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: http://xmm.vilspa.esa.es/sas/6.0.0/doc/emldetect/node3.html
Дата изменения: Thu Mar 18 23:21:31 2004 Дата индексирования: Sat Dec 22 10:14:12 2007 Кодировка: Поисковые слова: chandra |
For an input list of EBOXDETECT source locations, simultaneous maximum likelihood PSF fits to the source count distribution are performed in all energy bands of each EPIC telescope. A description of the main properties of the detection algorithm may be found in Cruddace, Hasinger, Schmitt (1988).
Free fit parameters are: Source location (alpha, delta), source extent (Gaussian sigma or beta model core radius), and source count rates in each energy band of each telescope. The source location and source extent are constrained to the same best-fit value in all energy bands of each EPIC instrument whereas the source count rates are adjusted to their individual best-fit value in each energy band of each EPIC instrument. Derived parameters are: Total source count rate, likelihood of detection (total and in each energy band), likelihood of source extent, and three hardness ratios.
All detection likelihoods are transformed to equivalent likelihoods
(Column DET_ML of the output source table), corresponding to the case of
two free parameters to allow comparison
between detection runs with different numbers of free parameters
(i.e., when different numbers of input images are used):
where is the incomplete Gamma function,
is the number of energy bands
involved,
is the number of degrees
of freedom of the fit (
if task parameter fitextent=yes
and
otherwise), and
with
as defined by Cash (1979).
Note, that
=1 in the case of the individual energy band detection
likelihoods listed in source table rows with ID_BAND
0 and
is equal
to the total number of energy bands in the ID_BAND=0 summary rows.
The transformed
detection likelihoods obey the simple relationship
where
is the probability for a random Poissonian fluctuation to
have caused the observed source counts. This is in agreement with
the detection likelihoods as defined in task eboxdetect
(column SIGMA
of eboxdetect
source lists).
Note that for very small numbers of source counts
(less than
9 counts, Cash 1979) this relation has to be treated
with caution.
Therefore it will
only give a rough estimate of the number of expected spurious sources.
If the parameter fitextent is set to ``yes'', the point spread function will
be convolved with a source extent model, that can be set to either a Gaussian
profile or a -model profile via the parameter extentmodel.
In the case of extentmodel=beta the surface brightness is calculated as
The value of the core radius is written to the column EXT of the
output source list. I the case of a Gaussian extent model
is written to
column EXT.
From version 4.27 also the extent likelihood values (EXT_ML) are corrected for the number of input images with the formalism described above.
If detection over several energy bands is performed, up to three hardness ratios are calculated from the source counts rates in the individual bands. The hardness ratios are defined as follows:
where B denotes the count rates in energy bands n and m, respectively. The energy bands n and m used to calculate the hardness ratios can be specified for each instrument via the parameters hrpndef, hrm1def, and hrm2def. The default band assignments (identical for all instruments) are given in the following table:
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1 | 1 | 2 |
2 | 2 | 3 |
3 | 3 | 4 |
All EPIC PN source count rates and fluxes written to the emldetect source list are corrected for photons arriving during readout of the PN CCDs and therefore are not detected on the nominal source position (out-of-time events). The correction factor is 1.0626 for PrimeFullWindow mode and 1.0223 for the PrimeFullWindowExtended mode. No correction is applied to data in other observing modes. From eexpmap v3.31 on the OOT events correction is applied to the exposure maps in all observing modes. Emldetect (v4.28 or higher) reads the keyword OOTCORR from the FITS header of the exposure maps. If OOTCORR is existing and set to ``true'', no further correction is applied by emldetect.
ML_ID_SRC | EMLDETECT source number |
BOX_ID_SRC | corresponding EBOXDETECT input source number |
ID_INST | instrument ID; 1: PN, 2: MOS1, 3: MOS2 |
ID_BAND | energy band number (band number 0: summary band) |
ID_CLUSTER | cluster id; sources fit simultaneously have same number |
SCTS | source counts |
SCTS_ERR | source counts error |
X_IMA | source image pixel X coordinate |
X_IMA_ERR | error of image pixel X coordinate |
Y_IMA | source image pixel Y coordinate |
Y_IMA_ERR | error of image pixel Y coordinate |
EXT | source extent, gaussian sigma or beta model core radius (arcsec) |
EXT_ERR | extent error error |
DET_ML | Likelihood of detection |
EXT_ML | Likelihood of extent |
BG_MAP | background at source location (counts/pixel) |
EXP_MAP | exposure at source location (seconds) |
FLUX | source flux (cgs units) |
FLUX_ERR | source flux error error |
RATE | source count rate (counts/sec) |
RATE_ERR | count rate error |
RA | source right ascension (degrees) |
DEC | source declination (degrees) |
RADEC_ERR | error (arcsec) |
LII | source galactic longitude (degrees) |
BII | source galactic latitude (degrees) |
RAWX | raw X source coordinate |
RAWY | raw Y source coordinate |
CCDNR | chip number |
HRn (n=1..3) | hardness ratios 1..3 |
HRn_ERR (n=1..3) | hardness ratio error |
CUTRAD | source cut out radius |
EFF | encircled energy fraction |
VIGNETTING | vignetting |
DIST_NN | distance to nearest neighbour (arcsec) |
VAL_FLAG | V&V flags |
VER_FLAG | V&V flags |
VER_COMM | V&V comment |
The band numbers n, and m are assigned to the individual bands by numbering the corresponding input images in the order in which they are given on the command line. It is therefore important that the ordering of the input images is consistent with the contents of hrdef to obtain meaningful hardness ratios.
