2006ApJ...637L.121B

In 2005 May-June, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observed the Be/X-ray binary system A0535+26 during a giant outburst, during which time its hard X-ray intensity reached several crab. This bright source presented a unique opportunity to search for redshifted neutron-capture lines from the surface of the neutron star. Such lines, if discovered, would strongly constrain the neutron star equation of state, motivating this search. An upper limit on the narrow, unredshifted line has been set at 6.5x10^–4 photons/cm2/s, while width-dependent limits on a broadened, redshifted line are set in the range of (4.0-10.5)x10^–4 photons/cm2/s. To our knowledge, these are the first measured upper limits on redshifted 2.2 MeV emission from an accreting neutron star.