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High-resolution multifrequency observations with the VLA and OSRT of the small-diameter flat-spectrum radio source G54.1 + 0.3 are presented. The filled-center brightness distribution, strong polarization at 6 cm, and flat radio spectrum (alpha of about -0.13) from 0.327 to 5 GHz confirm that G54.1 + 0.3 is a Crab-like supernova remnant.
Data from the UCSD/MIT Hard X-ray and Low-Energy Gamma-Ray Experiment on the first High Energy Astronomy Observatory (HEAO 1) were used to determine the spectrum of the Crab Nebula in the 10 keV-5 MeV energy range. The spectral change in the 100 keV-1 MeV region is characterized. When the combined spectrum is fitted to a broken power-law model, the data require a description with the following distinct spectral indices: a power-law slope of 2.08 in the 1-100 keV region and a slope of about 2.5 above 150 keV.
Recent observational and theoretical investigations of Galactic and extragalactic SNRs suspected of having neutron central stars are reviewed, with an emphasis on Einstein X-ray observations. Sections are devoted to the Crab Nebula and its twin SNR 0540-69.3, SNR with observationally well determined and indistinct central X-ray nebulae, SNR with weak and strong unresolved internal X-ray sources, binary systems, and the relationship between X-ray and radio luminosity and pulsar energy loss. Consideration is given to radio pulsars, SNR with filled-center X-ray emission attributed to thermal processes, radio plerions and composite SNRs, MSH 15-56, and SNRs in the LMC. Extensive contour maps, sample optical images, and tables of numerical data are included.
Previous relativistic pair plasma MHD wind models of the Crab Nebula indicate that the ratio of the wind's Poynting flux to particle kinetic energy flux must be very small. However, many current theories of pulsar magnetospheres and pulsar observations suggest that the spin-down luminosity should be primarily electromagnetic in origin and outward transport. For an oblique rotator, the asymptotic wind magnetic field near the rotational equator should consist of stripes of alternating toroidal field. A simple model of an initially high-sigma striped MHD wind is developed which describes the reconnection annihilation of the opposite polarity stripes as the wind flows radially outward. The initially dominant Poynting flux is converted into particle thermal and directed kinetic energy well within the inner standing shock which terminates the superfast wind.
The paper presents 1.644 micron forbidden-line Fe II and 1.570 micron continuum images of the Crab Nebula which are compared with optical H-alpha, forbidden-line O III, and 5100 A continuum data. While the same prominent filaments are generally apparent in all lines, no substantial variations within the nebula in the strength and morphology of forbidden-line Fe II relative to the other lines are found. The infrared to optical continuum ratio of the synchrotron nebula shows 15 - 20 percent variations which are dominated by features which appear related to the position of the pulsar. These are interpreted as variations of 0.15 in the spectral index and it is suggested that they are related to the radiative decay of energetic particles and the paths over which these particles travel from the pulsar throughout the nebula.
The angular resolution of gamma-ray detectors does not allow one to separate the nebula from the pulsar in the Crab. It is generally assumed that the steady emission of gamma rays comes from the nebula. Using the 'outer magnetospheric gap' model, an alternative mechanism in which the steady emission of gamma rays could come from a compact region, a couple of light cylinder radii beyond the pulsar.
Gamma ray pulsed emission from the Crab and Vela pulsars are discussed. Emphasis is placed on use of current and future Gamma Ray Observatory observations as diagnostics to examine and constrain theoretical models. Issues on the spectra and pulse profile are discussed. Specifically, the absence and possibly significant time variability of pulsed emission below 10 MeV from the Vela pulsar represents a serious challenge to current astronomical models. Theoretical implication of this 'peculiarity' is addressed.
The FIGARO II experiment (a large area, balloon borne, crystal scintillator detector working from 0.15 to 4.3 MeV) observed the Crab pulsar on 1990 Jul. 9 for about seven hours. The study of the pulse profile confirms some structures detected with a low significance during the shorter observation of 1986, and adds new important elements to the picture. In particular, between the two main peaks, two secondary peaks appear centered at phase values 0.1 and 0.3, in the energy range 0.38 to 0.49 MeV; in the same energy range, a spectral feature at 0.44 MeV, interpreted as a redshifted positron annihilation line, was observed during the same balloon flight in the phase interval including the second main peak and the neighboring secondary peak. If the phase interval considered is extended to include also the other secondary peak, the significance of the spectral line appears to increase.
Preliminary results are presented of the Compton Gamma Ray Observatory Oriented Scintillation Spectrometer Experiment (OSSE) observations of the Crab pulsar. The pulsar energy spectra and light curves are in general agreement with previous observations, validating the OSSE pulsar data acquisition modes and data analysis algorithms. The data suggest that the spectrum of the pulsar varies throughout the light curve. The 'interpulse' region has a slightly flatter spectrum in the approx. 60 to 250 keV region and a slightly steeper spectrum at higher energies than the two main pulses. No evidence was found for any lines in the spectra with a typical sensitivity of about 10(exp -4) photons/sq cm/s.
Observations are reported of the Crab pulsar made at radio frequencies concurrent with Oriented Scintillation Spectrometer Experiment (OSSE) observations from 15 to 27 May 1991. Using the 43 m telescope at Green Bank at 0.8 and 1.4 GHz, samples were made continuously for 10 hrs/day at intervals of 100 to 300 microsecs. The analysis of the radio data includes calculation of histograms of pulse intensities, absolute timing to about 20 microsec precision, and characterization of intensity variations on time scales from the 33 ms spin period to days. The most detailed analysis is presented made of giant pulses. The ultimate goal is to bin the radio data into giant and nongiant pulses and to form average waveforms of OSSE data for the corresponding pulse periods. A test is done to see whether the violet radio fluctuations (which are not seen in other radio pulsars to the same degree) are correlated with low energy gamma rays, yielding constraints on the radio coherence mechanism and the steadiness of the electron-positron outflow in the magnetosphere. Timing analysis of the radio data provides a well defined ephemeris over the specified range of epochs. The gamma ray pulse phase was predicted with an error of less than 70 microsecs.
