The Rossi X-ray Timing Explorer: Timing the Extreme Universe with Tomaso Belloni (INAF – Brera Astronomical Observatory, Italy)

The Rossi X-ray Timing Explorer (RXTE) was a satellite for X-ray astronomy launched by NASA on 30 December 1995 into a low-earth orbit. It operated for almost exactly sixteen years until its termination in January 2012. On board were three X-ray instruments a large-area Proportional Counter Array (PCA) built at NASA/GSFC, the High-Energy X-ray Timing Experiment (HEXTE) built at the University of California, San Diego, covering higher energies, an All-Sky Monitor (ASM), developed at MIT Cambridge.

Its characteristics, such as large collecting area, no imaging capability, high time resolution, broad energy coverage and very high flexibility of operation, together with the presence of an all-sky monitor to detect transient X-ray events, were aimed at the detailed study of bright galactic X-ray sources, in particular regarding their fast time variability. RXTE was indeed a giant step forward in our knowledge of variability of systems containing compact objects, the most powerful emitters of X-rays in our Galaxy and it allowed us to link their properties to physical models for accretion and to effects of General Relativity (GR) in the vicinity of a black hole or neutron star.

This was done through the discovery of millisecond quasi-periodic variability from accreting neutron stars in low-mass X-ray binaries, together with more elusive and slightly slower signals from black-hole binaries. These oscillations allowed us for the first time to obtain precise measurements of time scales in the accretion matter very close to a compact object, where the effects of GR are more extreme. Indeed, models based on GR frequencies have soon emerged and, although they have not been completely established, hold the promise to test GR in the strong field regime.

The long-sought missing link of neutron-star evolution was finally unveiled by RXTE: millisecond pulsations from accreting neutron stars in low-mass X-ray binaries. These systems were thought to be the progenitors of millisecond pulsars, but no pulsation was found and therefore we had no direct indication of their fast spinning. RXTE detected pulsations from several faint transient systems and sporadic pulsation from others. At the same time, evidences for neutron star rotation periods came from short-lived oscillations during nuclear explosions on the surface of the accreting neutron star. These provide powerful tools for studying properties of the compact object itself and have opened a promising avenue for putting tight constraints on the equation of state of neutron matter.

Sixteen years of operation mean a large discovery space: from the evolution of black-hole transients, to the discovery of extreme variability from peculiar objects, which can be linked to the complex phenomenon of ejection of relativistic collimated jets, to the long-term monitoring of the X-ray emission of active galactic nuclei and to the observation of signals from seismic waves on neutron stars in the case of an extremely powerful yet very rare flare. RXTE has changed our way we see accreting compact objects and has led the way to the current missions like NICER and Astrosat up to the future observatories like eXTP.

Tomaso Belloni is Research Executive at INAF – Brera Astronomical Observatory in Merate, Italy and Visiting Professor at the University of Southampton, UK. Although not directly involved in the mission, he has worked with RXTE data since its launch and continues today with the exploitation of its database, being also involved in the current Indian X-ray satellite Astrosat and the planning of the future mission eXTP. He is chair of COSPAR Commission E and vice-president of the Scientific Advisory Panel of INAF. His main research interests are in fast time variability from X-ray binaries, in particular black-hole binaries, and the search for GR effects through timing analysis.

Seminar was recorded on March 18, 2021