On the 110 keV Feature from A0535+26: Direct Evidence for a Near-critical Magnetic Field

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Abstract
A recent high-resolution measurement of an absorption line at ω 110 keV in the phase-averaged spectrum of A0535+26 (Grove et al.) and the conspicuous absence of a previously reported harmonic feature at ω ~ 50 keV (Kendziorra et al.) are indicative of cyclotron scattering in a magnetic field of about 1013 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 ω 57 keV. Higher field models (B 10.75 TG), however, yield significantly better fits (χlow2/χhigh2 ~ 71), thus strengthening the case for a near-critical field (Bcrit 44 TG). Phase-resolved OSSE spectra are not yet available but would greatly help in resolving this issue.
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