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

We analysed 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 10(-8) ergs cm(-2) s(-1) (3-30 keV), which corresponds to a luminosity of 2-8 x 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 (< 30 keV) in both states. Due 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 similar to 20 ms between 3-4 and 16-20 keV), and much smaller in the bright state (less than or similar to 2 ms). 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.