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Yu C.,CAS Yunnan Astronomical Observatory | Yu C.,Chinese Academy of Sciences
Astrophysical Journal | Year: 2012

We address a primary question regarding the physical mechanism that triggers the energy release and initiates the onset of eruptions in the magnetar magnetosphere. Self-consistent stationary, axisymmetric models of the magnetosphere are constructed based on force-free magnetic field configurations that contain a helically twisted force-free flux rope. Depending on the surface magnetic field polarity, there exist two kinds of magnetic field configurations, inverse and normal. For these two kinds of configurations, variations of the flux rope equilibrium height in response to gradual surface physical processes, such as flux injections and crust motions, are carefully examined. We find that equilibrium curves contain two branches: one represents a stable equilibrium branch, and the other an unstable equilibrium branch. As a result, the evolution of the system shows a catastrophic behavior: when the magnetar surface magnetic field evolves slowly, the height of the flux rope would gradually reach a critical value beyond which stable equilibriums can no longer be maintained. Subsequently, the flux rope would lose equilibrium and the gradual quasi-static evolution of the magnetosphere will be replaced by a fast dynamical evolution. In addition to flux injections, the relative motion of active regions would give rise to the catastrophic behavior and lead to magnetic eruptions as well. We propose that a gradual process could lead to a sudden release of magnetosphere energy on a very short dynamical timescale, without being initiated by a sudden fracture in the crust of the magnetar. Some implications of our model are also discussed. © 2012. The American Astronomical Society. All rights reserved..

He J.H.,CAS Yunnan Astronomical Observatory | Takahashi S.,Academia Sinica, Taiwan | Chen X.,Chinese Academy of Sciences
Astrophysical Journal, Supplement Series | Year: 2012

A northern subsample of 89 Spitzer GLIMPSE extended green objects (EGOs), the candidate massive young stellar objects, are surveyed for molecular lines in two 1GHz ranges: 251.5-252.5 and 260.188-261.188GHz. A comprehensive catalog of observed molecular line data and spectral plots are presented. Eight molecular species are undoubtedly detected: H 13CO +, SiO, SO, CH 3OH, CH 3OCH 3, CH 3CH 2CN, HCOOCH 3, and HN 13C. The H 13CO +3-2 line is detected in 70 EGOs, among which 37 also show the SiO6-5 line, demonstrating their association with dense gas and supporting the outflow interpretation of the extended 4.5 μm excess emission. Our major dense gas and outflow tracers (H 13CO +, SiO, SO, and CH 3OH) are combined with our previous survey of 13CO, 12CO, and C 18O1-0 toward the same sample of EGOs for a multi-line, multi-cloud analysis of linewidth and luminosity correlations. Good log-linear correlations are found among all considered line luminosities, the explanation of which requires a universal similarity of density and thermal structures and probably of shock properties among all EGO clouds. It also requires that the shocks be produced within the natal clouds of the EGOs. Diverse degrees of correlation are found among the linewidths. However, both the linewidth and luminosity correlations tend to progressively worsen across larger cloud subcomponent size scales, depicting the increase of randomness across cloud subcomponent sizes. Moreover, the linewidth correlations among the three isotopic CO1-0 lines show data scatter as linear functions of the linewidth itself, indicating that the velocity randomness also increases with whole cloud sizes in a regular way. © © 2012. The American Astronomical Society. All rights reserved.

Yu C.,CAS Yunnan Astronomical Observatory | Yu C.,Chinese Academy of Sciences
Astrophysical Journal | Year: 2011

Motivated by coronal mass ejection studies, we construct general relativistic models of a magnetar magnetosphere endowed with strong magnetic fields. The equilibrium states of the stationary, axisymmetric magnetic fields in the magnetar magnetosphere are obtained as solutions of the Grad-Shafranov equation in a Schwarzschild spacetime. To understand the magnetic energy buildup in the magnetar magnetosphere, a generalized magnetic virial theorem in the Schwarzschild metric is newly derived. We carefully address the question whether the magnetar magnetospheric magnetic field can build up sufficient magnetic energy to account for the work required to open up the magnetic field during magnetar giant flares. We point out the importance of the Aly-Sturrock constraint, which has been widely studied in solar corona mass ejections, as a reference state in understanding magnetar energy storage processes. We examine how the magnetic field can possess enough energy to overcome the Aly-Sturrock energy constraint and open up. In particular, general relativistic (GR) effects on the Aly-Sturrock energy constraint in the Schwarzschild spacetime are carefully investigated. It is found that, for magnetar outbursts, the Aly-Sturrock constraint is more stringent, i.e., the Aly-Sturrock energy threshold is enhanced due to the GR effects. In addition, neutron stars with greater mass have a higher Aly-Sturrock energy threshold and are more difficult to erupt. This indicates that magnetars are probably not neutron stars with extreme mass. For a typical neutron star with mass of 1-2 M ∞, we further explore the cross-field current effects, caused by the mass loading, on the possibility of stored magnetic field energy exceeding the Aly-Sturrock threshold. © 2011. The American Astronomical Society. All rights reserved.

Li Y.,CAS National Astronomical Observatories | Li Y.,CAS Yunnan Astronomical Observatory | Yuan Q.,CAS Institute of High Energy Physics
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2012

Recently some hints of the existence of γ-ray line around 130 GeV are reported according to the analysis of Fermi-LAT data. If confirmed it would be the first direct evidence to show the existence of new physics beyond the standard model. Here we suggest that using the forthcoming high energy resolution γ-ray detectors, such as CALET and DAMPE, we may test whether it is real line structure or just the background effect. For DAMPE like detector with designed energy resolution ~1.5%, a line significance will reach 11. σ for the same statistics as Fermi-LAT. For about 1.4 yr survey observation, DAMPE may detect a 5. σ signal of such a γ-ray line. © 2012 Elsevier B.V.

Yu C.,CAS Yunnan Astronomical Observatory | Yu C.,Chinese Academy of Sciences | Huang L.,Chinese Academy of Sciences
Astrophysical Journal Letters | Year: 2013

We propose a catastrophic magnetospheric model for magnetar precursors and their successive giant flares. Axisymmetric models of the magnetosphere, which contain both a helically twisted flux rope and a current sheet, are established based on force-free field configurations. In this model, the helically twisted flux rope would lose its equilibrium and erupt abruptly in response to the slow and quasi-static variations at the ultra-strongly magnetized neutron star's surface. In a previous model without current sheets, only one critical point exists in the flux rope equilibrium curve. New features show up in the equilibrium curves for the flux rope when current sheets appear in the magnetosphere. The causal connection between the precursor and the giant flare, as well as the temporary re-entry of the quiescent state between the precursor and the giant flare, can be naturally explained. Magnetic energy would be released during the catastrophic state transitions. The detailed energetics of the model are also discussed. The current sheet created by the catastrophic loss of equilibrium of the flux rope provides an ideal place for magnetic reconnection. We point out the importance of magnetic reconnection for further enhancement of the energy release during eruptions. © 2013. The American Astronomical Society. All rights reserved.

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