SAS watchout page

I heard that one column in MOS1 Timing Mode is affected by "high offset": can I still analyze the data?

Following a micrometeoritic impact in Rev.#961 (March 2005), a new hot column appeared, whose effects leak across the whole column RAWX=318. While a correction for this hot column is possible and implemented for MOS1 exposures taken in Imaging Mode, the offset value in MOS1 exposures taken in Timing Mode is too high for any meaningful correction to be possible. The affected column (and, at least, one neighboring columns on either side) needs to be removed from the extraction region of any scientific products.

Unfortunately, it is not enough to generate directly the effective area from the extraction region with excised columns to recover the proper source flux, because arfgen requires that spectral extraction regions are symmetric. The following steps outline a commented procedure to be followed to generate the correct effective area in this case. It makes use of standard SAS tasks, as well as of the LHEASOFT task addarf. In the following, it is assumed that:

• mos1.evt is the name of the MOS1 event list
• the source is centered on RAWX=310
• one wants to excise three column centered around RAWX=318, i.e. columns: RAWX=317, 318, and 319.

The procedure runs as follows:

1. Generate a spectrum without the excised columns

   evselect table='mos1.evt' withspectrumset=yes spectrumset='mos1_fullnoRAWX318.pi' specchannelmin=0 specchannelmax=11999 spectralbinsize=15 energycolumn=PI expression="#XMMEA_EM&&PATTERN==0&&(RAWX in [291:316] || RAWX in [320:329])"

2. Generate a spectrum (and the corresponding effective area) from the same RAWX range as in Step#1, however without excising any columns:

   evselect table='mos1.evt' withspectrumset=yes spectrumset='mos1_fullspectrum.pi' specchannelmin=0 specchannelmax=11999 spectralbinsize=15 energycolumn=PI expression="#XMMEA_EM&&PATTERN==0&&(RAWX in [291:329])"

   arfgen spectrumset='mos1_fullspectrum.pi' arfset='mos1_fullspectrum.arf' badpixlocation='mos1.evt'

3. Generate a spectrum (and the corresponding effective area) from a RAWX range, symmetric around RAWX=310, and whose upper limit is the column with the highest RAWX number to be excised (RAWX=319 in our example)

   evselect table='mos1.evt' withspectrumset=yes spectrumset='mos1_outerann.pi' specchannelmin=0 specchannelmax=11999 spectralbinsize=15 energycolumn=PI expression="#XMMEA_EM&&PATTERN==0&&(RAWX in [301:319])"

   arfgen spectrumset='mos1_outerann.pi' arfset='mos1_outerann.arf' badpixlocation='mos1.evt'

4. Generate a spectrum (and the corresponding effective area) from a RAWX range, symmetric around RAWX=310, and whose upper limit is the column with the highest RAWX number not to be excised (RAWX=316 in our example)

   evselect table='mos1.evt' withspectrumset=yes spectrumset='mos1_innerann.pi' specchannelmin=0 specchannelmax=11999 spectralbinsize=15 energycolumn=PI expression="#XMMEA_EM&&PATTERN==0&&(RAWX in [294:316])"

   arfgen spectrumset='mos1_innerann.pi' arfset='mos1_innerann.arf' badpixlocation='mos1.evt'

5. Calculate the effective area due to the excised columns

   addarf 'mos1_outerann.arf mos1_innerann.arf'' weights='0.5 -0.5' outfile='mos1_excluded.arf'

6. Find the final effective area without the excised columns

   addarf 'mos1_fullspectrum.arf mos1_excluded.arf' weights='1.0 -1.0' outfile='mos1_fullnoRAWX318.arf'

The effective area file mos1_fullnoRAWX318.arf shall be used together with the spectrum mos1_fullnoRAWX318.pi. The following plot shows to which extraction regions the different spectra used in this watchout item correspond.

Tests show that the flux recovered with the above procedure is accurate within 2.5%.