Timothy H. Hankins

Nanosecond radio bursts from strong plasma turbulence in the Crab pulsar

The Crab pulsar was discovered by the occasional exceptionally bright radio pulses it emits, subsequently dubbed ‘giant’ pulses. Only two other pulsars are known to emit giant pulses. There is no satisfactory explanation for the occurrence of giant pulses, nor is there a complete theory of the pulsar emission mechanism in general. Competing models for the radio emission mechanism can be distinguished by the temporal structure of their coherent emission. Here we report the discovery of isolated, highly polarized, two-nanosecond subpulses within the giant radio pulses from the Crab pulsar. The plasma structures responsible for these emissions must be smaller than one metre in size, making them by far the smallest objects ever detected and resolved outside the Solar System, and the brightest transient radio sources in the sky. Only one of the current models—the collapse of plasma-turbulent wave packets in the pulsar magnetosphere—can account for the nanopulses we observe.

Radio Emission Signatures in the Crab Pulsar

Our high time resolution observations of individual pulses from the Crab pulsar show that both the time and frequency signatures of the interpulse are distinctly different from those of the main pulse. Main pulses can occasionally be resolved into short-lived, relatively narrowband nanoshots. We believe these nanoshots are produced by soliton collapse in strong plasma turbulence. Interpulses at centimeter wavelengths are very different. Their dynamic spectrum contains regular, microsecond-long emission bands. We have detected these bands, proportionately spaced in frequency, from 4.5 to 10.5 GHz. The bands cannot easily be explained by any current theory of pulsar radio emission; we speculate on possible new models.

Multifrequency radio observations of the crab pulsar

Previously unseen profile components of the Crab pulsar have been discovered in a study of the frequency-dependent behavior of its average pulse profile between 0.33 and 8.4 GHz. One new component, 36 degrees ahead of the main pulse at 1.4 GHz, is not coincident with the position of the precursor at lower frequencies. Two additional, flat-spectrum components appear after the interpulse between 1.4 and 8.4 GHz. The normal interpulse undergoes a transition in phase and spectrum by disappearing near 2.7 GHz, and reappearing 10 degrees earlier in phase at 4.8 and 8.4 GHz with a new spectral index. The radio frequency main disappears for frequencies above 4.8 GHz, even though it is seen at infrared, optical, and higher energies. The existence of the additional components at high frequency and the strange, frequency-dependent behavior is unlike anything seen in other pulsars, and cannot easily be explained by emission from a simple dipole field geometry.Previously unseen profile components of the Crab pulsar have been discovered in a study of the frequency-dependent behavior of its average pulse profile between 0.33 and 8.4 GHz. One new component, 36 degrees ahead of the main pulse at 1.4 GHz, is not coincident with the position of the precursor at lower frequencies. Two additional, flat-spectrum components appear after the interpulse between 1.4 and 8.4 GHz. The normal interpulse undergoes a transition in phase and spectrum by disappearing near 2.7 GHz, and reappearing 10 degrees earlier in phase at 4.8 and 8.4 GHz with a new spectral index. The radio frequency main disappears for frequencies above 4.8 GHz, even though it is seen at infrared, optical, and higher energies. The existence of the additional components at high frequency and the strange, frequency-dependent behavior is unlike anything seen in other pulsars, and cannot easily be explained by emission from a simple dipole field geometry.

The brightest pulses in the universe: multifrequency observations of the Crab pulsar's giant pulses

We analyze the Crab pulsar at 10 frequencies from 0.43 to 8.8 GHz using data obtained at the Arecibo Observatory and report the spectral dependence of all pulse components and the rate of occurrence of largeamplitude ‘‘giant’’ pulses. Giant pulses occur only in the main and interpulse components that are manifest from radio frequencies to gamma-ray energies (known as the ‘‘P1’’ and ‘‘P2’’ components in the high-energy literature). Individual giant pulses reach brightness temperatures of at least 10 32 K in our data, which do not resolve the narrowest pulses, and are known to reach 10 37 K in nanosecond-resolution observations (Hankins et al. 2003). The Crab pulsar’s pulses are therefore the brightest known in the observable universe. As such, they represent an important milestone for theories of the pulsar emission mechanism to explain. In addition, their short durations allow them to serve as especially sensitive probes of the Crab Nebula and the interstellar medium. We identify and quantify frequency structure in individual giant pulses using a scintillated, amplitude-modulated, polarized shot-noise (SAMPSN) model. The frequency structure associated with multipath propagation decorrelates on a timescale � 25 s at 1.5 GHz. To produce this timescale requires multipath propagation to be strongly influenced by material within the Crab Nebula. We also show that some frequency structure decorrelates rapidly, on timescales less than one spin period, as would be expected from the shot-noise pattern of nanosecond-duration pulses emitted by the pulsar. We discuss the detectability of individual giant pulses as a function offrequency and provenance. Taking into account the Crab pulsar’s locality inside a bright supernova remnant, we conclude that the brightest pulse in a typical 1 hr observation would be most easily detectable in our lowest frequency band (0.43 GHz) to a distance � 1.6 Mpc at 5 � . We also discuss the detection of such pulses using future instruments such as LOFAR and the SKA.

