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Evidence for an association near the Vela pulsar rests on the H-R diagram. Definite bunching occurs around the mean line. However this evidence is not supported by correlation of proper motions in the region. If the Vela pulsar is a member of this association, a rather large mass is implied, about 10 solar masses.
A classification of compact stars, depending on the electron distribution in velocity space and the density profiles characterizing their magnetospheric plasma, is proposed. Fast pulsars, such as NP 0532, X-ray sources such as Sco-X1, and slow pulsars are suggested as possible evolutionary stages of similar objects. The heating mechanism of Sco-X1 is discussed in some detail.
Two separate theoretical models of X-ray sources were proposed, based on the premise that a pulsar can be hidden underneath a thick plasma cloud. The X-ray emission would come secondarily from the pulsar energy from the cloud underneath. Upper limits to the pulsed fraction for those conditions causing the most difficult detection of fluctuations are shown, along with power density and source of power spectra.
The effect of radiative reaction on the motion of particles is evaluated for pulsars with electromagnetic fields as strong as 10 to the twelfth power G, assuming that the pulsars are sources of high-energy particles. The analysis is limited to the specific motion of the particles. A simple method of solution, suitable for solving other similar problems, is proposed. The significance of particle motion to astronomical problems is examined.
Cooling rates were calculated for neutron stars of about one solar mass and 10 km radius, with magnetic fields from zero to about 10 to the 14th power gauss, for extreme cases of maximum and zero superfluidity. The results show that most pulsars are so cold that thermal ionization of surface atoms would be negligible. Nucleon superfluidity and crystallization of heavy nuclei were treated quantitatively, and more realistic hadron star models were chosen. Cooling rates were calculated for a stable hyperon star near the maximum mass limit, a medium weight neutron star, and a light neutron star with neutron-rich heavy nuclei near the minimum mass limit. Results show that cooling rates are a sensitive function of density. The Crab and Vela pulsars are considered, as well as cooling of a massive white dwarf star.
The research in the program for designing and fabricating payloads for sounding rockets for use in X-ray astronomy is reported. Published papers concerning six Aerobee flights are presented. The results include: (1) the establishment of the radio galaxy M87 as an X-ray source; (2) the studies of the pulsar NPO532, including its X-ray pulse shape, the determination of the synchronism of the optical and X-ray pulses and the codiscovery of its existence as an X-ray pulsar; (3) the determination of many X-ray source positions, and (4) the precise position of Cyg X-1 which led directly to the identification of a ninth magnitude BO supergiant.
Synchrotron emission is considered from individual particles which have small pitch angles and the general properties of synchrotron sources which mainly contain such particles, as well as the emissivities and degrees of circular polarization for specific source distributions. The limitation of synchrotron source models for optical pulsars and compact extragalactic objects are discussed, and it is shown that several existing models for the pulsar NP 0532 are inconsistent with the measured time variations and polarizations of the optical emission. Discussion is made also of whether the low frequency falloffs in the extragalactic objects PKS 2134 + 004, OQ 208, and NGC 1068 is due to emission from particles with small pitch angles or absorption by a thermal plasma or synchrotron self-absorption. It is concluded that the absorption interpretations cannot account for the turnover in the spectrum of PKS 2134 + 004. Measurements of polarization, angular structure, and X-ray flux are also described.
The stability of a circularly polarized electromagnetic wave that is strong enough to make plasma electrons, but not ions, relativistic is studied. Small perturbations are considered which propagate parallel to the large-amplitude driver. A relativistically strong wave can be unstable on time scales as short as twice its own oscillation period, and decays into a forward-going plasma oscillation and either one or two electromagnetic waves. Ion motion introduces an additional instability which can be important at short perturbation wavelengths, where the driver would otherwise be stable. The unstable ion and electron modes both have potential for producing anomalously large acceleration of relativistic particles, as well as significant amounts of backscattered light. These effects may be important in two applications: (1) the use of intense lasers to heat or compress plasma, and (2) the plasma surrounding a pulsar, if the pulsar is losing energy by radiation of electromagnetic waves at its rotation frequency. Instability persists in the nonrelativistic regime, reducing to stimulated Raman scattering as a special case.
Radio beacons with distinguishing signatures exist in nature as pulsating radio sources (pulsars). These objects radiate well determined pulse trains over hundreds of megahertz of bandwidth at radio frequencies. Since they are at known positions, they can also be used as navigation beacons in interplanetary space. Pulsar signals are weak and dispersive when viewed from earth. If an omnidirectional antenna is connected to a wideband receiver (200 MHz bandwidth centered at 200 MHz) in which dispersion effects are removed, nominal spacecraft position errors of 1500 km can be obtained after 24 h of signal integration. An antenna gain of 10 db would produce errors as low as 150 km. Since the spacecraft position is determined from the measurement of the phase of a periodic signal, ambiguities occur in the position measurement. Simultaneous use of current spacecraft navigation schemes eliminates these ambiguities.
