Pile-up in the RGS: how to prevent it, evaluate its existence and make corrections

Introduction This thread illustrates: How to evaluate if your RGS observation is affected by pile-up. How to correct piled-up data in some special cases. How to prevent the occurrence of pile-up. Expected Outcome Plot comparing the 1st and 2nd order spectra for each RGS for a given source. If 1st and 2nd order spectra agree, pile-up is not a problem for this source. SAS Tasks to be Used rgsproc rgsfluxer rgsrmfgen Prerequisites How to reduce RGS data and extract spectra of point-like sources Useful Links Pile-up in the RGS spectra Caveats

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Procedure

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 vary between CCD locations due to the 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.
One should consider potential pile-up if the total source flux per CCD approaches the following levels:

		2% pile-up flux			2% pile-up flux
		(erg cm-2 s-1)			(erg cm-2 s-1)
RGS1	CCD1	7.1E-10	RGS2	CCD1	3.1E-10
RGS1	CCD2	5.0E-10	RGS2	CCD2	2.1E-10
RGS1	CCD3	3.8E-10	RGS2	CCD3	1.6E-10
RGS1	CCD4	3.3E-10	RGS2	CCD4	-
RGS1	CCD5	2.9E-10	RGS2	CCD5	1.5E-10
RGS1	CCD6	3.2E-10	RGS2	CCD6	1.4E-10
RGS1	CCD7	-	RGS2	CCD7	1.9E-10
RGS1	CCD8	7.1E-10	RGS2	CCD8	3.7E-10
RGS1	CCD9	20.0E-10	RGS2	CCD9	14.1E-10
Double-node readout			Single-node readout

In rough terms, any individual CCD with a total count rate in all orders of more than 12 cts/s in RGS1 and 6 cts/s in RGS2 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. This is why the RGS2 limit is half the RGS1 limit.

In case that the count rates above are present in an observation, potential effects should be considered and suitable diagnostics executed, such as inspection of ratio of 1st and 2nd order fluxed spectra. The RGS calibration of the effective area ensures that the fluxed spectra of 1st and 2nd 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 likely to be suffering from pile-up.

The strong emission lines in Capella give the opportunity to make a reliable quantitative assessment of the pile-up fraction. The next three figures show the effects of pile-up in a RGS2 single-node observation of Capella.



The left panel shows 1st (black) and 2nd (red) order counts on CCDs 5 and 6. The counts in the 2nd order have been multiplied by a factor 10 to ease the comparison. The 2nd order data are a mixture of genuine 2nd order &lambda/2 events and 1st order pile-up migrations.

The middle panel shows 1st (black) and 2nd (red) order counts on CCDs 8 and 9. In this case, the counts in the 1st order have been multiplied by a factor 0.3 to ease the comparison. The effects of pile-up are obvious: e.g. the pile-up in the strong lines at 15 and 17 Å (see left panel) cause spurious lines in the 2nd order at 7.5 and 8.5 Å(middle panel).

The right panel shows the pile-up fraction estimated at the positions of the 4 brightest lines (black stars) along with the frame count rates for all pixels above the threshold, which naturally cluster around the strong lines.

How to compare 1st and 2nd order spectra

1. Obtain calibrated spectra and response matrices for the 1st and 2nd order spectra of RGS1 and RGS2 (see thread: How to reduce RGS data and extract spectra of point-like sources). If you followed the previous thread to the end, you will already have as a product fluxed spectra with names PxxxxxxyyyyOBX000fluxed1000.FIT and PxxxxxxyyyyOBX000fluxed2000.FIT (following the naming convention of the above specified thread) and you can skip the next point.


2. Run rgsfluxer to obtain fluxed spectra in 1st and 2nd order:
   
    rgsfluxer pha='PxxxxxxyyyyR1zeeeSRSPEC1001.FIT PxxxxxxyyyyR2zeeeSRSPEC1001.FIT' rmf='PxxxxxxyyyyR1zeeeRSPMAT1000.FIT\
     PxxxxxxyyyyR2zeeeRSPMAT1000.FIT' file=flux_1storder.fits
   
    rgsfluxer pha='PxxxxxxyyyyR1zeeeSRSPEC2001.FIT PxxxxxxyyyyR2zeeeSRSPEC2001.FIT' rmf='PxxxxxxyyyyR1zeeeRSPMAT2000.FIT\
     PxxxxxxyyyyR2zeeeRSPMAT2000.FIT' file=flux_2ndorder.fits


3. Plot the fluxed spectra in 1st and 2nd order:
   
    dsplot table=flux_1storder.fits (or dsplot table=PxxxxxxyyyyOBX000fluxed1000.FIT)
    dsplot table=flux_2ndorder.fits (or dsplot table=PxxxxxxyyyyOBX000fluxed2000.FIT)


4. Compare the fluxed spectra:
   
   
   
   The plots above show the 1st order (left panel), a detail of the 1st order (middle panel) and the 2nd order (right panel) fluxed spectra of Cygnus X-2. An excess is evident in the 2nd order fluxed spectra at 11-12 Å, which is not present in the 1st order fluxed spectra at the same wavelength. This excess is probably due to piled-up events from the Oxygen edge at 22-24 Å in 1st order spectra.

How to deal with piled-up spectra

Repair of piled-up spectra is limited and can currently 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)

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 the flux expected per CCD.

Last Updated: 16 April 2010

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Caveats None

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