Simultaneous Dual-Frequency Observations of Giant Pulses from the Crab Pulsar

Simultaneous measurements of giant pulses from the Crab pulsar were taken at two widely spaced frequencies using the real-time detection of a giant pulse at 1.4 GHz at the Very Large Array to trigger the observation of that same pulse at 0.6 GHz at a 25-m telescope in Green Bank, WV. Interstellar dispersion of the signals provided the necessary time to communicate the trigger across the country via the Internet. About 70% of the pulses are seen at both 1.4 GHz and 0.6 GHz, implying an emission mechanism bandwidth of at least 0.8 GHz at 1 GHz for pulse structure on time scales of one to ten microseconds. The arrival times at both frequencies display a jitter of 100 microseconds within the window defined by the average main pulse profile and are tightly correlated. This tight correlation places limits on both the emission mechanism and on frequency dependent propagation within the magnetosphere. At 1.4 GHz the giant pulses are resolved into several, closely spaced components. Simultaneous observations at 1.4 GHz and 4.9 GHz show that the component splitting is frequency independent. We conclude that the multiplicity of components is intrinsic to the emission from the pulsar, and reject the hypothesis that this is the result of multiple imaging as the signal propagates through the perturbed thermal plasma in the surrounding nebula. At both 1.4 GHz and 0.6 GHz the pulses are characterized by a fast rise time and an exponential decay time which are correlated. The pulse broadening with its exponential decay form is most likely the result of multipath propagation in intervening ionized gas.

Five Years of Pulsar Flux Density Monitoring: Refractive Scintillation and the Interstellar Medium

We have monitored the radio flux density of 21 pulsars on a daily basis for five years. The 610 MHz flux density time series for these pulsars range from nearly constant for the most distant and heavily scattered pulsars to rapidly varying, saturated time series for more nearby pulsars. The measured stability of the flux density from the most distant pulsars (variations less than 5%) implies that the average radio emission from pulsars, before it has been affected by propagation through the interstellar medium, is constant in strength on timescales of a few hours to several years. The modulation index of the flux density variations never exceeds 0.5, ruling out a density inhomogeneity spectrum with a steep power-law exponent (β > 4). The flux density variations for 15 of the pulsars are consistent with a Kolmogorov turbulence spectrum over a range of more than 5 orders of magnitude in scattering strength, with no detectable presence of an inner scale. For these lines of sight we constrain the inhomogeneity slope to be in the range 3.5 ≤ β ≤ 3.7, which brackets the Kolmogorov value of β = 3.67. The flux density variations are greater than predicted by this model for six pulsars—including the Crab and Vela—but this group is consistent with a Kolmogorov spectrum and an inner scale of ≈1010 cm. The lines of sight to three of the other pulsars in this group pass through H II regions around young, hot stars. For six pulsars we have found a change in the slope of the intensity structure function, which could be connected with a change in the slope of the inhomogeneity power spectrum at a scale of ≈1013 cm.

The eclipsing millisecond pulsar PSR 1957 + 20

Information obtained over the past year on the eclipsing millisecond pulsar PSR 1957 + 20 and its orbiting companion is discussed. The pulsar is found to be similar in many ways to other millisecond pulsars: its spin parameters are extremely stable, its period derivative is very small, its profile has a strong interpulse, and its radio spectrum has a steep power-law index. The orbit is nearly circular, and the mass function implies a companion mass not much greater than 0.022 solar. Eclipses last for approximately 56 and 50 min at 318 and 430 MHz, respectively, corresponding to a nu exp - 0.41 + or - 0.09 dependence of eclipse duration on frequency. The available evidence points strongly toward a system in which the radiation from the pulsar heats the companion to the point of ablation, thereby driving a stellar wind that trails outward and behind the companion. 28 refs.

Polarimetric Properties of the Crab Pulsar between 1.4 and 8.4 GHz

New polarimetric observations of the Crab pulsar at frequencies between 1.4 and 8.4 GHz are presented. Additional pulse components discovered in earlier observations are found to have high levels of linear polarization, even at 8.4 GHz. No abrupt sweeps in position angle are found within pulse components; however, the position angle and rotational phase of the interpulse do change dramatically between 1.4 and 4.9 GHz. The multifrequency profile morphology and polarization properties indicate a nonstandard origin of the emission. Several emission geometries are discussed, but the one favored locates sites of emission both near the pulsar surface and in the outer magnetosphere.

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 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.