Title: Target of Opportunity Observations of an Outburst in A 0535+26
Proposal ID: 0120115
Subject category: Compact Object
Principal investigator: Heindl
Institute: University of California, San Diego
Abstract
We propose two 100ksec observations in response to a normal (200mCrab, 2 to 10keV) and a giant (1 Crab)
outburst in the Be/X-ray binary pulsar A0535+26. The first observation will measure the broad-band,
phase-average spectrum with high significance to ~200keV. We will confirm the presence of a 50 keV cyclotron
line at half the energy of the known 100keV line. With the excellent energy resolution of the SPI and
IBIS/ISGRI, we will study in detail the line shapes. Previous instruments (mostly scintillators) have lacked
the energy resolution for such studies. The second observation, when A0535+26 is significantly brighter
than the Crab below 200 keV, will allow us to perform detailed phase-resolved spectral studies. By
comparing the phase variable spectrum to Monte-Carlo simulations of cyclotron line formation, we hope to
infer details of the emission region, such as the overall geometry, the optical depth, and the viewing angle
of the observer to the magnetic dipole axis. Finally, we will search for redshifted gamma-ray lines from
nuclear processes (particularly 2.2 MeV photons from deuterium formation) at the neutron star polar cap.
This has the exciting possibility of determining the redshift at the neutron star surface, thereby
constraining the neutron star equation of state.