Astron. Nachr. 320 (1999) 4/5 333 X-ray light curve of SN1987A observed with ASCA M. Itoh 1 , K. Asai 2 , S. Uno 3 , H. Inoue 4 , K. Nomoto 5 , T. Suzuki 5 , K. Masai 6 , F. Marshall 7 , S. Kumagai 8 and Y. Tanaka 9 1 Kobe University, Tsurukabuto 3-11, Nada-ku, Kobe 657-8501, Japan 2 Kanagawa University, Rokkakubashi, 3-27-1, Kanagawa-ku, Yokohama 221-8686, Japan 3 Nihon Fukushi University, Higashihaemi-cho 26-2, Handa-shi, Aichi 475-0012, Japan 4 ISAS, Yoshinodai 3-1-1, Sagamihara, Kanagawa 229-8510 , Japan 5 University of Tokyo, Hongo 7-3-1, Bunkyo Ward, Tokyo 113-0033, Japan 6 Tokyo Metropolitan University, Minami-Ohsawa 1-1, Hachioji, Tokyo 192-0397, Japan 7 Code 662, NASA/GSFC, Greenbelt, MD 20771, USA 8 Nihon University, Kanda-surugadai 1-8, Chiyoda-ku, Tokyo 101-8308, JAPAN 9 Max-Planck-Institut f¨ ur Extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany 1. Introduction The ejecta of the SN1987A is expanding with a speed of ∼10000 km sec -1 currently interacting with the gas in- side the inner ring of the circumstellar material (CSM). ASCA has been observing SN1987A almost once a year, and significant flux has been detected since 1995. Here, we briefly report the results of the X-ray observations with ASCA up to November, 1998. 2. Results X-ray light curve is shown in Fig.1. The flux has been steadily increasing, which is consistent with the ROSAT result reported by Hasinger et al. (1996). The observed light curve is consistent with the model assuming con- stant density of the CSM. From comparison with the model by Nomoto and Suzuki (1997), CSM density is estimated to be 40-50 amu cm -3 . Thermal bremsstrahlung fit to the spectrum from each observation for this period gives the best fit tem- perature of ∼ 2 keV, and no statistically significant vari- ation has been observed. Sum of the SIS spectra obtained in 1995-1998 is shown in Fig.2. The spectrum can be better fit with a thermal emission model with the ionization non- equilibrium effect than with the equilibrium model. In Fig.2, the best fit model is shown where the metal abun- dances are fixed to be 1/3 of solar. The best fit param- eters are kT=1.8 (1.3, 2.5) keV, log(nt/sec· cm -3 )=9.8 (9.5, 10.0), and N H =4.9 (4.2, 5.9)×10 21 cm -2 , where the values in the parentheses indicate the 90% confi- dence regions. The χ 2 /d.o.f.=1.26 for this model. To check the possibility that the X-rays are from a possible pulsar, we searched for periodicity in the GIS signal. The result so far is negative. Further monitoring of the X-ray will give us unique information on the distribution of the CSM. It is also important because the ejecta is reaching the ring in the near future. Spectroscopic observations with higher sen- sitivity by ASTRO-E will give us valuable information such as the elemental abundances of the X-ray emitting plasma and its bulk motion. Fig. 1. X-ray light curve (0.5-10keV) of SN1987A observed with ASCA. Fig. 2. Average X-ray spectrum of SN1987A observed with ASCA for the period of 1995-1998. References Hasinger, G., Aschenbach, B., Tr¨ umper, J.: 1996 Astron. As- trophys. 312, 9 Nomoto, K., Suzuki, T.: 1993, in X-ray Imaging and Spec- troscopy of Cosmic Hot Plasmas, eds. F. Makino, K. Mit- suda, (Universal Academy Press, Tokyo), p. 347