Engineering topics

Shaham, Jacob

Publications and source records attributed to Shaham, Jacob.

18 records

NASA NTRS1995

Physics of systems containing neutron stars

This grant deals with several topics related to the dynamics of systems containing a compact object. Most of our research in 1994 dealt with systems containing Neutron Stars (NS's), but we also addressed systems containing a Black Hole (BH) or a White Dwarf (WD) in situations relevant to NS systems. Among the systems were isolated regular pulsars, Millisecond Pulsars (MSP's) that are either Single (SMP's) or in a binary (BMP's) Low Mass X-Ray Binaries (LMX's) and Cataclysmic Variables (CV's). We also dealt with one aspect of NS structure, namely NS superfluidity. A large fraction of our research dealt with irradiation-driven winds from companions. These winds turned out to be of some importance in the evolution of LMXB's and MSP's, be they SMP's or BMP's. While their role during LMXB evolution (i.e. during the accretion phase) is not yet clear, they may play an important role in turning BMP's into SMP's and also in bringing about the formation of planets around MSP's.

Shaham, Jacob↗

NASA NTRS1994

Orbital period variability in the eclipsing pulsar binary PSR B1957+20: Evidence for a tidally powered star

Recent observations indicate that the eclipsing pulsar binary PSR B1957+20 undergoes alternating epochs of orbital period increase and decrease. We apply a model developed to explain orbital period changes of alternating sign in other binaries to the PSR B1957+20 system and find that it fits the pulsars observations well. The novel feature of the PSR B1957+20 system is that the energy flow in the companion needed to power the orbital period change mechanism can be supplied by tidal dissipation, making the companion the first identified tidally powered star. The flow of energy in the companion drives magnetic activity, which underlies the observed orbital period variations. The magnetic activity and the wind driven by the pulsar irradiation results in a torque on the spin of the companion. This torque holds the companion out of synchronous rotation, causing tidal dissipation of energy. We propose that the progenitor had a approximately 2 hr orbital period and a companion mass of 0.1-0.2 solar mass, and the system is evolving to longer orbital periods by mass and angular momentum loss on a timescale of 10(exp 8) yr.

Applegate, James H.↗

NASA NTRS1994

GRO: Black hole models for gamma-ray bursts

The possibility of creating gamma ray bursts (GRB's) from accretion flows on to black holes is investigated. The mechanism of initial energy release in the form of a burst is not understood yet. The typical time scales involved in this energy release and the initial distribution of photons as a function of energy are studied. As a first step the problem is formulated in the Minkowski spacetime for a homogeneous and isotropic burst. For an arbitrary initial distribution of photons, the equations of relativistic kinetic theory are formulated for nonequilibrium plasmas which can take into account various particle creation and annihilation processes and various scattering processes.

Shaham, Jacob↗

NASA NTRS1993

GRO: Black hole models for gamma ray bursts

This grant deals with the production of gamma-ray bursts (GRB's) close to horizons of black holes (BH's), mainly via accretion of small chunks of matter onto extreme Kerr BH's. In the past year, we laid the ground work for actual calculations close to Kerr BH's. Because of technical reasons, actual work has only started very recently. Following the detailed list of research subprojects as per our original proposal, we have performed research in the following areas: spectrum calculation; burst dynamics; tidal capture and primordial cloud collapse; halo density profile; and capture of other objects.

Shaham, Jacob↗

NASA NTRS1993

Planet formation around millisecond pulsars

We present a model for the formation of planets in circular orbits around millisecond pulsars. We propose that the planets originate from a circumbinary excretion disk around a binary millisecond pulsar and show how physical conditions in such a disk lead to the eventual formation of planets.

Banit, Menashe↗

NASA NTRS1992

Orbital angular momentum loss in PSR 1957 + 20

It is suggested that the companion winds, excited by the radiation from the neutron star in PSR 1957 + 20 form only through the combined action of the radiation heat on the companion's atmosphere and the radiation force on the slowly lifting wind. Ballistic simulations suggest that these winds leave only from selected areas of the illuminated surface of the companion; surface currents channel into these regions relatively hot (but altogether cooler than the companion escape velocity) 'coronal' matter from the whole illuminated area. Under suitable conditions, wind particles spend some time trailing the companion at close distances before taking off to escape from the system. This can torque the binary into angular momentum loss that will be as efficient as the one recently observed in PSR 1957 + 20 if the companion is bloated to dimensions close to that of the Roche lobe.

