A two day Workshop entitled PULSAR STUDIES IN AFRICA was held at the Hartebeesthoek Radio Astronomy Observatory on the 3rd and 4th December 2001

Photographs of the meeting may be viewed.
Abstracts of the presentations are given below.

The full text of the articles is available at the African Skies web edition. Note that you must click on the article names on the second (contents) page of this pdf file to reach the articles.

Programme 3rd Dec 2001

Abstracts

Binary evolution: Spin-up, spin-down and non-thermal (thermal) emission from accreting compact objects.

 Author: Hannes Calitz  
 e-mail: calitzjj@sci.uovs.ac.za 

 Affiliation: Department of Physics, University of the Free State, 
              Bloemfontein, South Africa
 
 Co-Author: Pieter.J. Meintjies 
 e-mail: MeintjPJ@sci.uovs.ac.za

 Affiliation: Department of Physics, University of the Free State, 
              Bloemfontein, South Africa

 Abstract: The spin-up or spin-down of accreting objects can be used as a
 diagnostic tool to investigate the mass transfer mechanism between the
 primary and secondary star in binary systems, as well as the evolution of
 the system. This can put tight contstraints on the non-thermal and thermal
 emission in the system. By getting a global view of system, potential
 non-thermal emitters can be identified for future studies by HESS and other
 new-generation gamma-ray telescopes.  

AE Aquarii, the “binary milli-second radio pulsar” of Cataclysmic Variables

   
 Author: Pieter.J. Meintjies  
 e-mail: MeintjPJ@sci.uovs.ac.za

 Affiliation: Department of Physics, University of the Free State, 
              Bloemfontein, South Africa
 
 Abstract: This is an investigation of the evolution of the binary system  
 AE Aquarii and the relation it has on the spin-up and spin-down of the
 compact companion of the system, as well as the thermal and non-thermal
 emission in the system. The mechanisms causing the rapid non-thermal
 outbursts are investigated and constraints are put on the binary system
 parameters. 

Observations, Past and Future, of X-ray pulsars from South Africa

  
 Author: David A.H. Buckley  
 e-mail: dibnob@saao.ac.za

 Affiliation: South African Astronomical Observatory, Cape Town,        
              South Africa

 Abstract: This talk covers two separate, but related, topics concerning
 X-ray emitting pulsars. Most of the talk concentrates on a description of an
 on-going optical/IR/Xray observational program on Be/X-ray binaries: early
 type Be stars whose mass loss powers the accretion energy of X-ray pulsars.
 This class of object is the most common of early spectral type X-ray
 binaries. Optical spectroscopy and IR photometry can help probe the
 circumstellar disks of Be stars, which are the mass "reservoirs" powering
 the accreting neutron stars. Understanding the states of these disks can
 help predict the X-ray activity of these, often transient, sources. Unlike
 the more massive systems, where Roche lobe overflow powers the accretion,
 neutron stars in Be/X-ray binaries are principally accreting from either a
 strong stellar wind or a circumstellar disk. In eccentric systems, the
 accretion rate can vary significantly over the orbit. Similarly the extent
 of the disk itself determines the X-ray properties. Recent work on these
 sources in the Magellanic Clouds will be discussed, particularly in
 relevance to the apparent discrepancies in populations between the two
 Clouds.

 The second part of the talk covers the interesting group of isolated
 accreting neutron stars, as typified by the 8.39 s spin period system, RX
 J0720.4-3125. This nearby system, although sharing some properties of the
 "anomolous 6 s pulsars", is clearly much less luminous and accreting from
 the ISM, rather than a remnant Thorne-Zytkow object, as proposed for the
 latter. Optical counterparts to this object, and a similar, but non-pulsing
 object (RX J1856.5-3754), have recently been identified. The paucity of
 similar objects has been at odds with the predictions of ~2000 isolated
 neutron stars supposed to be detectable from their hot thermal (soft X-ray)
 emission. I end with a discussion of the future role of SALT in this
 research, and the possible capabilities of instruments, including new
 generation photon detectors, with energy discrimination, like
 Superconduction Tunnel Junction (STJ) devices. 

