For piled-up observations, it is sometimes useful to analyze the events in the transfer streak. We are working on the calibration of ACIS used in this mode, but some things are known already.
The gain of the ACIS transfer streak is slightly higher than the normal timed-event mode data. The gains have been measured by two investigators.
For the S3 chip, see the memo by T. J. Gaetz, here: http://cxc.harvard.edu/cal/Hrma/psf/wing_analysis.ps See especially Appendix B, where he quotes a gain correction of +2.5%.
For the I array, see Heinke et al, ApJ, 590, 809 (2003). For this observation, a joint fit of Chandra and RXTE data produced a gain correction of +7±2%.
Both of these fits turn on the observation of the Ir M edge near 2 keV, so it's
not entirely clear that a linear correction is appropriate. Further work on targets
with more spectral features at a variety of energies is underway, with results
expected in 2007.
Last modified: 11/15/10
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