Circinus X-1 is an X-ray binary star system that includes a neutron star. Observation of Circinus X-1 in July 2007 revealed the presence of X-ray jets normally found in black hole systems; it is the first of the sort to be discovered that displays this similarity to black holes.
Location and 16.6d x-ray variability 
On June 14, 1969, an Aerobee 150 rocket, launched from Natal, Brazil, obtained X-ray data during a scan of the Norma-Lupus-Circinus region that detected a well-isolated source at ℓ = 321.4±0.9° b = -0.5±2° (galactic), RA 15h 14m Dec -57° 49′ within the constellation Circinus and referred to as Circinus XR-1 (Cir XR-1). A 16.6 day x-ray period was found by Kaluzienski et al.
Other spectral regions 
The radio and a possible visual counterpart were identified by Whelan et al. Its infrared counterpart was located and found to flare with a 16.6d period by Glass. A (heavily reddened) precise optical counterpart (now known as BR Cir) was identified by Moneti.
The binary nature of Cir X-1 has been established.
Theoretical X-ray astrophysics 
See also 
- Margon B, Lampton M, Bowyer S, Cruddace R (Oct 1971). "A Pulsing X-Ray Source in Circinus". Ap J. 169 (10): L23–5. Bibcode:1971ApJ...169L..23M. doi:10.1086/180806.
- Kaluzienski, L. J.; Holt, S. S.; Boldt, E. A.; Serlemitsos, P. J. (1976). "Evidence for a 16.6 day period from Circinus X-1". Astrophysical Journal 208: L71–L75.
- Whelan, J. A. J.; Mayo, S. K.; Wickramasinghe, D. T.; Murdin, P. G.; Peterson, B. A.; Hawarden, T. G.; Longmore, A. J.; Haynes, R. F.; Goss, W. M.; Simons, L. W.; Caswell, J. L.; Little, A. G.; McAdam, W. B. (1977). Monthly Notices of the Royal Astronomical Society 161: 259–271.
- Glass, I.S. (1978). "Variations of Circinus X-1 in the infrared". Monthly Notices of the Royal Astronomical Society 183: 335–340.
- Moneti, A. (1992). "Optical and infrared observations of Circinus X-1". Astronomy and Astrophysics 260: L7–L10.
- Jones C, Tananbaum H, Giacconi R (1973). 141st Meeting Amer Astron Soc (Tucson, Arizona).
- Mignani RP, De Luca A, Caraveo PA, Mirabel IF (2002). "HST observations rule out the association between Cir X-1 and SNR G321.9-0.3". Astron Astrophys. 386 (2): 487–91. arXiv:astro-ph/0202268. Bibcode:2002A&A...386..487M. doi:10.1051/0004-6361:20020224.
- "Circinus X-1: Neutron Stars Join the Black Hole Set". Chandra X-Ray Observatory. Harvard University / National Aeronautics and Space Administration. 2007. Retrieved 8 January 2009.
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