Detection of hard X-Ray pulsations and a strong iron Kβ emission line during an extended low state of GX 1+4

Abstract

We present here results obtained from a detailed timing and spectral analysis of three BeppoSAX observations of the binary X-ray pulsar GX 1+4 carried out in 1996 August, 1997 March, and 2000 August. In the middle of the 2000 August observation, the source was in a rare low-intensity state that lasted for about 30 hr. Although the source does not show pulsations in the soft X-ray band (1.0-5.5 keV) during the extended low state, pulsations are detected in the 5.5-10.0 keV energy band of the MECS detector and in the hard X-ray energy bands (15-150 keV) of the PDS instrument. Comparing the 2-10 keV flux during this low state with the previously reported low states in GX 1+4, we suggest that the propeller regime in GX 1+4 occurs at a lower mass accretion rate than reported earlier. Broadband (1.0-150 keV) pulse-averaged spectroscopy reveals that the best-fit model comprises a Comptonized continuum along with an iron Kα emission line. A strong iron Kβ emission line is detected for the first time in GX 1+4 during the extended low state of the 2000 observation with an equivalent width of ~550 eV. The optical depth and temperature of the Comptonizing plasma are found to be identical during the high- and low-intensity states, whereas the hydrogen column density and the temperature of the seed photons are higher during the low state. We also present results from pulse-phase-resolved spectroscopy during the high- and low-flux episodes.