Looking at A 0535+26 at low luminosities with NuSTAR

¹ Dr. Karl-Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: ralf.ballhausen@sternwarte.uni-erlangen.de
² Department of Physics and Center for Space Science and Technology, UMBC, Baltimore, MD 21250, USA
³ CRESST and NASA Goddard Space Flight Center, Astrophysics Science Division, Code 661, Greenbelt, MD 20771, USA
⁴ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
⁵ European Space Astronomy Centre (ESA/ESAC), Science Operations Department, Villanueva de la Cañada (Madrid), Spain
⁶ Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720-7450, USA
⁷ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁸ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Lyngby, Denmark
⁹ Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
¹⁰ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
¹¹ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

Received: 22 March 2017
Accepted: 10 July 2017

Abstract

We report on two NuSTAR observations of the high-mass X-ray binary A 0535+26 taken toward the end of its normal 2015 outburst at very low 3–50 keV luminosities of ~1.4 × 10³⁶ erg s^-1 and ~5 × 10³⁵ erg s^-1, which are complemented by nine Swift observations. 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 from 0.75 ± 0.04 to 0.56^+0.01_-0.04, 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.