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Evolution des Champs Magnétiques des Etoiles à Neutrons

L. W. Kebede
Department of Physics, Addis Ababa University, P.O. Box 1176,Addis Ababa, Ethiopia

RÉSUMÉ

L'évolution des champs magnétiques à la surface des étoiles à neutrons est discutée en utilisant la théorie relativiste de diffusion
du plasma des champs magnétiques de telles étoiles, théorie mise au point par l'auteur (Kebede, 1996). On montre que ces
champs sont dépendants de la température et se dissipent par suite des processus de refroidissement des neutrinos et des
photons, donnant des résultats en bon accord avec l'observation. Suivant cette théorie, l'indice de freinage est dépendant du
temps et est < 3 pour des pulsars tels que le Crabe.


Neutron Star Magnetic Field Evolution

SUMMARY
Based on the relativistic plasma diffusion theory for neutron star magnetic fields developed by the author (Kebede, L.W., 1996,
MNRAS, 282, 845), the evolution of surface magnetic fields of neutron stars is discussed. In particular, it is shown that neutron
star magnetic fields are dependent on the internal-temperature and hence dissipate as a result of neutrino and photon cooling
processes. During the first 105 years, neutrino emission is the dominant cooling mechanism. However, since the magnetic field
is still strong in this early phase, the temperature does not seem to decrease as fast. The slope of the log T – log t plot of the
cooling curve, in this case, can be shown to be about –1/6. The decay law corresponding to this branch of the cooling curve
indicates that the initial surface magnetic fields, which, according to the present theory, are ~ 1014 gauss, decrease to about 2 – 3
× 1012 gauss, in the first 104 – 105 years. This is in good agreement with observational results on the magnetic fields of young
pulsars. Also, it is shown that, according to this theory, the braking index, η, is strictly time-dependent. For Crab-like pulsars η
is calculated to be < 3.


Contraintes sur les Paramètres des Pulsars à partir des Observations Optiques
des Nébuleuses B1706–44 et Vela

R.R. Sefako and O.C. de Jager
Space Research Unit, Potchefstroom University, Potchefstroom 2520, South Africa

RÉSUMÉ

La cascade de paires au-dessus de la calotte polaire du pulsar avec l'énergie venant du vent du pulsar résulte en un plasma de
paires qui subit des chocs en interagissant avec l'environnement, conduisant à une émission synchroton issue des paires
« thermalisées ». C'est perçu comme une nébuleuse X compacte s'étendant peut-être dans la partie optique du spectre. Pour les
pulsars Vela et PSR B1706–44, nous montrons comment sont déduits la multiplicité des paires, la puissance de spin-down et les
paramètres de magnétisation du vent. Vela et PSR B1706–44 ont été observés aux fréquences optiques en utilisant un imageur
CCD à SAAO, couvrant une surface plus grande que la taille de la nébuleuse X compacte. Les résultats montrent que l'excès
optique du plérion peut avoir été résolu pour Vela, et qu'il y a évidence d'une raie Hα à l'onde de choc appelée « lame d'étrave »
(bow shock). Pour les deux objets, en s'appuyant sur des observations multi-longueurs d'onde, un modèle convectif est utilisé
pour contraindre la multiplicité (M) de production des paires et le paramètre de magnétisation (σ) au niveau du vent du pulsar
soumis aux chocs. M est en accord avec un modèle de production de paires magnétiques au-dessus d'une surface stellaire
chaude (T / 106 ~ K), alors que σ est égal à 1 pour Vela, mais inférieur à 0,7 pour PSR B1706–44, comme il le faut pour
expliquer pourquoi PSR B1706–44 est intrinsèquement aussi brillant que Vela, mais avec une puissance de spin-down plus petite.


Constraints on Pulsar Parameters from Optical Observations of the B1706–44 and Vela Nebulae

SUMMARY
The pair cascade above the pulsar polar cap, combined with the energy from the pulsar wind, results in a pair plasma which is
injected into the environment around a pulsar. This wind, which consists of particles and fields, is shocked by the environment,
resulting in synchrotron emission from “thermalised” pairs. We see this as a typical compact X-ray nebula which may extend
into the optical part of the spectrum, depending on the pair multiplicity, spindown power and wind magnetization parameter.
We show how such parameters are derived for the Vela and PSR B1706–44 pulsars.




