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The HEASARC and NuSTAR teams are greatly saddened by the sudden passing of Katja Pottschmidt. Most recently Katja was the lead scientist for the NuSTAR Guest Observer Facility (GOF), a role she had supported for many years. During her science career she worked on many other high energy astrophysics missions and played an integral role in advancing our knowledge of the universe. She was a wonderful colleague and friend and will be keenly missed by all who knew her.
The Salyut 4 Space Station
Mission Overview
The Salyut-4 space station was launched on 26 December 1974. It was in an orbit with an apogee of 355 km and a perigee of 343 km. The orbital period was 91.3 minutes. It was at an inclination of 51.6 degrees. The X-ray telescope was switched on on 15 January 1975. Salyut-4 reentered the Earth's atmosphere on 2 February 1977.
Instrumentation
The Salyut-4 X-ray instrument, often called the Filin telescope, consisted of four gas flow proportional counters, three of which have a total detection surface of 450 sq cm in the energy range 2-10 keV, and one of which has an effective surface of 37 sq cm for the range 0.2-2 keV. The field of view was limited by a slit collimator to 3 deg x 10 deg FWHM. The instrumentation also included optical sensors which were mounted on the outside of the station together with the X-ray detectors, and power supply and measurement units which were inside the station. Ground based calibration of the detectors is considered along with in-flight operation in three modes: inertial orientation, orbital orientation, and survey. Data could be collected in 4 energy channels: 2-3.1 keV, 3.1-5.9 keV, 5.9-9.6 keV, and 2-9.6 keV in the larger detectors. The smaller detector had discriminator levels set at 0.2 keV, 0.55 keV, and 0.95 keV.
Science
Among others, observations of Sco X-1, Cir X-1, Cyg X-1, and A0620-00 were published from the Filin data. A highly variable low energy (0.6-0.9 keV) flux was detected in Sco X-1. Cir X-1 was not detected at all during a 5 July 1975 observation, providing an upper limit on the emission of 3.5e-11 erg/sq-cm/s in the 0.2-2.0 keV range. Cyg X-1 was observed on several occasions. Highly variable flux, in both the time and energy domains, was observed.