The Evolution of the Accretion Disk around 4U 1820-30 During a Superburst

Accretion from a disk onto a collapsed, relativistic star -- a neutron star or black hole -- is the mechanism widely believed to be responsible for the emission from compact X-ray binaries. Because of the extreme spatial resolution required, it is not yet possible to directly observe the evolution or dynamics of the inner parts of the accretion disk where general relativistic effects are dominant. Here, we use the bright X-ray emission from a superburst on the surface of the neutron star 4U 1820-30 as a spotlight to illuminate the disk surface. The X-rays cause iron atoms in the disk to fluoresce, allowing a determination of the ionization state, covering factor and inner radius of the disk over the course of the burst. The time-resolved spectral fitting shows that the inner region of the disk is disrupted by the burst, possibly being heated into a thicker, more tenuous flow, before recovering its previous form in ~1000 s. This marks the first instance that the evolution of the inner regions of an accretion disk has been observed in real-time.Comment: 5 pages, 2 figures, accepted by ApJ Letter

Introducing a fifth pedagogy: Experience-based strategies for facilitating learning in natural environments

Educators have identified four categories of 'productive pedagogies' that are considered to lead to authentic student engagement and learning in the classroom. This study was designed to explore and extend these pedagogies in the context of learning in natural environments, in particular, through the programmes of Queensland environmental education centres. In-depth interview and observation data were collected from students, classroom teachers and centre teachers who had participated in 12 environmental education programmes across Queensland, in order to identify the strategies that are most effective in facilitating learning in the natural environment. A fifth productive pedagogy category, 'experience-based learning', is proposed. Experience-based learning is particularly important in addressing students' environmental attitudes and actions. The implications for the delivery of environmental education programmes both within and outside the classroom are discussed