Banit, Menashe↗

NASA NTRS1992

Orbital period change of the low-mass X-ray binary EXO 0748-676

The transient low-mass X-ray binary, EXO 0748-676, discovered with EXOSAT, is known to exhibit eclipses of a 492-s duration with a 3.82-hr period, intensity dips at pre-eclipse phases and type-I X-ray bursts. We observed this source with Ginga in 1989 March, 1990 December, 1991 January, and 1991 August and determined nine eclipse center times. Combining these eclipse center times with the previous result of the EXOSAT observations, we find that the orbital period of this source is not decaying monotonically, contrary to the previously reported suggestion. Instead, it shows a more complex behavior. A quadratic fit to the eclipse data yields a positive rate of change in orbital period with an approximate rate of 0.9 x 10 exp 7/yr, although the EXOSAT observations made in 1985 do not fit this trend. A sinusoidal function gives a better fit to the observed orbital period changes with a period of about 12 yr and an amplitude of about 44 lt-s, although the period is much longer than the observation interval of about 6.5 yr. Possible mechanisms for the orbital period change are discussed.

Asai, Kazumi↗

NASA NTRS1991

On the nature and evolution of the windy binary pulsar PSR 1744 - 24A

It is suggested that the companion of the pulsar PSR 1744 - 24A is an approximately 0.1 solar mass main-sequence star that slightly underfills its Roche lobe and is sending off a cool wind due to excitation by radiation from the pulsar. Unlike in the similarly excited system of pulsar PSR 1957 + 20, pulsar radiation pressure here almost balances wind pressure at the companion distance, leading to vastly fluctuating eclipse patterns and to occasional attempted accretion episodes that are aborted by the propeller effect and result in X-ray bursts. If this picture is correct, multiwavelength continuous observations of PSR 1744 - 24A will give valuable information about the accretion process onto neutron stars rotating with millisecond periods.

Shaham, Jacob↗

NASA NTRS1989

Physics of systems containing neutron stars

The following is a summary of work done during the period of Mar. to Oct. 1989. Three major topics were extensively looked into during this time: the reported 2,000 Hz optical signal from the direction of SNR1987A, the possibility that neutron stellar surface magnetic fields do not decay except when the star is accreting, and the 6 Hz QPOs of LMXBs.

Shaham, Jacob↗

NASA NTRS1989

Does SN 1987A contain a rapidly vibrating neutron star

If the recently reported 0.5 ms-period pulsed optical signal from the direction of Supernova 1987A originated in a young neutron star, its interpretation as a rotational period has difficulties. The surface magnetic field would have to be much lower than expected, and the high rotation rate may rule out preferred nuclear equations of state. It is pointed out here that a remnant radial vibration of a neutron star, excited in the supernova event, may survive for several years with about the observed (gravitationally redshifted) period. Heavy ions at the low-density stellar surface, periodically shocked by the vibration, may efficiently produce narrow pulses of optical cyclotron radiation in a surface field of about a trillion gauss.

Wang, Q.↗

NASA NTRS1989

Origin of pulsed emission from the young supernova remnant SN 1987A

To overcome difficulties in understanding the origin of the submillisecond optical pulses from SN 1987A a model similar to that of Kundt and Krotscheck for pulsed synchrotron emission from the Crab was applied. The interaction of the expected ultrarelativistic e(sup + or -) pulsar wind with the pulsar dipole electromagnetic wave reflected from the walls of a pulsar cavity within the SN 1987A nubula can generate pulsed optical emission with efficiency at most eta(sub max) is approximately equal to 10(exp -3). The maximum luminosity of the source is reproduced and other observational constraints can be satisfied for an average wind energy flow is approximately equal to 10(exp 38) erg/(s steradian) and for electron Lorentz factor gamma is approximately equal to 10(exp 5). This model applied to the Crab yields pulsations of much lower luminosity and frequency.

