Publications (15)38.48 Total impact
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Chapter: Near simultaneous optical and TeV observations of AE Aquarii
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ABSTRACT: We present results of near simultaneous optical and TeV observations made on Sept. 14, 1990 of the novalike cataclysmic variable AE Aquarii. The optical was in a flaring state resulting in a bolometric luminosity which may have been high enough to give detectable TeV radiation during the TeV observation. During the latter we have seen periodic TeV emission at a frequency redshift of –0.7% relative to the spin frequency which confirms our earlier detection during 1989 at the 99.5% level. Both the optical and TeV did show evidence of a changing frequency with time which appears to be describing a single event. We also show that the shock above the white dwarf's polar cap may be collissionless so that protons may be accelerated to energies above 1 TeV. If the part of the accretion stream from the companion which reprocesses incident X-rays to optical pulsations is magnetized and dense enough, it may trap incident TeV particles for a long enough time to give TeV -rays through decay with the same period as in optical. The frequency will be constant if the spot of reprocessing is fixed in the binary frame, but shifted if the spot moves.01/2006: pages 173-182; -
Article: Propeller spin‐down and the non‐thermal emission from AE Aquarii
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ABSTRACT: To explain the observed multiwavelength emission from the cataclysmic variable star AE Aquarii, we propose a model based upon the assumption that the compact component of this binary consists of a white dwarf with a surface field strength of 1 MG, rotating through a clumpy ring near the circularization radius. We argue that MHD instabilities like the Kelvin–Helmholtz instabilities will result in the white dwarf field mixing with the gas in this region and becoming highly sheared. The Poynting flux pumped into the region, and hence angular momentum transferred in the process, is enough to accelerate material centrifugally from the system. We also show that magnetic reconnection in this region will result in runaway electron acceleration to energies of 300 MeV or more, which will radiate through the synchrotron process up to frequencies of the order of ν∼1015 Hz. We show that, because the radiation is emitted in a gaseous medium, the spectrum will be suppressed below the so-called Razin–Tsytovich frequency, which is νR∼2000 GHz, which agrees with the observations. Magnetic shear in the ejection region will also induce field-aligned currents in the magnetosphere. Huge potential differences (double layers) can be generated in the circuit where the gas density becomes very low. This will occur close to the white dwarf. In this region potentials of the order of Φ∥=300 TV (TV = teravolt) can be generated. Synchrotron losses on electrons will restrict them to energies of the order of ℰ50 keV. As protons and ions are not affected by synchrotron losses, energies in excess of 1 TeV are possible. Very high-energy (VHE) -ray emission can be produced if these proton and ion beams collide with the clumpy gas in, or outside, the ejection zone. We show that pulsed VHE -ray emission is possible, if the gas particle density of the target matter Ng>1013 cm−3, which is compatible with the typical gas particle densities Ng∼1016 cm−3 of the blobs inside the clumpy accretion stream from the secondary star. We show that the VHE -ray bursts that were reported by two groups can be explained in terms of an exploding double layer, which will result in a cataclysmic burst of VHE protons and ions over short time-scales. The total power released in the process can be P,b∼1034 erg s−1, which is equal to the spin-down power of the white dwarf, and is enough to explain the reported VHE -ray bursts. This implies that the whole reservoir of spin-down power has gone into the production of VHE particles and -ray emission.Monthly Notices of the Royal Astronomical Society 04/2002; 311(3):611 - 620. · 4.90 Impact Factor -
Article: Simultaneous optical and TeV gamma-ray observations of the cataclysmic variable AE Aquarii
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ABSTRACT: In this paper we report on findings of simultaneous optical and TeV gamma-ray observations (eight nights, 32 hr in total) of the magnetic cataclysmic variable AE Aquarii which contains the most rapidly spinning white dwarf (P = 33.08 sec) known to date. The combined results of all the TeV observations (1992 July-1993 June) independently confirm previously identified optical and TeV gamma-ray periodicities at the 10-3 level without any period search, and confirm the finding that the observed duty cycle of these periodicities is approximately 10%. The strongest TeV signal appeared to be confined to the time just before the onset of a strong optical flare. The observed pulsar-like spindown of the white dwarf would push the accretion disk outside the the corotation radius, which gives a natural explanation of why the observed optical TeV gamma-ray periodicities are slightly redshifted relative to the spin period. Following an optical flare, we observed two rapid bursts (approximately 1-3 minutes duration) with three independent telescopes, within 25 minutes of each other. The significance of the largest burst was 4.6 sigma per minute. The probability of finding two such bursts given all the trials is 5.3 x 10-4. The observed spin-down power (Omega d Omega/dt approximately 6 x 1033 ergs/s) may account for the energy released by a TeV accelerator in AE Aquarii.The Astrophysical Journal 09/1994; 434:292-305. · 6.02 Impact Factor -
Article: TeV gamma-ray properties of VELA X-1
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ABSTRACT: Evidence is presented, from 5 years of observation, that Vela X-1 is a persistent emitter of pulsed TeV gamma-rays at the expected X-ray period. The significance of this effect is 99.3%, and an average, constant, pulsed signal of 2.3% of the cosmic-ray background was detected from 1986 to 1990. The light curve is a mixture of the fundamental period (283 s) and the second harmonic. No orbital modulation was detected, and there are indications of gamma-ray emission during X-ray eclipse. Enhancements in the signal strength of the order of seven and a duration of up to eight pulsar rotations occurred. These enhancements occurred on average once every 12.9 hr. We conclude that the TeV gamma-ray emission mimics the X-ray behavior of the source. The gamma-rays are probably produced during periods of higher accretion when the density of the accreting material produces a sufficiently dense target for the high-energy protons produced at the poles of the neutron star. Such a scenario can also explain the emission during X-ray eclipse.The Astrophysical Journal 05/1994; 428:777-784. · 6.02 Impact Factor -
