Silvia Zane (MSSL) - The LOFT Large Area Detector 

High-time-resolution X-ray observations of compact objects provide direct access to strongfield
gravity, black hole masses and spins, and the equation of state of ultra-dense matter.
LOFT, the large observatory for Xray timing, is specifically designed to study the very rapid Xray
flux and spectral variability that directly probe the motion of matter down to distances very
close to black holes and neutron stars. A 10 m2-class instrument in combination with good
spectral resolution (<260 eV around 6 keV) is required to exploit the relevant diagnostics and
holds the potential to revolutionise the study of collapsed objects in our galaxy and of the
brightest supermassive black holes in active galactic nuclei.
LOFT will carry two main instruments: a Large Area Detector (LAD, to be built at MSSL/UCL
with the collaboration of the Leicester Space Research Centre for the collimator) and a Wide
Field Monitor (WFM). The ground-breaking characteristic of the LAD (that will work in the
energy range 2-30keV) is a mass per unit surface in the range of ~10 kg/m2, enabling an
effective area of ~10 m2 (@10 keV) at a reasonable weight and improving by a factor of ~20
over all predecessors. This will allow timing measurements of unprecedented sensitivity,
allowing for instance the capability to measure the mass and radius of neutron stars with ~5%
accuracy, or to reveal blobs orbiting close to the marginally stable orbit in active galactic
nuclei.
In this talk, I will summarize the characteristics of the LAD instrument and give an overview of
the expectations for its capabilities.