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The Smooth Cyclotron Line in Her X-1 as Seen with Nuclear Spectroscopic Telescope Array

Fürst, Felix and Grefenstette, Brian W. and Staubert, Rüdiger and Tomsick, John A. and Bachetti, Matteo and Barret, Didier and Bellm, Eric C. and Boggs, Steven E. and Chenevez, Jerome and Christensen, Finn E. and Craig, William W. and Hailey, Charles J. and Harrison, Fiona and Klochkov, Dmitry and Madsen, Kristin K. and Pottschmidt, Katja and Stern, Daniel and Walton, Dominic J. and Wilms, Jörn and Zhang, William (2013) The Smooth Cyclotron Line in Her X-1 as Seen with Nuclear Spectroscopic Telescope Array. Astrophysical Journal, 779 (1). Art. No. 69. ISSN 0004-637X. doi:10.1088/0004-637X/779/1/69.

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Her X-1, one of the brightest and best studied X-ray binaries, shows a cyclotron resonant scattering feature (CRSF) near 37 keV. This makes it an ideal target for a detailed study with the Nuclear Spectroscopic Telescope Array (NuSTAR), taking advantage of its excellent hard X-ray spectral resolution. We observed Her X-1 three times, coordinated with Suzaku, during one of the high flux intervals of its 35 day superorbital period. This paper focuses on the shape and evolution of the hard X-ray spectrum. The broadband spectra can be fitted with a power law with a high-energy cutoff, an iron line, and a CRSF. We find that the CRSF has a very smooth and symmetric shape in all observations and at all pulse phases. We compare the residuals of a line with a Gaussian optical-depth profile to a Lorentzian optical-depth profile and find no significant differences, strongly constraining the very smooth shape of the line. Even though the line energy changes dramatically with pulse phase, we find that its smooth shape does not. Additionally, our data show that the continuum only changes marginally between the three observations. These changes can be explained with varying amounts of Thomson scattering in the hot corona of the accretion disk. The average, luminosity-corrected CRSF energy is lower than in past observations and follows a secular decline. The excellent data quality of NuSTAR provides the best constraint on the CRSF energy to date.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Fürst, Felix0000-0003-0388-0560
Grefenstette, Brian W.0000-0002-1984-2932
Tomsick, John A.0000-0001-5506-9855
Bachetti, Matteo0000-0002-4576-9337
Barret, Didier0000-0002-0393-9190
Bellm, Eric C.0000-0001-8018-5348
Boggs, Steven E.0000-0001-9567-4224
Chenevez, Jerome0000-0002-4397-8370
Christensen, Finn E.0000-0001-5679-1946
Harrison, Fiona0000-0003-2992-8024
Madsen, Kristin K.0000-0003-1252-4891
Pottschmidt, Katja0000-0002-4656-6881
Stern, Daniel0000-0003-2686-9241
Walton, Dominic J.0000-0001-5819-3552
Wilms, Jörn0000-0003-2065-5410
Zhang, William0000-0002-1426-9698
Additional Information:© 2013 American Astronomical Society. Received 2013 June 11; accepted 2013 September 20; published 2013 November 26. 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 (NuSTAR-DAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). We would like to thank John E. Davis for the slxfig module, which was used to produce all figures in this work. F.F. would also like to thank the Remeis-Observatory Bamberg for their hospitality. J.A.T. acknowledges partial support from NASA Astrophysics Data Analysis Program grant NNX13AE98G. M.B. was supported by the Centre National d’Études Spatiales (CNES).
Group:NuSTAR, Space Radiation Laboratory
Funding AgencyGrant Number
Centre National d’Études Spatiales (CNES)UNSPECIFIED
Subject Keywords:accretion, accretion disks; pulsars: individual (Her X-1); stars: neutron; X-rays: binaries
Issue or Number:1
Record Number:CaltechAUTHORS:20140121-073921852
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Official Citation:The Smooth Cyclotron Line in Her X-1 as Seen with Nuclear Spectroscopic Telescope Array Felix Fürst et al. 2013 ApJ 779 69
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:43441
Deposited By: Ruth Sustaita
Deposited On:21 Jan 2014 21:58
Last Modified:10 Nov 2021 16:37

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