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NuSTAR Discovery of a Luminosity Dependent Cyclotron Line Energy in Vela X-1

Fürst, Felix and Pottschmidt, Katja and Wilms, Jörn and Tomsick, John A. and Bachetti, Matteo and Boggs, Steven E. and Christensen, Finn E. and Craig, William W. and Grefenstette, Brian W. and Hailey, Charles J. and Harrison, Fiona and Madsen, Kristin K. and Miller, Jon M. and Stern, Daniel and Walton, Dominic J. and Zhang, William (2014) NuSTAR Discovery of a Luminosity Dependent Cyclotron Line Energy in Vela X-1. Astrophysical Journal, 780 (2). Art. No. 133. ISSN 0004-637X.

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We present NuSTAR observations of Vela X-1, a persistent, yet highly variable, neutron star high-mass X-ray binary (HMXB). Two observations were taken at similar orbital phases but separated by nearly a year. They show very different 3–79 keV flux levels as well as strong variability during each observation, covering almost one order of magnitude in flux. These observations allow, for the first time ever, investigations on kilo-second time-scales of how the centroid energies of cyclotron resonant scattering features (CRSFs) depend on flux for a persistent HMXB. We find that the line energy of the harmonic CRSF is correlated with flux, as expected in the sub-critical accretion regime. We argue that Vela X-1 has a very narrow accretion column with a radius of around 0.4 km that sustains a Coulomb interaction dominated shock at the observed luminosities of L_x ~ 3 × 10^36 erg s^−1. Besides the prominent harmonic line at 55 keV the fundamental line around 25 keV is clearly detected. We find that the strengths of the two CRSFs are anti-correlated, which we explain by photon spawning. This anti-correlation is a possible explanation for the debate about the existence of the fundamental line. The ratio of the line energies is variable with time and deviates significantly from 2.0, also a possible consequence of photon spawning, which changes the shape of the line. During the second observation, Vela X-1 showed a short off-state in which the power-law softened and a cut-off was no longer measurable. It is likely that the source switched to a different accretion regime at these low mass accretion rates, explaining the drastic change in spectral shape.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Bachetti, Matteo0000-0002-4576-9337
Boggs, Steven E.0000-0001-9567-4224
Harrison, Fiona0000-0003-2992-8024
Madsen, Kristin K.0000-0003-1252-4891
Stern, Daniel0000-0003-2686-9241
Additional Information:© 2014 American Astronomical Society. Received 2013 September 16; accepted 2013 November 19; published 2013 December 18. 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). We would like to thank John E. Davis for the slxfig module, which was used to produce all figures in this work. We would like to thank Fritz Schwarm for the helpful discussions about cyclotron line shapes. 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). We would like to thank the anonymous referee for the useful comments. Facility: NuSTAR
Group:Space Radiation Laboratory, NuSTAR
Funding AgencyGrant Number
Centre National d'Études Spatiales (CNES)UNSPECIFIED
Subject Keywords:accretion; accretion disks; radiation: dynamics; stars: neutron; X-rays: binaries; X-rays: individual (Vela X-1)
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory2014-04
Record Number:CaltechAUTHORS:20140218-153815613
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Official Citation:NuSTAR Discovery of a Luminosity Dependent Cyclotron Line Energy in Vela X-1 Felix Fürst et al. 2014 ApJ 780 133
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:43863
Deposited By: Jason Perez
Deposited On:19 Feb 2014 00:12
Last Modified:10 Oct 2017 21:13

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