LOFT is specifically designed to exploit the diagnostics of very rapid X-ray flux and spectral variability that directly probe the motion of matter down to distances very close to black holes and neutron stars. In order to achieve its main scientific goals, two instruments will be comprised in the scientific payload:

The Large Area Detector, LAD: a collimated experiment, reaching an effective area of ~10 m2@8 keV, which will provide a total of ~280'000 cts/s for a 1Crab source (about 3000 cts/s are expected from the background) and a spectral resolution of ~260 eV in the energy band 2-30 keV. The  operational energy band will extend up to 80 keV, but only a coarse energy resolution (2 keV) is expected in the energy range 30-80 keV.

The Wide Field Monitor, WFM: this instrument will complete the scientific payload of LOFT and has the main scope of catching good triggering sources to be pointed with the LAD. Its large field of view will permit to observe about 50% of the sky available to the LAD in the same energy band at any time. The WFM is designed also to catch transient/bursting events down to a few mCrab fluxes and provide for them the position and trigger time within ~30 seconds from the event (through the LOFT Burst Alert System, LBAS). For triggered events, data with fine spectral (up to ~300 eV in the 2-50 keV energy range) and timing resolution (up to 10 μsec) will be made available to the ground within a few hours. During normal operations, only a limited amount of data will be made available (including rate-meters, images in at least 8 energy bands and spectra accumulated every 300 s). The total energy coverage of the WFM extends up to 80 keV, but data in the energy range 50-80 keV are considered of lower priority and might not be available for all observations.

A brief summary of the properties of the LAD and the WFM is provided in the two following tables. More details are reported in the following sections of this page.


In the sections below:


Conceptual scheme of the LOFT satellite.

Effective area of the LAD ("goals") compared with that of other existing and planned X-ray misions.

Current WFM performance specification
Current LAD performance specification
Value (WFM)
Energy Range
2-50 keV (50-80 keV extended)
Geometric Area (10  cameras)
1820 cm2
Peak Effective Area (on-axis)
>80 cm2
Energy Resolution FWHM@6 keV  
< 500 eV
(EOL at -30°C)
Field of View at Zero Response
180° x 90° +
90°x90° toward night hemisphere
Angular Resolution
Point Source Location Accuracy (10σ)
On-axis sensitivity at 5σ in 3 s (Galactic Center)
270 mCrab
On-axis sensitivity at 5σ in 58 ks (1 day observation of the Galactic Center) 
2.1 mCrab
Value (LAD)
Energy range
2-30 keV (30-80 keV larger energy binning)
Effective Area
10 m2 (@8 keV)
Field of View
Energy resolution
260 eV at 6 keV (EOL)
Time resolution
≤1% for 1 Crab source
~10 mCrab
Maximum average source flux
500 mCrab
Maximum peak source flux
15 Crab


The LOFT payload is an extensive array of X-ray detectors with a total geometric area of ~18 m2. A preliminary evaluation of the mission has identified a LOFT configuration based on 6 deployable panels, connected by hinges at the optical bench located at the top of a tower. This arrangement allows the stowing of satellite inside the launch vehicle fairing, with the Wide Field Monitor (WFM) hosted on the top of the tower.

The satellite will operate in a low equatorial earth orbit (~600 km, <5° deg inclination) in order to reduce the background and the radiation damage effect of South Atlantic Anomaly. The science return of the mission has been preliminary evaluated assuming a medium-small class and a Vega launcher, though a Soyuz launcher was recently considered as a preferable option with very marginal changes on the overall LOFT envelope.  