Note that the source extent can only be determined reliably for relatively bright objects. If the likelihood of the source extent falls below a threshold, point source parameters are derived.
For each detected source the output source table contains one row for each energy band of each instrument. In addition, summary rows list combined results per instrument and total. The summary rows over the energy bands for each instruments (ID_BAND = 0) contain sums of the entries in the individual energy bands, where appropriate (counts, count rates, fluxes, and detection likelihoods). Spatial parameters (positions and extent values) are identical for all energy bands and repeated in the summary row.
The individual source rows are identified through the column entries ID_INST and ID_BAND in the output table where ID_INST refers to the EPIC instrument (1: PN, 2: MOS1, 3: MOS2, 0: summary row) and ID_BAND is the energy band number as defined by the ordering in which the energy bands are given on the command line. An ID_BAND value of 0 again refers to the summary information. An ID_BAND value of 9 stands for the XID energy band (0.5 - 4.5 keV) which will only be present if the input parameter withxidband has been set to true. The upper and lower bounds of each energy band are available in the header keywords aa_n_ELO and aa_n_EHI where aa stands for the EPIC camera (PN, M1, or M2) and n stands for the energy band number as given in table column ID_BAND. Additional keywords N_INST, aa_BNDS, and XID_BND specify the number of EPIC cameras, number of energy bands for each EPIC camera, and whether XID band information is present in the source table. Note, that the energy bands which constitute the XID band have to be specified for each instrument separately using the parameters xidpndef, xidm1def, xidm2def if the default values (bands 2 and 3, as defined in ID_BAND column) are not appropriate.
Simultaneous fitting of data from different instruments (i.e., all EPIC pn and MOS data) or different exposures is supported. The PSF fitting may either be performed in single source or in multi-source mode. In multi-source mode sources with overlapping PSFs are fitted simultaneously. Up to six neighbouring sources may be fitted simultaneously. Selection of sources for simultaneous fitting is controlled by a distance parameter (scut) and the maximum number of sources to be fit simultaneously (parameter nmaxfit. Sources fit simultaneous are identified in the the output table through the ID_CLUSTER table column. It is also possible to fit several PSFs (up to three) for each input source position by setting parameter nmulsou to the appropriate value.
If parameter usecalpsf is set to true (this is the default)
PSF fitting is performed using a tabulated energy and position
dependent PSF as provided in the calibration database
(accuracy level = medium). Alternatively, an internal,
hardcoded PSF as specified in section ``Algorithm'' is used.
The hardcoded PSF consists of a superposition
of four Gaussian profiles with an off-axis dependent width.
It is axially symmetric and currently has no energy dependence.
Right ascension and declination of the optical axis positions used for the
calculation of the off-axis angles are stored in keywords with the
names OARAaann and OADEaann where ``aa'' designates the EPIC camera
(PN, M1, or M2) and ``nn'' is a running number.
Note that the -model source extent function (extentmodel=beta)
can only be used with the CAL PSF (usecalpsf=true).
Two parameters determine the image region, on which a source fit is performed:
the parameter ecut determines the size of the subimage used for fitting
a source. The parameter scut determines the radius around each source,
in which other input sources are considered for multi-PSF fitting, if
parameter nmulsou is . Both ecut and scut
are given as encircled energy fraction of the calibration PSF. The actual radii
in pixel units therefore change slightly with energy band and source position.
The actual value for the cutout radius for each sources is listed in the
column CUTRAD of the output source list.
From version 4.32 the maximum value of the extent fit parameter can be given via the task parameter maxextent. The unit is image pixels. Large values of maxextent can in some cases lead to spurious detection of extended sources.
XMM-Newton SOC/SSC -- 2004-03-18