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ACIS pile-up mimicking bad aspect


This document illustrates a case where a flare in a moderately piled ACIS point source can mimic the effect of bad aspect. The case in point is a 20 ksec observation yielding a total of 3800 photons. At around 9 ksec into the observation the X-ray image of the source appears to get much broader, spanning up to 6 arcsec for a period of about 2 ksec, and remaining a bit broader for 3 ksec more. This coincides with a flare in the source. After that the image appears normal again. We discuss a number of possible causes for the apparent degradation, and conclude that pile-up is the likely cause.


The figure below shows photon sky position (basically RA and Dec with units of ACIS pixels) versus time (seconds), and clearly shows the apparent degradation of the image PSF at around 9 ksec:

The light curve, source counts versus time, is shown below. Note the flare where the apparent source count rate is a factor of two higher in the 9 to 10 ksec time span.

Possible causes

Poor aspect solution

The aspect quality plots for this observation show no problems. These plots use the aspect solution to de-dither the ACA star images, and we can conclude that the aspect solution is an accurate time history of the ACA pointing attitude. The fid lights show no unusual behavior. There remains the possibility of a high-frequency disturbance which is outside the bandpass of the PCAD system.

The plot below illustrates that the image degradation is most likely unrelated to the ground aspect solution. This shows the photon positions in detector X coordinates (DETX) as a function of time. No aspect correction has been applied to these data. At early times the source photons are tightly bunched in the dither pattern, but starting around time = 9 ksec the photons are noticably dispersed about the mean dither position.


The light curve of the source shows a flare exactly during the time when the PSF appears to degrade. Increasing the flux in a moderately piled source will put more photons into the wings and remove photons from the core because they are rejected on-board. In this case, the initial count rate is 0.20 cts/sec, which gives a pileup fraction of about 23%.

The plot above shows the PSF profile before the flare (black line) and during the flare (red line). Both are in units of counts/sec/pixel. The red line shows the classic signature of pile-up. Very strongly piled sources typically look like a donut.

Gyro bias jog

The gyro bias data were examined and no bias jog is apparent.

HRMA/spacecraft resonances excited by reaction wheels

This was a concern in the early PCAD design audits, and prompted TRW to insert a stage of vibration isolation between each RWA and the spacecraft. The RWA rates during this observation show no unusual features. The wheels are slowly spinning up or down at a steady rate. This doesn't rule out the case of a resonance being excited, but it remains unlikely since no other examples have appeared.

Background flare causing ACIS instrumental anomaly

The background rate is steady.
Tom Aldcroft
Last modified: 06/29/11

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