Discovery and Modeling of a Flattening of the Positive Cyclotron Line/Luminosity Relation in GX 304-1 with RXTE

TL;DR

This study analyzes RXTE observations of GX 304-1, revealing a flattening in the cyclotron line energy-luminosity relation at high luminosities, indicating complex accretion physics and magnetic field characteristics.

Contribution

It provides the first detailed spectral analysis showing the flattening of cyclotron line parameters with luminosity in GX 304-1, advancing understanding of accretion regimes in X-ray binaries.

Findings

Cyclotron line energy flattens at high luminosity.

Surface magnetic field estimated at ~60 keV.

Accretion occurs in the collisionless shock braking regime.

Abstract

The 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 disk accretion that yield the surface magnetic field to be ~60 keV. Since HEXTE cluster A was fixed aligned with the PCA field of view and cluster B was fixed viewing a background region 1.5 degrees 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.