摘要：We report on twoNuSTARobservations of the high-mass X-ray binaryA 0535+26taken toward the end of its normal 2015 outburst at very low 3–50 keV luminosities of~1.4 × 1036erg s-1and~5 × 1035erg s-1, which are complemented by nineSwiftobservations. The data clearly confirm indications seen in earlier data that the source’s spectral shape softens as it becomes fainter. The smooth exponential rollover at high energies seen in the first observation evolves to a much more abrupt steepening of the spectrum at 20–30 keV. The continuum evolution can be nicely described with emission from a magnetized accretion column, modeled using the compmag model modified by an additional Gaussian emission component for the fainter observation. Between the two observations, the optical depth changes from0.75 ± 0.04to, the electron temperature remains constant, and there is an indication that the column decreases in radius. Since the energy-resolved pulse profiles remain virtually unchanged in shape between the two observations, the emission properties of the accretion column reflect the same accretion regime. This conclusion is also confirmed by our result that the energy of the cyclotron resonant scattering feature (CRSF) at~45 keV is independent of the luminosity, implying that the magnetic field in the region in which the observed radiation is produced is the same in both observations. Finally, we also constrain the evolution of the continuum parameters with the rotational phase of the neutron star. The width of the CRSF could only be constrained for the brighter observation. Based on Monte Carlo simulations of CRSF formation in single accretion columns, its pulse phase dependence supports a simplified fan beam emission pattern. The evolution of the CRSF width is very similar to that of the CRSF depth, which is, however, in disagreement with expectations.
关键词：enX-rays: binariespulsars: individual: A 0535+26stars: neutronaccretion, accretion disksstars: magnetic field
