Joanna M. Rankin

Toward an empirical theory of pulsar emission. VI - The geometry of the conal emission region

This paper presents an empirical analysis of the geometry of the conal emission region for a total population of some 150 pulsars for which an adequate body of observations now exists. It continues the analysis begun in a previous paper that explored the geometry of the core emission region and provided a means of estimating the angle α between the rotation and magnetic axes of pulsars with core components. The various pulsars are thus divided into groups according to their morphological classification, and those species that have core components are treated first. Special consideration is given to the five-component (M) class and to those other, entirely conal species which are closely related to it, the conal triple (cT) and the conal quadruple (cQ)

Pulsar Magnetospheric Emission Mapping: Images and Implications of Polar Cap Weather

The beautiful sequences of drifting subpulses observed in some radio pulsars have been regarded as among the most salient and potentially instructive characteristics of their emission, not least because they have appeared to represent a system of subbeams in motion within the emission zone of the star. Numerous studies of these drift sequences have been published, and a model of their generation and motion was articulated long ago by Ruderman & Sutherland; but thus far, efforts have failed to establish an illuminating connection between the drift phenomenon and the actual sites of radio emission. Through a detailed analysis of a nearly coherent sequence of drifting pulses from the pulsar B0943+10, we have in fact identified a system of subbeams circulating around the magnetic axis of the star. A mapping technique, involving a cartographic transform and its inverse, permits us to study the character of the polar cap emission map and then to confirm that it, in turn, represents the observed pulse sequence. On this basis, we have been able to trace the physical origin of the drifting subpulse emission to a stably rotating and remarkably organized configuration of emission columns, in turn traceable possibly to the magnetic polar cap gap region envisioned by some theories.

Is Pulsar B0656+14 a Very Nearby Rotating Radio Transient?

The recently discovered RRAT sources are characterized by very bright radio bursts that, while being periodically related, occur infrequently. We find bursts with the same characteristics for the known pulsar B0656+14. These bursts represent pulses from the bright end of an extended smooth pulse-energy distribution and are shown to be unlike giant pulses, giant micropulses, or the pulses of normal pulsars. The extreme peak fluxes of the brightest of these pulses indicate that PSR B0656+14, were it not so near, could only have been discovered as an RRAT source. Longer observations of the RRATs may reveal that they, like PSR B0656+14, emit weaker emission in addition to the bursts.

Probing drifting and nulling mechanisms through their interaction in PSR B0809+74

Both nulling and subpulse drifting are poorly understood phenomena. We probe their mechanisms by investigating how they interact in PSR B0809+74. We find that the subpulse drift is not aliased but directly reflects the actual motion of the subbeams. The carousel-rotation time must then be over 200 s, which is much longer than theoretically predicted. The drift pattern after nulls differs from the normal one, and using the absence of aliasing we determine the underlying changes in the subbeam-carousel geometry. We show that after nulls, the subbeam carousel is smaller, suggesting that we look deeper in the pulsar magnetosphere than we do normally. The many striking similarities with emission at higher frequencies, thought to be emitted lower too, confirm this. The emission-height change as well as the striking increase in carousel-rotation time can be explained by a post-null decrease in the polar gap height. This offers a glimpse of the circumstances needed to make the pulsar turn off so dramatically.

Null-induced mode changes in PSR B0809+74

We have found that there are two distinct emission modes in PSR B0809+74. Beside its normal and most common mode, the pulsar emits in a signicantly dierent quasi-stable mode after most or possibly all nulls, occasionally for over 100 pulses. In this mode the pulsar is brighter, the subpulse separation is less, the subpulses drift more slowly and the pulse window is shifted towards earlier longitudes. We can now account for several previously unexplained phenomena associated with the nulling-drifting interac- tion: the unexpected brightness of the rst active pulse and the low post-null driftrate. We put forward a new interpretation of the subpulse-position jump over the null, indicating that the speedup time scale of the post-null drifting is much shorter than previously thought. The speedup time scale we nd is no longer discrepant with the time scales found for the subpulse-drift slowdown and the emission decay around the null.

