Post on 13-Apr-2021
P09
The X-ray Universe 2017, Rome, 6− 9 June 2017
Digging deeper: Towards a catalogue of detections from stacked XMM-Newton observations
Iris Traulsen, Axel Schwope, Georg Lamer (Leibniz-Institut für Astrophysik Potsdam, Germany)
on behalf of the XMM-Newton Survey Science Centre Consortium
Exposure time [ks]Exposure time [ks]
0 50 100 150
−135−90−4504590135180
−90
−60
−30
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90
• data not available
• public & processed
Potential for stacked source detection: overlapping observations in the XMM-Newton sky (blue),
with exposure time coded as brightness. Hammer-Aitoff projection in galactic coordinates.
Expanding 3XMM source detection to overlapping observations
About one third of the sky area covered by the XMM-Newton X-ray telescopes was ob-
served more than once. The XMM-Newton Survey Science Centre Consortium SSC
has generated catalogues of individual detections in all publicly available XMM-
Newton observations and published the most recent incarnation 3XMM-DR7
on June 1. To achieve ultimate sensitivity in repeatedly observed regions of
the XMM-Newton sky, we have now developed a new stan-
dardized approach to source detection on images of mul-
tiple pointings, making use of a maximum likelihood fitting
procedure as for 3XMM. Source parameters are derived
from stacks of the empirical point spread function per in-
volved instrument, energy band, and off-axis position (il-
lustrated to the left). The method has entered the Science
Analysis System SAS as a new task. It aims at higher
detection reliability and improved source parameters and
will be used to publish a catalogue of stacked detections.
Testing stacked source detection on “pseudo-mosaics”
In order to investigate the performance of
our stacking method, we construct “pseudo-
mosaics” by splitting long observations into arti-
ficial sub-exposures and performing source de-
tection on each sub-exposure and on their stack.
As intended, more and fainter sources are de-
tected in the stacks than in the individual sub-
exposures, the detection likelihood increases,
and the source parameters are determined with
higher precision. Because of statistical effects,
combining a long with a short observation may
result in a loss of detections anyhow. A system-
atic analysis of stacks with different exposure
time ratios reveals an increase in total detection
likelihood from about 40 % on, when using all
three instruments and five energy bands.
full observation: 132
2 parts 3 parts 4 parts
of similar length
5 parts 6 parts 1 long +
1 short
2 long +
1 short
0
50
100
150
Num
ber
of final dete
ctions
10
8/1
13
11
2/1
17
82
/86
89
/92
86
/88
63
/65
79
/82
83
/87
72
/75
58
/60
70
/72
72
/74
79
/80
59
/61
52
/53
70
/71
60
/61
66
/67
72
/74
52
/53
12
8/1
40
9/9
10
1/1
05
92
/93
9/9
130
126115
111 107
106 100
97 92
91
108
107
108
103
Decreasing number of detections with increasing number of
pointings for constant total exposure time. Sub-exposures
(light blue) of similar duration and of very different duration
have been extracted from a 100 ks observation of the Chandra
Deep Field South and stacked (dark blue). Numbers denote all
detections and those with valid 3XMM counterpart. Shaded:
Detections without 3XMM match, which thus might be spurious.
White: Percentage of total effective time per sub-exposure.
Para
mete
rssta
ck
/lo
ng
[%]
60
80
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120
140
Nu
mb
er
of
de
tectio
ns
Total det. likelihood > limit
At least one pointing > limit 60
80
100
120
140
Me
d.
de
tectio
n lik
elih
oo
d0 20 40 60
60
80
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120
140
Me
dia
n E
PIC
flu
x e
rro
r
0 20 40 60
60
80
100
120
140
Me
dia
n p
ositio
n e
rro
r
Exposure time short / long [%]
Detection efficiency and accuracy of the source parameters
depend on exposure time ratios: stacked source detection
combining a long sub-exposure of 50 % or 60 % of the total
time with a short one of increasing duration. The tests are
based on 55 XMM-Newton observations of at least 70 ks net.
A proto-catalogue of detections in largely overlapping observations
2 3 4 5 6 7 8 9 10 11 12
0
50
100
150
200
Nu
mb
er
of
sta
cks
Number of pointings per stack
Contents of the proto-catalogue.
Example detection image. Blue circles:
Stacked source detection. Thin circles:
Individual pointings. Boxes: 3XMM.
Stacked source detection uses a differ-
ent method to model the background.
For our first proto-catalogue, we have selected
full-frame observations with a minimum over-
lap of 80% in area, low background and more
than 99 % usable chip area. 736 individual
observations, grouped into 278 stacks, match
the selection criteria. We detect about 26 000
unique sources, an increase of at least 5%
over the joined detections of individual point-
ings before disregarding non-overlapping off-
axis areas and spurious detections. The proto-
catalogue shows that the new approach yields
higher sensitivity, improved source parameters
and likely fewer spurious detections.
0
20
40
60
80
100
Perc
enta
ge o
f dete
ctions
<=1 >=2 <=1 >=2
3XMM SUM_FLAG STACK_FLAG
matc
hed
3X
MM
only
matc
hed
3X
MM
only
matc
hed
sta
ck o
nly
matc
hed
sta
ck o
nly
Detection reliability: matching our proto-
catalogue with unique sources in 3XMM.
Those not recovered by stacked detection
are more probably spurious than matching
ones. Stack flags are set automatically only,
3XMM flags after additional visual screening.The full catalogue of detections from overlapping pointings will be processed and made available soon. It will be based on the
same set of OBS_IDs as 3XMM-DR7, selecting observations in full-frame mode only with reasonably low background level
which overlap by 20 % or more in area, and comprise order of 1800 fields.