PSPC Calibration Files for Standard Data Processing

The PSPC as part of the focal plane instrumentation of the XRT\ has been thoroughly calibrated using ground based as well as in-flight measurements. These results are used during the SASS processing (cf. module CT of the TEL processing in Sect. 8.2.1) to correct the recorded raw event information (RAWX, RAWY, and PHA = RAW_AMPL) for temporal and positional variations, saturation effects, and non-linearities of the PSPC and its following electronics yielding the corrected event (XDET, YDET, and PI = AMPL). A complete record of an event delivered by the standard processing is then labeled (RDF format) together with a corrected event time (see Sect. 3.8) and sky coordinates determined from the attitude solution (see Sect. 2.2) as (TIME, XDET, YDET, AMPL, XPIX, YPIX).

shortcut subroutine description and dependencies calibration file(s) used

BAL CTA bin number of Al K line (Prescott fit) steering database (INGRES )

NLC CADC correction for ADC non-linearity p$manpar:adc_bins.dat

GSC CGS correction for gain saturation (none)

TGC CTG correction for temporal gain variations (none)

EPC DCORE electronic position correction p$manpar:para_c.dat

p$manpar:senk_c.dat

p$manpar:para_b.dat

p$manpar:senk_b.dat

SGC DCORG correction for spatial gain variations p$manpar:gain_kor3_c.dat

p$manpar:gain_kor3_b.dat

p$manpar:gnampl_new.dat

WC DCORW window correction p$manpar:tabx_093_c.dat

p$manpar:taby_093_c.dat

p$manpar:tabx_093_b.dat

p$manpar:taby_093_b.dat

p$manpar:scal3_new.dat

FC FCOR field correction v$manpar:onax.dat

Table F.1: Corrections to the raw detector output information, listed in the order in which the corrections are applied.

Symbol

here SASS CT RDF key description

A RAW_AMPL ADC pulse height channel of event (integer in [1,256])

AN pulse height of event, corrected for ADC non-linearity

ANG pulse height of event, corrected for ADC non-linearity and gain saturation

ANGT pulse height of event, corrected for ADC non-linearity, gain saturation, and temporal variations

ANGG AMPL pulse height of event, corrected for ADC non-linearity, gain saturation, temporal and spatial variations (PI channel)

IX_EV RAWX raw detector x coordinate (integer number in [1,8192])

IY_EV RAWY raw detector y coordinate (integer number in [1,8192])

XE, YE detector coordinates (electronic position correction applied)

XW, YW detector coordinates (electronic position correction and window correction applied)

XC, YC XDET, YDET detector coordinates (electronic position correction, window correction and correction for XRT field distortion applied)

FAK PHA channel of the Al K line (BAL ) (from Prescott fit)

GAIN correction factor for temporal gain variations (close to 1; gain saturation is taken into account)

r RAN random number (real number in [0.0,1.0[)

SASS convention description

corresponds to

corresponds to

x' corresponds to a simple transformation of  x  (`modified x')

Table F.2: Parameters and conventions used by SASS

The raw event is actually triggered when an incoming X-ray photon scattered off the XRT mirror has passed through the PSPC window framed by the support structure. This process is described by the effective area of the XRT and the window transmission of the PSPC. Then the photon (depending on its energy) is absorbed by the counter gas of the PSPC\ (cf. Fig. 3.1) and yields a (primary) electron which is thermalized and causes secondary ionization of the counter gas. The electron cloud drifts through the K1 cathode grid (25 strips) towards the A1 anode grid. When close enough to an anode wire the charge cloud can be amplified by a (gain) factor of

because the electric field strength becomes sufficiently high. This avalanche of charge onto the anode leads to a charge pulse at the anode A1 and an induced signal is detected at the cathode K1 and K2 which are aligned perpendicular to each other to determine the position of the photon inside the detector. Typically two to five wires are excited by a signal which is digitized by means of pulse shapers, peak detectors, and ADCs. After validity checking the event position is calculated by a center of gravity determination. The strength of the signal caused by the photon inside the detector is described by the quantum efficiency of the counter gas and the gain, which depends on the gas state and the HV of the anode.

SASS processing stores the raw event information in the file TRGEV.TEL and module CT converts this into the files CTEV.TEL (for target events), CCEV.TEL (for calibration events), NOSEV.TEL (for non-standard events like filter-wheel-closed position) (see Sect. 8.2.1).

The following sections summarize comprehensively the present state of the correction, whose individual steps are listed in Table F.1 [Freyberg et al.1997]. Parameters and conventions often used in these sections are collected in Table F.2.