Документ взят из кэша поисковой машины. Адрес оригинального документа : http://hea-www.harvard.edu/XJET/fluxmap.txt Дата изменения: Sat Apr 24 02:44:31 2010 Дата индексирования: Mon Oct 1 20:37:08 2012 Кодировка: Поисковые слова: universe

file: fluxmap.txt
crea: deh
date: 2008 Jul 29
tlm : 2010 Apr 23
Subj: Content for Xjet webpage on fluxmaps


1.0 GENERAL
2.0 FLUXMAPS & PHOTOMETRY
3.0 RADIO MAPS
4.0 EVENT FILE
5.0 DOCUMENTATION
6.0 CAVEATS

1.0 GENERAL

From 2008.5, we are adding Chandra X-ray fluxmaps and associated files
to the downloadable images. This is in accord with our long term
policy of providing processed data products and will permit users to
perform X-ray photometry on their local systems in a relatively
straightforward manner. During spot checking, we reproduced published
values to within a factor of two, often within 20%. We feel that this
is satisfactory since the CALDB has changed and we are using
reprocessed data from the archives (2008). Furthermore, our products
are observed values whereas published fluxes and flux densities are
normally 'unabsorbed' fluxes.

There are 6 maps provided for each source: 3 fluxmaps (soft, medium,
and hard), a smoothed fluxmap constructed by adding the 3 band maps
and applying a Gaussian smoothing, a radio map used for registration
of the X-ray files, and an 'evt2' file which has been registered so as
to align the nuclear X-ray emission with the corresponding radio peak.
Pixel randomization has been removed from the evt file and the
original data were obtained from the Chandra archive after the
reprocessing of 2007.

In 2010 we decided to regenerate the fluxmaps using CIAO 4.2.1 and
CALDB 4.2.2. With the release (2009 Dec; CALDB 4.2) of a new model
for the contamination buildup on the ACIS filters, it became obvious
that our soft fluxmaps were compromised (with increasing time from
2004, the low energy effective area becomes more and more
overestimated meaning soft fluxes became more and more
underestimated). CALDB 4.2 contains a new contamination model which
corrects this situation. At the same time, we have devised a method
to replace the label "No thumbnail" with labels describing the type of
file. Thus whenever you encounter these labels ("registered event
file", "soft flux map", etc) you may be assured that the fluxmaps were
constructed with exposure maps using the new contamination model.


2.0 FLUXMAPS & PHOTOMETRY

2.1 The processing of fluxmaps is performed in 5 steps:

1) unpack the archival data and update the ardlib file for the badpix
information.

2) check for high background intervals and time-filter if necessary.
Unlike an 'official' pipeline, we do not have an algorithm for this
step. Rather it is a subjective decision based on the total available
exposure time as well as how badly the background is affected.

3) Remove pixel randomization from the evt file and apply the latest gain map.

4) construct a regridded smoothed array of the central source and use
this to determine the position of the nuclear emission. Shift the evt
file to align the X-ray peak/centroid with the radio peak by doing an
'hedit' of the key words RA_NOM, DEC_NOM, TCRVL11 & 12.

5) Produce the three band maps by constructing monochromatic exposure
maps and dividing them into regridded arrays (1024x1024) of each band.

band nominal energy band
____ ______________ _____________
soft 0.8 0.5 - 1 keV
med 1.4 1 - 2 keV
hard 4 2 - 7 keV

We also multiply each band map by h*nu where nu corresponds to the
nominal energy. Thus the units for the flux maps are erg/cm^2/s per
logical pixel.

Summing the 3 band maps produces the total fluxmap which we smooth
with a Gaussian and provide for visual purposes. It should not be
used for photometry.

Regridding is usually a factor of 4 (converting 0.492" native pixels
to 0.123" logical pixels), but may differ so as to include a larger
area of the sky in the fixed 1024 format ("f2"), or to improve
resolution for sources which are "event rich".

2.2 Photometry can be performed by the user with the tool of his
choice (e.g. DS9) by measuring any user-defined aperture on each or
all of the band maps. The result requires only a single correction to
accomodate the fact that all events in each band were assigned an erg
value based on the nominal energy. This is most easily done by using
'dmstat' in ciao on the evt file to determine the actual mean energy
of the events within the aperture. Then the measured flux is
multiplied by /nominal_energy. Since this correction is
normally only a few percent, it can be omitted for users without
access to CIAO. An example for the soft band:

dmstat "120NrR_evt2.fits[sky=region(k4ciao.reg),energy=500:1000][cols energy]" > k4.meanE






3.0 RADIO MAPS

Radio maps have been obtained from gererous colleagues, from the NRAO
VLA Archive Survey [NVAS; http://www.aoc.nrao.edu/~vlbacald/], from
the DRAGN website [http://www.jb.man.ac.uk/atlas/], from the MERLIN
archive [http://www.merlin.ac.uk/archive/], from NED
[http://nedwww.ipac.caltech.edu/], or by processing archival data.
Where possible we include the origin in the ASCII file accompanying
each fits image. We have used AIPS to obtain an rms value for the map
and the beam area in pixels, thereby allowing radio intensities to be
obtained with any imaging tool.


4.0 EVENT FILE

The event file we provide should be identical to the evt2 file in the
Chandra archives except for the removal of pixel randomization and the
registration (shifting the sky coordinates to be the same as the
reference radio map).



5.0 DOCUMENTATION

The construction of filenames is based on our established guidelines, and is a
compromise between including useful information and ending up with unwieldly,
long names. Each image whether generated by us or contributed by users should
have an ASCII descriptor file.

6.0 CAVEATS

We have made no effort to merge multiple observations. Normally we choose
the longest available obsid.

ACIS contamination buildup which reduces the effective area at lower energies
is dealt with via the use of exposure maps. However, it must be remembered
that the flux in the soft band will be reduced by the galactic column density
and if there is additional absorption at the source, other bands may also be
affected.

Pileup is not normally a problem for jet knots and hotspots, but it can occur.