A qualitative description of the interior of a neutron star is presented, giving attention also to the validity of the 'isotropic fluid' approximation. The atmosphere and surface of a neutron star are considered together with aspects concerning nuclear and solid state physics in the crust, the liquid interior, the hyperon core, neutron star models, and pulsar observations. Accretion processes are also investigated, taking into account such topics as the Eddington limit, accretion rates, the death of pulsars, changes of the surface composition of neutron stars by accretion, questions of X-ray emission, and aspects of gamma radiation emission.
Present applications of precise time and frequency (T/F) technology can be grouped as follows: (1) Communications systems which require T/F for time division multiplexing and for using spread spectrum techniques. (2) Navigation systems which need T/F for position fixing using a timed signal. (3) Scientific-Metrological applications which use T/F as the most precisely reproducible standard of measurement. (4) Astronomical-Space applications which cover a variety of the most demanding applications such as pulsar research, Very Long Baseline Interferometry (VLBI) and laser/radar ranging. In particular, pulsar time-of-arrival measurements require submicrosecond precision over a period of one-half year referred to an extraterrestrial inertial system, and constitute the most stringent requirements for uniform timekeeping to date.
Study illustrating how the presence of a high-intensity pulse of radiation can distort its own passage through a plane differentially shearing medium. It is demonstrated that the distortion is a sensitive function of the precise, and detailed, variation of the medium's refractive index by considering a couple of simple examples which are worked out numerically. In view of the high-intensity pulses observed from pulsars (approximately 10 to the 30th ergs per pulse), it is believed that the present calculations are of more than academic interest in helping unravel the fundamental properties of pulse production in, and propagating through, differentially sheared media - such as pulsars' magnetospheres within the so-called speed-of-light circle.
A survey of the instruments developed for gamma ray astronomy is given together with a brief summary of the current status of the observational results. These include the studies of galactic gamma ray emission, the diffuse, presumably extragalactic, gamma radiation, and localized gamma ray sources. The study of the spatial distribution of galactic gamma radiation is beginning to provide a new means for the study of galactic structure and dynamics. The diffuse emission may provide evidence of gamma ray emission in the cosmological past, although improved observations must be obtained before the picture can be clarified. The study of localized sources has shown NP0532, the Crab radio pulsar, to be a gamma ray pulsar also and strong emission from Vela may be due to supernova produced cosmic rays interacting with the remnant gas.
On 1973 July 23, the Cornell gamma-ray telescope had a second exposure to the Crab Nebula and pulsar via balloon flight from Palestine, Texas. On the previous flight, 1971 October 6, the signal from the pulsar had been unmistakably clear, especially at the highest energies (above 800 MeV), and an unpulsed component of similar average power had also been discernible. On the second flight, despite higher altitude, longer exposure, and improved electronics, the pulsed signal was barely detectable and the dc component not observable at all. The drop in intensity seems to increase with energy, being only a factor of 2 at 200 MeV but an order of magnitude at 1 GeV. There is some indication (not compelling) that the gamma-ray flux may have changed even within the 6-hour exposure. The first flight may have viewed the Crab in a state of enhanced high-energy emission correlated with the glitches that occurred on 1971 August 1 and October 25.
We have obtained medium-resolution spectra in the yellow of seven stars in the error box of the binary radio pulsar PSR 1913 + 16, using the image-tube scanner at the Lick 3-m telescope. None of the objects is spectroscopically abnormal and thus a strong candidate for membership in the system. The data are used to establish the extinction in the field, which is modest for this low galactic latitude. For three of the stars, limits of the order V exceeding 23 mag on optical pulsations with the frequency of the radio pulsar are derived. X-ray observations of the field have been obtained in the 2.5-7.5 keV band, using the UCL experiment aboard OAO Copernicus. No source is detected, with a 2 sigma upper limit on the time-averaged flux of 1.0 times 10 to the -10th power ergs per sq cm per sec.
Papers are presented dealing with galactic structure drawing on all branches of galactic astronomy with emphasis on the implications of the new gamma ray observations. Topics discussed include: (1) results from the COS-B gamma ray satellite; (2) results from SAS-2 on gamma ray pulsar, Cygnus X-3, and maps of the galactic diffuse flux; (3) recent data from CO surveys of the galaxy; (4) high resolution radio surveys of external galaxies; (5) results on the galactic distribution of pulsars; and (6) theoretical work on galactic gamma ray emission.
In October 1975 the high energy gamma ray flux from the Vela pulsar was measured by COS-B to be 1.6 to 2.1 times higher than the flux measured by SAS-2 in 1973. The existence is confirmed of a second region of enhanced radiation in the galactic anticenter in addition to that from the Crab pulsar.
Recent results in ground based very high energy gamma ray astronomy are reviewed. The various modes of the atmospheric Cerenkov technique are described, and the importance of cosmic ray rejection methods is stressed. The positive detections of the Crab pulsar that suggest a very flat spectrum and time-variable pulse phase are discussed. Observations of other pulsars (particularly Vela) suggest these features may be general. Evidence that a 4.8 hr modulated effect was detected from Cyg X-3 is strengthened in that the exact period originally proposed agrees well with a recent determination of the X-ray period. The southern sky observations are reviewed, and the significance of the detection of an active galaxy (NGC 5128) is considered for source models and future observations.