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Abteilung Astronomie

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[ROSAT spacecraft]

ROSAT Data Analysis -
Standard procedures for the analysis of ROSAT data


Table of Contents

General
Retrieving ROSAT data files
Images
Spectral Analysis
Lightcurves

General

Analysis of ROSAT data is usually done using the ROSAT account on the AIT VAX cluster (username: ROSAT, ask the system manager for the password). On this account ESO MIDAS and the MPE EXSAS package can be used:

invoke MIDAS (November 1994 release):
ait321,Rosat> nov94
start EXSAS:
Midas 001> set/context EXSAS

The utilities described in this document (.prg, .cmd, .par files) can be found in the directory [ROSAT.WORK.UTILS].

ROSAT calibration files are stored under the logical directory name EXSAS_CAL:.


Retrieving ROSAT data files

Archival data can be retrieved via WWW from the ROSAT public FTP archive. Select the directory of the requested ROSAT observation or use the archive browser to search for observations of your favourite object.

Download the files.

Unzip the files : gzip -d *.*-gz

Depending on the date of (re)processing different file formats may be found and different EXSAS commands have to be used to transform the data into MIDAS/EXSAS format:


format    | old MPE format | old US format  | rationalized FITS
----------+----------------+----------------+------------------
extensions|  .tfits        | .cas           | .fits
          |  .ifits        | .so            |
          |  .mt           | .evr           |
          |                | ...            |
command   |  intape/rod    | intape/usrod   | intape/rdf

Midas 002> intape/rod
Midas 002> intape/usrod DISK <detector (HRI/PSPC)>
Midas 002> intape/rdf DISK <root name of disk files>

New PI data received on tape have to be read from DAT or EXABYTE tape using the intape/rdf command.
Midas 002> intape/rdf <tape-device>

Now you should have at least the following files:
events.tbl: photon event file,
eventrates.tbl: eventrates for photon and particle events, and
attitude.tbl: spacecraft attitude information.


Images

Binning an image: Use the binima.prg utility
Midas 003> @@ binima
to get the image image.bdf with 15'' pixels from events.tbl.

Display image:
Midas 004> create/display
Midas 005> load/image image cuts=0,10

Identify your source by reading cursor coordinates:
Midas 006> get/coordinates equatorial 2000.0

Estimate detected counts in cursor rectangle:
Midas 007> statistics/image ? cursor

Measure image coordinates of object and possibly contaminating sources:
Midas 008> center/gauss


Spectral Analysis

Select events in source and background areas and bin spectra using the select_spec.cmd command file: Insert the world coordinate positions measured with center/gauss into the command file at line select/ring center=(x,y) ...
Run projection:
Midas 009> make/projection select_spec
-> raw spectrum, file name in this example: sourback.tbl

Correct the spectrum for incomplete extraction of the point spread function, deadtime losses, and spatial variations of the effective areas (vignetting correction) using the correct_spec.par parameter file:
Midas 010> correct/spectrum correct_spec
-> The resulting file (e.g. sourback_c.tbl contains a corrected effective area table, which has to be used later for fitting a spectral model.

Perform rebinnig and background subtraction:
Midas 011> prepare/spectrum sourback spectrum
produces the binned countrate spectrum spectrum.tbl. During the run you are requested to specify channels to be omitted and binning information, the first 10 channels (<0.1 keV) should be omitted, the nominal energy range of the PSPC ends at channel 240 (2.4 keV).

The channels of the detector response matrix have to be rebinned according to the binning of the file spectrum.tbl, for observations prior to the 11th of October 1991 (PSPC gain change) use the DRM EXSAS_CAL:DRMPSPC_06.BDF, for later observations use the file EXSAS_CAL:DRMPSPC.BDF.
Midas 012> bin/detector spectrum EXSAS_CAL:DRMPSPC drmb
-> binned DRM drmb.bdf

Fit a model to the spectrum:
Fitting a simple powerlaw using the parameter file pwl.par.
Midas 013> model/spectrum pwl
-> resulting file pwlspec.tbl, contains countrate and model photon spectrum, fit parameters are stored in the descriptor header.

Plot the spectrum:
Midas 014> plot/fit pwlspec
Or use the faster IDL program:
IDL> exsasplot,'pwlspec'


Lightcurves

PSPC lightcurves

It is convenient to perform the necessary corrections already during the selection process. This is done by switching the projection software into correction mode:
Midas 009> set/projection correct=EXSAS_CAL:effarea_pspcc,eventrates
-> correction mode is set, the specified files are used for vignetting correction and deadtime correction. Note that no PSF correction can be applied in correction mode.

Select source and background events and bin into lightcurve using command file select_light.cmd:
Edit select_light.cmd and insert your source coordinates and binsize.
Midas 010> make/pro select_light

Background subtraction:
Midas 011> prepare/lightcurve <table> <output-column> <source-column,back-column>
-> background subtracted, corrected lightcurve in column <output-column>
-> start time of bins in column :S_TI

Plot the lightcurve:
Midas 012> plot/lightcurve <table> ? <output-column>

Writing the lightcurve as an ASCII file:
Midas 013> assign/print file <filename>
Midas 014> print/table <table> :S_TI <output-column>

Hint: Due to the 400 second spacecraft wobble and the support structure of the PSPC entrance window the PSPC lightcurves show periodic dips. When highest precision of the lightcurve is required, use 400 sec binning in order to cover a full wobble period with each bin. Periodic features on timescales of tens of seconds to 400 seconds may be spurious.

HRI lightcurves

The make/projection correct command cannot be applied to HRI data. Therefore the necessary corrections have to be applied using the correct/lightcurve command.

Selection of events:
As for the PSPC the select_light.cmd command file may be used, but due to the better angular resolution of the HRI a smaller extraction radius (e.g. 50 sky pixel = 25'') is appropriate:
Midas 009> make/projection select_light
-> raw source and background light curves in table lightcurve.tbl (columns :slight, :blight).

Apply vignetting and deadtime correction using the parameter file correct_light_hri.par :
Midas 010> correct/lightcurve correct_light_hri
-> New column in table lightcurve containing corrected source light curve (column label :c_slight).

Background subtraction:
Midas 011> prepare/lightcurve lightcurve :lc :c_slight,:blight
-> background subtracted, corrected lightcurve in column :lc of table lightcurve.tbl.

Plotting and ASCII output:
see previous section PSPC lightcurves.


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Last modified 21 May 2008
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