Spectral and timing analysis of the bursting pulsar GRO J1744-28 with RXTE observations

TL;DR

This study analyzes RXTE observations of the pulsar GRO J1744-28, revealing a flux-dependent transition in spectral and timing properties, including changes in spectral hardness, pulsed fraction, and X-ray lags, linked to accretion structure variations.

Contribution

It presents the first detailed analysis of spectral-timing transitions in GRO J1744-28, identifying flux thresholds and associated changes in pulsar behavior.

Findings

Spectral hardness increases with flux in the faint state.

Pulsed fraction correlates positively with flux and energy.

Hard X-ray lag is significant only in the faint state.

Abstract

We analyzed RXTE/PCA observations of the bursting pulsar GRO J1744-28 during its two outbursts in 1995-1997. We have found a significant transition, i.e., a sharp change, of both spectral and timing properties around a flux of $\rm 10^{-8}\ ergs\ cm^{-2}\ s^{-1}$ (3-30 keV), which corresponds to a luminosity of 2--8$\rm \times 10^{37}\ ergs\ s^{-1}$ for a distance of 4--8 kpc. We define the faint (bright) state when the flux is smaller (larger) than this threshold. In the faint state, the spectral hardness increases significantly with the increasing flux, while remaining almost constant in the bright state. In addition, we find that the pulsed fraction is positively related to both the flux and the energy (<30keV) in both states. Thanks to the very stable pulse profile shape in all energy bands, a hard X-ray lag could be measured. This lag is only significant in the faint state (reaching $\sim$ 20\,ms between 3-4 keV and 16-20 keV), and much smaller in the bright state ($\lesssim$2ms). We speculate that the discovered transition might be caused by changes of the accretion structure on the surface of the neutron star, and the hard X-ray lags are due to a geometry effect.