106
P. Esposito et al.
Quasi Periodic Oscillations and Seismology of Magnetars
The tail of the 2004 giant flare from SGR 1806–20 displayed clear QPO
signals at about 18, 30, 93, 150, 625 and 1840 Hz [117, 231]. QPOs around
frequencies of 28, 54, 84 and 155 Hz were detected in the tail of the 1998
giant flare of SGR 1900+14 [199], while hints for a signal at ∼43 Hz were
found in the 1979 event from SGR 0526–66 [12]. Some QPOs were excited
simultaneously, others were detected only once in a very narrow time interval,
some faded and were re-excited several times. All the detected QPOs are
dependent on the phase of the spin period and show large variations of the
amplitude with time. Their similarities suggest that the production mechanism
is the same and the most obvious responsible are seismic vibrations induced
by the giant flares. This is very exciting, as the QPOs provide a window on
the neutron-star and magnetic field structures, and even on the dense matter
equation of state.
The QPOs, in accordance with early theoretical suggestions [55], were
initially interpreted in terms of torsional shear modes of the neutron star crust.
However, it did not take long to realize that neutron stars can sustain many
types of oscillation and that the identification of oscillatory modes of magnetars is conceptually (and computationally) extremely challenging because of
the magnetic coupling between the crust and the core [82, 138]. Indeed, at the
moment, the potential of magnetar asteroseismology is dampened by the high
degeneracy because of the many uncertainty associated with the magnetic
field and the superfluid state of matter (see for example [81, 139, 140] for
detailed discussions and [219] for an overview of the current understanding
of the field), but also by the paucity of new data. This motivated searches for
QPOs in the more bountiful short bursts. Only one signal, an unusually broad
but strong peak at 260 Hz, was identified in a burst, a 0.5-s-long event from
1E 1547.0–5408, while several candidates from ∼60 to 130 Hz were found in
data sets combining many short busts [107–109]. Unfortunately, after the end
of the RossiXTE mission and the non selection of LOFT [68] by ESA, the
possibility of collecting a large number of photons with good time resolution
and without saturation problems in case of exceptional events in the near
future seems rather remote.
3.2.2.2 Short Bursts
The SGR/magnetar short bursts are the hallmark of magnetars and thousands of
them have been recorded and studied, both individually and as samples [5, 87,
90, 110, 119, 220]. In the last few years, and in particular since the launch of
Swift (which pairs a sensitive hard X-ray instrument with large field of view
P. Esposito et al.
Quasi Periodic Oscillations and Seismology of Magnetars
The tail of the 2004 giant flare from SGR 1806–20 displayed clear QPO
signals at about 18, 30, 93, 150, 625 and 1840 Hz [117, 231]. QPOs around
frequencies of 28, 54, 84 and 155 Hz were detected in the tail of the 1998
giant flare of SGR 1900+14 [199], while hints for a signal at ∼43 Hz were
found in the 1979 event from SGR 0526–66 [12]. Some QPOs were excited
simultaneously, others were detected only once in a very narrow time interval,
some faded and were re-excited several times. All the detected QPOs are
dependent on the phase of the spin period and show large variations of the
amplitude with time. Their similarities suggest that the production mechanism
is the same and the most obvious responsible are seismic vibrations induced
by the giant flares. This is very exciting, as the QPOs provide a window on
the neutron-star and magnetic field structures, and even on the dense matter
equation of state.
The QPOs, in accordance with early theoretical suggestions [55], were
initially interpreted in terms of torsional shear modes of the neutron star crust.
However, it did not take long to realize that neutron stars can sustain many
types of oscillation and that the identification of oscillatory modes of magnetars is conceptually (and computationally) extremely challenging because of
the magnetic coupling between the crust and the core [82, 138]. Indeed, at the
moment, the potential of magnetar asteroseismology is dampened by the high
degeneracy because of the many uncertainty associated with the magnetic
field and the superfluid state of matter (see for example [81, 139, 140] for
detailed discussions and [219] for an overview of the current understanding
of the field), but also by the paucity of new data. This motivated searches for
QPOs in the more bountiful short bursts. Only one signal, an unusually broad
but strong peak at 260 Hz, was identified in a burst, a 0.5-s-long event from
1E 1547.0–5408, while several candidates from ∼60 to 130 Hz were found in
data sets combining many short busts [107–109]. Unfortunately, after the end
of the RossiXTE mission and the non selection of LOFT [68] by ESA, the
possibility of collecting a large number of photons with good time resolution
and without saturation problems in case of exceptional events in the near
future seems rather remote.
3.2.2.2 Short Bursts
The SGR/magnetar short bursts are the hallmark of magnetars and thousands of
them have been recorded and studied, both individually and as samples [5, 87,
90, 110, 119, 220]. In the last few years, and in particular since the launch of
Swift (which pairs a sensitive hard X-ray instrument with large field of view