Side view of the LOFT satellite (from the anti-Sun direction).
The LOFT satellite folded in the Vega launcher.
The study of the energy-resolved timing properties of the X-ray emission of cosmic sources requires the accurate measurement of the time-of-arrival and energy of the largest number of photons from the target source. The unambiguous identification of the target source in this type of experiments (e.g., the PCA onboard RXTE; Jahoda et al. 2006, ApJS, 163 401) is most effectively achieved by narrowing the field of view by means of an aperture collimator, down to a level (typically ~1°) large enough to allow for pointing uncertainties yet small enough to reduce the aperture background (cosmic diffuse X-ray background) and the risk of source confusion (i.e., two or more sources simultaneously in the field of view).
In this type of instruments, the knowledge of the impact point of the photon on the detector array is not needed (if not for the use of proper detector calibration data), so there is no need for position sensitive detectors. Instead, detector read-out segmentation is necessary to reduce the effects of pile-up and dead time. The development of a 10 m2–class experiment is now made possible by the recent advancements in the field of large-area silicon detectors, which are able to time tag an X-ray photon with an accuracy <10 µs and an energy resolution of about 260 eV (FWHM, Full Width at Half Maximum), and capillary-plate X-ray collimators.
With these characteristics, the LAD is specifically designed to exploit the diagnostics of very rapid X-ray flux and spectral variability (already known to exist) that directly probe the motion of matter down to distances very close to black holes and neutron stars. Its factor of ~20 larger effective area than RXTE’s PCA (the largest area X-ray instrument ever flown, see below) is crucial in this respect. LOFT/LAD’s much improved energy resolution (better than 260 eV) compared to that of RXTE/PCA will also allow the simultaneous exploitation of spectral diagnostics, in particular the relativistically broadened 6-7 keV Fe-K lines. The timescales that LOFT will investigate range from submillisecond quasi-periodic oscillations (QPOs) to years long transient outbursts, and the relevant objects include many that flare up and change state unpredictably, so relatively long observations, flexible scheduling and continuous monitoring of the X-ray sky are essential elements for success.
Top view of the LOFT satellite with the 6 LAD panels.
Structure of one panels of the LAD.
Each panel comprises 21 "modules".
Each module includes 16 SDD with the electronics mounted below and the collimator above (in grey).
Scanning electron microscope image of a sample LAD collimator.
The Large Area Detector (LAD) of LOFT is designed as a classical collimated experiment. The key feature of the LOFT design that allows reaching for the first time a very large effective area and a improved energy resolution is the low mass per unit area enabled by the solid-state detectors and capillary plate collimators. The basic set-up of the instrument is a set of 6 Detector Panels tiled with ~2000 Silicon Drift Detectors (SDDs), which operate in the energy range 2-30 keV and have an energy resolution of ~260 eV (energy coverage in the 30-80 keV will be also provided but only with a coarse energy resolution). The modular structure (see figures above) ensures a high level of redundancy and the robustness of the instrument against single units failures. The field of view of the LAD is limited to ≤1° by X-ray collimators. These are developed by using the technique of micro-capillary plates, the same used for the micro-channel plates: a 6 mm thick sheet of Lead glass is perforated by a huge number of micro-pores, ~100μm diameter, ~20 μm wall thickness. The stopping power of Pb in the glass over the large number of walls that off-axis photons need to cross is effective in collimating X-rays below 50 keV (the energy range 50-80 keV is mainly used for exceedingly bright events from outside the instrument filed of view, outshining through the collimator walls). The required stability of the instrument response is ensured by a combination of the collimator response and the attitude and orbital control system (AOCS) parameters. This will avoid any significant spurious modulation of the detected source flux. 

A summary of the presently established requirements and goals of the LAD are reported in the table below.

Effective area
4 m2 @ 2 keV
8 m2 @ 5 keV
10 m2 @ 8 keV
1 m2 @ 30 keV
5 m2 @ 2 keV
9.6m2 @ 5 keV
12 m2 @ 8 keV
1.2 m2 @ 30 keV
Calibration accuracy area
Energy range
2 – 80 keV
1.5 – 80 keV
Energy resolution
260 eV @ 6 keV
200 eV (singles, 40%)
2 keV above 30 keV (allows for binning)
200 eV @ 6 keV
160 eV (singles, 40%)
Knowledge energy scale
0.8 10-2
Total available observing time
40.2 Ms (mission duration 4 yr)
52.2 Ms (mission duration 5 yr)
Accessible sky fraction (daytime)
Galactic center visibiliy
Collimated FoV (FWHM)
<1 degree
<0.5 degree
Transparency of collimator
~1% at 30 keV
0.5% at 30 keV
Time resolution
10 μs
7 μs
Absolute time
1 μs
1 μs
Dead time
< 1% @ 1 Crab,
< 0.5% @ 1 Crab
< 10 mCrab
< 5 mCrab
Background knowledge
1% at 5-10 keV
Max flux (continuous, no loss of info)
> 500 mCrab
> 750 mCrab
Max flux (continuous, re-binned)
15 Crab
30 Crab
Onboard memory (transmitted over more orbits)
15 Crab, 300 minutes
30 Crab, 300 minutes



The LOFT baseline WFM is a coded aperture imaging experiment designed on the heritage of the SuperAGILEexperiment  successfully operating in orbit since 2007. With the ~100 µm position resolution provided by its Silicon microstrip detector, SuperAGILE demonstrated the feasibility of a compact, large-area, light, and low-power high resolution X-ray imager, with steradian-wide field of view.

