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 similar to 7 x 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-cyc = 47.7 +/- 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.