Scorpius X-1

Lore & Background

The possible existence of cosmic soft X-rays was first proposed by Bruno Rossi to Martin Annis. Following his urging, a contract from the United States Air Force was obtained to explore the lunar surface and incidentally detect galactic X-ray sources. Scorpius X-1 was discovered in 1962 by a team under Riccardo Giacconi, who launched an Aerobee 150 sounding rocket carrying a soft X-ray detector designed by Frank Paolini. The rocket trajectory was slightly off course but detected significant soft X-rays not coming from the Moon, making Scorpius X-1 the first X-ray source discovered outside the Solar System. The angular resolution of the detector did not initially allow accurate positioning, leading to suggestions it might be near the Galactic Center, but it was eventually realized to lie in Scorpius. The rocket launched on June 12, 1962 or June 19, 1962, and detected the source at J1950 coordinates RA 16h 15m Dec −15.2°.

In 1967, before the discovery of pulsars, Iosif Shklovsky examined X-ray and optical observations and correctly concluded that the radiation comes from a neutron star accreting matter from a companion. Scorpius X-1 shows regular variations of up to 1 magnitude in its intensity with a period of around 18.9 hours, and varies irregularly in optical wavelengths, though these changes are not correlated with the X-ray variations. The neutron star's intense gravity draws material off its companion into an accretion disk, where it falls onto the surface, releasing tremendous energy. The measured luminosity is consistent with a neutron star accreting matter at its Eddington limit. The system is classified as a low-mass X-ray binary; the neutron star is roughly 1.4 solar masses, while the donor star is 0.42 solar masses. The origin is debated: studies based on orbit reconstruction suggest the binary may have formed by a close encounter inside a globular cluster, but it is not clear how to reconcile this with the circularisation of the orbit.

Reader's Guide

Scorpius X-1 holds a foundational place in X-ray astronomy as the first extrasolar X-ray source discovered, a milestone achieved fortuitously during a rocket flight intended to study the Moon. Its discovery opened the field of cosmic X-ray sources and demonstrated that powerful X-ray emission occurs outside the Solar System. The source remains notable as the strongest apparent non-transient X-ray source in the sky aside from the Sun, with an X-ray output about 60,000 times the total luminosity of the Sun. The correct interpretation by Iosif Shklovsky in 1967—that the radiation originates from a neutron star accreting matter from a companion—preceded the discovery of pulsars and established the accretion-powered binary model. The system's classification as a low-mass X-ray binary, with a neutron star of roughly 1.4 solar masses and a donor star of 0.42 solar masses, provides a key example for studying mass transfer and accretion processes. The ongoing debate about its origin, possibly involving formation in a globular cluster, highlights unresolved questions in binary evolution. Scorpius X-1 thus serves as both a historical landmark and an active subject of astrophysical research.

Did You Know?

• It was the first extrasolar X-ray source discovered, detected fortuitously when the rocket trajectory was slightly off course.
• Its X-ray output is 2.3×10^31 W, about 60,000 times the total luminosity of the Sun.
• The system's origin is debated; studies suggest it may have formed by a close encounter inside a globular cluster.