BeppoSAX observations of GRO J1744-28

TL;DR

This paper analyzes BeppoSAX observations of GRO J1744-28, revealing its X-ray spectrum features, including a cyclotron line indicating magnetic field strength, and discusses the nature of burst emissions and thermal components.

Contribution

It provides the first detection of a cyclotron line in GRO J1744-28, estimating the magnetic field, and explores the spectral characteristics during outburst decay.

Findings

Detection of a cyclotron line at ~4.5 keV indicating a magnetic field of ~3.7 x 10^11 G.

Identification of a soft thermal component during bursts possibly linked to thermonuclear flashes.

Persistent X-ray continuum consistent with a cutoff power law typical for accreting pulsars.

Abstract

We present an analysis of BeppoSAX observations of the unique transient bursting X-ray pulsar GRO J1744-28. The observations took place in March 1997 during the decay phase of the outburst. We find that the persistent broadband X-ray continuum of the source is consistent with a cutoff power law typical for the accreting pulsars. We also detect the fluorescence iron line at 6.7 keV and an absorption feature at ~4.5 keV, which we interpret as a cyclotron line. The corresponding magnetic field strength in the line forming region is ~3.7 x 10^11 G. Neither line is detected in the spectra of the bursts. However, additional soft thermal component with kT ~2 keV was required to describe the burst spectrum. We briefly discuss the nature of this component and argue that among other possibilities it might be connected with thermonuclear flashes at the neutron star surface which accompany the accretion-powered bursts in the source.