Søren Brandt & Margarita Hernanz On behalf of the WFM team Wide Field Monitor

2 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 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)

3 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 ( 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 talk by Enrico Bozzo and the LOFT white papers

The WFM Camera 1 camera contains 4 Si drift detector modules –25μ Be window above the detector plane protects against orbital debris, μ-meteorites, soft protons 1.5D position resolution –Fine position resolution in one direction –Coarse position resolution in drift direction 2 crossed cameras constitute a WFM unit/pair 90 x 90 degree zero response field of view 32 x 32 degree fully illuminated field of view

WFM Summary The WFM is a coded mask instrument Energy range 2-50 keV

6 Detecting X-ray photons An X-ray interaction creates a charge cloud drifted towards the anodes –Max drift length: 35 mm –Anode pitch: 145 µm –448 anodes on each SDD half –Read-out by 7 ASICs on each side We measure for each photon (E,X,Y,T): –Photon energy is proportional to the collected charge –X-position is the center of the charge cloud (<60µm) –Y-position is proportional to the width of the charge cloud (<8mm) –Time is recorded by the Back End Electronic at the trigger (<10µs) 1 of the 4 SDD tiles in the WFM camera detector plane

The WFM Detector Module Silicon Drift Detector –Similar to the LAD, with smaller pitch, charge divided on several anodes Fitting center, width and total change content –Providing X, Y, E ParameterValue Si thickness450 µm Si tile geometric size77.4 mm x 72.5 mm Si tile active area65.1 mm x 70.0 mm = cm 2 Anode pitch145 µm Number of read-out anodes per tile448 x 2 rows = 896 total Drift length35 mm

8 WFM optical design 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 –Can be done by FFT WFM detector plane is based on the same Silicon Drift Detector (SDD) technology as LAD, but providing 2D photon positions 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. Note: 25% open fraction may be open for trade-off. Several coded mask instruments are 50% open

9 A compact camera design The fine position resolution in one direction offers a very compact camera design 4 SDD modules in the detector plane Detector plane

10 WFM imaging Each WFM camera produces a shadowgram convolved sky image with PSF ~5 arcmin x 5 degrees (we often refer to it as a1.5D 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 Note: positions and intensities are independently determined in each camera in a pair

11 WFM camera

12 WFM ESA M3 configuration The WFM consisted of 10 identical and independent cameras –Organized in 5 pairs with 90°x90° FoV [-60°,-15°,+15°,+60°] + anti-Sun –Covering half of the sky accessible to the LAD in one pointing (all in 2 pointings) –Peak sensitivity in the LAD pointing direction –Including coverage in the anti-Sun hemi- sphere (extended field of regard) Optical bench

13 WFM sky coverage ~1.75 π steradian (~44% of the sky) at zero response ~1.33 π steradian (~33% of the sky) at 20% of peak response LAD pointing at Galactic Center, effective area, peak at ~120 cm 2

14 WFM ESA M4 configuration The WFM consisted of 8 identical and independent cameras –Organized in 4 pairs each with 90°x90° FoV –3 pairs cover 180°x90° with some overlap –covering half of the sky accessible to the LAD in one pointing –Peak sensitivity in the LAD pointing direction –Including coverage in the anti-Sun hemi-sphere (LAD extended field of regard) Optical bench

Science Requirements 15

Science Requirements II 16

17 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 – but too good to let it pass! 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

WFM technical budgets Mass does not include support structure Streamlining the mechanical design of support structure, BEE box, and camera housing will enable a reduction of WFM mass during assessment phase. Current estimates are therefore conservative 18 Note: budgets include 20% margins

19 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 –Alternative will depend on a real time burst alert requirement

Conclusion The WFM offers a (LEGO block) 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) The WFM design is studied in detail to phase A level 20