Neutron Star Magnetic Field Evolution

 
 Author: Legesse W. Kebede  
 e-mail: Observatory.aau@telecom.net.et (Subject of mail must be Legesse
         Kebede as the address is shared with others.)

 Affiliation: Department of Physics, Addis Ababa University, Ethiopia

 Abstract: Based on the relativistic plasma diffusion theory for neutron
 star magnetic fields developed by the author, the evolution of surface
 magnetic fields of neutron stars is discussed. It is particularly shown that
 neutron star magnetic fields are temperature dependent and hence dissipate
 as a result of neutrino and photon cooling mechanisms. The decay law for the
 process seems to give results which are in good agreement with observations.
 Consequences of the decay law in relation to certain important astrophysical
 measurements are also discussed. 

Implications of time dependant parameters for braking indices

 
 Author: Adrian Tiplady  
 e-mail: g9731642@campus.ru.ac.za

 Affiliation: Physics Department, Rhodes University, Grahamstown, South
              Africa

 Co-Author: Fabio Frescura   
 e-mail: frescura@physnet.phys.wits.ac.za 

 Affiliations: Research Fellow in the Department of Physics, Rhodes University,
               Grahamstown, South Africa;  Centre of Theoretical Physics,
               University of the Witwatersrand, South Africa; Research
               Associate at the Hartebeesthoek Radio Astronomy Observatory.

 Abstract: The determination of braking indices via experimental results has
 been allowed by a certain degree of assumption. An analytical approach is
 taken in exploring the nature of the braking index and its possible
 different forms by introducing a time varying parameter. This is a first
 step into understanding the reason for our measurement of differing braking
 indices. 

Radio Observations and the Challenges of the Neutron Star Interior

     
 Author: Johnson O. Urama presented by Augustine E. Chukwude  
 e-mail: johnsonurama@yahoo.com

 Affiliation: Department of Physics and Astronomy, University of Nigeria, 
              Nsukka, Nigeria 
 
 Abstract: Radio pulsars are, by far, the most common observable
 manifestation of the fascinating neutron star. The timing observations of
 the emitted radio pulses remain one of the best probes of the neutron star
 interior. Such observations have led to a number of models describing the
 various components of the neutron star interior and the manner in which
 they couple with each other. This paper discusses the progress made in this
 regard.  

The HartRAO Glitch-Monitoring Project: Part I – Catching Glitches

  
 Author: Claire Flanagan   
 e-mail: 111flan@cosmos.wits.ac.za    

 Affiliation: Johannesburg Planetarium, University of the Witwatersrand,
              South Africa and the Hartebeesthoek Radio Astronomy 
              Observatory, South Africa

 Abstract: The HartRAO glitch-monitoring project was set up in 1984, in
 response to the need for good observational coverage of the response of the
 Vela pulsar to spin-ups. We describe the (at the time unique) technique used
 to obtain post-glitch observations (commencing in one case within seven
 minutes of a glitch). New insight into the interior of the pulsar obtained
 from these observations is presented, and value of extending the project
 into the future is discussed. 

Ground Based Observations with Third Generation Ground-Based Gamma-Ray Telescopes

   
 Author: Okkie C. de Jager  
 e-mail: okkie@fskocdj.puk.ac.za

 Affiliation: School of Physics, Potchefstroom University, South Africa

 Co-Authors: The MAGIC Pulsar Working Group (Magic-pulsar@ulises.gae.ucm.es)
             and the H.E.S.S. Pulsar Working Group (HESS-all@mpi-hd.mpg.de)

 Abstract: Pulsar polar cap models predict gamma-ray/pair cascades resulting
 in a high multiplicity of pairs in the pulsar magnetosphere. This large
 multiplicity is also required to explain the plerionic emission from such
 systems. The consequence is that emergent gamma-ray energies are
 constrained to be well below a TeV. Emergent maximum energies up to 30 GeV
 (pulsed photons) were seen by EGRET from PSR B1706-44 and PSR B1951+32,
 whereas observations above these energies by past ground-based pulsars only
 resulted in upper limits, which confirm the existence of such spectral
 cutoffs.

