On the 110 keV feature from A0535+26: Direct evidence for a near-critical magnetic field

A recent high-resolution measurement of an absorption line at omega similar or equal to 110 keV in the phase-averaged spectrum of A0535+26 (Grove et al.) and the conspicuous absence of a previously reported harmonic feature at omega similar to 50 keV (Kendziorra et al.) 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 omega similar or equal to 57 keV. Higher field models (B similar or equal to 10.75 TG), however, yield significantly better fits (chi(low)/(2) chi(high)(2) similar to 71), thus strengthening the case for a near-critical field (B-crit similar or equal to 44 TG). Phase-resolved OSSE spectra are not yet available but would greatly help in resolving this issue.