arXiv subjects

Jonathan Arons

Publications and source records attributed to Jonathan Arons.

24 records · Page 2Linked to original sources

arXiv2000

Polarization Sweeps in Rotation Powered Pulsars

We re-examine the characteristic polarization angle sweep of rotation-powered pulsars and calculate the expected deviations from this sweep caused by aberrational effects and by polar-cap current flow. We find that in addition to the previously known phase shift of the entire sweep by $ΔΦ= -4 r/R_L$, aberration shifts the polarization angle itself by $ΔΨ= -(10/3) (r/R_L) \cos α$. Similarly, current flow above the polar cap shifts the polarization sweep by $ΔΨ= (10/3) (r/R_L) (J/J_{GJ}) \cos α$, potentially providing a method of directly measuring the magnitude of the current. The competition between these two effects produces a potentially observable signature in the polarization angle sweep. Although these effects may appear similar to orthogonal mode shifts, they are an independent phenomenon with distinct observational characteristics.

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arXiv1999

Pulsar Death at an Advanced Age

I summarize the theory of acceleration of non-neutral particle beams by starvation electric fields along the polar magnetic field lines of rotation powered pulsars, including the effect of dragging of inertial frames which dominates the acceleration of a space charge limited beam. I apply these results to a new calculation of the radio pulsar death line, under the hypotheses that pulsar ``death'' corresponds to cessation of pair creation over the magnetic poles {\it and} that the magnetic field has a locally dipolar topology. The frame dragging effect in star centered dipole geometry does improve comparison of the theory with observation, but an unacceptably large conflict between observation and theory still persists. Offsetting the dipole improves the comparison, but a fully satisfactory theory requires incorporating magnetic conversion of inverse Compton gamma rays, created by scattering thermal photons from the surface of old neutron stars ($t > 10^8 $ years) kept warm ($T \geq 10^5$ K) by friction between the rotating core and the crust. The result is a ``death valley'' for pulsars; offsets of the dipole center from the stellar center in the oldest stars $\sim (0.7-0.8) R_*$ suffice. The resulting theory predicted the existence of rotation powered pulsars with these advanced ages, a prediction confirmed by the recent discovery that PSR J2144-3933 actually has a rotation period of 8.5 seconds.

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arXiv1999

Discovery of kHz Fluctuations in Centaurus X-3: Evidence for Photon Bubble Oscillations (PBO) and Turbulence in a High Mass X-ray Binary Pulsar

We report the discovery of kHz fluctuations, including quasi-periodic oscillations (QPO) at ~330 Hz and ~760 Hz and a broadband kHz continuum in the power density spectrum of the high mass X-ray binary pulsar Centaurus X-3. These observations of Cen X-3 were carried out with the Rossi X-ray Timing Explorer (RXTE). The fluctuation spectrum is flat from mHz to a few Hz, then steepens to $f^{-2}$ behavior between a few Hz and ~100 Hz. Above a hundred Hz, the spectrum shows the QPO features, plus a flat continuum extending to ~1200 Hz and then falling out to ~1800 Hz. These results, which required the co-adding three days of observations of Cen X-3, are at least as fast as the fastest known variations in X-ray emission from an accreting compact object (kHz QPO in LMXB sources) and probably faster since extension to ~1800 Hz is indicated by the most likely parameterization of the data. Multi-dimensional radiation hydrodynamics simulations of optically thick plasma flow onto the magnetic poles of an accreting neutron star show that the fluctuations at frequencies above 100 Hz are consistent with photon bubble turbulence and oscillations (PBO) previously predicted to be observable in this source. For a polar cap opening angle of 0.25 radians, we show that the spectral form above 100 Hz is reproduced by the simulations, including the frequencies of the QPO and the relative power in the QPO and the kHz continuum. This has resulted in the first model-dependent measurement of the polar cap size of an X-ray pulsar.

