7 Examples

7.1 3C 120

3C 120 is a nearby radio galaxy (z=0.033) with a prominent radio jet leaving the core at PA $\approx 270^{\circ}$. The ROSAT HRI observation was obtained in two segments, each of which had aspect problems. Since the average source count rate is 0.8 count/s, the X-ray emission is known to be highly variable (and therefore most of its flux must be unresolved), and each segment consisted of many OBIs, we used these observations for testing the dewobbling scripts.

7.1.1 Segment A: Two aspect solutions, both found multiple times

  The smoothed data (Fig. 2) indicated that in addition to the X-ray core, a second component was present, perhaps associated with the bright radio knot 4$^{\prime\prime}$ west of the core. When analyzing these two components for variability, it was 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 usually coincided with OBI boundaries. The guide star configuration table showed that a reacquisition coincided with the change of solution.

  

Figure 2: The original data for 3C 120 (segment A, rh702080n00), 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. Contour levels are 1, 10, 20, 30, ... 90% of the peak value, with the 50% contour, doubled. The nominal roll angle is $-167^{\circ}$ and the wobble direction is at PA = 122$^{\circ}$. The FWHM of this smoothed image is $11.6^{\prime\prime}\times 7.4 ^{\prime\prime}$

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 Sect. 4.1. The results are shown in Fig. 3. It can be seen that a marked improvement has occurred, but some of the E-W smearing remains.

  

Figure 3: The results after dewobbling 3C 120A, smoothed with a Gaussian of $FWHM = 3^{\prime\prime}$. The peak value on the map is now 104.8 counts per 0.5$^{\prime\prime}$ pixel. Contour levels are 1, 10, 20, 30, ... 90% of the peak value, with the 50% contour, doubled. The FWHM of this smoothed image is 8.1$^{\prime\prime}~\times$ 6.7$^{\prime\prime}$

7.1.2 Segment B: A single displaced OBI

  The second segment of the 3C 120 observation was obtained in 1997 March. In this case, only one OBI out of 17 was displaced. It was positioned 10$^{\prime\prime}$ to the north of the other positions, producing a low level extension (see Fig. 4). After dewobbling, that feature is gone, the half power size is reduced, and the peak value is larger (Fig. 5).

  

Figure 4: The original data of 3C 120 (segment B, rh702080a01), smoothed with a Gaussian of $FWHM = 3^{\prime\prime}$. The peak value on the map is 45.8 counts per 0.5$^{\prime\prime}$ pixel. The contour levels are the same percentage values as those of Fig. 2. The roll angle is 8$^{\circ}$ and the wobble PA is 127$^{\circ}$. FWHM for this image is 8.0$^{\prime\prime}~\times$ 6.7$^{\prime\prime}$

  

Figure 5: The results of 3C 120 (segment B) after dewobbling. The contour levels are the same percentage values as those of Fig. 4, but the peak is now 55.4. The FWHM is 7.2$^{\prime\prime}~\times$ 6.5$^{\prime\prime}$

7.2 M 81

M 81 is dominated by an unresolved nuclear source. The count rate is 0.31 count/s. The observation has 14 OBIs for a total exposure of 19.9 ks. Figure 6 shows the data from SASS processing. After running the "OBI by OBI" method, the source is more circularly symmetric, has a higher peak value, and a smaller FWHM (Fig. 7).

  

Figure 6: The original M 81 data (rh600739), smoothed with a Gaussian of $FWHM = 3^{\prime\prime}$. The peak value on the map is 15.3 counts per 0.5$^{\prime\prime}$ pixel. The contour levels are 1, 10, 20, 30, 40, 50 (the 50% contour, doubled), 60, 70, 80, and 90 percent of the peak value. The nominal roll angle is 135$^{\circ}$ and the wobble direction is 0$^{\circ}$. The FWHM of this smoothed image is 10.4$^{\prime\prime}~\times$ 7.5$^{\prime\prime}$

  

Figure 7: The results after dewobbling of M 81 smoothed with a Gaussian of $FWHM = 3^{\prime\prime}$. The peak value on the map is 22.5 counts per 0.5$^{\prime\prime}$ pixel. The contour levels are 1, 10, 20, 30, 40, 50 (the 50% contour, doubled), 60, 70, 80, and 90 percent of the peak value. Ten phase bins have been used. The FWHM of this smoothed image is 7.2$^{\prime\prime}~\times$ 6.5$^{\prime\prime}$

7.3 NGC 5548

This source was observed from 25 June to 11 July 1995 for a livetime of 53 ks with 33 OBIs. The average count rate was 0.75 counts/s and the original data had a $FWHM = 8.2^{\prime\prime}\times 6.8^{\prime\prime}$. Most of the OBIs appeared to have a normal PRF but a few displayed high distortion. After applying the OBI by OBI method, the resulting FWHM was 6.3$^{\prime\prime}$ in both directions and the peak value on the smoothed map increased from 138 to 183 counts per 0.5$^{\prime\prime}$ pixel.

7.4 RZ Eri

The observation of this star was reduced in MIDAS/EXSAS. The source has a count rate of 0.12 count/s. The reduction selected only a group of the OBIs which comprised a "stable roll angle interval"; almost half the data were rejected. The original smoothed image had a $FWHM = 8.4^{\prime\prime}\times 6.6^{\prime\prime}$. After dewobbling, the resulting FWHM was $6.9^{\prime\prime}\times 5.8^{\prime\prime}$.