The LOFT WFM applies the same concept, with improvements provided by the higher performance (low energy threshold and energy resolution) Silicon Drift Detectors (SDDs) in place of the Si microstrips. The working principle of the WFM is the classical sky encoding by coded masks and is widely used in space borne instruments (e.g. INTEGRAL, RXTE/ASM, Swift/BAT). The mask shadow recorded by the position-sensitive detector can be deconvolved by using the proper procedures and recover the image of the sky, with an angular resolution given by the ratio between the mask element and the mask-detector distance.  By using SDDs, with a position resolution <100 µm, a coded mask at ~200 mm provides an angular resolution <5 arc min. The coded mask imaging is the most effective technique to observe simultaneously steradian-wide sky regions with arc min angular resolution.
As a first approach, each WFM camera can be considered a one-dimensional coded mask imager. This means that after the proper deconvolution is applied to the detector images, the image of a sky region including a single point-like source will appear as a single peak over a flat background. The position of the peak corresponds to the projection of the sky coordinates onto the WFM reference frame. The width of the peak is the point spread function, of the order of a few arc minutes in the LOFT WFM. If more than one source is present in the observed sky region, the image will show a corresponding number of individual peaks, whose amplitude will depend on the intensity of the source and on the exposed detector area at that specific sky location. By observing simultaneously the same sky region with two cameras oriented at 90° to each other (such pair composing one WFM Unit), one can derive the precise 2D position of the sources, by intersecting the two orthogonal 1D projections.
The overall configuration of the WFM envisages a set of 5 units. Each unit is composed of 2 co-aligned cameras oriented at 90° to each other (see figure below). The 5 units are off-set one to other and rotated in order to optimize the coverage of the region of the sky accessible to the LAD (50% at a time) and minimize the images systematic. The cameras of the LOFT WFM are not purely one-dimensional, as the LOFT detectors have also a coarse position capability in the second dimension (drift direction). While the fine position is of the order of 30-50 μm, the second coordinate of each photon can be located with an accuracy of ~5 mm. Each camera is thus individually able to locate a source in a strip that is few arc min wide and few degrees long, largely reducing the confusion limit for crowded fields. This is particularly useful for the spectral analysis of individual sources.
The WFM is visible on the top of the satellite.
5 units (10 cameras) configuration for the WFM (the green bottom structure is the LOFT optical brench).
Detailed view of a single unit (2 cameras) and the corresponding mounting structure.
Detailed CAD model of a WFM camera (large version).
Field of view of the WFM (5 units).
Given the orbit altitude (~600 km) and inclination of LOFT (<5°), a theoretical network of about 12 VHF stations will be able to cover the entire orbit, as for the HETE-II case. This will represent about one third of the network that is currently foreseen for SVOM (36 stations, at 10° elevation limit).
The WFM will be endowed with an on-board detection capability for bright events, i.e. the so-called LOFT Burst Alert System (LBAS). An on-board VHF transmitter will broadcast the position and trigger time of the brightest events detected by the WFM within about 20-30 seconds to a ground system of VHF receivers. These will cover the equatorial region around the Earth and will guarantee a full coverage of the LOFT orbit. Upon reception of an alert, the VHF ground antennas will broadcast the trigger times and positions communicated from the LBAS to the LOFT Burst Alert Center (LBAC) and possibly to ground and space based robotic facilities to ensure quick heads-up and proper fast follows-up. Once human verified, the alerts will also be broadcasted further to the science community at large through fast communication systems (emails, sms, Atels, GCNs).
For each of the triggers, the WFM will collect photons by photons data. These data will be made available on the ground for further investigation within a few hours from the trigger. The relatively short orbit of LOFT (~90 minutes) and the availability of at least two ground stations for telemetry reception, will ensure that about 20% of the satellite orbit is endowed with a near-real time transfer capabilities. This guarantees that for 20% of the triggered events, the corresponding event-by-event data will be available for "immediate" inspection and further broadcasting to the science community at large.  
Apart from triggers, during normal operations, the WFM will deliver data on a best effort basis. The current baseline is that the WFM should be able to transmit at least images in 8 (requirement) or 16 (goal) energy bands and spectra integrated every 300 s, plus the detector rate meter data. Depending on the overall telemetry download capabilities that will be available at the end, these performaces are expected to be significantly improved. 

A summary of the presently established requirements and goals of the WFM are reported in the table below.





