1. WFM/eXTP: sensitivity and sky visibility trade off Jean in 't Zand, with help of Margarita Hernanz, Søren Brandt, Laura Alvarez, Yuri Evangelista, Riccardo Campana

2. Modular basis Same as for ESA M4 One camera
1-dimensional imaging ('1.5-d')
Detector SSD area 4 x 6.5 x 7.0 cm = 182 cm2
Mask 26 x 26 cm, 1-dimensional, 22% open  39 cm2
Focal length 20 cm
Field of view 90 x 90 square degrees (2.1 sr, 17% sky)
One unit
2 cameras co-aligned but perpendicularly oriented
2-dimensional imaging of same FOV
Combined area 78 cm2
eXTP / WFM sensitivity and coverage trade off / Beijing, October 2015

3. Single detector layout
Net collecting area 182 cm2 within a total area 210 cm2
eXTP / WFM sensitivity and coverage trade off / Beijing, October 2015

4. Two-unit example: detector exposures
eXTP / WFM sensitivity and coverage trade off / Beijing, October 2015

5. Tested camera configurations
ONE
TWO
TWO ASYM
Courtesy Laura Alvarez and Margarita Hernanz
THREE
FOUR
FIVE
eXTP / WFM sensitivity and coverage trade off / Beijing, October 2015

6. Single pointing photon collecting area configuration
eXTP / WFM sensitivity and coverage trade off / Beijing, October 2015

7. Performance calculations input parameters
Point sources
3-12 keV ASM catalog
Averaged over 15 year mission: 22 Crabs over 265 sources
Brightest Sco 12 Crab
Renormalize all sources to Crab, and multiply with expected 2-60 keV Crab flux of 2.7 c/s/cm2
No spectra nor timing
CXB 7.5 c/s/cm2/sr (2-60 keV) following Marshall et al. (1983). Not truly different from Gruber et al. (1999)
Mask = grey filter with transparancy 21.6%. Mask vignetting included; assumed thickness/size ratio is No imaging simulated!
Detector = 100% efficiency, dead zones included
Declination-dependent earth obscuration assuming H=600 km, i=0 deg
Observation program 15-yr RXTE mission
Sun angle constraint: between 60 and 120 deg (+30/-30 deg constraint)
eXTP / WFM sensitivity and coverage trade off / Beijing, October 2015

8. One day exposure coverage, 1 or 8 hr cadence
eXTP / WFM sensitivity and coverage trade off / Beijing, October 2015

9. 1 year exposure map
eXTP / WFM sensitivity and coverage trade off / Beijing, October 2015

10. 1 year sensitivity map
eXTP / WFM sensitivity and coverage trade off / Beijing, October 2015

11. Results
Parameter \ Configuration
ONE
TWO
-30,+30
TWO ASYM (-20,+60)
THREE
-60,0,+60
FOUR
(ESA-M4)
FIVE
(ESA-M3)
Number of units (1 unit = pair of 1.5d cameras) 1 2 3 4 5
Area on axis in system (=LAD) [cm2] 75 68 61 87
127
FOV per camera [sr] (FWZR)
2.1
Sky coverage per pointing 10 cm2/ZR)
17%
23%
25%
31%
40%
41%
Crab on-axis in 1 camera [cps]
106
Crab on axis in system (=LAD), all cameras [cps]
136
131
186
216
419
CXB per camera [cps]
400
1-yr exposure GC [Ms] with area>10% max
4.6
6.0
6.1
7.1
10.5
10.6
Total average photon rate, incl. data gaps [cps]
880
1707
1644
2378
3226
3678
3-sec 5σ sensitivity [Crab] on axis
0.49
0.59
0.50
0.36
1-day 5σ sensitivity [mCrab] on gc
3.8
3.9
2.8
1-yr GC [mCrab]
0.48
0.42
0.46
0.39
0.32
1-yr optimum [mCrab]
0.25
0.17
0.14
0.13
0.12
eXTP / WFM sensitivity and coverage trade off / Beijing, October 2015

12. Discussion The important issue is instanteneous FOV
Sky window is 360 x 60 degrees = 50% of sky. Cover this instantaneously?
WFM primarily for core science (limited no. of X-ray binary transients), but alert is somewhat relaxed to be issued within 1-2 d. With frequent slews and scattered targets, a smaller instantaneous FOV is required
Large iFOV essential for observatory science regarding short duration transients (<1 hr): GRBs, thermonuclear X-ray (super)bursts which in LOFT was considered important
Larger iFOV
LOFT does it through multiple units, but what else is possible?
By larger mask or shorter focal length  more background  less sensitivity
By smaller detector  less area  less sensitivity
By larger mask and larger detector to compensate for larger background  larger cameras  more space, like with multiple units
eXTP / WFM sensitivity and coverage trade off / Beijing, October 2015

13. Conclusions
What configuration is best? Judge by iFOV, sensitivity and countrate
In sensitivity and iFOV? FIVE
In iFOV? FOUR
THREE has 30% less coverage, but nearly same sensitivity as FOUR
TWO and ONE seem unfavorable
If instantaneous coverage is less important, requirement on no. Units relaxes
Most rewarding science: short transients (thermonuclear X-ray bursts, GRBs) and phenomena  iFOV and sensitivity; monitoring  c/s
The only way to substantially increase sensitivity is to build more collecting area
WFM in comparison to other ASMs:
If conf>ONE, iFOV better than *any* other <6 keV instrument ever flown  excellent sensitivity to sub-hour transients
If conf>TWO, countrate WFM better than any previouys ASM (WFM >200 c/s, ASM 75 c/s, WFC 280 c/s)
eXTP / WFM sensitivity and coverage trade off / Beijing, October 2015