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).
|Figure 1: The LECS FOV drawn to scale. The outer circle indicates the 37 diameter sky FOV which contains the standard 16 diameter source extraction region (shaded) and the two background semi-annuli. The positions of the two 55Fe radioactive sources are shown as crosses|
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 25 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.
|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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