Cyclotron emission, absorption, and the two faces of X-ray pulsar A 0535+262

Abstract

Deep NuSTAR observation of X-ray pulsar A 0535+262, performed at a very low luminosity of $\sim 7\times 10^{34}$ erg s${}^{-1}$, revealed the presence of two spectral components. We argue that the high-energy component is associated with cyclotron emission from recombination of electrons collisionally excited to the upper Landau levels. The cyclotron line energy of $E_{\mathrm{cyc}}=47.7\pm 0.8$ keV was measured at the luminosity of almost an order of magnitude lower than what was achieved before. The data firmly exclude a positive correlation of the cyclotron energy with the mass accretion rate in this source.

5 pages, 3 figures, accepted by MNRAS Letters

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