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At least 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Pulsar fluctuation spectra and the generalized drifting-subpulse phenomenon. II

Pulsar observations at meter wavelengths have been analyzed to investigate pulse-to-pulse variations and to identify regions of the pulse profile which display distinct statistical properties. Fluctuation spectra of eight recently discovered pulsars are presented along with a description of the drifting-subpulse phenomenon in several objects with quasiperiodic responses in their fluctuation spectra. A quantitative analysis of the drifting-subpulse phenomenon in PSR 0031-07 and PSR 0809+74 is given which emphasizes the broad-band nature of the phenomenon. A summary of the memory exhibited by pulsars with time scales of about 50 periods is given.

Backer, D. C.↗
NASA NTRS1975

SAS-2 high-energy gamma-ray observations of the Vela pulsar

The Second Small Astronomy Satellite (SAS-2) high-energy (in excess of 35 MeV) gamma-ray telescope has detected pulsed gamma-ray emission at the radio period from PSR 0833-45, the Vela pulsar, as well as an unpulsed flux from the Vela region. The pulsed emission consists of two peaks following the single radio peak by about 13 ms and 48 ms. The luminosity of the pulsed emission above 100 MeV from Vela is about 0.1 that of the pulsar NP 0532 in the Crab nebula, whereas the pulsed emission from Vela at optical wavelengths is less than 0.0002 that from the Crab. The relatively high intensity of the pulsed gamma-ray emission, and the double peak structure, compared with the single pulse in the radio emission, suggest that the high-energy gamma-ray pulsar emission may be produced under different conditions from those at lower energies.

Thompson, D. J.↗
NASA NTRS1976

A mechanism for the production of pulsar radio radiation

Recent theoretical studies have suggested that avalanches of electron-positron pairs are produced by spark discharges near the magnetic poles of pulsars. This pair production mechanism may result in a double-peaked particle distribution function. We investigate collective effects caused by a high-energy electron-positron beam traversing a low-energy electron-positron plasma for an astrophysically interesting range of physical parameters. The principal result of this paper is to identify transverse unstable waves whose phase propagation may occur at any arbitrary angle (other than zero) with respect to the magnetic field but whose group velocity is along the magnetic field and approximately the speed of light. It is suggested that the observed radio pulses from pulsars may be caused by the focusing of these transverse waves along the open field lines extending from a pulsar's polar cap to the light cylinder.

Hardee, P. E.↗
NASA NTRS1978

Neutrinos from binary pulsars

It is shown that binary systems containing moderately young pulsars may emit high-energy neutrinos (between 1 and 100 TeV) at detectable levels. The pulsars are assumed to have total luminosities of the order of 10 to the 38th erg/sec. The neutrinos are produced by high energy particles (e.g. protons) from the pulsar striking the companion. Cyg X3 may be detectable in high-energy neutrinos if it emits greater than about 10 to the 35th erg/sec in high-energy protons. There may be a whole class of objects like Cyg X3, but obscured by thick accretion clouds.

Eichler, D.↗
NASA NTRS1980

The X-ray pulsars 4U 1145-61 and 1E 1145.1-6141

Observations of the neighboring X-ray pulsars 4U 1145-61 and 1E 1145.1-6141 obtained by OSO 8, HEAO 1 and the Einstein Observatory between 1975 and 1979 are reported. A temporal analysis of Einstein Solid State Spectrometer and Monitor Proportional Counter data, OSO 8 A and C detector data and HEAO 1 A-2(3) experiment data confirms the presence of two periodicites at 291 and 297 sec in the region, with the shorter period located within 3 arcmin of 4U 1145-61. Observations indicate the spectrum of 4U 1145-61 to obey a power law with an index of 1.5 out to at least 60 keV, with little variation with pulse phase, and a 500-eV EW iron line at 6.7 keV. The spectrum of the other source is found to be more absorbed. Similarities between the pulsars are noted, and it is suggested that they may be the result of pulsar formation in a homogeneous cloud of progenitors.

White, N. E.↗
NASA NTRS1981

Navigation Using X-Ray Pulsars

Approximately one dozen X-ray pulsars are presently known which emit strong stable pulses with periods of 0.7 to approximately 1000 s. By comparing the arrival times of these pulses at a spacecraft and at the Earth (via an Earth orbiting satellite), a three dimensional position of the spacecraft can be determined. One day of data from a small onboard X-ray detector yields a three dimensional position accurate to approximately 150 km. This accuracy is independent of spacecraft distance from the Earth. Present techniques for determining the two spacecraft coordinates other than range measure angles and thus degrade with increasing spacecraft range. Thus navigation using X-ray pulsars will always be superior to present techniques in measuring these two coordinates for sufficiently distant spacecraft. At present, the break even point occurs near the orbit of Jupiter. The Crab pulsar can also be used to obtain one transverse coordinate with an accuracy of approximately 20 km.

