large Array (VLA) at λ6 and 2 cm of 17 sources, almost all of which were suspected to have extended emission only on one side of the nucleus. Five of them are still one-sided, three appear unresolved, while seven have radio lobes on both sides of the nucleus. The outer components in the double-lobed sources, however, have significantly different surface brightness or are very asymmetrically located with respect to the nucleus.
pp 475-482 December 1984
We present total-intensity and linear-polarization observations made with the VLA at 5 GHz of 1400+ 162, a BL Lac object in a group of galaxies. It has a misaligned triple structure with a prominent radio jet towards the east. There is evidence of a weak counter-jet towards the western component, which also has the more prominent warm-spot.
We discuss possible explanations for some of the observed features of this source. Although interaction with the cluster medium is possibly partly responsible for the observed distortion, we suggest that the large observed misalignment could also be due to amplification of a smaller misalignment by projection effects. In the relativistic beaming model, where BL Lac objects arise when the relativistic jets are seen end-on, we suggest that 1400+ 162 is more oblique to the line of sight than most members of this class.
pp 483-493 December 1984
Pulsars show intensity variations over timescales ranging from a few microseconds to a few years. Short-term intensity variations,i.e. those having timescales of a few minutes to a few hours had been difficult to study as their timescales are similar to those due to interstellar scintillations. We present here a method to separate the autocorrelation function of the short-term broadband intensity variations from that of the interstellar scintillations and thus overcome the above difficulty. The method assumes that the intrinsic variations are correlated over a bandwidth much larger than the decorrelation bandwidth for scintillations. Hence the ratio of the power in the variations due to the two causes depends on the bandwidth used. By applying the method to the intensity variations of 24 pulsars, we show that the presence of short-term intrinsic variations is very common in the radiation of pulsars. Quasi-periodicities were detected in the intensity variations of many pulsars, but their origin is not clear.
pp 495-506 December 1984
Over the past fifteen years, observations of some quasars with the techniques of very-long-baseline interferometry have shown that the angular separation between pairs of radio-emitting regions in their cores is increasing year after year. If the quasars are indeed as far away as implied by Hubble’s law, then these angular motions translate into linear speeds several times the speed of light. Several theoretical scenarios have been proposed to show that the observed motions are illusory. The leading contender in this field — the relativistic beam model — and an alternative offered by the concept of a gravitational screen are described and compared in the light of recent observational data.
Volume 41, 2020
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