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ASCA Guest Observer Facility

Recent ASCA IAU Circulars

6415	CYGNUS X-1		T. Dotani, H. Negoro, K. Mitsuda,
                                H. Inoue, and F. Nagase
"We observed Cyg X-1 with ASCA during May 30.23- 31.14 UT with a net exposure time of about 33 000 s. Cyg X-1 was in a high, soft state during the observations. The GIS intensity in the range 1-10 keV was about 1000 counts/s (after deadtime correction), which corresponds to about 1.3 Crab. The observed flux in 1-10 keV was 3.2 x 10E-8 erg sE-1 cmE-2. The energy spectrum can be approximated by a blackbody with a hard tail. The best-fitting temperature of the blackbody is 0.34 +/- 0.02 keV, and the hard tail can be approximated by a power law with a photon index of 2.4 +/- 0.1. The blackbody and the power-law components cross over around 2.3 keV, above which the power-law component becomes dominant."

6481	GRB 960720		T. Murakami, R. Shibata, H. Inoue,
                                F. Nagase, A. Yoshida, N. Kawai, 
                                C. Otani, F. Tokanai,  and J. Heise,
"We observed the error region of the SAX-BATSE gamma-ray burst (IAUC 6467) with ASCA on Sept. 19 with a net exposure of 26~000 s. Only one weak x-ray source was detected within the SAX-WFC error region, whose location is consistent with the reported x-ray source of the ROSAT all-sky survey (IAUC 6469), to an accuracy of within 1' radius. The observed GIS countrate (0.7-10 keV) was very weak, (2.8 +/- 0.6) x 10E-3 count/s. Assuming a power-law model with index -2, and a galactic absorption toward this direction of 2.5 x 10E20 cmE-2, the observed flux was 1 x 10E-13 erg cmE-2 sE-1 in the band 0.5-2 keV and 2 x 10E-13 erg cmE-2 sE-1 in the band 0.5-10 keV. The flux at 0.5-2 keV is roughly consistent with the flux observed with ROSAT before the burst that occurred during 1990 Aug. 22-28 (IAUC 6469)." Corrigenda to IAUC 6480. Line 22, for (2.5 +/- 0.5) read (0.9 +/- 0.2). Line 27, for about 1 x 10E-13 read about 4 x 10E-14. Line 28, for 1 and 3 x 10E-13 read 5 and 10 x 10E-14.
6585	1RXS J170849.0-400910 	M. Sugizaki and F. Nagase, S. Yamauchi,
                                K. Koyama, K. Matsuzaki, University of 

				and K. Torii, Osaka University; and 
                                T. Kotani, Riken
"During the galactic ridge survey with ASCA, we detected a relatively bright, soft source on 1996 Sept. 3.021-3.590 UT at R.A. = 17h08m46s, Decl. = -40o09'27" (equinox 2000.0). This source is identified with the ROSAT source 1RXS J170849.0-400910 within the ASCA position determination accuracy of radius 0'.5. From timing analysis of the source, we discovered coherent pulsations with a barycentric pulse period P = 10.997 +/- 0.001 s. The x-ray pulse profile shows a broad sinusoidal shape with a pulse fraction of about 30 percent. The energy spectrum in the range 1-10 keV is very soft and can be fitted by a power-law model with photon index 3.5 and an absorption column density 1.8 x 10E22 cmE-2. The energy spectrum can also be fitted with an optically thin thermal bremsstrahlung model with kT = 1.8 keV, although no emission line of iron is found with an upper limit of equivalent width 500 eV (either for a 6.4-, 6.6-, or 6.9-keV line; 90-percent confidence limit). The pulse-phase averaged flux in the range 1-10 keV is about 1.2 x 10E-10 cmE-2 sE-1 after correcting for soft x-ray absorption. From the 11-s period and the soft spectrum, we suspect this source to be the sixth member of a class of braking x-ray pulsars (Mereghetti and Stella 1995, Ap.J. 442, L17; van Paradijs et al. 1995, A.Ap. 299, L41)."

A. Yoshida, N. Kawai, C. Otani, and F. Tokanai, H. Inoue,

T. Murakami, F. Nagase, R. Shibata, Y. Ueda, S. Holt, L. Piro,

E. Costa J. Heise, and J. Hughes

"ASCA observed the SAX burst position of GRB 970228 (IAUC 6572) on Mar. 7.028-7.486 UT with a net exposure time of 20 000 s. The data displayed a weak excess in both GIS and SIS images of the revised BeppoSAX error region (IAUC 6576). Image fitting was done to both the GIS and SIS data sets, assuming a point source. The source's location was found to be consistent with that of SAX J0501.7+1146, considering statistical errors and the attitude uncertainty of ASCA. The 2-10- keV flux and its 1-sigma statistical error are estimated to be (9.0 +/- 2.6) x 10E-14 erg cmE-2 sE-1 for the GIS and (7.2 +/- 2.1) x 10E-14 erg cmE-2 sE-1 for the SIS, assuming a power-law spectrum with a photon index of 1.4."

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