Mixing Problem Based Learning And Conventional Teaching Methods In An Analog Electronics Course

This study, undertaken at the Walter Sisulu University of Technology (WSU) in South Africa, describes how problem-based learning (PBL) affects the first year ‘analog electronics course’, when PBL and the lecturing mode is compared. Problems were designed to match real-life situations. Data between the experimental group and the control group that related to attitudinal effect; the amount of reflection and learning outcome effects, were compared. A strong correlation was found between the students’ attitudes and project marks for those who used the problem-based learning method. It was found that students who followed the PBL method learned to do research, learned better how to work in groups and developed greater confidence. Also what they learned was more of a practical value and they had more positive attitudes and reflected more, but there were no significant improvements in their learning. This research is in response to the real need to address gaps between employer expectations and higher education outcomes in South Africa.

On the Association of γ\gamma-Ray Bursts with Supernovae

The recent discovery of a supernova (SN 1998bw) seemingly associated with GRB~980425 adds a new twist to the decades-old debate over the origin of gamma-ray bursts. To investigate the possibility that some (or all) bursts are associated with supernovae, we performed a systematic search for temporal/angular correlations using catalogs of BATSE and BATSE/{\it Ulysses} burst locations. We find no associations with any of the precise BATSE/ Ulysses locations, which allows us to conclude that the fraction of high-fluence gamma-ray bursts associated with known supernovae is small ($<$0.2%). For the more numerous weaker bursts, the corresponding limiting fraction of 2.5% is far less constraining due to the imprecise locations of these events. This fraction (2.5%) of bursts corresponds to $\sim$30% of the recent supernovae used as a comparison data set. Thus, although we find no significant evidence to support a burst/supernova association, the possibility cannot be excluded for weak bursts

On the nature of XTE J0421+560/CI Cam

We present the results of the analysis of RXTE, BATSE and optical/infrared data of the 1998 outburst of the X-ray transient system XTE J0421+560 (CI Cam). The X-ray outburst shows a very fast decay (initial e-folding time ~0.5 days, slowing down to ~2.3 days). The X-ray spectrum in the 2-25 keV band is complex, softening considerably during decay and with strongly variable intrinsic absorption. A strong iron emission line is observed. No fast time variability is detected (<0.5 % rms in the 1-4096 Hz band at the outburst peak). The analysis of the optical/IR data suggests that the secondary is a B[e] star surrounded by cool dust and places the system at a distance of >~ 2 kpc. At this distance the peak 2-25 keV luminosity is ~4 x 10^37 erg/s. We compare the properties of this peculiar system with those of the Be/NS LMC transient A 0538-66 and suggest that CI Cam is of similar nature. The presence of strong radio emission during outburst indicates that the compact object is likely to be a black hole or a weakly magnetized neutron star.Comment: Accepted for publication on The Astrophysical Journal, July 199

A Search for Non-triggered Gamma Ray Bursts in the BATSE Data Base

We describe a search of archival data from the Burst and Transient Source Experiment (BATSE). The purpose of the search is to find astronomically interesting transients that did not activate the burst detection (or ``trigger'') system onboard the spacecraft. Our search is sensitive to events with peak fluxes (on the 1.024 s time scale) that are lower by a factor of 2 than can be detected with the onboard burst trigger. In a search of 345 days of archival data, we detected 91 events in the 50--300 keV range that resemble classical gamma ray bursts but that did not activate the onboard burst trigger. We also detected 110 low-energy (25--50 keV) events of unknown origins which may include activity from SGR 1806-20 and bursts and flares from X-ray binaries. This paper gives the occurrence times, estimated source directions, durations, peak fluxes, and fluences for the 91 gamma ray burst candidates. The direction and intensity distributions of these bursts imply that the biases inherent in the onboard trigger mechanism have not significantly affected the completeness of the published BATSE gamma ray burst catalogs.Comment: Fixed incorrect values of and added erratum sectio

Limits on the cosmological abundance of supermassive compact objects from a millilensing search in gamma-ray burst data

A new search for the gravitational lens effects of a significant cosmological density of supermassive compact objects (SCOs) on gamma-ray bursts has yielded a null result. We inspected the timing data of 774 BATSE-triggered GRBs for evidence of millilensing: repeated peaks similar in light-curve shape and spectra. Our null detection leads us to conclude that, in all candidate universes simulated, $\Omega_{SCO} < 0.1$ is favored for $10^5 < M_{SCO}/M_{\odot} < 10^9$, while in some universes and mass ranges the density limits are as much as 10 times lower. Therefore, a cosmologically significant population of SCOs near globular cluster mass neither came out of the primordial universe, nor condensed at recombination.Comment: 14 pages including 3 figures, appeared 2001 January 2

A New Type of Transient High-Energy Source in the Direction of the Galactic Centre

Sources of high-energy (greater than 20 keV) bursts fall into two distinct types: the non-repeating gamma-ray bursters, several thousand of which have been detected but whose origin remains unknown, and the soft gamma-ray repeaters (SGRs), of which there are only three. The SGRs are known to be associated with supernova remnants, suggesting that the burst events most probably originate from young neutron stars. Here we report the detection of a third type of transient high-energy source. On 2 December 1995, we observed the onset of a sequence of hard X-ray bursts from a direction close to that of the Galactic Center. The interval between bursts was initially several minutes, but after two days, the burst rate had dropped to about one per hour and has been largely unchanged since then. More than 1,000 bursts have now been detected, with remarkably similar light curves and intensities; this behaviour is unprecendented among transient X-ray and gamma-ray sources. We suggest that the origin of these bursts might be related to the spasmodic accretion of material onto a neutron star