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The Wide Field Monitor

eXTP mission design coordination meeting, IHEP, CAS, Beijing – 21-23 March 2017

Figures of eXTP from XU Yupeng presentation

Margarita Hernanz, ICE (CSIC-IEEC) Søren Brandt, DTU

On behalf of the WFM team

WFM Primary Goals •  Provide triggers for target of opportunity

observations of pointed instruments, core science (0.5-1 day reaction time, or better) –  Detection of new, rare transient X-ray sources with

~1 arcmin accuracy •  Black hole transients (Strong Gravity)

–  Detection of recurrent transient X-ray sources •  Primarily Neutron Stars (Equation of State)

–  Detection of state changes in persistent X-ray sources

•  Neutron Stars and Black Holes •  Therefore we need as wide a field of view as

possible to catch rare events

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Note: The sources in the 2 latter categories almost all already have positions known to ~1 arcsec determined by earlier observations in X-rays, optical, or radio (XMM-Newton, Chandra or other observatories)

WFM Secondary Goals (observatory science)

•  Imaging of the LAD field of view to determine if there is source confusion/contamination

•  Monitor the long term behavior of X-ray sources •  Detect short (0.1-100 s) bursts and transient

events and record data with full resolution •  Option to transmit the position of burst sources

to ground in real time: BAS – Burst Alert System •  For a full list of observatory science goals,

see the LOFT & eXTP white papers

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Science Requirements

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Item Requirement Anticipated performance M3

Anticipated performance M4/eXTP

Notes

Location accuracy <1 arcmin <30 arcsec for P/L

<1 arcmin <30 arcsec for P/L

<1 arcmin <30 arcsec for P/L

Requirement on camera pair

Angular resolution <5 arcmin (FWHM) <4.5 arcmin (FWHM) <4.5 arcmin (FWHM) Requirement on camera pair

Peak sensitivity in LAD direction (5 σ)

1 Crab (1 s) 5 mCrab (50 ks)

<0.6 Crab (1s) <3 mCrab (50 ks)

<0.8 Crab (1s) <4 mCrab (50 ks)

Peak effective area reduced from 156 cm2 to 86 cm2

Absolute flux accuracy 20 % <20% <20% Requirement on detector Field of view 1 π steradians around the

LAD pointing 1.75 π steradians at 0% response, 1.33 π steradians at 20% of peak camera response

1.75 π steradians at 0% response, 1.33 π steradians at 20% of peak camera response

Depends on configuration. Current M4 leaves FoV at zero response unchanged

Energy range 2 – 50 keV 2 – 50 keV

2 – 50 keV Requirement on detector

Energy resolution 500 eV (FWHM) eV @ 6 keV

<300 eV @ 6 keV <300 eV @ 6 keV Requirement on detector

Energy scale 4% <2% <2% Requirement on calibration

Energy bands for compressed images

>=8 >=8, <=64 >=64 Reduced data rate. Better understanding of compression and actual data rate

Time resolution 300 sec for images 10 µsec for event data

<= 300 sec for images, < 10 µsec for event data

<= 300 sec for images, < 10 µsec for event data

Parameter definition

Absolute time calibration 2 µsec 1 µsec for P/L

<2 µsec <1 µsec for P/L

<2 µsec <1 µsec for P/L

Requirement on electronics

Burst trigger scale 0.1 sec - 100 sec 10 msec - 300 sec 10 msec - 300 sec Requirement on SW Rate meter data 16 msec <= 16 ms <= 16 ms Parameter definition Availability of triggered WFM 3 hours <3 hours <3 hours Mission requirement

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•  WFM cameras are organized in pairs with 90º x 90º FoV •  Configuration with 3 Camera Pairs (-60⁰, 0, 60⁰)

WFM on eXTP

Heritage from LOFT feasibility study for ESA-M3 and ESA-M4 proposal:

a coded mask instrument

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•  WFM cameras are organized in pairs with 90º x 90º FoV •  Configuration with 3 Camera Pairs (-60⁰, 0, 60⁰)

