FLIX - Upper Limit Service

Purpose

The main purpose of FLIX is to produce estimates of the upper-limit to the X-ray flux at a given point in the sky for there to have been no source detected there in the 2XMMi survey. This upper-limit flux is found empirically using the algorithm described by Carrera et al (2007) The XMM-Newton Serendipitous Survey, IV. The AXIS X-ray source counts and angular clustering. A copy of the preprint may be found in astro-ph at http://www.eprintweb.org/S/article/astro-ph/0703451 - the algorithm appears in Appendix A. These upper-limits use the DETMSK to find which pixels in the image are valid, EXPMAP data to find the exposure time, and BKGMAP data to determine the fitted background level.

In addition FLIX provides other information:

It computes a crude estimate of actual flux near the specified position by summing counts in a circle centred there, using the background level from the BKGMAP data. The equivalent flux is converted using an approximation to the point-spread function in the calibration files. Energy conversion factors can be found in Mateos et al. 2009 (astro-ph:0901.4026).
It scans the 2XMMi catalogue and lists the nearest source (FITS output) and up to 5 sources (HTML output) together with their offsets.
It produces a thumbnail image by overlaying images for the whole band from all active instruments (MOS1, MOS2, PN). Further thumbnails can be obtained by clicking on the SRCID numbers which link to the source summary pages in the LEDAS facility.

Results are returned as a web page with an HTML table per celestial position, and a FITS file is also produced with a binary table containing one row per observation which contains useful information. This may be downloaded as soon as the HTML output is complete, and examined with utilities such as FV or TOPCAT.

Running FLIX

The program is a CGI script which can be run from http://www.ledas.ac.uk/flix/flix.html

Input Parameters

The following parameters may be entered on the web form:

Position of interest and source-name: for a single position enter it in the text box, for multiple sources use any text editor to create a source file and upload it using the flix browser button. In either case the format is the same, and explained below.
Detection likelihood threshold for upper-limit estimation: the default is 10.0 which is the value used in the 2XMMi catalogue, but you may choose a higher value.
Radius of the circle used for actual flux estimation (arc-seconds): the default value is 30 arcseconds (7.5 pixels) but other values may be chosen within reason.
Wavebands for which HTML results are required: the FITS file output contains results for all bands, these selections only affect the HTML results. Selecting all bands may produce a results page which is much wider than your screen.
Thumbnail images: the size of this may be selected, ohttp://www.star.bris.ac.uk/~mbt/topcat/r you may choose to have no images at all to speed up display when using a large number of positions.
Image overlays: you may choose to have the image with no overlays, or with a green circle around the position of interest at the specified radius, or with also 2XMMi sources marked, and with SRCID numbers listed.

Likelihood Thresholds

For those who prefer to think in terms of N-sigma (assuming a 1-dimensional Gaussian distribution) then the table below shows approximate conversions from N-sigma to the likelihood values, which are actually -log_e(likelihood).

N-sigma	-log_e(likelihood)
3	6.6
4	10.4
5	15.1
6	20.7
7	27.4
8	35.0

Note that the likelihood threshold for the 2XMM surveys (and the default value for FLIX) is 10, corresponding to around 4 sigma.

Input file format

The input file is a plain text file with one position per line. This should contain an RA, a DEC; optionally a source name or identifier may follow. The RA and DEC can be given in decimal degrees, or sexagesimal hours, minutes, seconds (for RA) and degrees, arc-minutes, and arc-seconds (for DEC). The sexagesimal components may be separated by a space or colons, and the RA and DEC may be separated by spaces, a comma, or a vertical bar. Thus all of the following formats are allowed:

 8.51332233     -5.40595761     decimal degrees
 12:30:45.12345  -12:34:56.789  sexagesimal hms
 12 30 45.12345  -12 34 56.789  with spaces rather than colons
 12 30.75 -12 34.95             if hours and mins only need decimal minutes

 8.51332233,     -5.40595761     decimal degrees
 12:30:45.12345,  -12:34:56.789  sexagesimal hms
 12 30 45.12345,  -12 34 56.789  with spaces rather than colons
 12 30.75,  -12 34.95             if hours and mins only need decimal minutes

75 59 59 89 59 59 near maximum dec
75 59 59 00 00 12 mostly zeros for dec
0 0 57 -0 00 12  negative sign for dec

Results

Note that for each position the program has to read around compressed 40 data product files before performing the flux calculations, which takes from 5 to 10 seconds per position typically.

For positions in the sky which XMM did not cover a null row is produced in the HTML table, but no rows are written to the FITS table. For some positions there may be multiple observations containing data, up to 30 in a few cases. The FITS file contains one row per observation matching each position, so it may have more rows than there are positions in the input text file.

The FITS file will not be downloadable until the processing is completed; it will be retained on the server for at least 24 hours before being deleted.

The columns in the FITS table are described below; the columns in the HTML table are similar, but also contain a thumbnail image if requested.

RA	Right ascension of point of interest (decimal degrees)
DEC	Declination of point of interest (decimal degrees)
NAME	Name from input list, if any
OBS_ID	Observation identifier
REVOLUT	Revolution i.e. orbit number
RA_PNT	Pointing axis R.A. (decimal degrees)
DEC_PNT	Pointing axis declination (decimal degrees)
OFFAXIS	Approximate distance of point of interest from pointing axis (arcmins)
OBJECT	Target name of the original observation
OBSERVER	Name of principal investigator of original observation
DATE_OBS	Date and time at which observation was started
DATE_END	Date and time at which observation ended
M1/M2/PN_ACTIVE	Duration for which instrument was active (seconds)
M1/M2/PN_FILTER	Filter name for each instrument
M2/M2/PN_SUBMODE	Data submode - see XMM handbook for more details
M1/M2/PN_MASKFRAC	Fraction of pixels providing data around this position
M1/M2/PN_8_EXP	Exposure time for whole band (seconds)
M1/M2/PN_band_FUP	Flux upper limit for each instrument and bands 1-9 (erg/sq cm/sec)
M1/M2/PN_band_FES	Flux estimate for each instrument and band using counts in circle and fitted background level (erg/sq cm/sec)
M1/M2/PN_band_FER	Standard error on above flux estimate (erg/sq cm/sec)
SRCID	Source identifier (note not DETID) of the nearest source in the 2XMMi survey
SRCDIST	Distance of this source from the specified position (arcsecs)

In the HTML table the SRCID and SRCDIST values for the five nearest sources are shown, but only the nearest is listed in the FITS file to avoid format complications.

Author: Clive Page, University of Leicester. email: cgp (at) star.le.ac.uk
Date: 2009 January 27.