Using Technology to Encourage Self-Directed Learning: The Collaborative Lecture Annotation System

The rapidly-developing 21st century world of work and knowledge calls for self-directed lifelong (SDL) learners. While higher education must embrace the types of pedagogies that foster SDL skills in graduates, the pace of change in education can be glacial. This paper describes a social annotation technology, the Collaborative Lecture Annotation System (CLAS), that can be used to leverage existing teaching and learning practices for acquisition of 21st Century SDL skills. CLAS was designed to build upon the artifacts of traditional didactic modes of teaching, create enriched opportunities for student engagement with peers and learning materials, and offer learners greater control and ownership of their individual learning strategies. Adoption of CLAS creates educational experiences that promote and foster SDL skills: motivation, self-management and self-monitoring. In addition, CLAS incorporates a suite of learning analytics for learners to evaluate their progress, and allow instructors to monitor the development of SDL skills and identify the need for learning support and guidance. CLAS stands as an example of a simple tool that can bridge the gap between traditional transmissive pedagogy and the creation of authentic and collaborative learning spaces

Temporal properties of the short gamma-ray bursts

A temporal analysis has been performed on a sample of 100 bright gamma-ray bursts (GRBs) with T90<2s from the BATSE current catalog. The GRBs were denoised using a median filter and subjected to an automated pulse selection algorithm as an objective way of idenitifing the effects of neighbouring pulses. The rise times, fall times, FWHM, pulse amplitudes and areas were measured and the frequency distributions are presented here. All are consistent with lognormal distributions. The distribution of the time intervals between pulses is not random but consistent with a lognormal distribution. The time intervals between pulses and pulse amplitudes are highly correlated with each other. These results are in excellent agreement with a similar analysis that revealed lognormal distributions for pulse properties and correlated time intervals between pulses in bright GRBs with T90>2s. The two sub-classes of GRBs appear to have the same emission mechanism which is probably caused by internal shocks. They may not have the same progenitors because of the generic nature of the fireball model.Comment: 4 pages, 7 figure

Magnetically-dominated jets inside collapsing stars as a model for gamma-ray bursts and supernova explosions

It has been suggested that magnetic fields play a dynamically-important role in core-collapse explosions of massive stars. In particular, they may be important in the collapsar scenario for gamma-ray bursts (GRB), where the central engine is a hyper-accreting black hole or a millisecond magnetar. The present paper is focussed on the magnetar scenario, with a specific emphasis on the interaction of the magnetar magnetosphere with the infalling stellar envelope. First, the ``Pulsar-in-a-Cavity'' problem is introduced as a paradigm for a magnetar inside a collapsing star. The basic set-up of this fundamental plasma-physics problem is described, outlining its main features, and simple estimates are derived for the evolution of the magnetic field. In the context of a collapsing star, it is proposed that, at first, the ram pressure of the infalling plasma acts to confine the magnetosphere, enabling a gradual build-up of the magnetic pressure. At some point, the growing magnetic pressure overtakes the (decreasing) ram pressure of the gas, resulting in a magnetically-driven explosion. The explosion should be highly anisotropic, as the hoop-stress of the toroidal field, confined by the surrounding stellar matter, collimates the magnetically-dominated outflow into two beamed magnetic-tower jets. This creates a clean narrow channel for the escape of energy from the central engine through the star, as required for GRBs. In addition, the delayed onset of the collimated-explosion phase can explain the production of large quantities of Nickel-56, as suggested by the GRB-Supernova connection. Finally, the prospects for numerical simulations of this scenario are discussed.Comment: Invited paper in the "Physics of Plasmas" (May 2007 special issue), based on an invited talk at the 48th Annual Meeting of the APS Division of Plasma Physics (Oct. 30 - Nov. 3, 2006, Philadelphia, PA); 24 pages, 7 figure

Gamma Ray Bursts from the early Universe: predictions for present-day and future instruments

Long Gamma Ray Bursts (GRBs) constitute an important tool to study the Universe near and beyond the epoch of reionization. We delineate here the characteristics of an 'ideal' instrument for the search of GRBs at z>6-10. We find that the detection of these objects requires soft band detectors with a high sensitivity and moderately large FOV. In the light of these results, we compare available and planned GRB missions, deriving conservative predictions on the number of high-z GRBs detectable by these instruments along with the maximum accessible redshift. We show that the Swift satellite will be able to detect various GRBs at z>6, and likely at z>10 if the trigger threshold is decreased by a factor of ~2. Furthermore, we find that INTEGRAL and GLAST are not the best tool to detect bursts at z>6: the former being limited by the small FOV, and the latter by its hard energy band and relatively low sensitivity. Finally, future missions (SVOM, EDGE, but in particular EXIST) will provide a good sample of GRBs at z>6 in a few years of operation.Comment: 6 pages, 2 figures, MNRAS in pres

