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ASCA Guest Observer Facility

Recent ASCA IAU Circulars

This is a continuation of the list started in ASCANews #1 and continued in #2.

X-ray Nova in Scorpius
H. Inoue, F. Nagase, M. Ishida, T. Sonobe, and Y. Ueda

This object was observed with ASCA during Aug. 23.47-23.93 UT. The location of the x-ray source is R.A. = 16h53m57s.0, Decl. = -39o50'55" (equinox 2000.0), close to the positions of the optical nova (IAUC 6050) and radio emission (IAUC 6055), within the systematic uncertainty of the ASCA image (error circle radius 1'). The flux in the range 1-10 keV changed by a factor of four, starting at 80 mCrab and increasing from 2 to 8 x 10E-9 erg cmE-2 sE-1 during the observation. No apparent pulsation was found in the period range 1-1000 s. The spectrum in the range below 7 keV of the low-intensity state is represented by a power-law spectrum with photon index of about 2.1 and interstellar absorption column density of 5 x 10E21 cmE-2. An iron absorption edge is visible at 7.1 keV. An iron emission line around 6-7 keV and fine structures in the range 1-4 keV are possibly seen in the spectrum. The spectrum in the high-intensity state is similar but a little harder than that in the low-intensity state.

GRS 1915+105 and X-ray Nova in Scorpius
F. Nagase, H. Inoue, T. Kotani, and Y. Ueda

We observed the two x-ray novae GRS 1915+105 and GRO J1655-40 with ASCA on Sept. 27-28, with exposure times of 20000 s each. Both sources were bright in the ASCA soft x-ray band during the observations. Preliminary analyses reveal that the superluminal x-ray transient source GRS 1915+105, which was observed during Sept. 27.03-27.54 UT, showed an intensity variation that was small within +/-10 percent of the average flux over time scales from minutes to hours. The average flux in the range 2-10 keV was 1.0 x 10E-8 erg cmE-2 sE-1 (or about 500 mCrab). The GIS and SIS spectra in the range 1-10 keV can be roughly represented by a power-law model with a photon index of about 2.2 and an absorption column density of 4.3 x 10E22 cmE-2, with significant spectral softening at high energy. A weak edge-like structure is visible around 7 keV; however, no conspicuous lines are seen in the spectra.

The x-ray nova in Scorpius (GRO J1655-40) was again observed with ASCA during Sept. 27.54-28.18. The intensity variation was small during the observation (< +/-20 percent), in contrast with the large intensity variation in the previous observation on Aug. 23 (IAUC 6063). The average flux in the range 2-10 keV was 7.7 x 10E-9 erg cmE-2 sE-1 (or about 400 mCrab). The GIS and SIS spectra in the range 1-10 keV can be roughly represented by a power-law spectrum with a photon index of about 2.0 and an interstellar absorption column density of 4.4 x 10E21 cmE-2, with significant spectral softening at high energy. A weak but significant structure is seen around 6-7 keV.

EXO 1846-031
H. Inoue, F. Nagase, M. Ishida and T. Sonobe

Following the report by Zhang et al. (IAUC6096), ASCA observed EXO 1846-031 between Oct. 22.326 and 22.785 UT with an exposure time of about 12 000 s. No bright x-ray source was seen in the ASCA GIS field within 20' of R.A. = 18h49m.1, Decl. = -3 deg 08' (equinox 2000.0). The 5-sigma upper limit of the 2-10-keV flux at the position of EXO 1846-031 (Parmar et al. 1993, A.Ap. 279, 179) is 0.01 mCrab (2x10**-13 erg/cm**2/s, assuming the Crab Nebula spectrum). The upper limit increases to 0.04 mCrab at the rim of the GIS field. This suggests either that the outburst of EXO 1846-031 decayed very quickly or that the outburst detected by BATSE was not from this source.

OJ 287
H. Inoue, F. Nagase, F. Makino, C. Otani, R. Fujimoto, and Y. Ueda, M. Tashiro, T. Takahashi, and E. Idesawa

We observed OJ 287 with ASCA between Nov. 18.42 and 19.62 UT, with an exposure time of about 40~000 s, and detected significant x-ray emission from the source. The flux was then (4.2 +/- 0.2) x 10E-12 erg cmE-2 sE-1 in the range 2-10 keV, which is smaller by about a factor of two than when seen with the Einstein Observatory in 1979 and 1980 (Madjeski and Schwartz 1988, Ap.J. 330, 776), in spite of the current high luminosity at optical bands (IAUC 6095). The intensity variation during our observation was < 30 percent on time scales from 100 min to a day. The 0.5- to 10-keV spectrum is fit by a power-law spectrum with photon index 1.7 +/- 0.1 and with interstellar-absorption column density < 1 x 10E21 cmE-2.

R. Corbet and A. Smale, P. Charles and K. Southwell

The recurrent x-ray transient A0538-66 (16.65-day period) remains active although at a low level. ASCA observations on Feb. 3 (predicted x-ray maximum) and 4 gave GIS count rates of about 0.1 and 0.05 counts/s, respectively. The estimated luminosity of 2 x 10E36 erg/s is much less than the largest outbursts previously observed. Photometry from the South African Astronomical Observatory yielded the following (conservative errors of 0.03 mag): Jan. 31, V = 15.05; Feb. 1, 14.67; 2, 14.99; 3, 14.86; 4, 14.98; 5, 15.04; 6, 15.04. Spectra obtained at the Anglo-Australian Telescope on 1994 Nov. 29, shortly after a previous predicted outburst, showed variable Balmer emission but not the He II 468.6-nm emission that accompanies large outbursts. Optical observations around future predicted maxima are encouraged.

Markarian 421
T. Takahashi, F. Makino, T. Kii, M. Tashiro, H. Kubo, K. Makishima, T. Ohashi, N. Y. Yamasaki, and G. Madejski,

The BL Lac object Markarian 421 was observed from the x-ray astronomy satellite ASCA during Apr. 25.8-26.1 UT. The preliminary energy flux obtained from GIS and SIS abroad ASCA was 1.8 x 10E-10 erg cmE-2 sE-1 (2-10 keV), which is almost the same as that observed in the previous x-ray/TeV flare in May 1994 (IAUC 5993, 5996). The photon index was about 2.5. Further x-ray observations were planned for Apr. 28, 29, May 1, 3, 5, 6, and 8. Observations at optical, infrared, radio, and other wavelengths are encouraged. For more details of the x-ray observation schedule, contact T. Takahashi (e-mail

GRS 1915+105
K. Ebisawa, N. E. White, T. Kotani, and A. Harmon

The Galactic superluminal jet source GRS 1915+105 (Mirabel and Rodriguez 1994, Nature, 371, 46) was observed to be unusually bright with the ASCA satellite on Apr. 20.4-20.9. The flux in the band 2-10 keV was about 5 x 10E-9 erg cmE-2 sE-1 or 0.26 Crab. The spectrum is well fit by a power-law with photon index 1.4 between 1 and 5 keV, and it is significantly absorbed by cold matter corresponding to a hydrogen column density of 5 x 10E22 cmE-2. Above 5 keV, the spectrum decays exponentially with an e-folding energy of 5 keV. The near simultaneous BATSE flux in the band 20- 100 keV was < 50 mCrab, confirming the strong high-energy cut-off. The ASCA spectrum and the BATSE flux are similar to those in the 1994 Sept. observations, and the overall spectrum seems much softer than that during the 1992-1993 outburst, when the source was bright in both soft and hard x-rays. Follow-up x-ray and radio observations are encouraged.

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