Kotani, T and Dotani, T and Nagase, F and Greenhill, JG and Pravdo, SH and Angelini, L, ASCA and Ginga Observations of GX 1+4, The Astrophysical Journal, 510, (Jan 1) pp. 369-378. ISSN 0004-637X (1999) [Refereed Article]
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 L2-20 keV = 2.7 × 1036 ergs s-1 in 1991, after the peak activity of L2-20 keV = 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 L2-20 keV = 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.