The Be/X-ray Binary Swift J1626.6-5156 as a Variable Cyclotron Line Source

TL;DR

This study identifies a variable cyclotron resonance scattering feature in the X-ray spectrum of Swift J1626.6-5156, indicating a magnetic field of approximately 8.6 x 10^11 G and revealing phase-dependent variability and correlation with luminosity.

Contribution

First detection of a cyclotron line in Swift J1626.6-5156, establishing it as a new candidate for cyclotron line sources with phase-dependent variability.

Findings

Detected a ~10 keV cyclotron line in the spectrum.

Estimated magnetic field strength of ~8.6 x 10^11 G.

Observed phase-dependent variability of the cyclotron line.

Abstract

Swift J1626.6-5156 is a Be/X-ray binary that was in outburst from December 2005 until November 2008. We have examined RXTE/PCA and HEXTE spectra of three long observations of this source taken early in its outburst, when the PCA 2-20 keV count rate was >70 counts/s/PCU, as well as several combined observations from different stages of the outburst. The spectra are best fit with an absorbed cutoff power law with a ~6.4 keV iron emission line and a Gaussian optical depth absorption line at ~10 keV. We present strong evidence that this absorption-like feature is a cyclotron resonance scattering feature, making Swift J1626.6-5156 a new candidate cyclotron line source. The redshifted energy of ~10 keV implies a magnetic field strength of ~8.6(1+z) x 10^11 G in the region of the accretion column close to the magnetic poles where the cyclotron line is produced. Analysis of phase averaged spectra spanning the duration of the outburst suggests a possible positive correlation between the fundamental cyclotron energy and source luminosity. Phase resolved spectroscopy from a long observation reveals a variable cyclotron line energy, with phase dependence similar to a variety of other pulsars, as well as the first harmonic of the fundamental cyclotron line.