Core and conal component analysis of pulsar B1237+25

This paper provides a new analysis of this famous five-component (M) pulsar. In addition to the core-active ‘abnormal’ mode of the star, we find two distinct behaviours within its ‘normal’ mode, a ‘quiet-normal’ mode with regular 2.8-period subpulse modulation and little or no core activity, and a ‘flare-normal’ mode, where the core is regularly bright and a nearly four-period modulation is maintained. The flare-normal mode appears to be an intermediate state between the quiet-normal and abnormal behaviours. Short 5‐15-pulse flare-normal-mode ‘bursts’ and quiet-normal intervals alternate with each other quasi-periodically, making a cycle some 60‐80 pulses in duration. Abnormal-mode intervals are interspersed within this overall cycle, usually persisting for only a few pulses, but occasionally lasting for scores or even many hundreds of pulses. Within subsequences where the core is exceptionally quiet, the pulsar provides a nearly ‘textbook’ example of a central sightline traverse ‐ with a polarization-angle sweep rate measured to be at least some 180 ◦ deg −1 .O nthis basis it is shown that the sightline impact angle β must be ∼ 0. ◦ 25 or some 5 per cent of the outer conal beam radius. The core component of the star is found to be incomplete, despite the fact that the full antisymmetric circularly polarized signature of the core is present. The visible core component aligns with the trailing right-hand (RH) portion of the circular signature. Measures either of the circular signature or of the trailing half of the core, however, reiterate the roughly 2.6 ◦ angular diameter of the polar cap of the star. We find that the PA traverse of the star is disrupted through the action of orthogonally polarized linear power in the longitude range of the core component. This circumstance can be seen in the ‘hook’ under the core component which usually entails four sense reversals of the PA, with its centre falling at the same PA as the extrema of the overall traverse. This behaviour is modelled to show its effect. Finally, the star provides two well-defined fiducial points from which emission-height estimates can be computed, the respective centres of both the linear PA traverse and the zerocrossing point of the circular polarization signature. We thus find that the outer and inner cones are emitted at heights of some 340 ± 79 and 278 ± 76 km, respectively, such that their field-line ‘feet’ are some 78 ± 7 and 53 ± 5 per cent of the polar cap radius. We also find evidence that the core emission occurs at a height of some 60 km.

ARECIBO MULTI-FREQUENCY TIME-ALIGNED PULSAR AVERAGE-PROFILE AND POLARIZATION DATABASE

We present Arecibo time-aligned, total intensity profiles for 46 pulsars over an unusually wide range of radio frequencies and multi-frequency, polarization-angle density diagrams, and/or polarization profiles for 57 pulsars at some or all of the frequencies 50, 111/130, 430, and 1400 MHz. The frequency-dependent dispersion delay has been removed in order to align the profiles for study of their spectral evolution, and wherever possible the profiles of each pulsar are displayed on the same longitude scale. Most of the pulsars within Arecibos declination range that are sufficiently bright for such spectral or single pulse analysis are included in this survey. The calibrated single pulse sequences and average profiles are available by web download for further study.

Pulsar PSR B2303+30: a single system of drifting subpulses, moding and nulling

Analyses of multiple pulse sequences of the pulsar PSR B2303+30 reveal two distinct emission modes. One mode (B) follows a steady even-odd pattern and is more intense. The second mode (Q) is characteristically weak, but has intermittent drift bands with a periodicity of approximately 3P 1 /cycle, and nulls much more frequently than the B mode. Both modes occur with roughly equal frequency, and their profiles have a similar single-humped form with a slight asymmetry. Our observations and analyses strongly suggest that the subpulse drift rates in both modes are linked in a series of cycles, which can be modelled as relaxing oscillations in the underlying circulation rate.

A 24 Hr Global Campaign to Assess Precision Timing of the Millisecond Pulsar J1713+0747

The radio millisecond pulsar J1713+0747 is regarded as one of the highest-precision clocks in the sky and is regularly timed for the purpose of detecting gravitational waves. The International Pulsar Timing Array Collaboration undertook a 24 hr global observation of PSR J1713+0747 in an effort to better quantify sources of timing noise in this pulsar, particularly on intermediate (1-24 hr) timescales. We observed the pulsar continuously over 24 hr with the Arecibo, Effelsberg, GMRT, Green Bank, LOFAR, Lovell, Nancay, Parkes, and WSRT radio telescopes. The combined pulse times-of-arrival presented here provide an estimate of what sources of timing noise, excluding DM variations, would be present as compared to an idealized \sqrt{N} improvement in timing precision, where N is the number of pulses analyzed. In the case of this particular pulsar, we find that intrinsic pulse phase jitter dominates arrival time precision when the signal-to-noise ratio of single pulses exceeds unity, as measured using the eight telescopes that observed at L band/1.4 GHz. We present first results of specific phenomena probed on the unusually long timescale (for a single continuous observing session) of tens of hours, in particular interstellar scintillation, and discuss the degree to which scintillation and profile evolution affect precision timing. This paper presents the data set as a basis for future, deeper studies.

Arecibo 21 centimeter polarimetry of 64 pulsars - A guide to classification

1400 MHz polarimetric profile observations, made with the Arecibo instrument, are reported for a group of 56 weaker pulsars (including three interpulsars). These results are combined with previously reported 21 cm polarimetry on 11 stronger pulsars. Each pulsar is discussed in terms of the classification system published by Rankin in 1983 and 1986 in order to explore the strengths and weaknesses of the scheme. All but two of the 64 pulsars were classifiable into one of six groups. These six species fall into two broad categories, one in which a core component was evident and another wherein no core emission was apparent. Of the 64 pulsars, 44 (69 percent) were identified with the three core-associated classes and 18 (28 percent) with the three conal species. Full period polarimetric measurements are also given for three interpulsars, and classifications are attempted for both their main pulse and interpulse profiles. 39 refs.