Magnetar

About: Magnetar is a research topic. Over the lifetime, 2905 publications have been published within this topic receiving 106806 citations.

Can X-ray emission powered by a spinning-down magnetar explain some GRB light curve features?

Abstract: Long duration gamma-ray bursts (GRBs) are thought to be produced by the core-collapse of a rapidly-rotating massive star. This event generates a highly relativistic jet and prompt gamma-ray and X-ray emission arises from internal shocks in the jet or magnetised outflows. If the stellar core does not immediately collapse to a black hole, it may form an unstable, highly magnetised millisecond pulsar, or magnetar. As it spins down, the magnetar would inject energy into the jet causing a distinctive bump in the GRB light curve where the emission becomes fairly constant followed by a steep decay when the magnetar collapses. We assume that the collapse of a massive star to a magnetar can launch the initial jet. By automatically fitting the X-ray lightcurves of all GRBs observed by the Swift satellite we identified a subset of bursts which have a feature in their light curves which we call an internal plateau -- unusually constant emission followed by a steep decay -- which may be powered by a magnetar. We use the duration and luminosity of this internal plateau to place limits on the magnetar spin period and magnetic field strength and find that they are consistent with the most extreme predicted values for magnetars.

QPOs during magnetar flares are not driven by mechanical normal modes of the crust

Abstract: QPOs have been observed during three powerful magnetar flares, from SGR 0526−66, SGR 1806−20 and SGR 1900+14. These QPOs have been commonly interpreted as being driven by the mechanical modes of the magnetar's solid crust which are excited during the flare. Here we show that this interpretation is in sharp contradiction with the conventional magnetar model. Firstly, we show that a magnetar crustal mode decays on the time-scale of at most 1 s due to the emission of Alfven waves into the neutron star interior. A possible modification is then to assume that the QPOs are associated with the magnetars' global modes. However, we argue that at the frequencies of the observed QPOs, the neutron star core is likely to support a continuum of magnetohydrodynamic normal modes. We demonstrate this on a completely solvable toy model which captures the essential physics of the system. We then show that the frequency of the global mode of the whole star is likely to have a significant imaginary component, and its amplitude is likely to decay on a short time-scale. This is not observed. Thus we conclude that either (i) the origin of the QPO is in the magnetar's magnetosphere, or (ii) the magnetic field has a special configuration: either it is expelled from the magenta's core prior to the flares, or its poloidal component has very small coherence length.

Physical mechanisms for the variable spin-down and light curve of SGR 1900+14

Abstract: We consider the physical implications of the rapid spin-down of soft gamma repeater SGR 1900+14 reported by Woods and colleagues in 1999 During an 80 day interval between 1998 June and the large outburst on 1998 August 27, the mean spin-down rate increased by a factor of 23, resulting in a positive period offset of ΔP/P = 10-4 A radiation-hydrodynamical outflow associated with the August 27 event could impart the required torque, but only if the dipole magnetic field is stronger than ~1014 G and the outflow lasts longer and/or is more energetic than the observed X-ray flare A positive period increment is also a natural consequence of a gradual, plastic deformation of the neutron star crust by an intense magnetic field, which forces the neutron superfluid to rotate more slowly than the crust Sudden unpinning of the neutron vortex lines during the August 27 event would then induce a glitch opposite in sign to those observed in young pulsars, but of a much larger magnitude as a result of the slower rotation The change in the persistent X-ray light curve following the August 27 event is ascribed to continued particle heating in the active region of that outburst The enhanced X-ray output can be powered by a steady current flowing through the magnetosphere, induced by the twisting motion of the crust The long-term rate of spin-down appears to be accelerated with respect to a simple magnetic dipole torque Accelerated spin-down of a seismically active magnetar will occur when its persistent output of Alfven waves and particles exceeds its spin-down luminosity or if particle flows modulate the ratio of conduction to displacement currents in the outer magnetosphere We suggest that SGRs experience some episodes of relative inactivity, with diminished , and that such inactive magnetars are observed as anomalous X-ray pulsars (AXPs) The reappearance of persistent X-ray emission from SGR 1900+14 within one day of the August 27 event provides strong evidence that the persistent emission is not powered by accretion

A Swift Gaze into the 2006 March 29 Burst Forest of SGR 1900+14

Abstract: In 2006 March the soft gamma-ray repeater SGR 1900+14 resumed its bursting activity after ~2 yr of quiescence. The Swift mission observed the source several times. We report on the intense burst "forest" recorded on March 29, which lasted for ~30 s, when Swift was pointing at the source with the narrow field of view instruments. More than 40 bursts were detected by BAT and XRT, 7 of which were rare intermediate flares (IFs). The BAT data were used to carry out time-resolved spectroscopy in the 14-100 keV range down to 8 ms timescales. BAT and XRT simultaneous data were used to characterize the broadband energy spectra of IFs and verify the results obtained from the BAT-only spectral fits. This unique data set allowed us to test the magnetar model predictions, such as the magnetically trapped fireball and twisted magnetosphere, over an unprecedented range of fluxes and with large statistics. We confirmed that a two-blackbody component adequately fits the time-resolved and integrated spectra of IFs. However, Comptonization models give comparably good reduced χ2. Moreover, we found a change of behavior, around ~1041 erg s−1, above which the softer blackbody shows a sort of saturation, while the harder one still grows to a few times 1041 erg s−1, and a rather sharp correlation between temperature and radii of the blackbodies (R2 ∝ kT−3), which holds for the most luminous parts of the flares (~Ltot ≥ 1041 erg s−1). Within the magnetar model, the majority of these findings are accounted for in terms of thermalized emission from the E-mode and O-mode photospheres. Interestingly, the maximum observed luminosity coming from a region of ~15 km matches the magnetic Eddington luminosity at the same radius, for a surface dipole field of ~8 × 1014 G (virtually equal to that deduced from the spin-down of SGR 1900+14).

The Collimation and Energetics of the Brightest Swift Gamma-ray Bursts

Abstract: Long-duration gamma-ray bursts (GRBs) are widely believed to be highly collimated explosions (bipolar conical outflows with half-opening angle θ ≈ 1°-10°). As a result of this beaming factor, the true energy release from a GRB is usually several orders of magnitude smaller than the observed isotropic value. Measuring this opening angle, typically inferred from an achromatic steepening in the afterglow light curve (a "jet" break), has proven exceedingly difficult in the Swift era. Here, we undertake a study of five of the brightest (in terms of the isotropic prompt γ-ray energy release, E_(γ,iso)) GRBs in the Swift era to search for jet breaks and hence constrain the collimation-corrected energy release. We present multi-wavelength (radio through X-ray) observations of GRBs 050820A, 060418, and 080319B, and construct afterglow models to extract the opening angle and beaming-corrected energy release for all three events. Together with results from previous analyses of GRBs 050904 and 070125, we find evidence for an achromatic jet break in all five events, strongly supporting the canonical picture of GRBs as collimated explosions. The most natural explanation for the lack of observed jet breaks from most Swift GRBs is therefore selection effects. However, the opening angles for the events in our sample are larger than would be expected if all GRBs had a canonical energy release of ~10^(51) erg. The total energy release we measure for the "hyper-energetic" (E_(tot) ≳ 10^(52) erg) events in our sample is large enough to start challenging models with a magnetar as the compact central remnant.