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Register a new userThe Crab Pulsar at Centimeter Wavelengths II: Single Pulses
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We have carried out new, high-frequency, high-time-resolution observations of the Crab pulsar. Combining these with our previous data, we characterize bright single pulses associated with the Main Pulse, both the Low-Frequency and High-Frequency Interpulses, and the two High-Frequency Components. Our data include observations at frequencies ranging from 1 to 43 GHz with time resolution down to a fraction of a nanosecond. We find at least two types of emission physics are operating in this pulsar. Both Main Pulses and Low-Frequency Interpulses, up to about 10 GHz, are characterized by nanoshot emission - overlapping clumps of narrow-band nanoshots, each with its own polarization signature. High-Frequency Interpulses, between 5 and 30 GHz, are characterized by spectral band emission - linearly polarized emission containing about 30 proportionately spaced spectral bands. We cannot say whether the longer-duration High-Frequency Component pulses are due to a scattering process, or if they come from yet another type of emission physics.
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We have observed the pulsar in the Crab Nebula at high radio frequencies and high time resolution. We present continuously sampled data at 640-ns time resolution, and individual bright pulses recorded at down to 0.25-ns time resolution. Combining our new data with previous data from our group and from the literature shows the dramatic changes in the pulsars radio emission between low and high radio frequencies. Below about 5 GHz the mean profile is dominated by the bright Main Pulse and Low-Frequency Interpulse. Everything changes, however, above about 5 GHz; the Main Pulse disappears, the mean profile of the Crab pulsar is dominated by the High-Frequency Interpulse (which is quite different from its low-frequency counterpart) and the two High-Frequency Components. We present detailed observational characteristics of these different components which future models of the pulsars magnetosphere must explain.
We present statistical analysis of a fluence limited sample of over 1100 giant pulses from the Crab pulsar, with fluence > 130 Jy ms at ~1330 MHz. These were detected in ~260 hours of observation with the National Centre for Radio Astrophysics (NCRA)-15m radio telescope. We find that the pulse energy distribution follows a power law with index $alphaapprox$-3 at least up to a fluence of ~5 Jy s. The power law index agrees well with that found for lower energy pulses in the range 3-30 Jy ms. The fluence distribution of the Crab pulsar hence appears to follow a single power law over ~3 orders of magnitude in fluence. We do not see any evidence for the flattening at high fluences reported by earlier studies. We also find that at these fluence levels, the rate of giant-pulse emission varies by as much as a factor of ~5 on time-scales of a few days, although the power law index of the pulse-energy distribution remains unchanged. The slope of the fluence distribution for Crab giant pulses is similar to that recently determined for the repeating FRB 121102. We also find an anti-correlation between the pulse fluence and the pulse width, so that more energetic pulses are preferentially shorter.
Optical observations provide convincing evidence that the optical phase of the Crab pulsar follows the radio one closely. Since optical data do not depend on dispersion measure variations, they provide a robust and independent confirmation of the radio timing solution. The aim of this paper is to find a global mathematical description of Crab pulsars phase as a function of time for the complete set of published Jodrell Bank radio ephemerides (JBE) in the period 1988-2014. We apply the mathematical techniques developed for analyzing optical observations to the analysis of JBE. We break the whole period into a series of episodes and express the phase of the pulsar in each episode as the sum of two analytical functions. The first function is the best-fitting local braking index law, and the second function represents small residuals from this law with an amplitude of only a few turns, which rapidly relaxes to the local braking index law. From our analysis, we demonstrate that the power law index undergoes instantaneous changes at the time of observed jumps in rotational frequency (glitches). We find that the phase evolution of the Crab pulsar is dominated by a series of constant braking law episodes, with the braking index changing abruptly after each episode in the range of values between 2.1 and 2.6. Deviations from such a regular phase description behave as oscillations triggered by glitches and amount to fewer than 40 turns during the above period, in which the pulsar has made more than 2.0e10 turns. Our analysis does not favor the explanation that glitches are connected to phenomena occurring in the interior of the pulsar. On the contrary, timing irregularities and changes in slow down rate seem to point to electromagnetic interaction of the pulsar with the surrounding environment.
Detecting and studying pulsars above a few GHz in the radio band is challenging due to the typical faintness of pulsar radio emission, their steep spectra, and the lack of observatories with sufficient sensitivity operating at high frequency ranges. Despite the difficulty, the observations of pulsars at high radio frequencies are valuable because they can help us to understand the radio emission process, complete a census of the Galactic pulsar population, and possibly discover the elusive population in the Galactic Centre, where low-frequency observations have problems due to the strong scattering. During the decades of the 1990s and 2000s, the availability of sensitive instrumentation allowed for the detection of a small sample of pulsars above 10$,$GHz, and for the first time in the millimetre band. Recently, new attempts between 3 and 1$,$mm ($approx$86$-$300$,$GHz) have resulted in the detections of a pulsar and a magnetar up to the highest radio frequencies to date, reaching 291$,$GHz (1.03$,$mm). The efforts continue, and the advent of new or upgraded millimetre facilities like the IRAM 30-m, NOEMA, the LMT, and ALMA, warrants a new era of high-sensitivity millimetre pulsar astronomy in the upcoming years.
The last six years have witnessed major revisions of our knowledge about the Crab Pulsar. The consensus scenario for the origin of the high-energy pulsed emission has been challenged with the discovery of a very-high-energy power law tail extending up to 400 GeV, above the expected spectral cut off at a few GeV. Now, new measurements obtained by the MAGIC collaboration extend the energy spectrum of the Crab Pulsar even further, on the TeV regime. Above 400 GeV the pulsed emission comes mainly from the inter-pulse, which becomes more prominent with energy due to a harder spectral index. These findings require gamma-ray production via inverse Compton scattering close to or beyond the light cylinder radius by an underlying particle population with Lorentz factors greater than 5 times 106. We will present those new results and discuss the implications in our current knowledge concerning pulsar environments.
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