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

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 (