Looking at A 0535+26 at low luminosities with NuSTAR

Ballhausen R, Pottschmidt K, Furst F, Wilms J, Tomsick JA, Schwarm FW, Stern D, Kretschmar P, Caballero I, Harrison FA, Boggs SE, Christensen FE, Craig WW, Hailey CJ, Zhang WW (2017)

Publication Status: Published

Publication Type: Journal article

Publication year: 2017

Journal

Astronomy & Astrophysics EDP Sciences

Publisher: EDP SCIENCES S A

Book Volume: 608

Article Number: ARTN A105

DOI: 10.1051/0004-6361/201730845

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 similar to 1.4 x 10(36) erg s(-1) and similar to 5 x 10(35) 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(-0.04)(+0.01), 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 similar to 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.

Authors with CRIS profile

Ralf Ballhausen Dr.Karl-Remeis-Sternwarte Bamberg, Astronomisches Institut Jörn Wilms Professur für Astronomie und Astrophysik Fritz-Walter Schwarm Dr.Karl-Remeis-Sternwarte Bamberg, Astronomisches Institut

Involved external institutions

European Space Astronomy Centre (ESAC) ES Spain (ES) California Institute of Technology (Caltech) US United States (USA) (US) Columbia University US United States (USA) (US) Technical University of Denmark / Danmarks Tekniske Universitet (DTU) DK Denmark (DK) University of Maryland, Baltimore County US United States (USA) (US) National Aeronautics and Space Administration (NASA) US United States (USA) (US) University of California, Berkeley US United States (USA) (US)

How to cite

APA:

Ballhausen, R., Pottschmidt, K., Furst, F., Wilms, J., Tomsick, J.A., Schwarm, F.-W.,... Zhang, W.W. (2017). Looking at A 0535+26 at low luminosities with NuSTAR. Astronomy & Astrophysics, 608. https://dx.doi.org/10.1051/0004-6361/201730845

MLA:

Ballhausen, Ralf, et al. "Looking at A 0535+26 at low luminosities with NuSTAR." Astronomy & Astrophysics 608 (2017).

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