ASCA and Ginga Observations of GX 1+4

The X-ray binary pulsar GX 1+4 was observed with Ginga every year from 1987 to 1991 and with ASCA in 1994. During the Ginga observations, GX 1+4 was in the steady spin-down phase, although the X-ray flux was not steady. Assuming a distance of 10 kpc, the absorption-corrected X-ray luminosity decreased down to L_2-20keV=2.7×10³⁶ ergs s^-1 in 1991, after the peak activity of L_2-20keV=1.2×10³⁷ ergs s^-1 in 1989. On the other hand, the absorption column density showed a drastic increase over the Ginga observation series. It was less than 10²³ cm^-2 at the beginning of the series, and it reached a maximum of (1.4+/-0.2)×10²⁴ cm^-2 in 1991, indicating a rapid accumulation of matter in the vicinity of the source. The center energy and equivalent width of the iron line were consistent with emission by isotropically distributed cold matter. The ASCA observation was performed on 1994 September 15, a month before the transition into a spin-up phase. The source brightened again to L_2-20keV=10³⁷ ergs s^-1. The absorption column density was observed to decrease for the first time to (2.08+/-0.02)×10²³ cm^-2. The ionization degree of iron in the absorbing matter was determined to be Fe I-Fe IV using the ratio of the line-center energy to the absorption-edge energy. The low ionization degree is consistent with an absorbing matter distribution extending ~10¹² cm from the source. We compared the results with optical observations and found that the optical data also supports the picture. Based on the geometrical model, possible causes of the bimodal behavior of the source are discussed.