XL-CALIBUR Data Analysis

• How the clean event files were created
• Using xselect to extract a spectrum from the cleaned event file

How the clean event files were created

For XL-Calibur, “golden events” comprise interactions where exactly one pixel has been triggered, with no coincident activity in the anticoincidence shield. Due to the configuration of the data acquisition system, a two-hit event (such as a cosmic ray penetrating two of the CZT walls) can occasionally become split into two one-hit events. To counteract this, each unique event is assigned an event flag, and consecutive events are only accepted if their event flags are unique. This cleaning of “split” events happens prior to any other analysis step.

Data from the relevant flight observations (Crab, Cygnus X-1) is carefully inspected for unexpected behavior. Examples include events recorded during periods where timing information could not be ascertained (due to an overheating of the on-board timing system), and events that are misread or have become corrupted (unphysical values, unused channels, etc.) Good timing intervals (GTIs) exclude such periods.

Based on pointing data (WASP status flags) and/or angular separation from the desired target, events can be grouped into on-source (“signal”) and off-source (“background”) observations offset by one degree from the given target. Transitions between the two categories occur every few minutes: Crab observations mostly have 25 minutes on-source and 5 minutes off-source while for Cygnus X-1, corresponding numbers are18 minutes and 12 minutes. The slewing time can be treated as background, as long as care is taken to exclude periods with low angular separation to the target (at the beginning or end of the transition), to prevent source events from contaminating the background dataset.

Apart from a pixel-dependent in-flight trigger (median threshold ∼15 keV, standard deviation ∼5 keV), a pixel-by-pixel software trigger is also applied, to prevent noise triggers. Events with multiplicity (number of hits in the event) greater than unity are discarded, as they are most likely caused by background (e.g., through-going cosmic rays). An energy cut is also applied to the remaining events: typically, 15-60 keV deposited energy. It is noted that the primary energy of a photon event is not measured, as the scattering is expected to take place in the passive beryllium element. A detailed modelling of the instrument response is thus needed to provide an estimate of the primary energy. Finally, any event with a coincident hit in the anticoincidence shield is discarded. Applying the shield cut only at this stage allows the functionality of the anticoincidence system to be monitored throughout the flight.

When determining the scattering angles in the detector, in order to construct a modulation curve or to calculate Stokes parameters, the position of the mirror focal spot must be taken into account. Any offsets from the center of the scattering element may result in systematic effects affecting the polarization measurements (Aoyagi, et al., 2024). To counteract this effect, an offset correction based on the mean photon scattering location is used, as reconstructed in the imaging CZT located underneath the beryllium scatterer. The mean location (in two dimensions) is determined as the average interaction site of valid (background-subtracted) entries in this CZT, calculated for events impinging within 6 mm of the center of the imager (corresponding to the radius of the scattering element). It is determined separately for each observation of each target (Crab/Cygnus X-1). Photons are assumed to scatter from this calculated location and into any one of the circum-adjacent CZT pixels, thus providing the scattering angle of each valid event. The angle is referenced relative to the direction to celestial north (positive towards east, following IAU conventions), and it also takes into account the instantaneous polarimeter rotation angle, as well as any possible banking of the gondola, e.g., due to a swinging motion of the payload underneath the balloon.

Once valid events have been selected for the on-source (“signal”) and off-source (“background”) observations, polarization properties can be determined using standard methods, such as employing Stokes parameters for subtracting background, and Bayesian techniques to determine marginalized polarization parameters that take into account the bias (positive-definiteness) of the measurement, etc.

Finally, for observations of the Crab, events where valid timing solutions were available also have their barycentered arrival time calculated and provided. This allows a phase-folding analysis for the pulsar, e.g., based on the ephemeris provided by the Jodrell Bank Observatory.

Using xselect to extract a spectrum from the cleaned event file

This page contains an example of using the HEASoft tools xselect to analyze XL-CALIBUR data. The XL-CALIBUR archival data may be analyzed with the HEASoft software package with the calibration files distributed with CALDB.

The HEASoft task xselect has been updated to include the XL-CALIBUR specifications. Below is an example of how to derive a spectrum using the cleaned event file xl2024010002_pon_cl.evt. As the example shows, a sprectrum file is created for each Stoke parameter (I, Q, and U).

xselect
 
                         **  XSELECT V2.5c  **
 
> Enter session name >[calet_lc] xlcalibur
xlcalibur:XRISM > read event xl2024010002_pon_cl.evt
> Enter the Event file dir >[.] 
Got new mission: XLCALIBUR1
> Reset the mission ? >[yes] 

 Notes: XSELECT set up for      XLCALIBUR1
 Time keyword is TIME       in units of s
 Default timing binsize =   16.000
 
Setting...
 Image  keywords   = COLUMN     Z_LAYER     with binning =    1
 Energy keyword   = PI                     with binning =    1
 
Getting Min and Max for Energy Column...
Got min and max for PI:     1   9900
 
Got the minimum time resolution of the read data:  0.12800E-05
MJDREF =  6.0310000800741E+04 with TIMESYS = TT
 Number of files read in:            1
 
******************** Observation Catalogue ********************
 
Data Directory is: /processing/jmr/xlcalibur/xselect/
HK Directory is: /processing/jmr/xlcalibur/xselect/
 
 
        TELESCOP    INSTRUME    DATAMODE    OBJECT      OBS_ID      DATE-OBS            DATE-END
      1 XLCALIBUR1  POLARIMETER POLARIMETER Crab        2024010002  2024-07-11T08:18:05 2024-07-11T16:37:48
 
xlcalibur:XLCALIBUR1-POLARIMETER-POLARIMETER > extract spectrum stokes=UNWEIGHTED
extractor v6.19    30 May 2025
 No image X-axis TCRPX, set to 0
 No image Y-axis TCRPX, set to 0
 No image X-axis TCRVL, set to 0
 No image Y-axis TCRVL, set to 0
 No image X-axis TCDLT, set to 1
 No image Y-axis TCDLT, set to 1
 Doing file: /processing/jmr/xlcalibur/xselect/xl2024010002_pon_cl.evt
100% completed
          Total      Good    Bad: Time     Phase     Grade       Cut
          26547     26532           15         0         0         0
===============================================================================
    Grand Total      Good    Bad: Time     Phase     Grade       Cut
          26547     26532           15         0         0         0
   in  15515.     seconds
 Spectrum         has   1.710     counts/sec
 ... wrote the PHA data Extension
 Spectrum         has -2.6518E-02 counts/sec
 ... wrote the PHA data Extension
 Spectrum         has -8.9323E-02 counts/sec
 ... wrote the PHA data Extension
xlcalibur:XLCALIBUR1-POLARIMETER-POLARIMETER > save spectrum myspec_25
Wrote spectrum to myspec_25I.pha
Wrote spectrum to myspec_25Q.pha
Wrote spectrum to myspec_25U.pha

If you need help, with the XL-CALIBUR data analysis please send your queries to the help desk of HEASARC or FTOOLS .