The crystalline color superconducting phase is believed to be the ground
state of deconfined quark matter for sufficiently large values of the strange
quark mass. This phase has the remarkable property of being more rigid than any
known material. It can therefore sustain large shear stresses, supporting
torsional oscillations of large amplitude. The torsional oscillations could
lead to observable electromagnetic signals if strange stars have a crystalline
color superconducting crust. Indeed, considering a simple model of strange star
with a bare quark matter surface, it turns out that a positive charge is
localized in a narrow shell about ten Fermi thick beneath the star surface. The
electrons needed to neutralize the positive charge of quarks spill in the star
exterior forming an electromagnetically bounded atmosphere hundreds of Fermi
thick. When a torsional oscillation is excited, for example by a stellar
glitch, the positive charge oscillates with typical kHz frequencies, for a
crust thickness of about one-tenth of the stellar radius, to hundreds of Hz,
for a crust thickness of about nine-tenths of the stellar radius. Higher
frequencies, of the order of few GHz, can be reached if the star crust is of
the order of few centimeters thick. We estimate the emitted power considering
emission by an oscillating magnetic dipole, finding that it can be quite large,
of the order of 1045 erg/s for a thin crust. The associated relaxation
times are very uncertain, with values ranging between microseconds and minutes,
depending on the crust thickness. The radiated photons will be in part absorbed
by the electronic atmosphere, but a sizable fraction of them should be emitted
by the star.Comment: 11 pages, 5 figures, corrected the emission power, added a discussion
on the temperature effects, added references. Almost matches the version
accepted for publication in Phys. Rev.
