Adv. Space Res. Vol. 25, No. 314, pp. 391-394.2000 0 2000 COSPAR. Published bv Elsevier Science Ltd. All rights reserved Pergamon www.elsevier.nl/locate/asr Printed in &eat Britain 0273-l 177100 $20.00 + 0.00 HI: SO273-1177(99)00765-6 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP The ASCA Observation of zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA GRO 51744-28 M. Nishiuchi’, K. Koyama’, Y. Maeda’, T. Dotani2, K. Asai2, Y. Ueda2,K. Mitsuda2, F. Nagase2, H. Inoue2, C. Kouveliotou3 ‘Dept. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of Physics, Iiyoto Univ., Kitashirakawa-Oiwake-Cho Sakyo Kyoto 606-8502, Japan mamiko@cr.scphys.kyoto-u.ac.jp 2 InstiW e of Space a,nd Astronautical Science, Yoshinodai, 3-l -l,Sagamihara, h’anagawa 2294510, Japan 3 Universities Space Research Association, at ES-&, NA SA /M SFC, Huntsville, AL 35812, U.S.A. ABSTRACT The ASCA results of the bursting X-ray pulsar GRO J1744-28 are reported. The observations were made twice, in February 1996 and March 1997. We detected 12 and 17 Type II bursts during the two observations, with mean bursting intervals of about 27 min and 37 min. Each burst is followed by an intensity dip with the depleted flux depending on the burst fluence. The energy spectra are approximated by an absorbed power law with additional structure around 6-7 keV, which is most probably due to iron and maybe reproduced by the disk line model with additional broadening mechanism. The absorption column is constant ((5 - 6) x 1O22 cmp2 ), independent of the observation dates and emission phase (persistent, burst and dip). This means the source may be actually located near the galactic center (8.5kpc). 0 2000 COSPAR. Published by Elsevier Science Ltd. 1 INTRODUCTION The discovery of GRO J1744-28 was made with the Burst And Transient Source Experiment (BATSE) on board the Compton Gamma-Ray Observatory (CGRO) on 1995 Dec. 2 (Fishman et al. 1995; Kouveliotou et al. 1996). Finger et al. (1996) concluded that GRO J1744-28 is a low mass X-ray binary system including a neutron star with a pulse frequency of 2.14 Hz and a binary orbital period of 11.83 days. Several estimation of the magnetic fields strength of GRO J1744-28 were made ; Finger et al. (1996) estimated the magnetic field to be < 6 x 1011 G, Cui et al (1996) reported to be - 2.4 x 1O’l G, assuming that the system was in the propeller phase, using the Proportional Counter Array(PCA) on Rossi X-ray Timing Explorer (RXTE). Zhang et al. (1996) discovered Quasi-Periodic Oscillations (QPO) of the X-ray intensity at 20 Hz, 40 Hz and 60 Hz in the PCA data. Since the discovery of GRO J1744-28, several thousands of Type II bursts have been observed. Following some bursts, the flux decreases below the persistent level for a few seconds to a few minutes (to be called hereafter dip), depending on the burst fluence (Swank et al. 1996). A shift in the arrival time of the pulse phase in the bursts was observed by the Oriented Scintillation Spectrometer Experiment (OSSE) on the CGRO (Blanc0 et al. 1996, Strickman et al. 1996) and the PCA on RXTE (Stark et al. 1996) and the BATSE (Koshut et al. 1998). In spite of the stimulating results on the timing analysis summarized above, only limited spectral information has been reported so far (Swank et al. 1996). We therefore performed ASCA observations with a better spectral resolution than so far reported. 2 OBSERVATION AND DATA REDUCTION The ASCA observations of GRO 51744-28 were made twice so far, the first was a TOO (target of opportunity) observation made on 26-27 Feb. 1996 (hereafter observation I), and the second was made on 16 Mar. 1997 (observation II). The bit assignment of GIS telemetry was changed to achieve higher time resolution (2msec). Because GRO J1744-28 was very bright. the deadtime effect became significant (- 391