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The "ACIS stowed" background measurements (Aug and Sep 2002) presented an opportunity to collect additional calibration data by running the HRC-I in "Next In Line" (NIL) mode with a large offset pointing. The constraints were:
RXJ1856.5-3754, an isolated neutron star, was chosen as the target:
Three observations were made, on Aug 6, Sep 3, and Sep 23 2002. The first was short (10 ksec), to test that all the ACIS parameters were OK. The second comprised the remainder of the nominal 70 ks total, but only the first 32 ksec of HRC-I data were useful because of flaring, which saturated the telemetry, leading to loss of event timing and therefore aspect corrections. (A bug in the AP assignment of otherwise valid event times was also noted and corrected by Mike Juda.) The third short observation (7 ksec, 5 ksec without flaring) was made on-axis for normalization.
|4286||25.62'||10 ks||0.984 +/- 0.008|
|4288||0||5 ks||1.803 +/- 0.020|
Quoted errors are statistical only, and do not include possible QE nonuniformity. Measured rates are in ~good agreement with predictions based on previous HRC-S/LETG observations, but spectral model uncertainties are still being investigated.
Measured vignetting factor for +25.62' is 0.546 +/- 0.008, versus raytrace prediction of 0.545.
70-ksec raytrace simulation, without background
32 ksec of data (obsid 4287). Ghost rays are barely visible.
Raytrace group is currently studying the PSF.
There are roughly 10000 counts in each of the two densest regions of the PSF, which is enough counts to study QE uniformity near 250 eV on few-mm scales near the HRC-I center. Analysis in progress...
Last modified: 11/02/10
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