Collapsars - Gamma-Ray Bursts and Explosions in "Failed Supernovae"

Using a two-dimensional hydrodynamics code (PROMETHEUS), we study the continued evolution of rotating massive helium stars whose iron core collapse does not produce a successful outgoing shock, but instead forms a black hole. We study the formation of a disk, the associated flow patterns, and the accretion rate for disk viscosity parameter, alpha ~ 0.001 and 0.1. For the standard 14 solar mass model the average accretion rate for 15 s is 0.07 solar masses per second and the total energy deposited along the rotational axes by neutrino annihilation is (1 - 14) x 10**51 erg, depending upon the evolution of the Kerr parameter and uncertain neutrino efficiencies. Simulated deposition of this energy in the polar regions results in strong relativistic outflow - jets beamed to about 1.5% of the sky. The jets remain highly focused, and are capable of penetrating the star in 5 - 10 s. After the jet breaks through the surface of the star, highly relativistic flow can commence. Because of the sensitivity of the mass ejection and jets to accretion rate, angular momentum, and disk viscosity, and the variation of observational consequences with viewing angle, a large range of outcomes is possible ranging from bright GRBs like GRB 971214 to faint GRB-supernovae like SN 1998bw. X-ray precursors are also possible as the jet first breaks out of the star. While only a small fraction of supernovae make GRBs, we predict that all GRBs longer than a few seconds will make supernovae similar to SN 1998bw. However, hard, energetic GRBs shorter than a few seconds will be difficult to make in this model.Comment: Latex, 66 pages including 27 figures (9 color), Submitted to The Astrophysical Journal, latex uses aaspp4.sty. Figures also available at http://www.ucolick.org/~andre

Selection effects shaping the Gamma Ray Burst redshift distributions

Long Gamma Ray Bursts hold the promise of probing star-formation and metal enrichment up to very high redshifts. The present GRB samples with redshift determination are largely incomplete and therefore a careful analysis of selection effects is mandatory before any conclusion can be drawn from the observed GRB redshift distribution. We study and compare three well defined samples of long GRBs detected by Swift, HETE2 and BeppoSAX. We find that Swift GRBs are slighly fainter and harder than BeppoSAX and HETE2 GRBs, as expected due to the higher energy range in which Swift GRBs are detected and localized, compared to BeppoSAX and HETE2. Gas and dust obscuration plays a role in shaping the GRB samples and the present samples of GRBs with redshift. We argue that the majority of the bright Swift GRBs without redshift might actually be z<~2 events therefore the present Swift GRB sample with redshift is biased against low-z GRBs. On the other hand, the detection of bright UV rest-frame afterglows from high-z GRBs, and even from those with large X-ray obscuration, implies a dust amount lower than in nearby GRBs,and/or a different dust composition. If this is the case, the Swift sample of GRBs with redshifts is probably a fair sample of the real high-z GRB population. The absence of high-z GRBs in the BeppoSAX and HETE2 samples of GRBs with redshifts is probably due to the fact at the time of BeppoSAX and HETE2 follow-up faint afterglows of high redshift GRBs will have weaken below the spectroscopic capabilities of even 10m class telescopes. The redshift distribution of a subsample of Swift GRBs with distributions of peak-fluxes, X-ray obscuration and optical magnitude at a fixed observing time similar to those of the BeppoSAX and HETE2 samples, is roughly consistent with BeppoSAX+HETE2 redshift distribution.Comment: 9 pages, back to A&A after referee repor

The star-formation rate in the host of GRB 990712

We have observed the host galaxy of GRB 990712 at 1.4 GHz with the Australia Telescope Compact Array, to obtain an estimate of its total star-formation rate. We do not detect a source at the position of the host. The 2 sigma upper limit of 70 microJy implies that the total star-formation rate is lower than 100 Msun/yr, using conservative values for the spectral index and cosmological parameters. This upper limit is in stark contrast with recent reports of radio/submillimeter-determined star-formation rates of roughly 500 Msun/yr for two other GRB host galaxies. Our observations present the deepest radio-determined star-formation rate limit on a GRB host galaxy yet, and show that also from the unobscured radio point-of-view, not every GRB host galaxy is a vigorous starburst.Comment: A&A Letters, in press, 5 pages; a high-resolution color gif version of the paper figure is also supplie