LOFT Large Area Detector RXTE X-ray target catalogue Observing time vs. PCA area

The NASA Rossi X-ray Timing Explorer (RXTE) mission (Bradt et al., 1990) which flew from late 1995 to early 2012 became one of NASA’s longest running astrophysics missions. RXTE contained three complimentary experiments - the Proportional Counter Array (PCA) provided by the Goddard Space Flight Center (GSFC), the High Energy X-ray Timing Experiment (HEXTE) provided by The University of California at San Diego (UCSD) and the All Sky Monitor (ASM) provided by the Massachusetts Institute of Technology (MIT). The large area, high time resolution, PCA provides the ‘bench-mark’ for many aspects of the Large Area Detector (LAD) experiment proposed on the ESA LOFT mission. The PCA comprised 5 notionally identical Proportional Counter Unit (PCU) detectors. These detectors were comprehensively described, particularly the calibration aspects, in Jahoda et al. (2006) (and references therein). Only a few of these details are directly relevant to the study reported here. The scientific data from the PCA (ignoring housekeeping, health & safety and commanding) was all produced by a single additional box known as the Experiment Data System (EDS). This box was also provided by MIT and although operated and monitored as a separate device by the MIT team was, for scientific purposes, effectively part of the PCA and was routinely operated by the staff of the Science Operation Facility (SOF) at GSFC. The Guest Observer Facility (GOF), also at GSFC, ran the proposal rounds and generated the final package of FITS format data files delivered to a successful Principal Investigator (PI) once their observations had been executed. The PI then had one year’s priority on the data before it became public. The RXTE spacecraft was operated from the Mission Operation Centre (MOC) which was located next to the SOF. Four of the five PCU’s developed high voltage (HV) break-down problems during the long mission. An on-board device was used to curtail a break-down by turning off the PCU which protected the detector from irreversible damage. These same break-down issues also led to PCU’s being ‘rested’ or switched off for periods of time in an effort to prolong their operational life. Both these variable factors overrode the routine X-ray data acquisition intervals existing in the observing schedule making it complicated to assess, from the command history, the true PCA geometric area exposed to any particular X-ray source. This study analyses all the available information to construct a table of PCA area vs. exposure time for the 941 identified unique point sources in the RXTE observing history. This information may serve as a guide to planning the LAD experiment observations on LOFT since this will essentially be observing the same set of target sources. Such follow-on observations by the LAD will have a ~16x increase in area (compared to the whole PCA) with corresponding increases in detected count rate and telemetry loading, much better energy resolution and more PHA channels.