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PS reprint
J. D. Silverman
Smithsonian Astrophysical Observatory,
Cambridge, MA 02138
K. Kearns
Wesleyan University,
Middletown, CT 06459
D. E. Harris
Smithsonian Astrophysical Observatory,
Cambridge, MA 02138
In response to a report (Arp 1995) which concluded that features on an X-ray map were caused by celestial UV emission rather than X-rays, we have initiated a re-evaluation of the so-called `UV-leak' in the ROSAT HRI (David et al. 1995). We believe Arp's results can be attributed to HRI gain effects although there remain unanswered questions about the `UV Leak'.
Based on the `UV leak' of Vega, Arp suggested that the high count rate in PHA 1--3 of spatial regions surrounding NGC 1097 was a result of UV emission rather than real X-rays. From two observations of NGC 1097 separated by six months, it is apparent that the count rate increase in specific regions of the detector is caused by the spatial variation of the gain (mean PHA) across the detector. Therefore, it is necessary to first evaluate the complete spectral distribution of the source or spatial feature. Looking only at the first three PHA channels (as in the case of NGC 1097) could result in a misleading analysis of the observations.
Figure 1 shows an image of the prelaunch gain map of the ROSAT HRI for the element CU.
Figure 1: The ground based spatial gain map for the spectral line of
copper (0.93 keV). This image has been blocked by a factor of
64 thereby producing a scale of 32 arcsec/pix. This image includes
the greyscale of the spatial variation of the gain overlayed with
the contour lines of the mean PHA channel. The contour lines begin
with a PHA of 4.0 in the central dark region an extend outward to a
PHA of 7.0 on the periphery.
Figure 1: PS 125 Kb
PHA distributions in the darker areas will be shifted to lower channels.
The observations of NGC 1097 provide an important means of distinguishing between the effects of the gain map on a detection and the `UV Leak'. Arp has analyzed an HRI detection of NGC 1097 observed in Jan. 1994.
Figure 2: Two observations of NGC 1097 separated by six months. A
smoothed image of a `soft map' for the observation in Jan (a) and
June (b) are shown.
Figure 2: (left) PS 272 Kb,
Figure 2: (right) PS 528 Kb
By looking at the features on a soft map (PHA 1--3, Figure 2a), he suggested that there appears to be emission from spatial features adjacent to the region of NGC 1097 (top right circle). Since these features are not detected in the more sensitive PSPC, Arp attributed the features to emission from UV celestial sources.
In a second observation of NGC 1097 taken six months later, in which
the detector is rotated about 180
from the first
observation, features which Arp interpreted as UV emission are no
longer apparent in that region of the sky. The higher count level on
the `soft map' has now shifted to the opposite side of the field (Figure 2b)
as expected because of the rotation of the detector.
We have compared the spectral distributions for the two opposite regions from the image of NGC 1097 for both observations.
Figure 3: Spectral distribution for the marked regions of the
two observations of NGC 1097. The solid line corresponds to
the top right region while the dashed line corresponds to the lower
left region. These results are consistent with the rotation of the
gain map by 180
.
Figure 3: (left) PS 124 Kb,
Figure 3: (right) PS 124 Kb
In Figure 3a, the top right region with the features described by Arp falls within the low gain area of the detector and shows an increase in the count rate for PHA 1--3. Then in Figure 3b, we see that the increase in low PHA channel counts now occurs on the other half (lower left corner) of the field.
Throughout the mission, there has been evidence that sources with strong UV emission cause higher count rates in the lower pulse height amplitude channels (PHA) of the HRI than predicted. The observed count rates for Vega (0.1 cts/sec) and Beta Car have been shown to significantly exceed the expected values for the HRI. Based on the spectral distribution from IUE measurements, Zombeck (1992) has predicted an HRI count rate of 4.7e-4 cts/sec for Vega. For the PSPC, however, the observed count rate of Vega agrees with the filter transmission measurement and Vega's excess UV emission. From preflight measurements, the peak UV transmission of the UV filter is within the 1500--1700 Angstrom band. Therefore, we might have expected the `UV Leak' photons to produce a PHA distribution which would be indistinguishable from that of the UV calibration lamp (1800 Angstroms). Since the mean PHA of Vega is less than that of the UV cal lamp (David et al.), we are not confident that Vega's observed high count rate can actually be caused by 1500--1700 Angstrom emission.
The `UV Leak' of the ROSAT HRI has motivated an analysis of the contributing factors that affect the spectral distribution of a source. We have shown that the gain map directly affects the PHA distribution of collected photons. Even though Vega and Beta Car have high count rates in the lower PHA channels, it is incorrect to assume that the excess counts in PHA 1--3 are always attributed to UV radiation because of the extremely limited spectral resolution of the HRI. Therefore, it is necessary to determine where on the detector the events occurred and how its count rate is affected by the variation of the gain.
Two questions still remain. Why is the predicted count rate of Vega different from the observed measurements? And why does the spectral distribution of Vega differ from the UV calibration lamp?
This work was supported by NASA contract NAS5-30934.
David, L. D., Harnden Jr., F. R., Kearns, K. E., & Zombeck, M. V. 1995, The ROSAT High Resolution Imager (HRI)
Zombeck M. 1992, internal memo