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

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 August 1996, March 1997, and August 2000. In the middle of the August 2000 observation, the source was in a rare low intensity state that lasted for about 30 hours. Though 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 5.5–10.0 keV energy band of the MECS detector and in 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. Broad-band (1.0–150 keV) pulse averaged spectroscopy reveals that the best-fit model comprises of 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 2000 observation with 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. Subject headings: stars : neutron — Pulsars : individual (GX 1+4) — X-rays : stars Department of Physics, University College Cork, Cork, Ireland, [email protected], [email protected] Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India, [email protected]