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An Example - 3C 120

We received the SASS output for the HRI observation of 3C 120 (a highly variable radio galaxy with a prominent jet leaving the core at PA $\approx 270^{\circ}$) and smoothed the data with a Gaussian of FWHM = 3'' (Figure 13.1). Our initial elation at discovering such strong x-ray emission from the radio jet faded rapidly when analysis demonstrated that most of the emission was unresolved, but that the aspect solution had at least two different solutions, and that the change from one to the other coincided with obi boundaries. The 24 obis comprising the 36.5 ksec exposure were obtained between 96Aug16 and 96Sep12. Because 3C 120 is close to the ecliptic, the roll angle hardly changed, and our first attempts at dewobbling divided the data into 2 'stable roll angle intervals'. This effort made no noticeable improvement. We then used the method described in section 13.2.4. The results are shown in Figure 13.2. It can be seen that a marked improvement has occurred, but some of the E-W smearing remains. Our normal procedure for evaluation is to measure the FWHM of the observed response on a map smoothed with a 3 $^{\prime\prime}$Gaussian. For the best data, we find something close to 5.7 $^{\prime\prime}$ FWHM. While there are many measures of source smearing, we prefer this approach over measuring radial profiles because there is no uncertainty relating to the position of the source center and we rely on visual inspection of the two dimensional image as a check on severe abnormalities. The FWHM of the original data (Figure 13.1) is 11.8 $^{\prime\prime}\times7.4^{\prime\prime}$ (PA $\approx 97^{\circ}$); after dewobbling (Figure 13.2) the width of the 50% contour is 7.6 $^{\prime\prime}\times6.3^{\prime\prime}$. Deconvolving to 'remove to first order' the widening caused by our smoothing function, we find that the largest size has dropped from 11.4 $^{\prime\prime}$ to 7.0 $^{\prime\prime}$ whereas in the orthogonal direction, 6.8 $^{\prime\prime}$ has been reduced to 5.5 $^{\prime\prime}$.

Figure 13.1: The original data, smoothed with a Gaussian of FWHM = 3 $^{\prime\prime}$. The peak value on the map is 70.9 counts per 0.5 $^{\prime\prime}$ pixel. In these units, contour levels are 1, 8, 17, 26, 35.5 (the 50% contour, doubled), 44, 53, and 62.

Figure 13.2: The results after dewobbling. The contour levels are the same as Figure 13.1, but since the peak is now 105.6, the 50% contour is 53.5, and 4 additional contours (always with an interval of 9) have been added.