Article: The Discovery of a Brake on the White Dwarf in Ae-Aquarii
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ABSTRACT: We report the discovery of a rapid spindown of the 33-s rotation period of the white dwarf in AE Aqr at a steady rate of 5.64 × 10-14 s s-1 over a baseline of at least 14.5 yr. The newly derived orbital period by Welsh, Home & Gomer is confirmed, and it is shown that the secondary star's absorption-line radial velocities provide the correct phasing for the 33-s oscillations if they originate at the white dwarf. The amplitude of the pulse-timing delays, αwdsin i=2.04s, yields a white dwarf mass of between 0.9 and 1.0 Msun. The spindown power of the white dwarf is --IΩΩdot=6 × 1033 I50 erg s-1, which exceeds the accretion luminosity of AE Aqr by a factor of ˜120. A significant fraction of --IΩΩdot may be converted to the acceleration of particles as in a pulsar, which may explain the observed radio synchrotron emission and the reported γ-ray emission. We also give evidence that the secondary lies above the main sequence since the secondary mass inferred from the mainsequence mass-radius relationship is too large.Monthly Notices of the Royal Astronomical Society 03/1994; 267:577. · 4.90 Impact Factor -
Article: TeV Gamma-Ray Observations of Compton Gamma-Ray Observatory Pulsars: Evidence against Inverse-Compton Controlled Outer Gaps
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ABSTRACT: We present the first results of TeV γ-ray observations of the Compton Gamma-Ray Observatory γ-ray pulsar PSR 1706-44, indicating a spectral cutoff between ˜20 GeV and 600 GeV. Improved upper limits are also presented for the Vela pulsar and PSR 1509-58. These and other TeV results confirm similar cutoffs for these γ-ray pulsars as well as for Geminga. The polar cap model predicts such a spectral cutoff due to magnetic pair production, whereas the outer gap model for Vela-type pulsars predicts a similar cutoff resulting from the maximum synchrotron energy in the outer gap. The spectral flattening below a certain minimum energy, seen from all of these pulsars, is also consistent with the outer gap prediction. The predicted fluxes of TeV γ-rays for Vela and Geminga are, however, much larger than the observed upper limits. These TeV observations place constraints on any pulsar model which employs the inverse-Compton scattering of soft photons by primary TeV electrons at distances relatively far from the stellar surface.The Astrophysical Journal 11/1993; 418:836. · 6.02 Impact Factor -
Article: Short optical bursts and acceleration to TeV energies in AE Aquarii
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ABSTRACT: In this paper we give the first report of short optical bursts with rise- and decay times of a few seconds and about l-minute duration from the novalike variable AE Aquarii. With such short timescales and rapid light variations with large amplitudes, which is most probably associated with dynamic activity in the magnetosphere of the spinning white dwarf, we expect large electric fields to be generated from the induction equation of Faraday. The corresponding potential drop may be larger than during quiescence, and the luminosity involved in such a burst exceeds the quiescent luminosity, which improves the probability for burst-like detections at TeV gamma-ray energies. A l-minute burst from AE Aquarii was earlier reported in TeV gamma-rays at a significance level of 6 sigma. This gamma-ray event also revealed other convincing evidence in favor of gamma-ray emission from AE Aquarii, such as about 100 percent modulation at the 16.5 s period. A factor 10 enhancement in the amplitude of this periodicity was also seen in one of the optical bursts.Astronomy and Astrophysics 01/1993; 268:L1-L4. · 4.59 Impact Factor -
Article: AE Aquarii - an emitter of pulsed TeV gamma rays resembling optical emission during flares
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ABSTRACT: Extensive optical and VHE data on AE Aquarii obtained over the last four years are described. The optical data are analyzed, and it is shown that AE Aqr shows definite emission patterns during flares which may be used to identify possible VHE emission. It is demonstrated that the total VHE data base show indications of emission similar to optical emission from AE Aqr during flares. Exhaustive tests show that these are not systematic effects. The temporal behavior of the VHE emission supports the notion of flarelike emission. Results of the two simultaneous optical and VHE observation are presented which show the correlation of the pulsed emission in the two energy bands.The Astrophysical Journal 11/1992; 401:325-336. · 6.02 Impact Factor -
Article: TeV γ-Ray Properties of X-Ray Binaries I: Vela X-1
07/1991; 1:328. -
Article: TeV γ-Ray Properties of X-Ray Binaries I - Cen X-3
07/1991; 1:340. -
Article: Pulsed TeV γ-Rays from AE AQUARII Resembling Optical Pulsations during Flares
07/1991; 1:360. -
Article: The Observed and Expected Orbital Modulation of TeV γ-Rays from PSR 1957+20
1:467. -
Article: Is AE Aquarii a Magnetically Driven System?
85:396. -
Article: Persistent Optical-like Pulsed TeV γ-Ray Emission from AE Aquarii
1:338. -
Article: Pulsed TeVγ-Ray Emission from Binary Radio Pulsars
1:334.
Top co-authors
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Institutions
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2002
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University of the Free State
- Department of Physics
Bloemfontein, Free State, South Africa
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1994
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South African Astronomical Observatory
Cape Town, Province of the Western Cape, South Africa
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