Upper Limit Server
Contents
The Upper Limit Server (ULS) is designed to return the upper limit to
a count rate and flux which could be produced by a source at the input position in a given image. It is not intended to be a source detection system and will return an
upper limit rather than a count rate for faint sources. For stronger sources
it will return a count rate but the automatic processing means that these will not be as
accurate as count rates available in the mission source catalogues.
To find the strengths of detected sources it is strongly recommended to
use a data archive such as the XMM-Newton Science Archive (XSA), LEDAS
or HEASARC.
The routine works by extracting images which contain
the entered celestial position from an image repository. Each image is
then processed by the
XMM-Newton SAS task eregionanalyse which finds the background subtracted
count rate at the position. The count rate is calculated by:
- Extract the counts from a circle of radius 30" about the position.
- Extract the background counts from an annulus of inner radius 45"
and outer radius 75" about the position.
- Perform a background subtraction, normalising for the areas of the
source and background regions.
- Calculate a background subtracted count rate at the source position.
- Calculate an upper limit at the source position, using the
Bayesian statistics of Kraft, Burrows and Nousek 1991 (ApJ 374, 344),
with a uniform prior. The significance (1,2 or 3-sigma) is user selectable
with the default set at 2-sigma (95.4%).
NB: The count rates are corrected for instrumental effects such as
the point spread function and vignetting.
If the background subtracted count rate is greater than twice its error
then the output shows the measured count rate with the error. Otherwise,
the upper limit
is displayed. This is conservative and means that low-significance, real
sources, will be shown as an upper limit rather than a measured count rate.
It should be stressed that the source catalogues should be consulted
to find the presence and strength of detected sources.
- Convert the count rate to a flux. The conversion uses the PIMMS software
(v 3.9b) with a fixed spectrum of a power-law of slope 1.7 absorbed by a column of
NH=3x1020 cm-2.
- Select missions to use (only XMM-Newton slew and pointed data
are currently available)
- Enter the RA, DEC in decimal or sexagesimal format of the required
sky position. Alternatively enter a list of coordinates from an ASCII file.
The coordinate list should be an ASCII file with one position per line
and optionally a source name or identifier, e.g.
54.260694 47.646995 AGN1
68.632096 +12.629393 AGN2
NB: Only decimal degrees are permitted in a list of sources at the moment.
The current implementation takes about 30-60 seconds execution time per source.
It is recommended to limit lists to around 20 sources.
The output is a block of results per position.
Each block has, as a header, the coordinates of the position and the
identifier or source name.
There is an output line for each observation containing the input position.
The columns are:
- OBSDATE : the data of the observation in YYYY-MM-DDTHH:MM:SS format,
e.g. 2004-03-05T14:00:00 being 14.00 UTC on 5th March 2004.
- COUNT RATE (0.2-12.0) : The count rate or upper limit to the count
rate in the full (0.2-12 keV) XMM-Newton energy band. Photons with event
pattern 0 only are used from 0.2-0.5 keV and event patterns 0-4 are used
at energies above 0.5 keV.
- COUNT RATE (0.2-2.0) : the soft energy band count rate or upper limit.
- COUNT RATE (2.0-12.0) : the hard energy band count rate or upper limit.
- EXP TIME (s) : the exposure time in the full (0.2-12 keV) energy
band at the given position
- FLUX (0.2-12.0) : the flux in the full energy band, calculated
from the count rate assuming a spectral model of a power-law of slope 1.7
absorbed by a Hydrogen column of 3x1020 cm-2,
units of ergs s-1 cm-2
- FLUX (0.2-2.0) : the flux in the soft energy band
- FLUX (2.0-12.0) : the flux in the hard energy band
- QFLAG : a quality flag. For the XMM-Newton slew data this indicates whether the count rate is so high that the source is piled-up. If this is set to 'N' then the given count rate is likely to be an underestimate of the true
value.
The same output as XMM-Newton slew data except that only the 0.2-12 keV
count rate and flux are shown.
If the mouse is placed over the count rate values, a box is displayed showing
the number of counts detected in the source box, the number of background
counts calculated from the number of counts in the background box multiplied
by the scaling factor (BSF) for the respective box sizes and
the encircled energy factor (EEF) which was used to correct for photons
scattered outside the finite source box.
count rate = ( source_counts - backgnd_counts ) * EEF / exposure_time
XMM-Newton slew data is taken by the EPIC-pn instrument with the 'Medium' filter.
The XMM-Newton count rate is converted to a flux using the PIMMS v3.9b
software. PIMMS v3.9b, assumes that the EPIC pn count rates contain
events of pattern 0-4 which is nearly compatible with the pattern
selection used in the creation of the slew images
(pattern 0 from 200-500 eV and patterns 0-4
at energies from 500-12000 eV).
The conversion is performed with a spectral model of
a power-law of slope 1.7
absorbed by a Hydrogen column of 3x1020 cm-2.
To convert to flux using a different model; take the count rate from the
output of the tool as the input count rate for a 'Medium' filter EPIC-pn
observation using event patterns 0-4. If using PIMMS or WebPimms please
remember that they currently require
that the input count rate is from an extraction radius of 15" (roughly 72%
of the total counts).
Due to space restrictions in the slew image archive the routine
currently uses the broad band (0.2-12 keV) exposure maps to find the
exposure time at a point. The exposure time of the soft band and hard band images will be slightly different to this value. An algorithm should be
implemented to correct for this in a future version as described in
section 6.3 of Saxton et al., 2008, A&A, 480, 611.
Occasionally two upper limits or count rates are given for a position
with the same observation date. This is usually due to the position falling on the boundary between two images and having exposure time in both.
In this case please use the larger upper limit or count rate and ignore the other one. A future release should automatically choose the image with the
larger exposure time.
The system retrieves "all-EPIC" images, which consist of a single,
0.2-12 keV image, prepared by combining the EPIC-pn, MOS-1 and MOS-2
individual camera images. This gives a global source measurement for
each observation.
For a more detailed upper limit, or flux, from each camera, use the
FLIX
tool developed by Clive Page at the University of Leicester.
- VERSION 2.0
- Added in support for XMM-Newton pointed data.
- Allow the user to choose the significance level for the upper limit
- Allow the user to choose the type of statistics.
- Add in more missions
- Resolve the problem of overlapping slew images, i.e. positions which
are present on the border between two slew images and currently give rise
to two upper limits from the same slew.
- Allow the spectral model for flux conversion to be user selectable
- Allow sexagesimal input for coordinates in the source list