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ASCA Guest Observer Facility

PSF in the Low Energy Region

H. Ezawa^1, T. Endo^2,
Y. Fukazawa^1, K. Kikuchi^3,
T. Takahashi^2, and the Image Analysis Working group

1 Department of Physics, University of Tokyo,
2 Institute of Space and Astronautical Science,
3 Department of Physics, Tokyo Metropolitan University,

It is well known that the XRT on ASCA has a complex point spread function (PSF) which depends strongly on position. The XRT PSF has been studied in detail based on the multi pointing observations of Cyg X-1 (Takahashi et al. 1995). Based on the comparison with the image of 3C273, they also pointed out that the image of Cyg X-1 in the low energy region (below 2 keV) may be considerably extended. This is probably due to the dust-scattering halo associated with the significant Galactic absorption along the line of sight to Cyg X-1 (see Table 1).

The effective area of ASCA XRT is relatively large below 2 keV, which causes the observed X-ray image to be dominated by low energy photons. It is an important issue to clarify the profile of the PSF in the low energy region for detailed analysis on point sources as well as extended objects. For the purpose, Her X-1 was selected for the calibration source. Her X-1 is a bright point source (at main-on status) located in a high Galactic latitude with small neutral hydrogen column density, which makes the absorption to be much less than that of Cyg X-1. The first Her X-1 observation for the PSF calibration was performed in April 1996, at the period when Her X-1 was at main-on status and in non-eclipse phase. The source position on the detector has been selected to coincide with one of the positions in the past Cyg X-1 observations, which is named `point 0' in Takahashi et al. (1995); hereafter we call it `Cyg X-1 (p0)'.

Table 1
				   Her X-1		 Cyg X-1 (p0)

	Date of Obs.		Apr. 11, 1996		Nov. 11, 1993
         Exposure		   ~20 ksec.		   ~24 ksec.
          (l,b)			(58.149, 37.523)	(71.335, 3.067)
	NH [cm^-2]		 < 4 x 10^20		   ~6 x 10^21

Table 1: Properties and observation log of Her X-1 and Cyg X-1 (p0). Her X-1 keeps higher galactic latitude and smaller HI column density than Cyg X-1. The exposure times are those of after data reduction described in the text.

The observed GIS data was filtered with cut off rigidity over 10 GeV/c and elevation from the earth greater than 5 degrees. The NXB and CXB is subtracted applying the method by Ikebe et al. (1995) after correcting the dead time. The data obtained when the space craft was near the SAA was also reduced to avoid flare-like events seen in the GIS data (Ikebe et al. 1995).

As the first approach, we extracted radial surface brightness profiles of Her X-1 and Cyg X-1 (p0) obtained by the GIS. Figure 1 shows the ratio of radial profiles of the two sources in several energy bands. One can clearly see that the Cyg X-1 image is broader than that of Her X-1 in the low energy region, especially around 1 keV. The source position on the detector of the two observations differs, as a result, for about 1 arcmin. The position difference does not affect the tendency shown in Figure 1; the effect on the radial profile ratio is estimated to be ~ 5%.

The encircled energy function (EEF) based on images from 0.5-1 keV band of Her X-1 and Cyg X-1 (p0) is compared in Figure 2. Each plot is normalized to 1.0 at 6 arcmin from the image center. The difference between the two EEFs can be estimated to be a few percents. Again, the effect of the 1 arcmin position difference is small to affect the difference shown in Figure 2. These two figures suggest that the Her X-1 image is more suitable than the Cyg X-1 image for modeling the low energy PSF, or determining whether the source is `point like', in the low energy region.

We have thus confirmed that the Cyg X-1 images are more extended, in low energies, than the actual PSF, presumably due to the dust scattering halo. In order to further reinforce this conclusion and better qualify the low energy PSF, we are planning to conduct four more pointings onto Her X-1, at those positions which were utilized in the Cyg X-1 observations (point 1, 4, 5, 6 in Takahashi et al. 1995). The observations are planned in February, March, and August 1997, aiming at main-on status of Her X-1.


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Ikebe, Y., et al . ASCA News 3, 13, 1995.
Takahashi, T., et al. ASCA News 3, 34, 1995.

Figure 1 Ratio of radial surface brightness profiles of Her X-1 over Cyg X-1 obtained by GIS. The ratio is normalized to 1.0 when averaged out to 16 arcmin in radius from the image center. Error bars represent the 1 sigma statistical errors. Positions of the sources differ by about 1 arcmin, while the effect caused by the position difference is estimated to be sim 5% in the plots.

Figure 2 Encircled energy function obtained from Her X-1 image (solid line) and Cyg X-1 (dashed line) by GIS in the 0.5-1.0 keV region. The plots are normalized to 1.0 at 6 arcmin in radius from the center. Note that the effect of the 1 arcmin position difference is not considered in the figure, while the effect is estimated to be smaller than the difference shown in the figure.

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