The Rossi X-ray Timing Explorer (RXTE) was a spacecraft launched in 1995 to study the variability of X-ray sources. The RXTE carried three main instruments: the All Sky Monitor (ASM), the Proportional Counter Array (PCA), and the High Energy X-ray Timing Experiment (HEXTE). The RXTE operated until 2012, when it ran out of fuel and was decommissioned.
The ASM on board the RXTE monitored about 80% of the sky at any given time and was used to provide real-time monitoring of more than 150 X-ray sources. The PCA consisted of five identical Proportional Counter Units (PCUs), each with a large field of view, that were sensitive to photons in the 2–60 keV energy range. The HEXTE consisted of two clusters, each containing four NaI/CsI scintillation detectors, that were used to measure photons in the 15–250 keV energy range.
The RXTE made many important discoveries during its 17 years of operation. These include finding evidence for the existence of black holes, discovering how matter behaves near a black hole’s event horizon, making precise measurements of neutron star spin rates, and providing insights into stellar evolution.