Example simulations

In the following, simulations of faked point-source are described. All neccessary steps are described in detail which are necessary to build the source and start the virtual observation.

SIMPUT Source creation
Virtual XMM observation
End products
Stepwise observation simulation
X-IFU calorimeter simulation

SIMPUT Source creation

Before starting an observation simulation, the sources need to be described in the SIMPUT format. The SIXTE software package contains tools to obtain such a file from several input files such as an XSPEC- or ISIS-parameter file for a spectrum, as well as for example an ASCII description of a lightcurve.

Here, we will start with a simple XSPEC spectrum and an ASCII lightcurve to construct the SIMPUT source.

The spectrum is described in the file example_spectrum.xcm, containing the model definition and parameter values:

model  phabs*powerlaw
           21.5      0.001          0          0     100000      1e+06
            1.5       0.01         -3         -2          9         10
            1.0       0.01          0          0      1e+20      1e+24

We define our lightcurve in an ASCII-file with two rows: The first row gives the time in seconds (relative to an arbitrary reference time), the second one is the flux relative to the reference flux of the source given in the source description. Note that the lightcurve is not required to be sampled evenly over the covered time period but will be interpolated by the simulation program. It should look like the example example_lightcurve.dat:

0.0           1.0
1.592568e+06  1.1710184667753536
4.746168e+06  1.3358780211173131
6.606792e+06  1.3293381514300098
         ...                 ...

We can now create a SIMPUT file from these data by calling:

simputfile RA=40.2 \
           Dec=12.8 \
           XSPECFile="example_spectrum.xcm" \
           LCFile=example_lightcurve.dat \
           MJDREF=50800.0 \
           Emin=0.5 \
           Emax=10.0 \
           srcFlux=2.3e-12 \

This will place a single point source at RA=40.2°, Dec=12.8° with a flux of 2.3e-12 erg/s/cm^2 in the 2.0 keV - 10.0 keV reference band and save it into the SRC_CAT-extension of the output file "example_source.simput".

The spectral model contained in example_spectrum.xcm will be converted into a list of energy-flux points, written as an entry in the SPECTRUM-extension of the output file, and assigned to the source.

The lightcurve in example_lightcurve.dat will be saved in the correct SIMPUT-format as the TIMING-extension of "example_source.simput". As discussed above, value of 1 in the lightcurve will result in a source flux of 2.3e-12 erg/s/cm^2 in the given reference energy band. The value of MJDREF is a reference time for the light curve. This means that at a simulation time of MJD=50800.0, the lightcurve starts with a time of 0. The usage if a lightcurve is optional, if none is assigned to a SIMPUT-source, its flux will be constant at the value of srcFlux.

If no value for srcFlux is given explicitely, the spectrum with its intrinsic normalization will be integrated in the reference energy band and the resulting flux is written to the source entry.

Virtual XMM observation

With a valid source in SIMPUT format, a virtual observation can be started by calling

runsixt .

For example, an observation of the source created above using a EPIC-pn camera-like CCD in fullframe mode for 100ks we call

runsixt RawData="example_raw.fits" EvtFile="example_evt.fits"\
        Mission="XMM" \
        Instrument="EPICPN" \
        Mode="FFTHIN" \
        XMLFile="$SIXTE/share/sixte/instruments/xmm/epicpn/fullframe_thinfilter.xml" \
        Simput="example_source.simput" \
        Exposure=100000. \
        RA=40.2 Dec=12.8 \

Here, example_raw.fits will be the raw output file listing each pixel event is listed, and example_evt.fits will be an output event file where photon patterns ("grades" in US-astronomy speak) have already been combined (see Sect. End products for more details). This corresponds to the generally known event files from a typical extraction software.

The value of MJDREF has to be set to the mission specific reference value (here for XMM Newton to MJD50814.0).

The instrument specification is contained in the input file fullframe_thinfilter.xml. XMM-Newton specific setup files can be downloaded here. They should be installed in the same way as described in Setup.

The example's output, drawn in RAWX, RAWY-coordinates, looks like this:

The example observation as picture in RAWX,RAWY-coordinates.

XMM event list

To create a fake XMM-like event list which can be used for analysis with standard XMM SAS tools, the programs epicpn_events, epicmos1_events and epicmos2_events can be used. (***Note: These tools are in the test phase, if you encounter problems, please contact us!***)

To get such a faked XMM-like event list for the observation above, we can call

epicpn_events EvtFile="example_evt.fits" \
which results in the output file example_epn_evts.fits. The input of epicpn_events is just the event file, which is the output of the runsixt call.

End products

The two main output files of the simulation are the raw data file and the event file (the latter can be constructed from the raw data file if none was created by the simulation).

Raw Data File

The raw data file contains a binary table extension called EVENT inwhich all individual pixel events are listed. Its header includes all necessary information to reconstruct the simulation. In the table, the events are listed with information about the position on the chip. The column PH_ID contains the simulation-internal indices of the photons which caused the respective event. The column is a vector column, where each field contains a integer vector of length 2. If an event is piled up, i.e., if more than one photon hit the pixel during one integration interval, the vector gives the indices of the first two photons which hit the pixel, otherwise the second number is 0. The column SRC_ID gives the indices of the input sources which caused the respective event. In the column PILEUP, each value for single photon events is 0, for piled up events a 1 is set.

Event file

The event file looks similar to the raw data file but lists not single pixel events but combined patterns like used for further analysis. It is similar to cleaned level 2 event files of mission such as Chandra, or Suzaku. In the header of the EVENT-estension, there are some additional interesting numbers:

  • NVALID gives the number of patterns which are graded as "valid" for further analysis.
  • NPVALID gives the number of patterns which are graded as "valid" for further analysis but which are in fact piled up.
  • NINVALID gives the number of patterns which are graded as "invalid" for further analysis.
  • NPINVALI gives the number of patterns which are graded as "invalid" for further analysis and which are in fact piled up.
  • Other keywords give closer detail about the individual grades.
Additionally, the FITS table now also contains the reprojected RA and DEC coordinates of the events.

SIXTE Workshop (20.-21. April 2016, IRAP)

SIXTE Simulator Manual

The first version of the SIXTE simulator manual is available for download below. It includes general description of SIXTE and the implementation of detectors such as the WFI, X-IFU, eROSITA and others. In addition, tutorials for Athena WFI and X-IFU simulations are included.

download: simulator_manual_v1.3.8.pdf

supplementary files for the X-IFU tutorial (730MB!): download

Talks presented at the Workshop

  • End-to-end introduction (J. Wilms): talk
  • SIXTE implementation of the WFI (T. Dauser): talk
  • SIXTE implementation of the X-IFU (P. Peille): talk
  • Bright Sources Capabilities of the WFI (T. Dauser): talk
  • Tessim Simulations (J. Wilms): talk
  • X-IFU Bright Source Studies (P. Peille): talk
  • XML-files and Other Missions (J. Wilms): talk

SIXTE team · Dr. Remeis-Sternwarte Bamberg · Sternwartstraße 7 · D-96049 Bamberg