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:

• SIM_Z = +70 to +100 mm
• HRC-I NIL telemetry limit = 3.5 events/sec
• total exposure time of ~70 ksec (100 ksec approved for AO4)

RXJ1856.5-3754, an isolated neutron star, was chosen as the target:

• Steady rate of ~1.5 cts/sec
• Spectrum is well calibrated by HRC-S/LETG (kT=61 eV, blackbody)
• Emission is peaked around the C edge where HRC-I QE is not well known

Objectives:

• Absolute QE calibration (~1% statistics) around C-K on HRC-I center
• Vignetting calibration at a large off-axis angle (~2% accuracy)
• Deep exposure for on-orbit off-axis PSF studies
• Additional timing data for pulsation searches (although unlikely to improve current limits)
• QE uniformity at C-K

Chosen parameters:

• 10-tap by 10-tap window---to limit background event rate to ~1.0 ct/sec
• SIM_Z = +75 mm--safely within the 70-100 mm range for ACIS BG
• Z_offset = +25.62'--put the target on the center of the HRC-I, with an offset at which interesting HRMA ghost rays might be visible.

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.

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.

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...

Tags

2002 : CfA : Chandra : CXC : HRC : HRC-I : HRMA : RXJ 1856 : Wargelin
[add a tag to this presentation]

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Last modified: 11/02/10

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