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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Physical implications of the eclipsing binary pulsar

The observed characteristics of the msec pulsar P1957+20, discovered in an eclipsing binary by Fruchter et al. (1988), are considered theoretically. Model equations for the stellar wind and optical emission are derived and used to estimate the effective temperature and optical luminosity associated with wind excitation; then the energy levels required to generate such winds are investigated. The color temperature of the pulsar-heated stellar surface calculated under the assumption of adiabatic expansion is 1000-10,000 K, in good agreement with the observational estimate of 5500 K.

Wasserman, Ira↗
NASA NTRS1989

Limits on an optical pulsar in supernova 1987A

Since March 1987 the optical flux from supernova 1987A for periodic pulsations has been sought. As of August 1988, after 38 separate observations, no pulsar has been detected. The typical upper limit placed on the pulsed fraction optical light from the supernova is 0.0002, for pulse frequencies in the range 0.03-5000 Hz. The best limit on the pulsed fraction of supernova light is 7 x 10 to the -6th, on January 22, 1988. On August 28, 1988 the faintest limit for the magnitude of the pulsar, dimmer than 20th mag is reached. These limits are based on Fourier transforms of up to 67 million points, covering a range of spindown rates.

Pennypacker, C. R.↗
NASA NTRS1989

Is the sub-millisecond pulsar strange?

The possibility that the submillisecond pulsar from supernova 1987A is composed of strange matter is theoretically discussed. It is shown that for a range of hadron parameters, the maximum rotation rate of secularly stable strange stars may exceed that of the half-millisecond pulsar and the nonrotating maximum mass is greater than 1.52 solar mass. The low-mass companion(s) to SN1987A, inferred from the periodic modulations of the optical signal, can be accounted for by stable strange-matter lump(s) ejected from the young strange star.

Frieman, Joshua A.↗
NASA NTRS1989

Pulsar-aided SETI experimental observations

The rotational frequencies of pulsars are used to select preferred radio frequencies for SETI. Pulsar rotational frequencies are converted into SETI frequencies in the 1-10 GHz Galactic radio window. Experimental observations using the frequencies are conducted for target stars closer than 25 parsecs, unknown targets in a globular cluster, and unknown targets in the Galaxy closer than 2.5 kpc. The status of these observations is discussed.

Heidmann, J.↗
NASA NTRS1990

Discovery of a cyclotron absorption line in the spectrum of the binary X-ray pulsar 4U 1538 - 52 observed by Ginga

A cyclotron absorption line near 20 keV has been found in the spectrum of the massive eclipsing binary X-ray pulsar 4U 1538 - 52 in observations with the Ginga observatory. The line is detected throughout the 529 s pulse cycle with a variable equivalent width that has its maximum value during the smaller peak of the two-peak pulse profile. It is found that the profile of the pulse and the phase-dependence of the cyclotron line can be explained qualitatively by a pulsar model based on recent theoretical results on the properties of pencil beams emitted by accretion-heated slabs of magnetized plasma at the magnetic poles of a neutron star. The indicated field at the surface of the neutron star is 1.7 (1 + z) x 10 to the 12th G, where z is the gravitational redshift.

Clark, George W.↗
NASA NTRS1991

Cyclotron harmonics in accreting pulsars and gamma-ray bursters - Effect of two-photon processes

The radiative transfer at the cyclotron first, second, and third harmonics is discussed for simplified accreting pulsar and gamma-ray burster emission regions. Two-photon scattering as well as two-photon emission, which play a major role in determining the line-strength ratio is included. Stimulated effects are also important for accurately modeling the optically thick line and continuum regions. These calculations are compared with recent observations of accreting pulsars and gamma-ray burst sources, showing a qualitative agreement. The cyclotron harmonic lines should be strongly polarized.

Alexander, S. G.↗
NASA NTRS1991

A model of unpulsed very high energy gamma rays from the Crab Nebula and pulsar

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.

Kwok, P. W.↗
NASA NTRS1991

On the evolution of pulsars

Data from a previous investigation on the angle chi between the axis of rotation and the magnetic dipole axis, determined from polarization observations, provides a complete catalog which makes it possible to carry out a detailed comparison of the theoretical results of this present investigation with the observed distribution of radio pulsars over the angel chi. Before such a comparison is made, the main features of a theory for pulsar evolution is described.

Beskin, V. S.↗
NASA NTRS1991

Observations of x ray pulsars from the Kvant module

The Roentgen international x ray observatory on the Kvant module of the Mir space station has been successfully operating since the beginning of June 1987. Many x ray sources were observed and among them were several x ray pulsars. Four telescopes mounted on board the Kvant module cover a wide energy range with good timing resolution. Timing analysis of the Kvant module data suffers from the presence of only short continuous intervals of source observations, separated by 90 min gaps (90 min is the orbital period of the Mir space station around the Earth). The presence of 90 min gaps leads to the appearance of beat frequencies v=v sub 0 + or - n/90 min (n = 1, 2, 3). Special analysis was applied to avoid this difficulty. Results are presented of the pulsation period measurements of the x ray pulsars Her X-1, Cen X-3, SMC X-1, Vela X-1, A0535 + 26 by the instruments on board the Kvant module in 1987 to 1989. The values of the periods are reduced to the solar system barycenter and to the binary system barycenter (excluding A0535 + 26).