Chester, T. J.↗
NASA NTRS1981

Pulsar and diffuse contributions to observed galactic gamma radiation

The first calculation of a gamma-ray production spectrum from pulsars in the Galaxy, along with a statistical analysis of data on 328 known radio pulsars, are presented. The implications of this point source contribution to the general interpretation of the observed galactic gamma-ray spectrum are indicated. The contributions from diffuse interstellar cosmic-ray induced production mechanisms are then re-examined, concluding that pulsars may be contributing significantly to the galactic gamma-ray emission.

Harding, A. K.↗
NASA NTRS1981

High energy X-ray observations of the 38-second pulsar

The results of observations of the 38-second pulsar obtained at high X-ray energies (13-180 keV) with the UCSD/MIT instrument aboard HEAO 1 are reported. The results include a measurement of the source location, measurement of the pulse profile, and determination of the average intensity and spectrum during each of three time intervals spanning a baseline of 1 year. The total intensity of the pulsar is seen to vary on a 6-month time scale. The spectrum is hard but, like other X-ray pulsars, steepens at energies above 20 keV.

Byrne, P. F.↗
NASA NTRS1981

The gamma-ray spectra of radio pulsars

A general expression for the gamma-ray spectra of pulsars is derived for models where the radiation is produced by particles accelerated along the open field lines of a neutron star. The accelerating potentials are assumed to be limited by pair production discharges and to decrease linearly with increasing number of electron-positron pairs created during these discharge processes. The corresponding energy distribution of particles is derived both for the case of strong radiation reaction and for that of no radiation reaction. The resultant formula for the gamma-ray spectrum fits well to the Crab and the Vela pulsar spectra assuming that radiation reaction is negligible in these two fast pulsars.

Ayasli, S.↗
NASA NTRS1981

Asymmetries in the galactic distribution of pulsars

The distribution of pulsars in galactocentric radius and z distance has been determined for opposite halves of the Galaxy, using data on 328 pulsars from three surveys. The distributions in galactocentric radius are found to be significantly different at positive and negative longitudes, although both show strong peaks between 5 and 6 kpc. There is also some indication that pulsars are located preferentially along spiral arms. Distributions in the z component of dispersion measured above and below the galactic plane also show asymmetry, with higher dispersion occurring at negative z. This may imply the existence of a narrow (approximately 100 pc), high electron density layer below the plane of the sun in the inner galaxy.

Harding, A. K.↗
NASA NTRS1982

A new, fast X-ray pulsar in the supernova remnant MSH 15-52

A pulsing X-ray source has been discovered within the shell of the supernova remnant MSH 15-52. The period is 0.150 s, and the rate of increase of period with time is the highest measured for any pulsar. These characteristics and the fact that the pulsar is surrounded by a small, bright nebula indicate that this object is very similar to the Crab pulsar.

Seward, F. D.↗
NASA NTRS1983

On neutron star structure and the millisecond pulsar

The recently discovered millisecond pulsar (PSR1937-214) is observed to be rotating close to the limit of dynamical instability for a neutron star. Despite its extremely rapid rotation, measurements of the period derivative put a stringent upper limit on the energy loss from gravitational radiation, thus requiring that the quadrupole moment be quite small. The pulsar must also be rotating below the critical frequency at which its equilibrium configuration would become non-axisymmetric, since the lifetime of this configuration against decay by gravitational radiation is very short. This critical frequency, given by the theory of rotating ellipsoids, imposes a restriction on the rotation rate more severe than the break-up frequency and may be used to set a lower limit, rho 2 x 10 to the 14th power g/cu cm, on the density of the star. If the mass is 0.5 - 1.5 solar mass, several of the stiffer neutron star equations of state may be ruled out, and the radius should be less than 16 km. The condition for axisymmetry also imposes an upper limit on the rotation rate to which neutron stars may be spun up by accretion disks in binary systems, a model recently proposed for the evolution of the millisecond pulsar.

