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: http://xmm.vilspa.esa.es/sas/5.4.1/doc/emldetect/node3.html
Дата изменения: Fri Jan 10 22:20:44 2003 Дата индексирования: Sat Dec 22 10:15:55 2007 Кодировка: Поисковые слова: m 103 |
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), 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).
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 |
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 (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.
XMM-Newton SOC/SSC -- 2003-01-10