The coordinates of the source selected as 'prime' in the
source list are wrong. How can I change them?
In the example shown below, the coordinates of the PROPOSAL
source are incorrect (click on the table to see an enlarged
version). They should be R.A.=184.61100 and Dec=+29.81267.
As a result of this, the extraction mask is far off the source
position and, after processing using the proposal coordinates, the
final spectrum is wrong (note the empty regions in the dispersion
vs energy image):
Wrong source coordinates
To change the coordinates of the prime source, the new coordinates
have to be entered, and the interactive pipeline has to be run
starting at least from the angles stage, to calculate the
correct aspect drift corrections.
where entrystage and finalstage are left to their
default values. withscr=yes is used to define a new prime
source by the parameters scrclabel, srcra and
srcdec. The label is the name that the new source will get in
the source list, coordinates are J2000 in decimal degrees.
Alternatively, skip the first rgsproc stage
(events), modify the existing source list, and start
rgsproc at the second stage (angles).
where ra, dec and label are similar to
srcra, srcdec and scrclabel, and
changeprime and userasprime have both to be set to
yes in order to change the primary source.
Correct source coordinates
There are several X-ray sources in the field of view! How is
this taken into account in the processing?
In case you find sources not included in the source list, you may
want to add them for processing (i.e. getting spectra, but not as a
primary source) and/or, more important, for excluding them from the
background extraction region.
This example shows an observation of YY Gem
(obsid. 01237101), in which the nearby Castor is also
visible at the top of the EPIC image (left). The spectrum of this
second source is also present in the RGS spatial image (right).
If the observation is processed with the default rgsproc
parameters, the spectrum of Castor would be included in the
background:
Wrong background extraction region
and therefore the net source spectrum would be underestimated.
The image shown above has been created with the command:
where procsrcsexpr='INDEX==1' indicates that the extraction
mask is computed for the first entry in the source list (i.e. the
PROPOSAL coordinates of YY Gem).
The new background is now correctly estimated, as the contribution
from Castor is not included any more:
Correct background extraction region
The extraction mask is too small for the size of my
source...
It may happen that the source observed is extended and fills part or
all of the aperture. The use of the default values for the size of
the source and the background extraction regions would lead to
erroneous results, both for underestimating the source countrate and
for overestimating the background.
We show in the image below the case of a moderately (1.5 arcmin)
extended Supernova Remnant N49 in the LMC. The standard
95%-of-the-PSF extraction region is clearly too small for the actual
size of the source (left panel), while a 98% region seems to be more
appropriate (right panel).
95% of the PSF
98% of the PSF
First, the size of the extraction region has to be changed in the
source list, and new extraction masks have to be computed:
where the last parameter, xpsfexcl=99, increases the exclusion
region around the source used to determine the background with
respect to the default value of 98%.
[Note that to load RGS extraction regions into ds9 they have first to
be converted to the appropriate format with the SAS task
cxctods9:
