XMM-Newton Science Analysis System: User Guide

5.6.1 Events, sources and selection criteria

In the default spectroscopy mode, the most important parameters for each detected photon in the event lists are the two spatial coordinates and one energy coordinate. These go through a series of improvements in the initial stages of rgsproc in which the initial raw set of

[CCDNR, CCDNODE, RAWX, RAWY, ENERGY]

is transformed using the detailed geometry of the grating and detector assembles into coordinate angles along the grating dispersion direction, $\beta$ or BETA, and the perpendicular cross-dispersion direction, $\chi$ or XDSP,

[BETA, XDSP, PHA]

which, in turn, are combined with knowledge of the history of the spacecraft's pointing and CCD gain and CTI characteristics to yield

[BETA_CORR, XDSP_CORR, PI]

in units of radians, radians and eV respectively. The detectors are almost invariably read out using a method which combines a $3\times3$ area of original CCD pixels into a single value, so-called $3\times3$ on-chip binning or OCB. BETA_CORR and XDSP_CORR reconstruct the angular distribution of photons emerging from the gratings and include randomisation of the quantised CCD coordinates in order to simulate a continuous distribution. The coordinates of the nominated source make a subtle appearence at this stage as they are used in the correction of BETA to BETA_CORR caused by variations in the angle of incidence on the gratings caused by spacecraft pointing jitter. The otherwise redundant PI energy column is used to distinguish between spatially overlapping orders.

Once the event list has been filtered for bad pixels of the various types signalled by rejflags in Table 7, the source list plays a central role. The source list here has 5 binary-table extensions

P0136540101R2S002SRCLI_0000.FIT:SRCLIST            1=PROPOSAL 2=ONAXIS 3=MKN421
P0136540101R2S002SRCLI_0000.FIT:RGS2_BACKGROUND    BETA_CORR-XDSP_CORR region
P0136540101R2S002SRCLI_0000.FIT:RGS2_SRC3_SPATIAL  BETA_CORR-XDSP_CORR region
P0136540101R2S002SRCLI_0000.FIT:RGS2_SRC3_ORDER_1  BETA_CORR-PI region
P0136540101R2S002SRCLI_0000.FIT:RGS2_SRC3_ORDER_2  BETA_CORR-PI region

which specify selection regions for the nominated source #3 and the orders 1 and 2 chosen by default and the background region. The calculation of the position and width of the selection regions depends on the source's celestial coordinates, which the observer has taken so much trouble to get right, and the fractions of the XDSP_CORR and PI response curves specified by the parameters xpsfbelow, xpsfabove, xpsfexcl and pdistincl. The events which contribute to source and background spectra are those that fall in selection regions defined, for example, for 1st order in RGS2 :

((BETA_CORR,PI) IN REGION(P0136540101R2S002SRCLI_0000.FIT:RGS2_SRC3_ORDER_1))
                                       &&
((BETA_CORR,XDSP_CORR) IN REGION(P0136540101R2S002SRCLI_0000.FIT:RGS2_SRC3_SPATIAL))

Finally, the simulated continuous BETA_CORR or MLAMBDA distributions of selected events are rebinned into channels according to instructions to yield the final binned counts spectra. Before SAS v8.0, spectra were always accumulated in intervals of the dispersion angle BETA_CORR. There is now the possibility of calculating spectra instead on a uniform wavelength grid through the parameter spectrumbinning. Wavelength-based spectra are now the default product in SAS, and offered in order to facilitate the combination of two or more spectra and their associated background and response files.