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Looking at A 0535+26 at low luminosities with NuSTAR

Ballhausen, Ralf and Pottschmidt, Katja and Fürst, Felix and Wilms, Jörn and Tomsick, John A. and Schwarm, Fritz-Walter and Stern, Daniel and Kretschmar, Peter and Caballero, Isabel and Harrison, Fiona A. and Boggs, Steven E. and Christensen, Finn E. and Craig, William W. and Hailey, Charles J. and Zhang, William W. (2017) Looking at A 0535+26 at low luminosities with NuSTAR. Astronomy & Astrophysics, 608 . Art. No. A105. ISSN 0004-6361. doi:10.1051/0004-6361/201730845.

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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 × 10^(36) erg s^(-1) and ~5 × 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^(+0.01)_(-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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Pottschmidt, Katja0000-0002-4656-6881
Fürst, Felix0000-0003-0388-0560
Wilms, Jörn0000-0003-2065-5410
Tomsick, John A.0000-0001-5506-9855
Stern, Daniel0000-0003-2686-9241
Kretschmar, Peter0000-0001-9840-2048
Harrison, Fiona A.0000-0003-2992-8024
Boggs, Steven E.0000-0001-9567-4224
Christensen, Finn E.0000-0001-5679-1946
Zhang, William W.0000-0002-1426-9698
Additional Information:© ESO, 2017. Received: 22 March 2017. Accepted: 10 July 2017. Published online 12 December 2017. We thank Sebastian Falkner, Matthias Kühnel, and Ingo Kreykenbohm for many fruitful discussions. We thank the Deutsches Zentrum für Luft- und Raumfahrt for support under contract 50 OR 1410. This work was supported under NASA Contract No. NNG08FD60C, and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software, and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). This work has made use of data from the European Space Agency (ESA) mission Gaia (, processed by the Gaia Data Processing and Analysis Consortium (DPAC, Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This research has made use of a collection of ISIS functions (ISISscripts) provided by ECAP/Remeis observatory and MIT (
Group:NuSTAR, Space Radiation Laboratory, Astronomy Department
Funding AgencyGrant Number
Deutsches Zentrum für Luft- und Raumfahrt50 OR 1410
Gaia Multilateral AgreementUNSPECIFIED
Subject Keywords:X-rays: binaries / pulsars: individual: A 0535+26 / stars: neutron / accretion, accretion disks / stars: magnetic field
Record Number:CaltechAUTHORS:20180104-163940808
Persistent URL:
Official Citation:Looking at A 0535+26 at low luminosities with NuSTAR Ralf Ballhausen, Katja Pottschmidt, Felix Fürst, Jörn Wilms, John A. Tomsick, Fritz-Walter Schwarm, Daniel Stern, Peter Kretschmar, Isabel Caballero, Fiona A. Harrison, Steven E. Boggs, Finn E. Christensen, William W. Craig, Charles J. Hailey and William W. Zhang A&A, 608 (2017) A105 DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:84113
Deposited By: George Porter
Deposited On:05 Jan 2018 15:43
Last Modified:15 Nov 2021 20:16

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