Scorpius X-1

V818 Sco or Scorpius X-1

Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Scorpius
Right ascension	16^h 19^m 55.07^s[1]
Declination	−15° 38' 24.8"[1]
Apparent magnitude (V)	12.2[1]
Distance	9,000 ly (2,800[2] pc)
Other designations
V818 Sco, H 1620-15, 2RE J161955-153824, 1XRS 16170-155, 2A 1616-155, INTEGRAL1 21, RE J1619-153, XSS J16204-1536, 3A 1617-155, INTREF 685, RE J161956-153814, X Sco X-1, 2EUVE J1619-15.6, KOHX 20, SBC7 569, EUVE J1619-15.6, 1M 1617-155, 2U 1617-15, AAVSO 1614-15, 1H 1617-155, 2MASS J16195506-1538250, 3U 1617-15, H 1617-155, 2RE J1619-153, 4U 1617-15
Database references
SIMBAD	data

Scorpius X-1 is an X-ray source located roughly 9000 light years away in the constellation Scorpius. Scorpius X-1 was the first extrasolar X-ray source discovered, and, aside from the Sun, it is the strongest apparent non-transient source of X-rays in the sky.^[3] The X-ray flux varies day-to-day, and is associated with an optically visible star, V818 Scorpii, that has an apparent magnitude which fluctuates between 12-13.^[4]

Discovery and early study

The possible existence of cosmic soft X-rays was first proposed by Bruno Rossi, MIT professor and board chairman of American Science and Engineering in Cambridge, Massachusetts to Martin Annis, president of AS&E. Following his urging, the company obtained a contract from the United States Air Force to explore the lunar surface prior to the launch of astronauts to the Moon, and incidentally to perhaps see galactic sources of X-rays.

Subsequently, Scorpius X-1 was discovered in 1962 by a team, under Riccardo Giacconi, who launched an Aerobee 150 sounding rocket carrying a highly sensitive soft X-ray detector designed by Frank Paolini. The rocket trajectory was slightly off course but still detected a significant emission of soft X-rays that were not coming from the Moon. Thus fortuitously, and as first pointed out by Frank Paolini, Scorpius X-1 became the first X-ray source discovered outside the Solar System. The angular resolution of the detector did not initially allow the position of Scorpius X-1 to be accurately determined. This led to suggestions that the source might be located near the Galactic Center, but it was eventually realized that it lies in the constellation Scorpius.^[4] As the first discovered X-ray source in Scorpius, it received the designation Scorpius X-1.

The Aerobee 150 rocket launched on June 12, 1962, detected the first X-rays from another celestial source (Scorpius X-1) at J1950 RA 16^h 15^m Dec −15.2°.^[5] Sco X-1 is a LMXB in which the visual counterpart is V818 Scorpii.

Although the above reference indicates the rocket launch was on June 12, 1962, other sources indicate the actual launch was at 06:59:00 UTC on June 19, 1962.^[6][7]

Historical footnote: "The instrumentation had been designed for an attempt to observe X-rays from the moon and was not equipped with collimation to restrict the field of view narrowly. As a result, the signal was very broad, and accurate definition of the size and position of the source was not possible. A similar experiment was repeated in October 1962 when the Galactic Center was below the horizon and the strong source was not present. A third attempt, in June 1963, verified the results of the June 1962 flight."^[8] The Galactic Center is < 20° RA and < 20° Dec from Sco X-1, the two X-ray sources are separated by ~20° of arc and may not have been resolvable in the June 1962 flight.^[8]

Scorpius XR-1 has been observed at J1950 RA 16^h 15^m Dec −15.2°.^[8]

In 1967 (before the discovery of pulsars), Iosif Shklovsky examined X-ray and optical observations of Scorpius X-1 and correctly concluded that the radiation comes from a neutron star accreting matter from a companion.^[9]

Characteristics

A broadband optical light curve for V818 Scorpii, adapted from Hynes et al. (2016)^[10]

