INSTRUMENTS ON-BOARD LOFT

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 ~12 m2@8 keV, which will provide a total of ~140'000 cts/s for a 500 mCrab 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 50 keV, but only a coarse energy resolution of ~2 keV is expected in the energy range 30-50 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 in the same energy range of the LAD about 50% of the sky at once. The WFM is designed also to catch transient/bursting events down to a few mCrab fluxes and will provide for them data with fine spectral (up to ~300 eV in the 2-50 keV energy range) and timing  resolution (up to 10 μsec).

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.

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
Parameter
Value (WFM)
Energy Range
2-50 keV
Geometric Area
  
1460 cm2
 
Peak Effective Area (on-axis)
>80 cm2
Energy Resolution FWHM
< 300 eV
Field of View at Zero Response
180° x 90°
Angular Resolution
5’ x 5’
Point Source Location Accuracy (10σ)
< 1’x1’
On-axis sensitivity at 5σ in 3 s (Gal. Center)
270 mCrab
On-axis sensitivity at 5σ in 58 ks (1 day Galactic Center)
2.1 mCrab
 
Parameter
Value (LAD)
Energy range
2-50 keV (30-50 keV larger energy binning)
Effective Area
10 m2 (@8 keV)
Field of View
~38 arc minutes
Energy resolution
~260 eV at 6 keV (EOL)
Time resolution
~7μs
Dead-time
~0.7% for 1 Crab source
Background
~10 mCrab
Maximum average source fux
500 mCrab
Maximum peak source flux
15 Crab

THE SPACECRAFT

Concept:

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 evaluated assuming a medium-small class and a Vega launcher. The maximum area achievable is a product of both the size of the detector array and the solar array size that can be accommodated, as power is an important limiting factor.

Side view of the LOFT satellite.
The LOFT satellite folded in the Vega launcher.

THE LARGE AREA DETECTOR (LAD)

Concept:

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. ApJS 163 401 2006) 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.
 
Scheme of the LOFT satellite.
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.
 

Design:

 
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 2-50 keV and have an energy resolution of ~260 eV. 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 ~40 arcmin by X-ray collimators. These are developed by using the technique of micro-capillary plates, the same used for the micro-channel plates: a 3 mm thick sheet of Lead glass is perforated by a huge number of micro-pores, ~20μm diameter, ~4-6 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. In order to accommodate for the internal misalignments of the instrument and for attitude uncertainties, the response of the collimator in the central ~10-15 arc min angle is flat (flat-top response) to avoid any 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.

Item
Requirement
Goal
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
15%
10%
Energy range
2 – 50 keV
1 – 50 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
10-2
0.8 10-2
Collimated FoV (FWHM)
1 degree
0.5 degree
Transparency of collimator
~1% at 30 keV
0.5% at 30 keV
Flat top
12 arcmin, ± 2%
12 arcmin, ± 1%
Time resolution
10 μs
7 μs
Absolute time
1 μs
1 μs
Dead time
< 1% @ 1 Crab,
< 10% @ 10 Crab
< 0.5% @ 1 Crab,
< 5% @ 10 Crab
Calibration knowledge deadtime
Less than the statistical precision of power spectrum for 1 day at 15 Crab (TBC)
Factor 2 better
Background
< 10 mCrab
< 5 mCrab
Background knowledge
10%
5%
Max flux (continuous, no loss of info)
> 500 mCrab
> 500 mCrab
Max flux (continuous, re-binned)
15 Crab
 
30 Crab
Onboard memory (transmitted over more orbits)
15 Crab, 3 orbits
30 Crab, 3 orbits

 

THE WIDE FIELD MONITOR (WFM)

Concept:

 

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.

Design:

The overall configuration of the WFM envisages a set of 4 units. Each unit is composed of 2 co-aligned cameras (see figure below). The 4 units are off-set one to other along one direction in order to provide the maximum coverage of the region of the sky accessible to the LAD.
 
The WFM is visible on the top of the satellite.
8 cameras configuration for the WFM.
Side view of the 8 cameras with sizes.
Expected field of view of the WFM.

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

Item

Requirement

Goal

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

50% of the accessible part of the sky of the LAD

Same, as improvement of the sensitivity is the prime goal

Energy range

2 – 50 keV

1 – 50 keV

Energy resolution

500 eV

300 eV

Energy scale knowledge

4%

1%

Number of energy bands for compressed images

8

16

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 - 60 sec

0.1 - 60 sec

Rate meter data

16 msec

8 msec

Transient event down-link

< 3 hours (2 orbits)

< 1.5 hour (1 orbit)

Availability of triggered WFM data

3 hours

1.5 hours

Onboard memory

5 min @ 100 Crab

10 min @ 100 Crab

 

An alternative option for the WFM

An alternative detector option is being considered for the WFM. The alternative employs 2D imaging cameras. Each camera is equipped with Double Side Strip Detectors (DSSD) for the detection plane and a coded mask with square elements (300 μm x 300 μm). With 2D imaging in each camera this option only need four cameras in the WFM assembly. Each camera detection plane is composed of four detectors offering an active area of 81 cm2. This alternative configuration is currenlty being studied by collaborators in CEA-Saclay and APC-Paris (France).  

 
LOGIN (LOFT teams only)



On-line now

We have 175 guests and 17 members online