 The next generation telescopes such as MAGIC (La Palma) and H.E.S.S.
 (Namibia) should have low enough thresholds to detect the high energy tails
 of pulsed emission from some of these pulsars. This opens a new window on
 Pulsar Astronomy, since the main contribution to the photon luminosity will
 be measured by such telescopes. This talk reviews the sensitivities of
 H.E.S.S., MAGIC and 5@5 for such detections within a single night of
 observation. 

HartRAO Long-term Pulsar Timing Observations – Relevance and Prospect

  
 Author: Augustine E. Chukwude  
 e-mail:aus_chukwude@yahoo.com; austine@hartrao.ac.za (until 21 Dec 2001)
 
 Affiliation: Department of Physics and Astronomy, University of Nigeria, 
              Nsukka, Nigeria; Hartebeesthoek Radio Astronomy Observatory, 
              South Africa

 Abstract: Regular timing observations of some radio pulsars have now been
 conducted at the Hartebeesthoek Radio Astronomy Observatory (HartRAO) for 
 more than a decade. Here we present some of the results of our current 
 work on timing activity in radio pulsars. Our results highlight the 
 relevance of such long-term timing program in our quest for better 
 understanding of the dynamical behaviour of neutron stars. Arguments are 
 also presented for the continuation and improvement of the current HartRAO 
 pulsar monitoring program.

Programme 4th Dec 2001

Abstracts

Effects of Pulsar Timing Noise and Glitches on Gamma-Ray Pulsar Observations

  
 Author: E. de Ona-Wilhelmi 
 e-mail: fskeow@puknet.puk.ac.za

 Affiliation: School of Physics, Potchefstroom University, South Africa
 
 Co-Authors: The MAGIC Pulsar Working Group (Magic-pulsar@ulises.gae.ucm.es)

 Abstract: Pulsed emission from a few Gamma-Ray pulsars may be detectable
 with Ground-Based Gamma-Ray Telescopes. The pulsar ephemerides from the
 archival data of HARTRAO and Princeton (between 1989 and 1998) provide an
 excellent opportunity to study the accuracy of extrapolations of such
 ephemerides to the present moment, if an appropriate time shift is
 introduced. The aim of this study is to investigate the smear in the
 gamma-ray pulse profile during a single night of observations, given the
 absence of contemporary radio pulsar observations during ON-line analyses
 when "quick look" procedures must be employed to determine the status of a
 candidate. Pulsars with typical gamma-ray parameters on the (P, Pdot)
 diagram will be employed in this study.   

Constraining Pulsar Pair Production Multiplicity and Wind Magnetization Parameters with Multiwavelength Observations.

 
 Authors: Rocky R. Sefako (fskrrs@puknet.puk.ac.za) 
          Okkie C. de Jager (okkie@fskocdj.puk.ac.za)

 Affiliation: School of Physics, Potchefstroom University, South Africa

 Co-Authors: A. Konopelko, R.A. Burger

 Affiliation: School of Physics, Potchefstroom University, South Africa

 Abstract: 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 "thermalized" 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 review
 these parameters for the Crab Nebula, but show how such parameters are
 derived for the Vela and PSR B1706-44 pulsars. The multiplicity derived for
 these two pulsars agree with the theoretical predictions. We also show how
 the total number of pairs contribute to the diffuse gamma-ray signal through
 inverse Compton scattering of the Cosmic Microwave Background, galactic dust
 emission and optical starlight emission. With these target photon fields
 known, we can verify the injection rate of pairs into the "plerion" by
 measuring the high energy to very high energy gamma-ray spectrum. We will
 apply this to three pulsars and show that such TeV signals should be
 detectable from a few pulsar/plerion systems with the H.E.S.S. Telescope
 system. 

Pulsars and Precession

  
 Author: Fabio Frescura 
 e-mail: frescura@physnet.phys.wits.ac.za 

 Affiliation: Research Fellow in the Department of Physics, Rhodes      
              University, Grahamstown, South Africa; Centre of Theoretical 
              Physics, University of the Witwatersrand, South Africa; 
              Research Associate at the Hartebeesthoek Radio Astronomy 
              Observatory.

 Abstract: We consider the implications of precession for pulsar timing
 measurements. 

