Astronomy and Astrophysics – Astrophysics
Scientific paper
Mar 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aipc..556..228d&link_type=abstract
EXPLOSIVE PHENOMENA IN ASTROPHYSICAL COMPACT OBJECTS: First KIAS Astrophysics Workshop. AIP Conference Proceedings, Volume 556,
Astronomy and Astrophysics
Astrophysics
2
Neutron Stars, Pulsations, Oscillations, And Stellar Seismology, Gamma-Ray Sources, Gamma-Ray Bursts
Scientific paper
Pure shear oscillations in neutron star crusts are strongly excited by magnetically-induced starquakes. These toroidal modes could play a key role in the physics of Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs). Toroidal modes drive relativistic Alfvén waves into the magnetosphere. Energy is lost along field lines that are blown open by this outflow pressure. This helps damp the crust vibrations while driving a relativistic wind of waves and particles away from the star. When channeled by a strong magnetic field, this outflow carries away angular momentum. Frequent small-scale fractures maintain a quasi-steady shear mode excitation in a SGR's crust, accelerating SGR spindown. During energetic bursts or flares, large-scale fractures excite the vibrations to much higher amplitudes. Shear oscillations with energy ~1044 E44 erg were probably excited during the 1998 August 27 event, comparable to the observed energy in hard photons. This inevitably drove transient accelerated spindown, with a net period shift ΔP/P=10-4E441/3. Such a period shift was found in SGR 1900+14 following the August 27 flare. RXTE observations made one day after the event show that the enhancement of the spindown rate had nearly decayed away by that time; this constrains the dipole field of the star to B*>~1014 Gauss. Anomalous X-ray Pulsars may be inactive magnetars, without strongly vibrating crusts. There is evidence that AXP 2259+586 experienced two active episodes during the past 20 years with crust excitations ~1039-1040 ergs during each episode, driving accelerated spindown by an amount ΔP/P~3×10-6. We describe how P(t) varies following a sudden, strong excitation. Because the excess Ṗ decays on a time scale τ~B*-2, SGR or AXP timing data following a strong excitation could be used to measure the stellar dipole field B*. .
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