The observed TeV flux from the Crab can be explained by inverse Compton scattering in the nebula if the average nebular field is indeed approximately equal to 0.0003 G as is predicted by the spectral break between radio and optical. The estimated spectral index at TeV energies also agrees with the observed value. The unpulsed high energy gamma-rays seen by COS-B are not explained by this inverse Compton component but can be explained by synchrotron radiation requiring electrons up to at least 10 exp 16 eV, most likely accelerated at the shock in the pulsar wind. The detection of a spectral depression at about 1 GeV would allow an estimate of the maximum electron energy. The flux estimates also predict the detection of a steady flux of unpulsed UHE gamma-rays from near the shock with sensitive detectors if the field strength increases downstream with increasing radius as is predicted by MHD theory.
The observations of transient gamma-ray line and sporadic continuum emissions from the Crab over the last two decades are reviewed, and these measurements are used to explore the emission mechanisms and regions. When examined in detail, the measurements require an emission region(s) considerably more complex than predicted by simple synchrotron models, suggesting behavior reminiscent of gamma-ray bursts.
The inverse Compton scattering model for the unpulsed TeV emission from the Crab Nebula is reexamined using the magnetic field distribution derived from MHD flow models of the nebula. It is shown that the observed flux can be explained if the average nebular field is indeed about 0.0003, as is predicted by the spectral break between radio and optical. The brightness distribution of the TeV gamma-ray signal is expected to extend out to about 1.5 arcmin from the pulsar. The present estimates predict a steady flux of unpulsed ultrahigh-energy gamma-rays due to the inverse Compton scattering of soft photons by shock-accelerated electrons and/or positrons in the vicinity of the shock.
This document is to serve as the requisite Final Technical Report on grant NAG5-1557 which was awarded under the NASA ROSAT Guest Investigator Program to Columbia University. In response to the NASA Research Anouncement describing the first round of Guest Investigations to be carried out under the U.S.-German ROSAT Program (AO-1), the PI submitted several proposals, three of which were accepted in part: (1) the x-ray population of globular clusters; (2) three crab-like SNR in the Large Magellanic Cloud; and (3) x rays from nearby radio pulsars. The status of these three programs as of 31 May 1993, the termination date of the grant, is reported.
We extend Kennel and Coroniti's (1984) spherical magnetohydrodynamic models for the Crab Nebula to include the pinching effect of the toroidal magnetic field. Since the bulk nebular flow is likely to be very submagnetosonic, a quasi-static treatment is possible. We show that the pinching effect can be responsible for the observed elongation of the pulsar wind bubble, as indicated by the surface brightness contours of optical synchrotron radiation. From the observed elongation we estimate a value for sigma, the ratio of Poynting flux to plasma kinetic energy flux in the free pulsar wind, which is consistent with previous results from spherical models. Using the inferred magnetic field configuration inside the pulsar wind bubble, combined with the observed dimensions of the X-ray nebula, we are able to constrain the particle distribution function. We conclude that, for a power-law injection function, the maximum energy has to be much larger in the pulsar equatorial region than in the polar region.
Observations of PSR 0531+21 with the High Speed Photometer on the HST in the visible in October 1991 and in the UV in January 1992 are presented. The time resolution of the instrument was 10.74 microsec; the effective time resolution of the light curves folded modulo the pulsar period was 21.5 microsec. The main pulse arrival time is the same in the UV as in the visible and radio to within the accuracy of the establishment of the spacecraft clock, +/- 1.05 ms. The peak of the main pulse is resolved in time. Corrected for reddening, the intensity spectral index of the Crab pulsar from 1680 to 7400 A is 0.11 +/- 0.13. The pulsed flux has an intensity less than 0.9 percent of the peak flux just before the onset of the main pulse. The variations in intensity of individual main and secondary pulses are uncorrelated, even within the same rotational period.
Using the radio through hard X-ray images of the Crab nebula to derive the spatial dependence of the electron spectrum and the magnetic field distribution from MHD flow models, we have rederived the high-energy spectrum of inverse Compton scattered gamma rays. We find agreement with the observed spectrum at TeV energies, but it is clear that the inverse Compton flux does not contribute significantly to the unpulsed nebular emission observed by COS-B from 50 to 500 MeV, which is consistent with a smooth continuation of the spectrum in hard X-rays. The emission at these energies must therefore be due to synchrotron radiation by electrons of at least PeV energies. It appears that the emission in the high-energy gamma-ray range, sensitive to the highest energy electrons in the nebula, can put interesting constraints on the acceleration mechanism.
We propose a model of a optical 'wisps' of the Crab Nebula, features observed in the nebular synchrotron surface brightness near the central pulsar, as manifestations of the internal structure of the shock terminating the pulsar wind. We assume that this wind is composed of ions and a much denser plasma of electrons and positrons, frozen together to a toroidal magnetic field and flowing relativistically. We construct a form of solitary wave model of the shock structure in which we self-consistently solve for the ion orbits and the dynamics of the relativistically hot, magnetized e(+/-) background flow. We ignore dispersion in the ion energies, and we treat the pairs as an adiabatic fluid. The synchrotron emission enhancements, observed as the wisps, are then explained as the regions where reflection of the ions in the self-consistent magnetic field causes compressions of the e(+/-).