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ASCA Guest Observer Facility

Recent ASCA IAU Circulars

6205  X-ray Nova 1994 in Scorpius   B. A. Harmon, W. S. Paciesas, 
				    and G. J. Fishman
The radio-jet transient GRO J1655-40, which has been outburst since late
July (IAUC 6196, 6201), reached a peak intensity in the band 20-100 keV of 650
+/- 30 mCrab on Aug. 1, decreasing to 80 +/- 20 mCrab as of Aug. 9.

H. Inoue, F. Nagase, and Y. Ueda ASCA will observe this source during Aug. 15.45- 16.18 UT, to investigate jet activity. Simultaneous optical and radio observations are encouraged.

6210 X-ray Nova 1994 in Scorpius H. Inoue, F. Nagase, and Y. Ueda ASCA observed GRO J1655-40 during Aug. 15.45-16.18 UT. The average flux in the range 2-10 keV was 3.4 x 10-8 erg cm-2 s-1 (about 1.6 Crab, remaining within +/- 10 percent of this throughout), and was about 3 times as bright as that in the previous ASCA observation in 1994 Sept. (IAUC 6094). The preliminary spectral analysis shows that the 1- to 10-keV spectrum can be reproduced by a power law with an exponential cutoff and with a photoelectric absorption. The best-fit photon index, folding energy, and absorption column are 0.5, 2 keV, and 8 x 1021 cm-2, respectively.

6241 GRO J1750-27 T. Dotani, R. Fujimoto, F. Nagase, and H. Inoue We observed the region of GRO J1750-27 (cf. IAUC 6222) with ASCA between Sept. 25.36 and 26.05 UT. The pulsar was detected near the edge of ASCA's GIS field-of-view (and outside the SIS field-of-view), near the northwest corner of the BATSE error box (source position R.A. = 17h49m12s.0, Decl. = -26o38'50"; equinox 2000.0; uncertainty radius 2'). The average x-ray flux of the source was 6 x 10-10 erg cm-2 s-1 in the band 2-10 keV. We detected clear pulsation with a period of 4.452 +/- 0.006 s (corrected for the solar-system barycenter), which confirms the source to be GRO J1750-27. The energy spectrum is approximated by a power law with a photon index of 0.8 and an absorption column of 2 x 1022 cm-2.

6260 AX J2315-592 K. Misaki, Y. Kamata, Y. Terashima, K. Isobe, H. Kunieda, Y. Tawara, Nagoya University; T. Tsuru, Y. Maeda, M. Sakano, M. Akiyama, and H. Sogawa, Kyoto University; and M. Ishida and R. Fujimoto ASCA discovered a new x-ray source during the observation of IRAS 23128-5919 carried out during Nov. 2.55-3.87 UT. The new source is located at R.A. = 23h15m18s, Decl. = -59o10'.7 (equinox 2000.0; error circle 1' radius). There are two optical stars (at 500 nm) of mag 16-17 within this circle in the online chart of 'Sky View' (provided by NASA). Unfortunately, the source was out of the field-of-view of the SIS, and only the GIS data are available. The average x-ray flux is 2.3 x 10-11 erg cm-2 s-1, or 1.1 mCrab, at 2-10 keV. No x-ray source with this intensity is known in the HEAO-1 catalogue, GINGA archival data, ROSAT WFC catalogue, or SIMBAD. The new source AX J2315-592 shows a strong periodic x-ray modulation with period 5360 +/- 100 s (or 1.49 +/- 0.03 hr). The lightcurve is characterized by a deep and narrow intensity minimum, which lasts for 20 percent of the period. The deficit of the countrate is approximately 90 and 70 percent of the peak intensity in the energy ranges 0.7-2.4 and 2.4-10 keV, respectively. The average spectrum is hard and is represented either by a power law with photon index between 1.3 and 1.4 or by thermal bremsstrahlung with a temperature of > 20 keV. The absorption is small and the line-of-sight hydrogen column density is < 1 x 1021 cm-2. A prominent iron emission line is detected at 6.79 +/- 0.06 keV with an equivalent width of about 1000 eV. The line is broad with 1- sigma width of 200-400 eV, suggestive of a blend of thermal and fluorescent emissions. From the lightcurve and spectral characteristics, AX J2315-592 is probably a new AM Her-type cataclysmic variable. Follow-up optical observations (spectroscopy and polarimetry) are encouraged.

6302 3C 279 F. Makino, H. Inoue, T. Kii, F. Nagase, and A. Yamashita, Following the report of a gamma-ray flare of 3C 279 (IAUC 6294), ASCA observed this region during Jan. 27-28. From a quick-look analysis, the 2- to 10-keV flux is estimated to be 1.2 x 10-11 erg cm-2 s-1. The spectrum is consistent with a power law with a photon index of about 1.7. According to the previous ASCA observations, the 2- to 10-keV flux was (6-9) x 10-12 erg cm-2 s-1 from the middle of 1993 to the beginning of 1995, and was (1.0-1.6) x 10-11 erg cm-2 s-1 in the middle of 1995. 3C 279 seems to have shown no large intensity change in the band 2-10 keV since the middle of 1995, in spite of the recent gamma-ray flaring.

