The method of operation of the LECS is similar to that of
conventional gas scintillation proportional counters.
An X-ray that passes through the ultra-thin (1.25 m) entrance
window and is absorbed in the 11.4 cm diameter Xe filled gas cell liberates
a cloud of electrons. A uniform electric field between the entrance window,
kept at -20 kV, and a grounded grid causes scintillation as the
electrons travel towards the grid. The UV light from these scintillations
is detected by a multi-anode photo-multiplier tube (PMT).
The LECS is also sensitive to cosmic rays since these can
ionize the counter gas and leave tracks through the cell.
The 2 cm diameter gas cell entrance window is supported by a tungsten
strongback and fine grid.
The strongback consists of a square grid of tungsten bars with a
separation of 4
. Each strongback square is further divided into
8 by 8 pixels by a fine grid. The overall effect of the strongback
and fine grid is to reduce the X-ray transmission by between 20-40%,
depending on position within the FOV. The LECS includes
two 55Fe radioactive sources which constantly illuminate regions
of the detector outside the sky FOV to provide monitoring of the
instrument performance (see Fig. 1).
The LECS mirror system consists of 30 nested,
Au coated mirrors with a double cone approximation to the
Wolter I geometry. The mirror focal length is 185 cm and the geometric
area 124 cm2 (Conti 1994, 1996).
The off-axis behavior of the mirrors
is complicated. Conti et al. (1994) demonstrate that X-rays within a cone
of solid angle 2 can reach the focal plane after reflection
off only one of the mirror surfaces.
As part of the BeppoSAX Science Verification
Phase, observations were performed with the Crab Nebula just
outside the LECS FOV. These show that at an offset of
60
, the Crab Nebula is visible
as an extended emission region offset in the direction of the source,
with an intensity (
of that
on-axis. The spectrum of the offset emission (a power-law with a
photon index,
, of
) is consistent with that
recorded from the Crab Nebula on-axis (
).
The LECS is only operated during satellite night time. Although
this reduces the observing efficiency considerably, it means
that that any contribution to the background from
scattered solar X-rays, e.g., as seen by the ROSAT Position Sensitive
Proportional Counter (PSPC) when the Sun-Earth-satellite angle is
<120 (e.g., Snowden & Freyberg 1993), is negligible.
The BeppoSAX observing schedule is designed to optimize the LECS
observing efficiency. This means that, in general, only time constrained
or Target of Opportunity observations include long intervals of dark Earth
pointing.
The LECS is not operated when the satellite passes
close to the South Atlantic Anomaly (SAA).
Due to the low BeppoSAX orbital inclination SAA passages result in between 5 and
12 min of data being lost per 96 min satellite orbit.
In order to maximize the observing efficiency, many BeppoSAX NFI
observations are made with a Sun-satellite-target angle close to 120
,which is as far from the solar direction as it is possible to operate.
The LECS energy resolution is 32% full-width half-maximum
(FWHM) at 0.28 keV and 8.8% FWHM at 6 keV.
The FOV is circular
with a diameter of 37. The position resolution
corresponds to 90% encircled energy within a radius of
2
5 at 1.5 keV. This is a factor
4 worse than that of the PSPC and comparable to that of the
ASCA Gas Imaging Spectrometer (GIS).
Figure 2 shows the on-axis LECS effective area.
A key scientific goal of the LECS is to study sources in the
energy band below the instrument's C edge at 0.28 keV. This typically
means that the absorption to a source must be
atom cm-2.
The prime advantage of the LECS, compared to previous high throughput
X-ray detectors, is its good low-energy spectral resolution and its low
background afforded by the imaging characteristics.
The 0.1-2.0 keV energy resolution is a factor
2.4
better that that of the PSPC,
while the effective area is between a factor
20 and 5 lower at
0.28 keV and
1.5 keV, respectively. The energy resolution is similar to that of the
GIS in the overlapping energy range, and comparable to that of the
ASCA Solid-state Imaging Spectrometer at energies of
0.5 keV.
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Figure 2: A comparison of the LECS (solid line), ASCA SIS (dashed line) and ROSAT PSPC (Dashed-dotted line) effective areas in the energy range 0.1-5.0 keV |
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