AFRICAN SKIES/CIEUX AFRICAINS, No 7, May 2002                                                                                    49
Vela and PSR B1706–44 were observed at optical frequencies using CCD imaging at SAAO, covering an area larger than the
size of the X-ray compact nebula. The results of these show that optical plerionic excess may have been resolved for Vela.
Evidence of an Hα feature was also seen at the bow shock of Vela. A convection model is used to constrain the pair
production multiplicity (M) and the magnetization parameter (σ) at the pulsar wind shock for both objects using the results
from multi-wavelength observations. The values of M are consistent with a model for magnetic pair production above a hot
stellar surface (T ≥ 106~K), whereas σ is consistent with unity for Vela, but less than 0.7 for PSR B1706–44. The lower σ for
the latter source is required to explain why PSR B1706–44 is intrinsically as bright as Vela, whereas it has a smaller spindown
power.


Le Projet de Surveillance des Glitches à HartRAO
                     1,2
Claire S. Flanagan
1
Johannesburg Planetarium, University of the Witwatersrand, PO Box 31149, Braamfontein 2017, South Africa
2
Hartebeesthoek Radio Astronomy Observatory, PO Box 443, Krugersdorp 1740, South Africa

RÉSUMÉ

Le projet de surveillance des glitches à l'Observatoire de Radioastronomie de Hartebeesthoek (HartRAO) fut entrepris en 1988
dans le but d'obtenir une bonne couverture temporelle des réponses de glitches du pulsar Vela ( PSR 0833–45). Le succès du
projet nous conduit à examiner dans quelle mesure il pourrait être utilement étendu à d'autres pulsars. Les pulsars dans le
groupe d'âge ~104 – 105 ans connaissent le taux le plus élevé de grands glitches (∆ν/ν ≈ 10–6). Dans ce groupe d'âge 13 pulsars
sont actuellement connus avec ν > 2,8 Hz et | ν |> 6 x 10-12 Hz sec–1, conditions requises pour une détection de glitches
couronnée de succès.


The HartRAO Glitch-monitoring Project

SUMMARY
Pulsar “glitches” are apparently sudden changes in pulsar spin-rate, and have been likened to “starquakes”. Observations of the
pulsar glitch-response are thus expected to be useful probes of the neutron star interior. The glitch-monitoring project at the
Hartebeesthoek Radio Astronomy Observatory (HartRAO) was initiated in 1988 with the aim of obtaining good time-coverage
of the glitch-response of the Vela pulsar (PSR 0833–45). The project has been successful in its initial aim: the pulsar response to
three large glitches can be described in terms of three components, of magnitude ∆ν /ν ≈ 20%, 2% and 0.4%, recovering on
timescales of 0.5 d, 5 d and 60 d respectively (Flanagan, C.S., 1995, PhD thesis, Rhodes University, South Africa). The last of
these components is generally the only one observed following glitches in other pulsars (e.g. Lyne, A.G., Shemar, S.L., Smith,
F.G., 2000, MNRAS, 315, 534), presumably because of the sparse coverage of post-glitch observations.

Since there are currently 1285 known pulsars (http://www.atnf.csiro.au/research/pulsar/catalogue/), we consider the extension of the
glitch-monitoring project to other pulsars. Potentially useful targets of a glitch-monitoring project should satisfy the following
criteria: they should experience frequent glitches, be able to trigger a “glitch alarm”, be observable from HartRAO, and we
should expect to obtain useful data from the observations. Although the number of known pulsars has nearly doubled in the
last five years or so, there is already an indication that pulsars in the age group ≈ 104 – 105 yr experience the highest rate of large
(∆ν/ν ≈ 10–6) glitches. In order to trigger a glitch alarm within a reasonable amount of time (say one hour after the glitch), the
pulsar should have a spin-rate ≥ 2.8 Hz. We would ideally like to observe the recovery in spin-down-rate | ν |, so candidates
should have large | ν |.

Simulations show that pulsar observations with a SNR of ~ 1 made every 20 min (a worst-case scenario) should yield a
measurement of ν with error δ ν ~ 10–12 if the data are averaged over 2 d. This would not generally allow observation of the
rapid component of the glitch response. If, however, the averaging time is extended to 5 d, the measurement error is reduced by
a factor of (5/2)–3.5, corresponding to an absolute measurement error of ~ 6 × 10–14 Hz s–1. This would allow measurement of a
~ 10 d recovery component of relative magnitude 1% for pulsars with | ν | > 6 × 10–12 Hz s–1.