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arXiv1998

On the Coupling of Rotation Powered Pulsars to Plerionic Nebulae

After discussion of observational constraints on the nature of the MHD wind coupling between the Crab Pulsar and the Crab Nebula, the theory of transverse relativistic shock structure is reviewed and applied to the interpretation of the wisps in the Nebula as the manifestation of the distributed wind termination shock structure, energetically dominated by heavy ions, accelerated in the rotational equator of the pulsar to energies comparable to the total voltage across the pulsar's open field lines and carrying a current comparable to the Goldreich- Julian current. New results on the variability of the shock structure are presented, which show that the gyrating ion bunches emit outwardly traveling finite amplitude compressional waves, in agreement with recent ground based observations. The implications of the theory for X-ray, $γ$-ray and high energy neutrino emission are briefly discussed, as are the problems of low magnetic energy density in the upstream wind and the origin of the Nebular radio emission. A brief discussion of other plerions leads to the conclusion that much more detailed observations are needed before these systems can be modeled with the same sophistication as can be done for the Crab Nebula.

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arXiv1998

General Relativistic Polar Particle Acceleration and Pulsar Death

I summarize the theory of acceleration of non-neutral particle beams by starvation electric fields along the polar magnetic field lines of rotation powered pulsars, including the effect of dragging of inertial frames which dominates the acceleration of a space charge limited beam. I apply these acceleration results to a new calculation of the radio pulsar death line, under the hypotheses that pulsar ``death'' corresponds to cessation of pair creation over the magnetic poles and that the magnetic field has a locally dipolar topology. While the frame dragging effect in star centered dipole geometry does improve comparison of the theory with observation, an unacceptably large fraction of the observed stars outside the bounds of pair creation theory still persists. Offsetting the dipole improves the correspondence between theory and observation. The result is a ``death valley'' for pulsars; acceptable comparison of observation and theory occurs if the boundary of death valley corresponds to offsets of the dipole center from the stellar center $\sim (0.7-0.8) R_*$. I also point out that pulsars are absent for magnetic moments corresponding to star centered polar fields in excess of $\sim 4 \times 10^{13}$ Gauss, and I suggest that this absence is due to pairs forming as bound positronium atoms in such strong fields, creating a neutral, relativistically outflowing gas which cannot participate in low altitude collective radio emission processes in such strongly magnetized objects.

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arXiv1996

Theory of high-energy emission from the pulsar/Be-star system PSR 1259$-$63 I: radiation mechanisms and interaction geometry

We study the physical processes of the PSR B1259-63 system containing a 47 ms pulsar orbiting around a Be star in a highly eccentric orbit. Motivated by the results of a multiwavelength campaign during the January 1994 periastron passage of PSR B1259-63, we discuss several issues regarding the mechanism of high-energy emission. Unpulsed power law emission from the this system was detected near periastron in the energy range 1-200 keV. We find that the observed high energy emission from the PSR B1259-63 system is not compatible with accretion or propeller-powered emission. Shock-powered high-energy emission produced by the pulsar/outflow interaction is consistent with all high energy observations. By studying the evolution of the pulsar cavity we constrain the magnitude and geometry of the mass outflow outflow of the Be star. The pulsar/outflow interaction is most likely mediated by a collisionless shock at the internal boundary of the pulsar cavity. The system shows all the characteristics of a {\it binary plerion} being {\it diffuse} and {\it compact} near apastron and periastron, respectively. The PSR B1259-63 cavity is subject to different radiative regimes depending on whether synchrotron or inverse Compton (IC) cooling dominates the radiation of electron/positron pairs advected away from the inner boundary of the pulsar cavity. The highly non-thermal nature of the observed X-ray/gamma-ray emission near periastron establishes the existence of an efficient particle acceleration mechanism within a timescale shown to be less than $\sim 10^2-10^3$ s. A synchrotron/IC model of emission of e\pm-pairs accelerated at the inner shock front of the pulsar cavity and adiabatically expanding in the MHD flow provides an excellent explanation of the observed time variableX-ray flux and spectrum from the PSR

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