omain Artigue (IRAP, France)

Constraining the equation of state of the dense matter from X-ray burst oscillations

We show that detailed modeling of X-ray burst oscillations can be an extremely powerful probe of the
neutron star structure (M, R), and thus the equation of state of ultra-dense matter. M and R are encoded in
the shape of the oscillations resulting from the spin modulation of the thermonuclear burst flux from the
neutron star. We have developed an expanding hot spot model for X-ray burst oscillations, taking into
account all relativistic effects affecting photon trajectories from the neutron star surface to the observer. We
have simulated light curves assuming different M, R, hot spot geometry and emission pattern. In comparison
to RXTE (Rossi X-Ray Timing Explorer) data, we show that an improvement by a factor of 20 in count
statistics (as achieved by LOFT) is required to detect the harmonic content of the signal, and whose
amplitude relates directly to the asymmetry of the waveform. Using a library of theoretical light curves, we
have then fitted the simulated light curves to evaluate the accuracy with which M and R could be recovered.
We show that M and R can be constrained within less than 10% from one single burst. Allowing several
bursts to be co-added, the statistical uncertainty in M and R can be further reduced within a few percent, as
required to become discriminant for the various solutions of the equation of state of dense matter.