There are currently 13 known pulsars in the age group 104 – 105 yr with ν > 2.8 Hz and | ν | > 6 ×10–12 Hz s–1. Of these, 10
have southern declinations >30 deg. Extension of the glitch monitoring project at HartRAO would obviously benefit
enormously from the acquisition of improved pulsar-observing equipment that can utilise more signal bandwidth (Tiplady &
Jonas, this volume). However, even with current equipment, it is likely that HartRAO can contribute more to the field of glitch-
monitoring.




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Nouvelles Technologies Prometteuses pour la Radioastronomie

Justin Jonas
Department of Physics & Electronics, Rhodes University, Grahamstown 6140, South Africa

RÉSUMÉ

L'augmentation de la capacité instrumentale est essentielle pour le développement de la science moderne. Il y a un consensus
selon lequel « un réseau d'un grand nombre d'unités N » est le meilleur moyen d'avoir un grand radiotélescope. Ces réseaux
fournissent sensibilité et résolution angulaire au plus bas coût. Les besoins technologiques de tels réseaux vont au-delà des
ressources physiques et financières qui sont disponibles aujourd'hui, mais il est raisonnable de croire que de tels réseaux
deviendront viables dans les dix prochaines années. Les technologies nécessaires incluent des éléments d'antennes innovateurs,
des circuits intégrés monolithiques en microondes, des réseaux logiques programmables sur le champ, des processeurs de
traitement numérique du signal et la photonique. Quelques-unes sont utilisées et testées dans l'instrumentation qui est
développée à HartRAO conjointement avec ses partenaires universitaires et industriels.


New Enabling Technologies for Radio Astronomy

SUMMARY
All areas of Radio Astronomy research, including observational pulsar astronomy, require an exponential growth in the
capabilities of the instrumentation if new science is to be developed. This growth has been maintained for the past six decades,
but the growth curve is flattening at the moment. Currently large telescope arrays are being built or planned in order to continue
the exponential growth. These instruments, which include ALMA, LOFAR and SKA, will rely on new technologies that are not
currently in widespread use in Radio Astronomy. Many countries are in the process of building new large antenna arrays, or
extending existing arrays, to provide demonstrators that will be used to benchmark these new technologies, while producing
scientific output. If South/Southern Africa were to consider building a new Radio Astronomy facility (e.g. a pulsar telescope),
then it would be prudent to follow this international trend.

There is a developing consensus that a “large-N array” is the best way to implement a large radio telescope. Such an
implementation provides the required collecting area (i.e. sensitivity) and physical extent (i.e. angular resolution) at the lowest
cost. It is implicit in their nature that such arrays require a large number of receiver elements, a vast network of signal
interconnections and a massive amount of signal processing. The technological requirements of such arrays extend beyond the
physical and financial resources available today, but there are reliable indications that the SKA will be a feasible proposition
within the next decade. Moore's law and the COTS (commodity off the shelf) principle will drive electronic instrumentation
into a region of the cost/performance parameter space that is necessary for, and accessible to, Radio Astronomy.

Specific technologies that will be required by an extensive, large-N array include: innovative antenna elements (e.g. Luneburg
lenses, Vivaldi dipoles, artificial magnetoelectrics); MMICS (Monolithic Microwave Integrated Circuits); FPGAs (Field
Programmable Gate Arrays); DSPs (Digital Signal Processors) and Photonics (e.g. digital and analogue signal conduits, optical
beam forming).

Some of these technologies are already being used and evaluated in instrumentation being developed at HartRAO in
conjunction with its academic and industrial partners, and this development work could be the basis for mutually beneficial
technology transfer between industry and science.

