Daniel R. Stinebring

Long-term pulsar flux monitoring and refractive interstellar scintillation

The flux densities of 14 pulsars have been observed for over 1 year at an observing frequency of 610 MHz, using an NRAO 26 m telescope in Green Bank, West Virginia. The observed time scales for long-term flux variation for seven pulsars are in good agreement with refractive scintillation theory. Four pulsars, including the Vela pulsar, have observed time scales that are in poor agreement with theoretical predictions. Pulsars predicted to have extremely long refractive scintillation time scales are observed to have stable fluxes. These pulsars therefore have intrinsically stable luminosities

Polarimetry of millisecond pulsars

The mean polarization profiles of several fast pulsars were measured. Observations of pulsar PSR 1937 + 21 are similar to previously published results, except that a shallower position angle was found to swing across the pulse at low frequency; for the first time, the occurrence of orthogonal mode switching at all frequencies is explicitly demonstrated. Contrary to previous reports, it is shown that PSR 1953 + 29 is substantially linearly polarized in its first and third components. In addition, the first polarization measurements of the recently discovered millisecond pulsar PSR 1957 + 20 are presented. No evidence is found that the polarization properties of these recycled pulsars differ from those of their slower counterparts. Available data on the spectral development of millisecond pulsar profiles is also reviewed. It is emphasized that millisecond pulsars severely test phenomenological models such as the one proposed by Rankin (1983, 1990). 27 refs.

Pulsar flux stability and refractive interstellar scintillation

Accurate flux densities of 25 pulsars at 310, 416, and 750 MHz have been measured for 43 consecutive days with the NRAO 91 m telescope. Pulsars covering a wide range of dispersion measures (DM) were chosen to distinguish intrinsic luminosity variations from diffractive and refractive interstellar scintillations (RISS). The pulsars PSR 1818 - 04, 1933 + 16, and 211 + 46 have such high DMs and/or low space velocities that their predicted RISS time scales at low frequencies are much greater than 43 days. These pulsars were observed to have quite constant flux densities, implying that their luminosities must be intrinsically stable on time scales from several minutes to several weeks. Since the DM is primarily a property of the interstellar medium along the line of sight to a pulsar, rather than of the pulsar itself or its environment, it is concluded that most pulsars have intrinsically stable luminosities. 30 refs.

A flexible data acquisition system for timing pulsars

We describe a flexible, inexpensive data acquisition system built for high‐precision timing observations of pulsars. The system is designed to interface with a wide variety of radio telescope receiver back ends; it permits standardized measurement techniques and data formats in work carried out at a number of different observatories. Copies of the basic ‘‘Mark III’’ system are now in regular use at the Arecibo Observatory, Green Bank, and the Very Large Array. We describe the specifications, hardware, and software implementation of the system, and briefly outline some of its current applications.

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.

Microstructure-determined pulsar dispersion measures and the problem of profile alignment

Time-aligned profile measurements for two pulsars, combining data from the Arecibo, Puerto Rico, and Pushchino, USSR, observatories over a seven-octave frequency interval between 25 and 5000 MHZ are analyzed along with several new microstructure dispersion values. DMA(A) values of 2.9701 + or {minus} 0.0003, and 4.8470 + or {minus} 0.0003 pc/cu cm for PSR 0950+08 and 1113 + 16, respectively are obtained, with small departures from alignment that appear traceable to changes in a profile shape with frequency. These subtle changes in the profile form are noticeable only by virtue of the high time resolution and broad frequency coverage. It is noted that small low-frequency delays result when the sets of profiles are aligned optimally according to the best available microstructure dispersion values. 19 refs.

Improved timing of the millisecond pulsar PSR 1937+21 using real-time coherent dedispersion

Profiles of the millisecond pulsar PSR 1937+21 have been obtained with 6-micron resolution using a real-time hardware dispersion removal device. This dedisperser has a potential resolution of better than 0.5 microsec and is immune to time-of-arrival jitter caused by scintillation-induced spectral gradients across the receiver passband. It significantly reduces the time-of-arrival residuals when compared with the timing technique currently in use. This increased timing accuracy, when utilized in a long-term timing program of millisec pulsars, will improve the solar system ephemeris and will substantially improve the detection limit of a gravitational wave background. 27 references.

Refractive interstellar scintillation and millisecond pulsar timing

Timing observations of the msec pulsar PSR 1937 + 21 now show significant LF noise. This noise is incompletely removed by a simple dispersion-correcting algorithm based on dual-frequency observations. A two-dimensional simulation of refractive interstellar scintillation propagation effects, with parameters appropriate to the pulsar timing experiment, is developed. These computations show good agreement with a power-law spectrum of interstellar turbulence. Moreover, a second roughly frequency-independent noise component is isolated from the data. This noise, with a very steep red spectrum, may be best explained as irregularities in the pulsar rotation or possibly as signature of a cosmological gravitational-wave background. 15 refs.

Eclipse Duration and Posteclipse Delay in PSR 1957 + 20

We have observed the eclipsing millisecond pulsar PSR 1957+20 on 32 occasions during the interval of March-September 1988 at the Arecibo Observatory. We have made radio observations at 318 MHz and 430 MHz, primarily, but we also have obtained a small amount of data a t 606 MHz. We report on these observations here, concentrating on the frequency dependence of the eclipse duration and the posteclipse delay. The basic parameters of the system are presented in the discovery paper, and polarization characteristics and differential Faraday delay observations are summarized by Thorsett et a1. All of these data were obtained with the Princeton-Arecibo millisecond pulsar timing system using coherent dispersion removal: Observations near 430 MHz were made with dual circularly polarized signals a t 425 MHz and 435 MHz with 400-kHz bandwidths a t each observing frequency. Observations with the 3 18-MHz feed were made with a single linear polarization and two 260-kHz-wide channels separated by up to 18 MHz. Synchronous pulse averages were recorded a t intervals as short as every 10 seconds, with 30 seconds being a more typical integration time. The diffractive scintillation time scale for this pulsar a t 430 MHz is about 120 seconds and the scintillation bandwidth is about 50 kHz, so scintillation causes strong signal-strength variations in our data. Individual averages were recorded with a ratio of signal (at the pulse peak) to root-mean-square off-pulse noise of greater than loo:], but a 15:l ratio was more typical, and scintillation minima were encountered in which the signal disappeared entirely for up to five minutes. Timing observations obtained a t 430 MHz far from eclipse are useful for determining the orbital parameters of the system. The full results of our timing solution will be reported elsewhere, but a number of newly determined parameters are particularly important. The spin-down rate of the pulsar has been determined to be dP/dt = (1.2 * 0.1) x lo-* s s-, implying a surface magnetic field of B = 1.4 x lo* gauss and a spin-down luminosity of L = 1 . 1 x lo3 ergs s-. This is the lowest surface magnetic field determined for any of the 450 known pulsars. It is consistent with the standard spin-up scenario for the creation of millisecond pulsars (e.g., Alpar et aL6)

PSR 1957+20. Polarization and the nature of the eclipsing medium

Polarization data for PSR 1957 + 20, collected at 321, 430, and 606 MHz are presented. The integrated profile of the pulsar showed that the main pulse is narrower than that of any known pulsar, with the slight broadening at 321 MHz, which is ascribed to scattering in the interstellar medium. In addition, circular polarization observations of three eclipses at 430 MHz were made in an attempt to investigate the time delay noted previously by Fruchter (1988, 1989). In plots of magnetic induction vs orbital phase, average parallel magnetic fields of about -2.7 and 0.7 gauss were found before and after the eclipse, respectively.