Its X-ray output is 2.3×10³¹ W, about 60,000 times the total luminosity of the Sun.^[2] Scorpius X-1 shows regular variations of up to 1 magnitude in its intensity, with a period of around 18.9 hours. The source varies irregularly in optical wavelengths as well, but these changes are not correlated with the X-ray variations.^[4] Scorpius X-1 itself is a neutron star whose intense gravity draws material off its companion into an accretion disk, where it ultimately falls onto the surface, releasing a tremendous amount of energy. As this stellar material accelerates in Scorpius X-1's gravitational field, X-rays are emitted. The measured luminosity for Scorpius X-1 is consistent with a neutron star which is accreting matter at its Eddington limit.^[2]

This system is classified as a low-mass X-ray binary; the neutron star is roughly 1.4 solar masses, while the donor star is only 0.42 solar masses.^[11] The two stars were probably not born together; recent research suggests that the binary may have been formed by a close encounter inside a globular cluster.^[12]

See also

• List of X-ray pulsars

References

1. Staff (March 3, 2003). "V* V1357 Cyg—High Mass X-ray Binary". Centre de Données astronomiques de Strasbourg. Retrieved 2009-12-21.
2. Bradshaw, C.F.; Fomalont, E.B.; Geldzahler, B.J. (1999). "High-Resolution Parallax measurements of Scorpius X-1". The Astrophysical Journal. 512 (2): L121 – L124. Bibcode:1999ApJ...512L.121B. doi:10.1086/311889.
3. Giacconi, R.; Gursky, H.; Paolini, F.R.; Rossi, B.B. (1962). "Evidence for X-rays from sources outside the solar system". Phys. Rev. Lett. 9 (11): 439–443. Bibcode:1962PhRvL...9..439G. doi:10.1103/PhysRevLett.9.439.
4. Shklovskii, Iosif S. (1978). Stars: Their Birth, Life, and Death. W.H. Freeman. ISBN 978-0-7167-0024-1.
5. Giacconi R (August 2003). "Nobel Lecture: The dawn of X-ray astronomy". Rev. Mod. Phys. 75 (3): 995–1010. Bibcode:2003RvMP...75..995G. doi:10.1103/RevModPhys.75.995.
6. Drake SA (September 2006). "A Brief History of High-Energy Astronomy: 1960–1964".
7. "Chronology—Quarter 2 1962". Archived from the original on 2010-01-18.
8. Bowyer S; Byram ET; Chubb TA; Friedman H (1965). Steinberg JL (ed.). "Observational results of X-ray astronomy". Astronomical Observations from Space Vehicles, Proceedings from Symposium No. 23 Held in Liège, Belgium, 17 to 20 August 1964. 23. International Astronomical Union: 227–39. Bibcode:1965IAUS...23..227B.
9. Shklovsky, I.S. (April 1967). "On the Nature of the Source of X-Ray Emission of SCO XR-1". Astrophys. J. 148 (1): L1 – L4. Bibcode:1967ApJ...148L...1S. doi:10.1086/180001.
10. Hynes, Robert I.; Schaefer, Bradley E.; Baum, Zachary A.; Hsu, Ching-Cheng; Cherry, Michael L.; Scaringi, Simone (July 2016). "Kepler K2 observations of Sco X-1: orbital modulations and correlations with Fermi GBM and MAXI". Monthly Notices of the Royal Astronomical Society. 459 (4): 3596–3613. arXiv:1605.00546. Bibcode:2016MNRAS.459.3596H. doi:10.1093/mnras/stw854.
11. Steeghs, D.; Casares, J. (2002). "The Mass Donor of Scorpius X-1 Revealed". The Astrophysical Journal. 568 (1): 273–278. arXiv:astro-ph/0107343. Bibcode:2002ApJ...568..273S. doi:10.1086/339224. S2CID 14136652.
12. Mirabel, I. F.; Rodrogues, I. (2003). "The origin of Scorpius X-1". Astronomy and Astrophysics. 398 (3): L25 – L28. arXiv:astro-ph/0301580. Bibcode:2003A&A...398L..25M. doi:10.1051/0004-6361:20021767. S2CID 2585509.