Développement d'un Instrument Numérique de Chronométrage de Pulsars

A.J. Tiplady and J.L.Jonas
Department of Physics and Electronics, Rhodes University, Grahamstown 6140, South Africa

RÉSUMÉ

On discute le développement d'un nouvel instrument numérique de chronométrage de pulsars (timer) à l'Observatoire de
Radioastronomie de Hartebeesthoek (HartRAO). L'utilisation du timer inclue la continuation et l'extension du programme actuel
de détection des glitches et la mesure du bruit (timing noise) du pulsar. Le timer est modulaire, entièrement souple, et conçu pour
être peu coûteux et scalable. C'est un banc de filtres extensible à double couche avec deux modules généraux: une carte de
distribution (DB) et une carte de traitement des données (DPB). La DB est une carte électronique personnalisée et chaque DPB
consiste en un unique FPGA Série Spartan-II de Xilinx et d'un microcontrôleur ARM. Les prototypes ont été testés avec un
microcontrôleur AVR au lieu du ARM. En utilisant un canal unique à 1 MHz, les deux pulsars Vela et PSR B1641-45 ont été
détectés après des intégrations de respectivement 30 et 180 secondes.




AFRICAN SKIES/CIEUX AFRICAINS, No 7, May 2002                                                                                  51
SUMMARY
The development of a new digital pulsar timer for the Hartebeesthoek Radio Astronomy Observatory (HartRAO) is discussed.
Applications for the timer include the continuation and extension of the present glitch detection program, and the measurement
of pulsar timing noise. The timer is designed to be inexpensive and scalable. Components used in the timer are cheaper than
those found in existing timers, are easily obtainable, and have better performance. Added advantages are that the timer can be
implemented in stages, is modular and entirely flexible, and extra bandwidth can be continually added in the future without the
need for any redesign.

The timer is designed as a dual layer filter bank which is easily expandable through the use of two general modules – a
Distribution Board (DB) and a Data Processing Board (DPB). The first layer is a distribution front-end made up of any number
of DBs connected to a common analogue signal, station signals and sampling clock. The DB is a custom designed printed
circuit board which incorporates mixer chips used in the cellular phone industry, video amplifiers and high speed analogue to
digital converters (ADC), all of which consume little power and are highly efficient, economical and mass produced. Each DB
mixes to baseband a 32 MHz analogue channel bandwidth, selected by choice of local oscillator, in quadrature DC. The
complex voltage signal is digitised by a dual 6-bit ADC which minimises net path delays and is clocked at a nominal 32 MHz.

This digital signal is fanned out to the second data processing layer, where a single DB has any number of DPB's hung off the
digital signal, depending on the channel bandwidth chosen. Each DPB consists of a single Xilinx Spartan II FPGA and an ARM
micro-controller. The FPGA accepts the 6-bit complex signal which is applied to a complex mixer. The resulting complex signal
passes through real and imaginary FIR low pass filters, used to select narrow bandwidth sub-channels within the 32 MHz
analogue channel, where the imaginary coefficients have been convolved with a Hilbert transform. Following the digital filters,
the signals are truncated to 5 bits and added (subtracted) to obtain upper (lower) sidebands. Signals are squared and integrated
for one bin period. These integrated 20-bit values are latched and read by an ARM micro-controller and stored in SRAM where
a pulse profile is integrated for a user defined period.

The micro-controller on each DPB communicates with a common data bus which is connected to a master controller. At the
end of an integration period the master controller will read integrated pulse profiles from each available DPB in order to
perform incoherent de-dispersion. The master controller will accept information via an ethernet network before an observation
to determine the parameters needed to perform a pulse integration, namely sampling rate, number of bins per pulse and number
of integrated pulse periods.

A prototype DB and DPB has been tested, with an AVR Microcontroller in place of the ARM. Using a single 1 MHz channel,
both the Vela pulsar and PSR B1641–45 were detected, after 30 second and 180 second integrations respectively.




                                    Pulsar Studies in Africa Workshop Participants
                Back row (left to right): George Nicolson, David Buckley, Okkie de Jager, Legesse Kebede, Adrian
                Tiplady, Marion West, Emma de Oña-Wilhelmi, Sharmila Goedhart, Jonathan Quick, Augustine
                Chukwude, Hannes Calitz, Louis Venter, Sarah Buchner, Mike Gaylard, Justin Jonas. Font row (sitting,
                left to right): Claire Flanagan, Beate Woermann, Christo Raubenheimer, Fabio Frescura, Pieter Meintjes.
                More photos of the Workshop may be viewed at: http://www.hartrao.ac.za/conferences/pulsars2001/
                pulsconf_pics.html.




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