Gilfanov, M.↗
NASA NTRS1992

Low Frequency Interstellar Scattering and Pulsar Observations

Radio astronomy at frequencies from 2 to 30 MHz challenges time tested methods for extracting usable information from observations. One fundamental reason for this is that propagation effects due to the magnetoionic ionosphere, interplanetary medium, and interstellar matter (ISM) increase strongly with wavelength. The problems associated with interstellar scattering off of small scale irregularities in the electron density are addressed. What is known about interstellar scattering is summarized on the basis of high frequency observations, including scintillation and temporal broadening of pulsars and angular broadening of various galactic and extragalactic radio sources. Then those high frequency phenomena are addressed that are important or detectable at low frequencies. The radio sky becomes much simpler at low frequencies, most pulsars will not be seen as time varying sources, intensity variations will be quenched or will occur on time scales much longer than a human lifetime, and many sources will be angularly broadened and/or absorbed into the noise. Angular broadening measurements will help delineate the galactic distribution and power spectrum of small scale electron density irregularities.

Cordes, James M.↗
NASA NTRS1992

Gamma rays from the Crab and Vela pulsars

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.

Ho, Cheng↗
NASA NTRS1992

Gamma rays from hidden millisecond pulsars

The properties were studied of a new class of gamma ray sources consisting of millisecond pulsars totally or partially surrounded by evaporating material from irradiated companion stars. Hidden millisecond pulsars offer a unique possibility to study gamma ray, optical and radio emission from vaporizing binaries. The relevance of this class of binaries for GRO observations and interpretation of COS-B data is emphasized.

Tavani, Marco↗
NASA NTRS1992

Crab pulsar giant pulses: Simultaneous radio and GRO observations

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.

Lundgren, Scott C.↗
NASA NTRS1992

Millisecond pulsar observation system at CRL

Millisecond pulsars have attracted attention as future reference clocks in place of present atomic clocks. The Communications Research Laboratory (CRL) has been developing an observation system to measure the pulse timing of a millisecond pulsar precisely, and has recently completed the basic part of the task. We observed PSR1937+21 at 1.5GHz band and got a pulse timing with a precision of 16 micro sec/tau by five days of observation.

Hanado, Yuko↗
NASA NTRS1992

On the power spectra of the wind-fed X-ray binary pulsar GX 301 - 2

A phenomenological model of accretion which is applied to the wind-fed X-ray binary pulsar GX 301 - 2 is developed, assuming that the accretion onto the neutron star does not occur from a continuous flux of plasma, but from blobs of matter which are threaded by the magnetic field lines onto the magnetic polar caps of the neutron star. These 'lumps' are produced at the magnetospheric limit by magnetohydrodynamical instability, introducing a 'noise' in the accretion process, due to the discontinuity in the flux of matter onto the neutron star. This model is able to describe the change of slope observed in the continuum component of the power spectra of the X-ray binary pulsar GX 301 - 2, in the frequency range 0.01 - 0.1 Hz. The physical properties of the infalling blobs derived in the model are in agreement with the constraints imposed by observations.

Orlandini, Mauro↗
NASA NTRS1992

Gamma-ray bursts and radio pulsar glitches

Upper limits to gamma-ray fluxes produced in conjunction with a radio pulsar glitch are presented. The glitch occurred on the Vela pulsar on December 24, 1988 and was the first to be observed as it occurred. Sensitive gamma-ray burst detectors aboard the Phobos 2 spacecraft were operating at this time, but recorded no significant burst at the time of the glitch. It is concluded that if a gamma-ray burst was generated in the energy range to which the Phobos detectors were sensitive, and if it was not beamed away from the spacecraft, the efficiency of glitch energy conversion into gamma-rays could not have exceeded 10 exp -4.

Hartmann, D.↗
NASA NTRS1992

Optical observations of low-mass X-ray binaries and millisecond pulsars

We review recent results from optical observations of low-mass X-ray binaries (LMXBs) and millisecond pulsars. We discuss the optical and X-ray properties of those LMXBs which have been suspected of harboring black holes, reviewing in particular recent work on GX 339-4. We also present preliminary results from new optical observations of the ablating millisecond pulsar system PSR 1957 + 20. The orbital modulation is smooth and symmetrical, and is at least four magnitudes deep in R. However, the overall shape of the light curve can be reproduced by a simple model assuming blackbody reradiation from the secondary.

Callanan, Paul J.↗
NASA NTRS1992

Formation of the dark bays in the Crab optical synchrotron nebula - Is the Crab pulsar wind bubble interacting with its progenitor's wind?

Models are presented for the formation of the two dark bays in the Crab optical synchrotron nebula, from which the optical synchrotron-emitting particles seem to be excluded. It is proposed that the bays are formed by the advance of the Crab pulsar's wind bubble into an ambient medium possessing a stratified density distribution with higher density in the plane of the bays than in other directions. The ambient density and degree of stratification required to produce the bays depends on whether the medium consists of a general interstellar medium, a progenitor stellar wind, or fast-moving supernova ejecta. In the case of an interstellar medium, the bays would be expected to move apart much more slowly than observed, and the required density would have to be implausibly high. It is concluded that the pulsar wind bubble is probably interacting with the slow wind from its progenitor, and it is shown that bays formed in this way agree with the observations reasonably well.

Li, Zhi-Yun↗