WFM on eXTP

WFM and sunshade – ESA M4

Heritage from LOFT feasibility study for ESA-M3 and ESA-M4 proposal:

a coded mask instrument

•  The M4 LOFT WFM included 8 identical cameras distributed by camera pairs - eXTP: 3 camera pairs (no antisun)

•  A Sunshade is required to protect the masks (and the detectors) from solar radiation: prevent direct solar illumination of the masks to keep mechanical stability

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Overall dimensions (mm)

FRONT VIEW LATERAL VIEW

WFM ESA M4 configuration

The WFM Camera •  Each camera contains 4 Si drift detector modules

–  25µm Be window above the detector plane protects against orbital debris, µ-meteorites, soft protons

•  1.5D position resolution –  Fine position resolution in anode direction –  Coarse position resolution in drift direction Ø see detector

•  2 crossed cameras constitute a WFM unit/pair –  2D position resolution

•  90 x 90 degree zero response field of view •  32 x 32 degree fully illuminated field of view

Detecting X-ray photons: same detector as the LAD but with smaller anode pitch, to get better spatial resolution

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1 of the 4 SDD tiles in the WFM camera detector plane

See presentation by Yuri Evangelista on SDD detectors

The WFM Camera •  Each camera contains 4 Si drift detector modules

–  25µm Be window above the detector plane protects against orbital debris, µ-meteorites, soft protons

•  1.5D position resolution –  Fine position resolution in anode direction –  Coarse position resolution in drift direction

•  2 crossed cameras constitute a WFM unit/pair –  2D position resolution

•  90 x 90 degree zero response field of view •  32 x 32 degree fully illuminated field of view

WFM design

Single WFM camera

WFM camera pair – overall dimensions in mm

WFM optical design – coded mask

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WFM mask with 150 µm thickness and pitch of 250 µm x 16.4 mm, matching the detector position resolution. ~25% of the mask elements are open (250 µm x 14 mm) for optimizing the sensitivity to weak sources over the cosmic diffuse background.

•  The X-ray source casts a “shadowgram” of the mask onto the detector plane

•  The sky image is derived from the “shadowgram” by cross-correlation

•  Reminder: WFM detector plane is based on the same Silicon Drift Detector (SDD) technology as LAD, but providing 2D photon positions

Note: 25% open fraction may be open for trade-off. Several coded mask instruments are 50% open

WFM imaging •  Each WFM camera produces a shadowgram •  convolved sky image with PSF ~5 arcmin x 5 degrees

(we often refer to it as a 1.5D image) –  Position accuracy in fine direction is <14 arcsec (1 sigma) –  2D position is found by combining the two independent

orthogonal positions found by the cameras in a pair –  Intensity is found by fitting source strength in each camera

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Note: positions and intensities are independently determined in each camera in a pair

A compact camera design - summary

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Camera Camera Pair

Optical design Mask-detector distance 202.9 mm Mask size 260 x 260mm2 Mask pitch 0.250 x 16.4mm2 Size of open Mask elements

0.250x14mm2

Active detector area 182 cm2 364 cm2 Peak Effective Area (on-axis, through mask)

>39 cm2 > 78 cm2

SDD spatial resolution (fine direction, FWHM)

< 60 µm

SDD spatial resolution (coarse direction, WHM)

< 8 mm

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lane

WFM camera design: main components components

WFM camera design: exploded view

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WFM functional diagram (M3) SDD = Silicon Drift Detector FEE = Front End Electronics

BEE = Back End Electronics

PDU = Power Distribution Unit

PSU = Power Supply Unit

DHU = Data Handling Unit

ASICs - Front-end electronics – Back-end electronics

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•  Stephanne Schanne CEA (ASICs) •  Yuri Evangelista (detectors and FEE) •  Chris Tenzer (BEE)

Instrument Control Unit (ICU)

Soren Brandt

WFM technical budgets

Mass does not include support structure

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M3 (4+1) eXTP (3) Nominal

Mass [kg] Nº Nominal Mass [kg] Nº Nominal

Mass [kg] WFM Camera 11.152 10 111.520 6 66.912 ICU 4.32 2 8.640 2 8.640 Camera-ICU harness

