FLIX - Sensitivity estimator

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 points where no source was detected in the 4XMM 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 https://arxiv.org/abs/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:

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 and examined with utilities such as FV or TOPCAT.

Input Parameters

The following parameters may be entered on the web form:

Note that the FITS file that is produced contains a complete set of columns including encircled flux in all XMM energy bands

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 -loge(likelihood).

N-sigma

-loge(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 4XMM 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 colons or h, m, s, d, and the RA and DEC must be separated by spaces. Thus the following formats are allowed:

 8.51332233 -5.40595761 decimal degrees
 12:30:45.12345 -12:34:56.789 sexagesimal hms
 12h30m45.1s -12d34m56.78s with h, m, s and d specified rather than colons

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 "no match" page is returned with the coordinates of the nearest XMM pointing. For some positions there may be multiple observations containing data, up to 50 in a few cases. The processing time in such case can take a bit more than a minute. Please be patient. 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 be retained on the server for at least 24 hours before being deleted.

The columns in the FITS table are described below; most of these columns are also in the HTML table.

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

inst_ACTIVE

Duration for which instrument was active (seconds)

inst_FILTER

Filter name for each instrument

inst_SUBMODE

Data submode - see XMM handbook for more details

inst_MASKFRAC

Fraction of pixels providing data around this position



inst_band_FUP

Flux upper limit for each instrument and band (erg/sq cm/sec)

inst_band_FES

Flux estimate for each instrument and band using counts in circle

and fitted background level (erg/sq cm/sec)

inst_band_FER

Standard error on above flux estimate (erg/sq cm/sec)

inst_band_BGMEAN

Background counts from BKGMAP summed over the circle of radius cutrad.

This is adjusted to allow for any missing pixels in the DETMSK array.

inst_band_BGCIRC

Background summed over a circle of radius radpix, not adjusted for missing pixels.

inst_band_TEXPOS

Exposure time (seconds) at the nearest pixel to the position, derived from EXPMAP product.

inst_band_IMCIRC

Sum of the image pixels over a circle of radius radpix, not adjusted for missing pixels

inst_band_ECF

Energy Conversion Factor (ECF) used to derive flux from count-rate (count sq cm/erg)

inst_band_EEF

Enclosed Energy Fraction (EEF) used to derive flux from count-rate

SRCID

Source identifier (note not DETID) of the nearest source in the 3XMM survey

SRCDIST

Distance of this source from the specified position (arcsecs)

Notes for the table above:

radpix is the circle radius chosen by the user in the HTML form, the default is 30 arcseconds.

In the column names: inst is a two-character abbreviation of the instrument, M1 = MOS-1, M2 = MOS2, PN = PN.

In the column names: band is a single digit band designator: bands 1 to 5 are the standard EPIC energy bands, band 8 is the overall band corresponding to bands 1-5. Band 6 is approx bands 1+2+3, band 7 is approx bands 4+5, band 9 is bands 3+4+5.

cutrad is a radius in pixels (4 arcsecs/pixel) which is energy band-dependent:

Band

1

2

3

4

5

6

7

8

9

cutrad

5.12

5.08

5.15

5.49

5.86

5.1

5.7

5.2

5.24



In the result page the SRCID and distance for the six nearest sources are shown, but only the nearest is listed in the FITS file to avoid format complications.

The flux estimate and its error from pixels in the image are calculated using these formulae:

Flux estimate, FEST = (IMCIRC - BGCIRC) / (TEXPOS * ECF * EEF)

Error on flux estimate, FERR = sqrt(IMCIRC + BGCIRC) / (TEXPOS * ECF * EEF)

Note that the IMCIRC and BGCIRC values result from summing flux and background only for valid pixels, i.e. pixels in chip gaps or other places where DETMSK is zero are simply ignored. This is because there really isn’t any satisfactory way of estimating the effect of these gaps, and it could make a large difference depending on whether the source happens to be centred in a chip-gap or not.