A critical view on the origin of broad iron lines in neutron star low-mass X-ray binaries

The recent claim of broad skewed iron lines from neutron star low-mass X-ray binaries has opened the exciting possibility of determining an upper limit to the radius of the neutron star, the most difficult parameter to obtain in order to constrain their equation of state. With the aim of studying further this possibility we performed a systematic analysis of XMM-Newton archival observations of bright neutron star low-mass X-ray binaries (LMXBs).
We performed a detailed analysis taking into account the systematic effects that arise as a consequence of the high count rates present in the observations of these sources. The properties of the iron lines differed from previous published analyses due to either incorrect pile-up corrections or different continuum parametrization. Most important, we could fit the iron line with a simple Gaussian component for all the sources. The lines did not show the asymmetric profiles that were interpreted as an indication of relativistic effects in previous analyses of these LMXBs. I will discuss the implications of these results for the origin of broad Fe lines in both neutron star and black hole LMXBs.