The first phenomenon is the observation of gravitationally-redshifted atomic lines during X-ray bursts
from the source EXO 0748–676 [37]. Figure 16 shows the values of the gravitational redshift from
the surface of neutron stars with different masses in second-order scalar-tensor theories with
different values of the parameter
[43
]. In this calculation, the parameter
was set to
zero and the neutron-star structure was calculated using the equation of state U [36]. The
hatch-filled area corresponds to neutron-star masses that are unacceptable for each value of the
parameter
, while the thick curve separates the scalarized stars from their general-relativistic
counterparts.
A dynamical measurement of the mass of EXO 0748–676 can rule out the possibility that the neutron
star in this source is scalarized, because scalarized stars have very different surface redshifts compared to
the general-relativistic stars of the same mass. The source EXO 0748–676 lies in an eclipsing binary system,
which makes it a prime candidate for a dynamical mass measurement. In the absence of such a
measurement, however, a limit on the parameter can be placed under the astrophysical constraint
that the baryonic mass of the neutron stars is larger than
. This is a reasonable
assumption, given that a progenitor core of a lower mass would not have collapsed to form a neutron
star. Combining this constraint with the measured redshift of z = 0.35 leads to a limit on the
parameter
, which depends only weakly on the assumed equation of state of neutron-star
matter [43].
|
A second set of phenomena that can lead to strong-field tests of gravity are the fast quasi-periodic
oscillations observed from many bright accreting neutron stars [171]. The highest known frequency of such
an oscillations is 1330 Hz, observed from the source 4U 1636–53 and corresponding to the Keplerian
frequency of the innermost stable circular orbit of a slowly-spinning neutron star. Figure 17
shows
the maximum Keplerian frequency outside a neutron star in the second-order scalar tensor theory for
different values of the parameter
. For small stellar masses, the limiting frequency is achieved at the
surface of the star, whereas for large stellar masses, the limiting frequency is reached at the innermost
stable circular orbit. Figure 17
shows that scalarized stars allow for higher frequencies than
their general-relativistic counterparts. Therefore, requiring the observed oscillation frequency to
be smaller than the highest Keplerian frequency of a stable orbit outside the compact object
cannot be used to constrain the parameters of this theory. On the other hand, the correlations
between the various dynamical frequencies outside the compact object depend strongly on the
parameter
and hence the gravity theory can be constrained given a particular model for the
oscillations [44].
http://www.livingreviews.org/lrr-2008-9 | ![]() This work is licensed under a Creative Commons License. Problems/comments to |