Ruderman, M.↗

NASA NTRS1989

Does mass accretion lead to field decay in neutron stars

The recent discovery of cyclotron lines from gamma-ray bursts indicates that the strong magnetic fields of isolated neutron stars might not decay. The possible inverse correlation between the strength of the magnetic field and the mass accreted by the neutron star suggests that mass accretion itself may lead to the decay of the magnetic field. The spin and magnetic field evolution of the neutron star was calculated under the hypothesis of the accretion-induced field decay. It is shown that the calculated results are consistent with the observations of binary and millisecond radio pulsars.

Shibazaki, N.↗

NASA NTRS1989

Self-sustained strong mass transfer without Roche lobe overflow - Cygnus X-3

It is proposed that the binary evolution of Cyg X-3 is driven by a self-excited wind from a solar composition companion star sustained by radiation from a neutron star primary. Observations would then imply that the companion is a white dwarf underfilling its Roche lobe, with mass between 0.01 and 0.03 solar masses. Cyg X-3 could then be in the late stage of very low mass X-ray binary evolution expected to result in a millisecond pulsar binary similar to the eclipsing system PSR 1957 + 20.

Tavani, Marco↗

NASA NTRS1989

Pulse sharpness and asymmetry in millisecond pulsars

The light curves of radiation from antipodal caps of rapidly rotating neutron stars are calcuated with and without gravity. The light curves depend strongly on the rotation speed and on the emission spectrum and could become quite sharp. Gravity generally flattens the light curves, but it does so less for rapidly rotating neutron stars and the newly discovered PSR 1957+20, and can never flatten them enough to explain the absence of pulsation from low-mass X-ray binaries. Rotation also causes an asymmetry in the pulses similar to that caused by interstellar medium absorption or scattering.

Chen, Kaiyou↗

NASA NTRS1988

Magnetospheric models for QPOs

Models of quasi-periodic oscillation (QPOs) based on the assumption that QPOs have magnetospheres are discussed. Evidence supporting the use of magnetospheric models is presented. The study of low frequency noise in the blob model is examined, stressing the question of the how much the blob-to-blob correlation is needed to suppress the low frequency noise to the levels observed in QPO sources. It is suggested that in low mass X-ray binaries, larger amounts of energy should be expected to be released at the magnetospheric boundary rather than on the neutron stellar surface. Also, examples of reproducing hardness ratio curves and QPO frequencies are given.

Shaham, Jacob↗

NASA NTRS1987

The beat frequency model for QPOs

The quasi-periodic oscillation (QPO) phenomenon has come to pose increasingly complex problems as sources have been discovered; these currently number 12, of which at least seven are X-ray binaries. The beat-frequency model (BFM), originally devised to characterize the first and simplest of the QPOs, GX5-1, is presently discussed with a view to its assumptions and applicability to other QPOs. The most important problem with the BFM is identified as the assumption that the neutron star involved rotates at an angular frequency of the order, in the case of GX5-1, of 10 msec.

Shaham, Jacob↗

NASA NTRS1987

Towards a model for QPOs

Models for explaining the phenomenon of quasi-periodic oscillations (QPOs) are reviewed, concentrating on the beat frequency model. It is suggested that the beat frequency model, which was developed to describe GX 5-1, is the best available model. The equations which this model uses to describe the QPO modulation are presented and problems of the beat frequency model are considered.

Shaham, Jacob↗

NASA NTRS1986

Free precession in quasi-periodic oscillators

The recent discoveries of quasi-periodic oscillations (QPOs) brought about a surge of theoretical work concerned with the disk-magnetosphere boundary in accreting neutron stars. Much of the detailed theoretical discussion deals with the beat-frequency model (BFM). The beat frequency (BF) spectrum resulting from applying the BFM to the most general freely rotating neutron star, i.e., a freely precessing neutron star whose angular momentum vector is, in addition, not perpendicular to the disk, is considered. It is found that in the course of free precession, the BF spectrum usually changes, with the various QPO lines changing in intensity. This allows, in principle, phenomena of frequency changing not due to changes in luminosity such as those observed in Cyg X-2 and, perhaps, in other QPOs. Such 'mode' changes may, in turn, reflect the nature of the disk-magnetosphere coupling in QPOs.

Shaham, Jacob↗