Cyclic behaviour in rotation and DM of PSR B1557-50

  
 Author: Augustine Chukwude
 e-mail:aus_chukwude@yahoo.com; austine@hartrao.ac.za (until 21 Dec 2001)
        
 Affiliation: Department of Physics and Astronomy, University of Nigeria, 
              Nsukka, Nigeria; Hartebeesthoek Radio Astronomy Observatory, 
              South Africa

 Abstract: Analysis of pulsar timing data collected at regular intervals over 
 a long timescale remains one the best probes of the long-term rotational 
 behaviour of neutron stars. Here we summarize results of the analysis of 
 13 years of pulse arrival time data of the pulsar PSR 1557-50 collected at 
 the Hartebeesthoek Radio Astronomy Observatory (HartRAO) between 1986 August 
 and 1999 May. Our results show that over this period, both the timing 
 residuals and dispersion measure of this pulsar exhibit sustained cyclic 
 variations. No few than two cycles with peak-to-peak separation of about 
 1500 days are evident in our data. The DM variation is more than 60% 
 anticorrelated with rotation parameters. The results are discussed in the 
 context of our current understanding of the dynamics of neutron stars.  

The HartRAO Glitch-Monitoring Project: Part II – Glitch-Monitoring By-Products

 
 Author: Claire Flanagan  
 e-mail: 111flan@cosmos.wits.ac.z
 
 Affiliation: Johannesburg Planetarium, University of the Witwatersrand,  
              South Africa and the Hartebeesthoek Radio Astronomy 
              Observatory, South Africa

 Abstract: The glitch-monitoring project described in an earlier paper
 results in the collection of a massive amount of data, whereas the
 targetted glitch transients endure for a few days or weeks once every three
 years or so. The insight this additional data gives us into the timing noise
 and underlying spin-down forces in the Vela pulsar is discussed. 

Future Radio Pulsar Observing – Threats and Opportunities

   
 Author: Michael J Gaylard   
 e-mail: mike@hartrao.ac.za

 Affiliation: Hartebeesthoek Radio Astronomy Observatory, South Africa
 
 Abstract: The increasing use being made of the existing 26-m radio telescope
 at Hartebeesthoek has steadily reduced the time available for pulsar
 observing. Even greater pressure on pulsar observing time is forseen from
 2002.

 It is therefore appropriate to investigate the possibilities for a new radio
 telescope optimised for pulsar research, as part of strategic planning for
 the future of astronomy in South Africa. This is particularly relevant in
 view of the increasing number of people in Africa involved in pulsar
 research.

 Technically, a telescope optimised for pulsar studies would likely operate
 at frequencies below the 1.6 GHz lower end of the 26-m telescope. We
 investigate the sensitivity required for observing different types of
 pulsars and look at trade-offs in telescope design that could produce an
 instrument that is desirable and useful but affordable in the face of the
 massive expenditure on SALT.  

Enabling Technologies for Radio Astronomy

 
 Author: Justin Jonas 
 e-mail: j.jonas@ru.ac.za

 Affiliation: Physics Department, Rhodes University, Grahamstown, 
              South Africa

 Abstract: Recent advances in digital electronics, photonics and material
 science have provided the radio astronomy community with new tools for the
 implementation of new generation telescopes. Artificial magneto-electric
 materials allow the fabrication of novel low-cost antennas. Fibre optic
 technology and versatile, high-speed digital electronic devices facilitate
 the implementation of "large-N" interferometric arrays. The applicability of
 various new technologies to a pulsar telescope will be discussed. A dominant
 topic will be the use of digital electronics in radio astronomy receivers. 

Development of a Digital Pulsar Timer

 
 Author: Adrian Tiplady  
 e-mail: g9731642@campus.ru.ac.za

 Affiliation: Physics Department, Rhodes University, Grahamstown, 
              South Africa

 Abstract: A new, digital pulsar timer which will replace the existing one at
 HartRAO is presently being developed at Rhodes University. The timer makes
 use of cell phone technology and FGPAs, allowing for relatively cheap and
 simple implementation as well as the possibility of replacing existing,
 expensive equipment with affordable circuitry. The modularised system is
 generalised in order to allow for easy, limitless expansion at any time
 without further design adjustments while the use of new signal processing
 techniques allows for maximum bandwidth usage and efficiency.
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