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Fig. 1:
The top panel shows a cross section through the star, with the solid
crust in yellow and the fluid core in grey. The patterns on the
surface show movement due to the vibrations. The lower panels
illustrate different types of crust vibrations. In panel (a) the
motion in the crust is primarily horizontal. This is the type of
movement associated with the torsional shear modes. In panel (b) the
motion is radial - these types of vibrations should be much harder to
excite.
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Fig. 2:
Frequencies of torsional shear modes for two different strange crust
models: one in which there is a thin crust of normal nuclear material
(top panel), and one in which strange matter exists in nuggets (lower
panel). In the top plot the black line shows the value predicted for
the lowest frequency mode (the fundamental), with the blue dashed line
showing the observed value. The red line shows the value predicted
for the first radial overtone, and the green dashed line the observed
value. For the strange nugget crust (lower panel) the models predict
a range of values, shown as shaded areas, depending on the strange
quark mass. For the thin nuclear crust, the fundamental is always
higher than the measured value; for the nugget crust it is always
lower. For the harmonic, the values predicted by our models are far
higher than the observed value. Varying the stellar mass, radius and
temperature does not alter the conclusions.
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The technique exploits observations of some of the most violent events
in the Universe, giant gamma-ray flares from a group of stars known as
Soft Gamma Repeaters. These are thought to be magnetars - neutron
stars with very strong magnetic fields. This field decays and twists,
becoming periodically unstable. Eventually the field lines snap and
shift, launching bursts of gamma-rays in the process.
The magnetic field lines are, however, fixed to the crust of the
neutron star and as they break they should also crack the crust. It
had long been suspected that this might trigger global seismic
vibrations in the star. Calculations showed that a particular type of
vibrations, torsional shear modes, were the most likely to be excited
(Figure 1). Observational evidence to support this idea came in late
2004, when high frequency oscillations were discovered in a
particularly energetic flare from the Soft Gamma Repeater SGR 1806-20.
Similar phenomena were then uncovered in data of an earlier giant
flare from a different star. The frequencies of the vibrations were
close to those predicted for the torsional shear modes of a neutron
star crust.
The frequencies of seismic vibrations can tell us about the
composition and depth of the crust (see Research Highlight, May 2006).
However they can also tell us about the matter deep in the core of the
star. The pressures are so high that neutrons will be squeezed out of
atomic nuclei, and may themselves dissolve into their constituent
quarks. Calculations in the 1980s by US scientist Ed Witten suggested
that the quarks might then react to form a stable fluid of up, down
and strange quarks which due to its enhanced stability would convert
the entire star into strange quark matter - making a strange star.
Much effort has since gone into working out how observations might
distinguish strange stars from neutron stars, but the question has yet
to be settled decisively.
One of the major differences between the two types of stars is crust
composition, which would modify the frequencies of seismic vibrations.
In addition strange star crusts should be much thinner, which has a
large effect on higher frequency harmonics. These differences led
Anna Watts (MPA) and her colleague Sanjay Reddy (LANL) to wonder
whether the magnetar observations might rule out strange star models.
They have now completed the first computation of torsional shear mode
frequencies for strange star crusts. Despite many poorly constrained
parameters, no strange star crust model matches the observations -
whereas neutron star models do (Figure 2).
"This is for us a very exciting result.", said Watts. "Our
calculations predict a dramatic difference between the frequencies of
a strange star crust and a neutron star crust. If the seismic
interpretation of the data is correct - and no other explanation comes
close at present - we may have found a robust way to rule out strange
stars".
Anna Watts and Sanjay Reddy
Publication
A.L.Watts & S. Reddy:
"Magnetar QPOs pose challenges for strange stars",
Physical Review Letters, submitted;
 astro-ph/0609364
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