Harding, A. K.↗
NASA NTRS1983

JPL pulsar timing observations. II - Geocentric arrival times

Monitoring of the behavior of naturally pulsating galactic radio sources, or pulsars, through regularly spaced measurements of pulse arrival times, has been conducted by several laboratories. A tabular presentation is here made of pulse arrival time measurements from the NASA Deep Space Network between late 1968 and early 1981. By expressing the measurements in ephemeris time, and referring them to the geocenter, usable tables of results have been generated for each pulsar listed in the first of the tables given. The considerations addressed by the tables are: (1) a necessary step in the study of pulsar dynamics is the reduction of topocentric arrival times to the barycenter of the solar system; (2) the tabulated data are accessible to all opinions as to the procedures to be used in interpreting arrival time data; and (3) different observing programs can usually be combined to produce an improvement in the total data set.

Downs, G. S.↗
NASA NTRS1983

Genesis stories for the millisecond pulsar

Theoretical models proposed to explain the origin of the millisecond pulsar (MP) PSR 1937+214 are reviewed, examining their ability to explain its low surface dipole magnetic field (B), its low birth temperature (less than 10 to the 8th K), the absence of a companion or remnant, and its low velocity perpendicular to the Galactic plane. The models discussed are a single isolated explosion forming a rapidly spinning neutron star, spin-up of a dead pulsar by accretion from a companion, collapse of an accreting spinning white dwarf, and fusion of a tight binary composed of two old neutron stars. Although all of the models have difficulties in explaining one or more of the MP characteristics, the second model is found to be most probable in the light of present knowledge. The lack of a companion is explained by its tidal disruption after it had fed the accreting pre-pulsar for 1 Gyr or more and its mass had decreased to about 0.01 solar mass. Neutron stars accreting in this way have been observed in Galactic-bulge X-ray sources.

Ruderman, M. A.↗
NASA NTRS1983

Self-modulational formation of pulsar microstructures

A nonlinear plasma theory for self-modulation of pulsar radio pulses is discussed. A nonlinear Schroedinger equation is derived for strong electromagnetic waves propagating in an electron-positron plasma. The nonlinearities arising from wave-intensity-induced particle-mass variation may excite the modulational instability of circularly and linearly polarized pulsar radiation. The resulting wave envelopes can take the form of periodic wave trains or solitons. These nonlinear stationary waveforms may account for the formation of pulsar microstructures.

Chian, A. C.-L.↗
NASA NTRS1984

Vortex creep and the internal temperature of neutron stars. II - Vela pulsar

The observed complex postglitch behavior of the Vela pulsar is explained as resulting from coupling of the crust to crustal neutron superfluid, specifically that part of the superfluid in which vortex lines are pinned to crustal nuclei. It is shown how the general theory of vortex creep provides an excellent fit to the timing observations of Downs which span the decade 1969-1979 and include four giant glitches. Relaxation times, inertial moments, and limits on superfluid pinning parameters are extracted for three distinct regions of vortex pinning in the star, with results which are consistent with microscopic theories of its internal structure. Relaxation times due to vortex creep are directly proportional to the internal temperature of the star, so that the limits obtained for pinning parameters translate to bounds on this temperature. It is concluded that the internal temperature of the Vela pulsar is about 10-million K and discuss the extent to which improved calculations of vortex pinning as well as soft X-ray observations of other stars will make possible an improved determination of the pulsar temperature.

Alpar, M. A.↗
NASA NTRS1984

Confinement of the Crab pulsar's wind by its supernova remnant

A steady state, spherically symmetric, magnetohydrodynamic model of the Crab nebula is constructed. A highly relativistic positronic pulsar wind is terminated by a strong MHD shock that decelerates the flow and increases its pressure to match boundary conditions imposed by the recently discovered supernova remnant that surrounds the nebula. If the magnetic luminosity of the pulsar wind upstream of the shock is about 0.3 percent of its particle luminosity, the pressure and velocity boundary conditions imposed by the remnant place the shock where it is inferred to be: near the outer boundary of an underluminous region observed to surround the pulsar. It is necessary to include the weak magnetization of the wind to satisfy the boundary conditions and to calculate the regular synchrotron radiation self-consistently.

Kennel, C. F.↗
NASA NTRS1985

The 1000 GeV gamma ray emission from radio pulsars

Radio pulsars have concentrated on long observations of the Crab pulsar and showed that it emits short intense bursts and a persistent weak periodic flux at gamma-ray energies 1000 GeV. It was shown that the light curve of the persistent emission was dominated by a single peak, coincident with the position of the radio and low energy gamma-ray main pulse. The results of a more detailed analysis of the structure of this main pulse are reported following an appraisal of the timing system. It is shown that at energies 1000 GeV the duration of the main pulse is not greater than 0.4 ms, which is less than that seen at all frequencies other than radio. Flux limits for the emission of 1000 GeV gamma-rays by seven other radio pulsars are reported

Chadwick, P. M.↗