6337 GRO J1744-28 T. Dotani, Y. Ueda, M. Ishida, F. Nagase, H. Inoue and Y. Saitoh, We observed GRO J1744-28 with ASCA between Feb. 26.42 and 27.09 UT with a net exposure time of about 20 000 s. We detected a bright bursting/pulsating source at a location consistent with the VLA position (IAUC 6323). The GIS intensity of the persistent component in the range 1-10 keV was about 130 counts/s (after deadtime correction). The energy spectrum of the persistent emission is hard, is contaminated by a large soft x-ray absorption, and can be approximated by a power law with photon index alpha = 1.22 and absorbing column density NH = 5.1 x 1022 cm-2. In addition, a structure is seen that appears as a broad hump around 6 keV and an edge at 7 keV and that can be attributed to a highly absorbed component. Assuming a cosmic abundance of iron, one requires a column density of 2 x 1023 cm-2 to explain the 7-keV edge structure. With the inclusion of this highly-absorbed component, the photon index of the power-law component becomes a little steeper, alpha about 1.36 +/- 0.02, with NH = (5.3 +/- 0.1) x 1022 cm-2. The average flux of the persistent component was 1.9 x 10-8 erg s-1 cm-2 in the range 2-10 keV, which corresponds to about 900 mCrab. The apparent barycentric pulse period was found to be 0.467033(5) s on Feb. 26.76 UT. The pulse profile is almost sinusoidal with a single peak, and the pulse fraction in the band 2-10 keV is about 10 percent (peak-to-peak). After correction for the orbital doppler effect using the parameters on IAUC 6286, the intrinsic pulse period at this epoch is estimated to be 0.467039 s. A total of nine bursts was detected during about 16 000 s of high- time-resolution data. The duration of bursts is about 10 s, and their peak flux is an order of magnitude larger than the persistent flux. After the decay of the bursts, the persistent flux decreased by 30 percent and then returned to the preburst level within a few hundred seconds. The energy spectrum of bursts is similar to, but slightly harder than, the persistent emission. The burst spectrum can be well fitted by a power law with alpha = 1.01 +/- 0.04 and NH = (6.1 +/- 0.2) x 1022 cm-2.

6339 GRO J1744-28 A. J. Castro-Tirado, R. Gonzalez-Delgado, E. Perez, and J. M. Rodriguez-Espinosa On Mar. 3 and 5, we observed the entire XTE error box of GRO J1744-28 (IAUC 6309) with the 3.5-m telescope (+ MAGIC in high-resolution mode, 0".31 per pixel) at the German- Spanish Calar Alto Observatory. Images were taken using two narrow-band filters centered at 2.166 microns (Brackett-gamma, width 0.022 microns) and 2.26 microns (continuum K, width 0.060 microns). Co-added images (total exposures 1560 s at 2.166 microns and 250 s at 2.26 microns) were obtained in order to search for Br- gamma emission-line objects within the XTE error box. No such objects brighter than K = 14 were found within the XTE error box or at the position of the variable radio source (IAUC 6307, 6323) that has been recently confirmed by ASCA (IAUC 6337) as the radio counterpart of GRO J1744-28. Taking into account the optical/ infrared astrometry (IAUC 6315, 6318) and the large N(H) value derived by ASCA, it seems that 'star C' (K = 13.3, IAUC 6318, 6321) is a foreground star about 0".5 away from GRO J1744-28 (similar to GX 17+2, whose optical counterpart is almost coincident with the position of a foreground G star; Naylor et al. 1991, MNRAS 252, 203). For a source distance of 7 kpc, we estimate 13.5 < K < 17.5. Therefore, only deep, high-resolution, and preferably thermal imaging of GRO J1744-28 while in outburst may reveal the infrared counterpart.

6340 GK PERSEI M. Ishida, A. Yamashita, H. Ozawa, F. Nagase, and H. Inoue ASCA observed GK Per in outburst during Mar. 4.33-4.87 UT in the energy band 0.5-10 keV, during the period that the optical magnitude was in the range V = 11.8-12.1. The mean counting rates of the SIS and GIS sensors were 1.1 and 1.3 counts/s, respectively. Sinusoidal intensity modulation is clearly seen with a period of 351 s, and the amplitude of the sinusoid is about 30 percent in the band 0.7-10 keV. A periodic intensity variation with a typical time scale of several thousands of seconds is also observed. The phase-averaged spectra of both the SIS and GIS are well represented by a thermal bremsstrahlung continuum with a temperature 18 (+6/-4) keV, plus a fluorescent iron emission line centered at 6.40 +/- 0.02 keV with an equivalent width of 100 +/- 30 eV. No evidence of thermal plasma emission lines is obtained. Both of the spectra show a heavy absorption feature that can be approximated by the sum of three components of different absorptions with hydrogen column densities of 50, 6, and 1 x 1022 cm-2. The covering fractions of the three components over the x- ray emission region are 75, 24, and 1 percent, respectively. The x-ray flux at 2-10 keV is 1.9 x 10-10 erg cm-2 s-1, corresponding to 10 mCrab; assuming a distance of 525 pc to GK Per, we obtain an x-ray luminosity of 6 x 1033 erg/s in this energy range. Correcting for the absorption, the bolometric luminosity is estimated to be as large as 3 x 1034 erg/s."

6368 GRO J1744-28 T. Dotani, Y. Ueda, F. Nagase, H. Inoue, and C. Kouveliotou Further analysis of ASCA data of GRO J1744-28 obtained between Feb. 26.42 and 27.09 UT yields the source position at R.A. = 17h44m31s.5, Decl. = -28o44'26" (equinox 2000.0), with a 90-percent-confidence error radius of 1'. The source position was also cross-checked with reference to the Einstein source 2E 1743.1-2842, which was detected in the same field of view. The result is fully consistent with the above coordinates, taking into account the IPC error radius of 51" for 2E 1743.1-2842. The ASCA error circle overlaps the XTE error region (IAUC 6309), but the VLA radio source (IAUC 6323) lies slightly outside the ASCA error circle.

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