0.247 20 4.940 12 2,964

Total WFM MASS

125.100 Total WFM MASS

78,516

M3 (4+1) eXTP (3) Nominal

Power [W] Nº Nominal [W] Nº Nominal

[W] WFM Camera 9.070 10 90.700 6 54.42 ICU 17.600 1 17.600 1 17.600

Total WFM POWER

108.300 Total WFM POWER

72.020

Budgets include 20% margins

WFM technical budgets: very detailed for M3

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WFM on eXTP summary The WFM is a coded mask instrument with energy range 2-50 (2-30) keV

Instrument Characteristic WFM Detector type Si Drift Mass, including margins 11.2 kg per camera

78,5 kg for 3 pairs, including ICU Nominal Power, 3 pairs + ICU 72 W Detector Operating T <-20°C Total Detector Effective Area (3 camera pairs)

180 cm2 per camera 3 pairs 1080 cm2

Energy range 2-50 keV Energy resolution [FWHM] <500 eV @ 6 keV Mask pixel size 250 µm x 16 mm Field of View Camera: 90° x 90° FWZR

3 pairs: 180° x 90° FWZR Angular Resolution <5 arcmin Typical/Max data rate 40/80 kbits/s(after compression)

WFM sky coverage (M4/eXTP conf)

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~1.4 π steradian (~35% of the sky) at zero response

~1.1 π steradian (~27% of the sky) at 20% of peak response

Pointing at Galactic Center, effective area, peak at ~120 cm2

WFM design: thermal model

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Temperature variation along the orbit Detector temperature Coded Mask temperature

•  The WFM assembly is protected by a Sun shade

•  Aim is to minimize temperature variations and maintain mechanical stability

•  Heat is conducted to the platform via a thermal strap

•  Final thermal design depends on detailed S/C design

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WFM design: thermal model Mask thermal requirements:

Results:

Mask temperature variation along the orbit fulfills the mask stability requirement

TEMPERATURE MASK Plane gradient < 5 ºC (TBC)

Orbit stability < 10 ºC(TBC)

Camera pair 0

Camera pair 1

Camera pair 2

Camera pair 3

Camera pair 4

LOFT orbit, LEO 550km with +2.5º Inclination, for SAA90

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Technology Development

•  The ASICs and SDD developments are common with the LAD

•  Development of coded mask technology – Trade-off of manufacturing method:

chemical etching preferred •  In the LOFT design the data handling was

based of ITAR restricted technology, but see Soren Brandt presentation

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WFM design:coded mask sample Sample characteristics: Made by chemical etching

Dimensions:100x100x0.1 mm A part of the coded mask pattern has been performed.

Pattern dimensions: Slit: 14 x 0.250 mm Space between slits: 2.4 mm

Enlarged view of the slits

Optional Burst Alert System •  The large field of view of the WFM (3 pairs) provides unique

opportunities for detecting Gamma Ray Burst (~100 GRBs per year) –  Not directly linked to top level science goals

•  BAS is modeled on the SVOM + heritage from IBAS/INTEGRAL •  Onboard Burst Trigger and localization

–  The localization will drive the onboard processing power, as count rate burst trigger is ‘easy’, while image deconvolution is ‘hard’

•  Onboard VHF transmitter required to transmit short message with time and sky position

•  Network of small ground stations to receive message (SVOM) •  Delivery of trigger time and burst position to end users within 30 s

for fast follow up of the fading GRB afterglow

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Product Tree M4

Conclusion •  The WFM offers a modular design

–  number of camera pairs can be adjusted to fulfill science goals (and resource constraints)

– Relative placement of cameras can customize the sensitivity over the total field of view

– Accommodation on spacecraft is flexible (as long as FoV is unobstructed)

– TM budget should be discussed (event-by-event format is preferred)

•  The WFM design is studied in detail to phase A level with robust performance calculations 29