MG13 - Talk detail

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Abstract

We examine two trigger mechanisms, one internal and the other external to the neutron star, that give rise to the intense soft gamma-ray repeater (SGR) giant flares. So far, three giant flares have been observed from the three out of the seven confirmed SGRs on 1979 March 5, 1998 August 27 and 2004 December 27, respectively. The last two events were found to be much more powerful than the first, and both showcased the existence of a precursor, which we show to have had initiated the main flare. In the internal mechanism, we propose that the strongly wound-up poloidal magnetic field develops tangential discontinuities and dissipates its torsional energy in heating the crust. The time-scale for the instability to develop coincides with the duration of the quiescent state that followed the precursor. Alternatively, we develop a reconnection model based on the hypothesis that shearing motion of the footpoints causes the materialization of a Sweet-Parker current layer in the magnetosphere. The thinning of this macroscopic layer due to the development of an embedded super-hot turbulent current layer switches on the impulsive Hall reconnection, which powers the giant flare. Again, we show that the thinning time is on the order of the pre-flare quiescent time. This model naturally explains the origin of the observed non-thermal radiation during the flares as well as the post-flare radio afterglows.

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Accepted

Yes

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Talk

Title

Abstract

Microlensing is generally studied in the geometric optics limit. However, diffraction may be important when nearby substellar objects lens occult distant stars. In particular the effects of diffraction become more important as the wavelength of the observation increases. Typically if the wavelength of the observation is comparable to the Schwarzschild radius of lensing object, diffraction leaves an observable imprint on the lensing signature. The commissioning of the Square Kilometre Array (SKA) over the next decade begs the question of whether it will become possible to follow up lensing events with radio observations because the SKA may have sufficient sensitivity to detect the typical sources, giant stars in the bulge. The detection of diffractive lensing in a lensing event would place unique constraints on the mass of the lens and its distance. In particular it would distinguish rapidly moving stellar mass lenses (e.g. neutron stars) from slowly moving substellar objects such freely floating planets. An analysis of the sensitivity of the SKA along with new simple closed-form estimates of the expected signal applied to local exemplars for stellar radio emission reveals that this effect can nearly be detected with the SKA. If the radio emission from bulge giants is stronger than expected, the SKA could detect the diffractive microlensing signature from Earth-like interstellar planets in the solar neighborhood.

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