Discovery and modelling of a flattening of the positive cyclotron line/luminosity relation in GX 304−1 with RXTE

• Published in: Monthly notices of the Royal Astronomical Society 2017-04, Vol.466 (3), p.2752-2779
• Main Authors: Rothschild, Richard E., Kühnel, Matthias, Pottschmidt, Katja, Hemphill, Paul, Postnov, Konstantin, Gornostaev, Mikhail, Shakura, Nikolai, Fürst, Felix, Wilms, Jörn, Staubert, Rüdiger, Klochkov, Dmitry
• Format: Article
• Language: English
• Subjects: Accretion; Astronomy; Clusters; Correlation; Cyclotrons; Flux; Luminosity; Mathematical models; Measurement; Normal distribution; Parameters; Spectrum analysis
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stw3222

Abstract The Rossi X-ray Timing Explorer (RXTE) observed four outbursts of the accreting X-ray binary transient source, GX 304−1 in 2010 and 2011. We present results of detailed 3–100 keV spectral analysis of 69 separate observations, and report a positive correlation between cyclotron line parameters, as well as other spectral parameters, with power-law flux. The cyclotron line energy, width and depth versus flux, and thus luminosity, correlations show a flattening of the relationships with increasing luminosity, which are well described by quasi-spherical or disc accretion that yield the surface magnetic field to be ∼5 × 1012 Gauss. Since HEXTE (High Energy X-ray Timing Experiment) cluster A was fixed aligned with the Proportional Counter Array field of view and cluster B was fixed viewing a background region 1 $_{.}^{\circ}$ 5 off of the source direction during these observations near the end of the RXTE mission, the cluster A background was estimated from cluster B events using HEXTEBACKEST. This made possible the detection of the ∼55 keV cyclotron line and an accurate measurement of the continuum. Correlations of all spectral parameters with the primary 2–10 keV power-law flux reveal it to be the primary driver of the spectral shape. The accretion is found to be in the collisionless shock braking regime.