Location accuracy

1 arcmin

0.5 arcmin

Angular resolution

5 arcmin

3 arcmin

Sensitivity (5 σ)

1 Crab (1 s)

5 mCrab (50 ks)

0.2 Crab (1s)

2 mCrab (50 ks)

Calibration accuracy (sensitivity)

20 %

15 %

Field of view

1π steradian around the LAD pointing including the antisolar direction

1.5π steradian around the LAD pointing including the antisolar direction

Energy range

2 – 50 keV (up to 80 keV in the extended mode)

1 – 80 keV

Energy resolution

<500 eV


Energy scale knowledge



Number of energy bands for compressed images



Time resolution

300 sec for normal

10 μsec for triggered

150 sec for normal

5 μsec for triggered

Absolute time calibration

1 μsec

1 μsec

duration for rate triggers

0.1 sec - 100 sec

0.001 - 100 sec

Rate meter data

16 msec

8 msec

10σ transient events position and trigger time

<30 sec for 65% of the events

<20 s for 65% of the events

Availability on the ground of triggered WFM data

3 hours

1.5 hours

Onboard memory

5 min @ 100 Crab

10 min @ 100 Crab



The LAD background has been analyzed and computed using Monte Carlo simulations of a mass model of the whole LOFT spacecraft and all known radiation sources in the LOFT orbit. The results reported in the figures below indicates that the anticipated LAD background over the full 2-30 keV energy band is compliant with the requirement of 10 mCrab, while being  <5 mCrab in the most important energy band of (2-10 keV). 
The background simulations show that the LAD background is dominated (>70%) by high energy photons of Cosmic X-ray Background (CXB) and Earth albedo “leaking” and scattering through the collimator structure, which becomes less efficient at high energies. These two radiation sources are very stable and  then predictable. Though, the rotation of the spacecraft
relative to the Earth during the orbital motion makes their viewing angle to change. Due to a different energy spectrum of the CXB and albedo components, the varying orientation produces a small orbital modulation of the background, which is however entirely due to a geometrical effect.
Indeed, this is the largest source of LAD background variaton. The effect of the other potentially varying source  - particle induced background - is greatly reduced by the  very stable particle environment offered by the equatorial orbit and by the fact that this component accounts for less than 6% of the overall background. Overall, the largest modulation of the total LAD background is estimated as <20% (for comparison, the background of the RossiXTE/PCA could vary by more than ~200% and it was largely due to the intrinsically variable particle-induced background) and can be effectively described by a geometrical model. The variation is slow (orbital timescale) and smooth as it depends only on the varying viewing angles of Earth and open sky. In fact, in the picture below (on the right) we show the results of a study in which the behavior of each background component was modelled along the satellite orbit. A geometrical model, properly calibrated using in-orbit flat fields, is expected to predict the background at the level of 1% or better (in 2-10 keV), which is the LAD science requirement. For comparison, the modelling of the RXTE/PCA reached 1%. However, as some of the LOFT science cases (especially the extragalactic science) will benefit from reaching a better background knowledge, we introduced a “blocked collimator” (a collimator with the same stopping power but no holes) for one Module of the LAD. This will continuously monitor all components of the LAD background, with the exception of the aperture background (about 10% or the total), accounting for 90% of the total background. In this way, in addition to the background modelling, the system will provide a continuous and independent benchmark of the background model. Preliminary simulations for different targets (i.e., different attitude configurations) show that an accuracy of ~0.1-0.3% in modelling the background variation can be achieved already over a few orbits timescale.
Of course, in whatever science case the absolute value of the background becomes important, the local background could be predictable down to the cosmic variance level only (order of 0.7%). A better knowledge of the absolute level of the background will require local blank fields, to be requested in the observing proposal.  
An extensive discussion of the LAD background modelling is reported in Campana et al. (2012) and will be soon updated in Campana et al. 2013 (submitted).


The LAD background and its various contributions. The spectrum of a 10 mCrab souce is also shown.
Preliminary model of the background orbital variation, as decomposed in all its individual components. Total background modulation is shown by the top black curve.
LAD sensitivity curves calculated assuming 1% systematics on the instrument background knowledge (in agreement with the present LAD requirements).
Same as on the left but assuming 0.5% systematics on the background knowledge. This is a conservative expected value of the systematics derived from instrument GEANT simulations.

As for other coded-mask instruments, the WFM background is dominated by the aperture CXB. The latter only depends on the instrument field of view. As for the LAD, detailed simulations of all contributions to the WFM background led to the figures reported below.

The WFM background and its various contributions.