- set up your SAS environment (following the SAS start-up
thread)
- start xmmselect
xmmselect table=PN.evt &
The xmmselect
call pops-up a window similar to that shown in Fig.1.
Fig.1: The main xmmselect
window
In this window, we identify:
- a data screening widget (top)
- a data column panel (middle)
- the buttons 1D region and 2D region, which
allow to translate selection expression defined in a grace or ds9 window,
respectively, into proper selectlib
expressions
- "action" buttons (bottom)
- extract a single event (i.e. pattern zero only), high
energy (E > 10 keV) light curve, to identify intervals of flaring
particle background
. This is done by:
- writing the following selection expression in the data
screening widget: '#XMMEA_EP && (PI>10000) &&
(PATTERN==0)' (cf. Fig.1)
- clicking on the radio button close to the TIME column
in the data column panel
- clicking on "OGIP rate curve"
This will pop-up another window: the evselect parameter user
interface (see Fig.2).
Fig.2: The main evselect
window
On this multi-panel window, one should at least:
- click on the Lightcurve menu (and this leads you to
the sub-panel shown in Fig.3)
- define the output file name in the rateset widget (e.g.:ratePN.fits)
- define the time bin size (in seconds) in the timebinsize
widget
- click Run
Fig.3: The Lightcurve panel in the evselect window
As customary for SAS task, each widget, button or menu in the evselect window
corresponds to a task parameter. The whole list of available evselect parameters, with
their description, is available at the evselect task
description.
Once Run is clicked, dsplot
is silently run on the created light curve, and the corresponding grace
window appears (see Fig.4)
- define a "low-background" interval on the light curve, using the
following steps in the grace
window:
- go to the item Edit/Region/Define
- define region type (e.g.: in Horiz. Range)
- click Define
- click oncewith the right mouse on the grace
window to define the starting time of the first interval, and once
to define the end time.
- repeat this as many times as needed. Just be aware that the
counter Define region: needs to be updated, if you want to
define 7.0.0 regions (i.e., not to overwrite the old ones)
Fig.4: A grace
window, displaying a light curve, and one interval created as explained
in text
Be aware that grace
allows you to define several different types of intervals. Among the
normally most useful: Above/below line, in Vert. range,Out
of Horiz/Vert. range, Left/Right of line
When you are happy with your definition, click the button 1D
region in the xmmselect
window. The selection region will be automatically transferred into the
data selection widget of the xmmselect
window, and properly translated into a selectlib expression
- extract an image (sky coordinates in this example; extraction in
detector - DET[XY] - coordinates is possible as well, and may
be preferable for some specific scientific needs)
. This is accomplished by:
- clicking the square checkbox besides X and Y
in the data column xmmselect
panel
- click on Image
- go the the Image sub-panel in the evselect window (see
Fig.5)
- change at least the file name in the imageset window (e.g.
to PNimage.fits)
- [remember to remove the selection expression you used to generate
the background light curve from the Selection expression widget]
- click Run
Fig.5: The Image panel in the evselect window
xmmselect will
automatically launch a ds9
window on the created image
- select the region, from which the source+background spectrum
shall be accumulated, using the Region/Circle menu in ds9
. Properties shall be set to Source (the green circle
on the ds9 window will appear as a solid line)
- select the region, from which the background spectrum shall be
accumulated, using the Region/Circle menu in ds9
. Properties shall be set to Background (the green
circle on the ds9 window will appear as a dashed line)
- click the function button OGIP Spectral Products in the
main xmmselect window
- a window pops-up, asking the user for the preferred method to
determine the source+background extraction region:
Fig.6: Spectrum extraction region optimization window in especget
- Use optimized region: allows the user to extract the
source+background spectrum from a circular region, which maximizes the
signal-to-noise ratio. The properties of this optimal region are
displayed in the window
- Use initial region: allows the user to extract the
source+background spectrum from the region initially chosen
- Start over: allows the user to start the whole
process again, from the definition of spectral extraction regions in the
field-of-view image (step 6. above)
- a 7.0.0 window pops-up, asking for confirmation of the input
parameters (see Fig.7)
Fig.7: Main especget
window
It is recommended to change the filestem parameter to a string
of user's choice. Be reminded that xmmselect
silently overwrites files with the same name. Given the choices in
Fig.7, the following files will be created:
- stem_srcspec.ds: source+background spectrum
- stem_bckspec.ds: background spectrum
- stem_srcspec.rmf: redistribution matrix
- stem_srcspec.arf: effective area vector
- prepare the source+background spectrum and link associated files
FTOOL: grppha: PHA filename: mypn_src.ds
output filename: PNsource_spectrum.grp
group min 25 ! as an example
exit
- fit the
spectrum
- NOTE ON PATTERN SELECTION IN PN SPECTRA:
For bright sources and sources with narrow lines it might be better to
extract two spectra and corresponding backgrounds, response and
ancillary files: one set for single pixel events (PATTERN==0)
and another set for doubles (PATTERN IN [1:4]).
Fitting these two spectra simultaneously will show if there are any
problems with pile-up (see also SAS task epatplot) and - as
the energy resolution for singles is slightly better than the one for
doubles - will show the line features at highest energy resolution in
the single events spectra.
However, in case of TIMING mode observations (where the rate of single
to double events depends on the source position) one should always
create and fit a spectrum of the combined single and double events.