Outburst of GX 304-1 monitored with INTEGRAL: positive correlation between the cyclotron line energy and flux

TL;DR

This study analyzes INTEGRAL data of GX 304-1 during its 2012 outburst, revealing a positive correlation between cyclotron line energy and flux, indicating a sub-Eddington accretion regime.

Contribution

First detailed measurement of cyclotron line energy variation with flux in GX 304-1, confirming positive correlation during an outburst.

Findings

Cyclotron line energy increases with flux in GX 304-1.

The source operates in a sub-Eddington accretion regime.

Spectral parameters vary systematically with luminosity.

Abstract

X-ray spectra of many accreting pulsars exhibit significant variations as a function of flux and thus of mass accretion rate. In some of these pulsars, the centroid energy of the cyclotron line(s), which characterizes the magnetic field strength at the site of the X-ray emission, has been found to vary systematically with flux. GX 304-1 is a recently established cyclotron line source with a line energy around 50 keV. Since 2009, the pulsar shows regular outbursts with the peak flux exceeding one Crab. We analyze the INTEGRAL observations of the source during its outburst in January-February 2012. The observations covered almost the entire outburst, allowing us to measure the source's broad-band X-ray spectrum at different flux levels. We report on the variations in the spectral parameters with luminosity and focus on the variations in the cyclotron line. The centroid energy of the line is found to be positively correlated with the luminosity. We interpret this result as a manifestation of the local sub-Eddington (sub-critical) accretion regime operating in the source.