Search for Redshifted 2.2 MeV Neutron Capture Line From A0535+262 in Outburst

TL;DR

This study searched for redshifted 2.2 MeV neutron capture lines from A0535+262 during outbursts using INTEGRAL data, setting the strongest upper limits to date and analyzing photon escape probabilities from the neutron star surface.

Contribution

First to set stringent upper limits on redshifted 2.2 MeV lines from A0535+262 and analyze neutron star surface transparency for these photons.

Findings

No significant detection of the neutron capture line.

Established the strongest upper limits on the line flux.

Found that photon escape probability is small but non-zero.

Abstract

The Be/X-ray binary system A0535+262 underwent a giant outburst in May-June 2005, followed by a dimmer outburst in August-September 2005. 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 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 - 11) x 10^(-4) photons cm^(-2) s^(-1). 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.