2009ApJ...694..593C

The Be/X-ray binary system A0535+262 underwent a giant outburst in 2005 May-June, followed by a dimmer outburst in 2005 August-September. This increased intensity provided an opportunity to search for redshifted neutron capture lines from the surface of the neutron star. If discovered, such lines would constrain the neutron star equation of state, providing the motivation of this search. The spectrometer (SPI) on board the INTEGRAL satellite observed the dimmer outburst and provided the data for this research. We have not yet detected a line with enough significance with the width-dependent upper limits on the broadened and redshifted neutron capture line in the range of 2-11x10^–4 photons/cm2/s. To our knowledge, these are the strongest upper limits on the redshifted 2.2 MeV emission from an accreting neutron star. Our analysis of the transparency of the neutron star surface for 2.2 MeV photons shows that photons have a small but finite chance of leaving the atmosphere unscattered, which diminishes the possibility of detection.