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Pile-Up in the RGS

PILE-UP IN THE RGS: HOW TO PREVENT IT, EVALUATE ITS EXISTENCE AND MAKE CORRECTIONS.


This thread illustrates how to evaluate if your RGS observation is affected by pile-up; if so, how to deal with it in some special cases; and how to prevent it occurring.

For a detailed explanation on these issues, the user is referred to the following document: CAL-TN-0075.

1. Effects of pile-up in the RGS


Pile-up occurs whenever more than one photon hits a single pixel during one integration time. Pile-up will cause two coincident first order
photons to combine into a single second-order event at the same spatial position on the detector but apparently with half the wavelength. In addition,
at locations on the CCD where multiple-pixel events are more likely, pile-up will have a higher chance to result in the type of complicated event patterns that get discarded by the on-board processing and thus removed from the data. Thus, pile-up may differ between RGS1 and RGS2 and may abruptly vary between CCD locations due to the nominal event pattern distribution variations over the detectors. The effects of pile-up on spectra are therefore two-fold:

   * Migration of photons from 1st to 2nd order (or from 2nd order to 3rd order)
   * Rejection of events with 'complicated patterns' by the on-board processing.

In piled-up observations, any spectral features which are identified can only be trusted if they are consistent between the different RGSs, and the piled-up 2nd order appears at half the wavelength.

2. How to evaluate whether pile-up may be a problem for a source in your RGS observation


One should consider potential pile-up if photon rates approach the range of 0.03-0.04 photons/cm2/s around the wavelength of maximum effective area (15 ц…). In rough terms, any individual CCD with a total count rate in all orders of more than 20 cts/s may suffer from pile-up. Starting in August 2007, RGS2 CCDs are read via a single node. RGS2 frame times are therefore twice as long as RGS1 time frames (readout time for RGS2 is 9.6 sec, i.e. 1.2 sec per CCD). Thus, for observations performed after August 2007 any RGS2 CCD with more than 10 cts/s may suffer from pile-up. In case that the above count rates are present in
an observation, potential effects should be considered and suitable diagnostics be executed, such as inspection of ratio of 1st, 2nd and higher order fluxed spectra.  The RGS calibration of the effective area ensures that the fluxed spectra of all the spectral orders of non-piled-up sources agree within a few percent. Therefore, sources for which the fluxed spectra of, say, 1st and 2nd order differ by more than 10% are very likely suffering from pile-up. In the next three figures spectra which suffer from pile-up (Figs. 1 and 2) are shown for comparison with spectra which are free of pile-up (Fig. 3)


FIG 1: Comparison (top) and ratio (bottom) between 1st (red) and 2nd (green) order of RGS2 for the source J1665 in a high, piled-up state.


FIG 2: Comparison (top) and ratio (bottom) between 1st (red) and 2nd (green) order of RGS2 for the source RS Oph, also in high state, piled-up.



FIG 3: Comparison (top) and ratio (bottom) between 1st (red) and 2nd (green) order of RGS2 for the source J1665 in a low, non piled-up state.


3. How to deal with piled-up spectra


Repair of piled-up spectra is limited and can only be done with ad hoc methods in some special cases, e.g., when the source spectrum is very soft and it can be safely assumed that the visible 2nd order spectrum is due only to piled-up first order photons. This is the case of the RGS observations of RS Oph taken during its 2006 outburst (see 'The SSS Phase of RS Ophiuchi Observed with Chandra and XMM-Newton. I. Data and Preliminary Modeling', Ness et al. 2007, ApJ 665, p. 1334)


4. How to prevent pile-up


The easiest remedy against pile-up is faster readout of the CCDs. If only one CCD is read instead of 8, the integration time per frame of 4.8s is reduced by a factor 8, thus reducing pile-up by the same amount. One can consider to have different readout modes for the two RGSs. Having one RGS in single CCD mode and the other in double CCD readout mode, the frame times of both RGS differ by a factor of two, but are still both faster than in 8-CCD readout. This difference will allow to make an estimate of pile-up. When there is minimal difference between the two RGS observed, pile-up is not important. Other modes, e.g. going from 4 to 2 to 1 CCD readout mode can be considered depending on count rate going from 0.04 to 1.0 photons/cm2/s.