The impulsive phase of magnetar giant flares: assessing linear tearing as the trigger mechanism

Giant $\gamma$-ray flares comprise the most extreme radiation events observed from magnetars. Developing on (sub)millisecond timescales and generating vast amounts of energy within a fraction of a second, the initial phase of these extraordinary bursts present a significant challenge for candidate trigger mechanisms. Here we assess and critically analyse the linear growth of the relativistic tearing instability in a globally twisted magnetosphere as the trigger mechanism for giant $\gamma$-ray flares. Our main constraints are given by the observed emission timescales, the energy output of the giant flare spike, and inferred dipolar magnetic field strengths. We find that the minimum growth time of the linear mode is comparable to the $e$-folding rise time, i.e. $\sim10^{-1}$ ms. With this result we constrain basic geometric parameters of the current sheet. We also discuss the validity of the presumption that the $e$-folding emission timescale may be equated with the growth time of an MHD instability.Comment: 15 pages, 4 figures, MNRAS in pres

Composing with Alternative Acoustic Guitar Techniques

Guitar techniques such as alternate tunings, capos, harmonics, and tapping have the potential to greatly expand both the tonal and percussive capabilities of the acoustic guitar. This work presents an examination of alternative techniques’ origins, recent developments, and contemporary applications. A historical thread of innovation and experimentation regarding alternative techniques is discovered by exploring works of composers such as Giovanni Paolo Foscarini, Fernando Sor, Andrés Segovia, Benjamin Britten, and Chet Atkins. Further contributing to the development of alternative techniques were advances in notation, specifically tablature. As a result, composers were able to express their creativity by incorporating alternative techniques such as alternate tunings, harmonics, and percussive performance – in which the performer uses their fingers and hands to strike the guitar at various locations to create a beat – with greater ease and intentionality. A final alternative technique discussed is the capo – full and partial. The use of one or more capos further expands the technical, harmonic, and sonic possibilities of the guitar. This work further presents in-depth analysis of various compositions by three composers: Andy McKee, Phil Keaggy, and the author. It is argued that when adequately understood and effectively implemented, alternative techniques provide a wealth of possibilities for even the most advanced composer

Washington Environmental Law Year in Review

We are proud to present the first installment of the Washington Environmental Law Year in Review. This feature, which will be published annually in the Fall issue, will track significant developments in the environmental laws and regulations of Washington, and present a summary of these changes organized by topic

Magnetar giant flare high-energy emission

High energy ($> 250$ keV) emission has been detected persisting for several tens of seconds after the initial spike of magnetar giant flares. It has been conjectured that this emission might arise via inverse Compton scattering in a highly extended corona generated by super-Eddington outflows high up in the magnetosphere. In this paper we undertake a detailed examination of this model. We investigate the properties of the required scatterers, and whether the mechanism is consistent with the degree of pulsed emission observed in the tail of the giant flare. We conclude that the mechanism is consistent with current data, although the origin of the scattering population remains an open question. We propose an alternative picture in which the emission is closer to that star and is dominated by synchrotron radiation. The $RHESSI$ observations of the December 2004 flare modestly favor this latter picture. We assess the prospects for the Fermi Gamma-Ray Space Telescope to detect and characterize a similar high energy component in a future giant flare. Such a detection should help to resolve some of the outstanding issues.Comment: 20 pages, 14 figure

Rotational phase dependence of magnetar bursts

The trigger for the short bursts observed in $\gamma$-rays from many magnetar sources remains unknown. One particular open question in this context is the localization of burst emission to a singular active region or a larger area across the neutron star. While several observational studies have attempted to investigate this question by looking at the phase dependence of burst properties, results have been mixed. At the same time, it is not obvious a priori that bursts from a localized active region would actually give rise to a detectable phase-dependence, taking into account issues such as geometry, relativistic effects, and intrinsic burst properties such brightness and duration. In this paper, we build a simple theoretical model to investigate the circumstances under which the latter effects could affect detectability of a dependence of burst emission on rotational phase. We find that even for strongly phase-dependent emission, inferred burst properties may not show a rotational phase dependence depending on the geometry of the system and the observer. Furthermore, the observed properties of bursts with durations short as 10-20% of the spin period can vary strongly depending on the rotational phase at which the burst was emitted. We also show that detectability of a rotational phase dependence depends strongly on the minimum number of bursts observed, and find that existing burst samples may simply be too small to rule out a phase dependence.Comment: 15 pages, 17 figure