On the 110 keV Feature From A0535+26: Direct Evidence For A Near-Critical Magnetic Field

Abstract

A recent high resolution measurement of an absorption line at 110 keV in the phase-averaged spectrum of A0535+26 (Grove et al. 1994) and the conspicuous absence of a previously reported harmonic feature at around 50 keV (Kendziorra et al. 1992, 1994) are indicative of cyclotron scattering in a magnetic field of about $10^{13}$G. However, controversy has risen over whether a lower field scenario may account for the alleged absence or weakness of a lower energy fundamental harmonic feature. This work explores these two alternatives through the generation of theoretical cyclotron spectra. For lower field models, a cylindrical geometry of the line forming region and the strong angle dependence of the cross section for resonant scattering conspire to fill in a first harmonic at around 57 keV. Higher field models (B $\simeq$ 10.75 TG), however, yield significantly better fits (${\cal X}^2_{low}/{\cal X}^2_{high} \sim 71$), thus strengthening the case for a near-critical field ($B_{crit} \simeq 44$ TG). Phase-resolved OSSE spectra are not yet available but would greatly help in resolving this issue.