Hercules X-1

Her X-1 is the prototype for the massive X-ray binaries although it falls on the borderline, {{Solar mass|~2|link=y}}, between high- and low-mass X-ray binaries.

An intermediate-mass X-ray binary (IMXB) is a binary star system where one of the components is a neutron star or a black hole. The other component is an intermediate mass star.{{cite journal |vauthors=Podsiadlowski P, Rappaport S, Pfahl E |title=Evolutionary Binary Sequences for Low- and Intermediate-Mass X-ray Binaries |date=2001 |doi=10.1086/324686 |journal=The Astrophysical Journal |volume=565 |issue=2 |pages=1107–1133 |arxiv=astro-ph/0107261 |bibcode=2002ApJ...565.1107P|s2cid=16381236 }}

Image:Herx1 lc.gif. This eclipse shows the 1.7-day orbital period of the system.]]

The source exhibits complex time variability, pulsing with a period of 1.24 s due to the rotation of the neutron star, eclipsing every 1.70 days with the period of the binary orbit, and also varying with a 35-day period believed associated with the precession of the accretion disk. From observations, a twisted accretion disk, in retrograde precession, modulates the X-rays illuminating HZ Her and Earth.{{ cite journal |vauthors=Priedhorsky WC, Holt SS |title=Long-term cycles in cosmic X-ray sources |journal=Space Sci. Rev. |volume=45 |issue=3–4 |date=1987 |pages=291–348 |bibcode=1987SSRv...45..291P |doi=10.1007/BF00171997|s2cid=120443194 }}

Image:Herx1 spin.gif

The 1.24 second pulsar period associated with Her X-1 is immediately evident from the data. The sharp cut-off at ~24 keV in the flat spectrum observed for Her X-1 in this exposure provided the first reported evidence for radiative transfer effects to be associated with a highly magnetized plasma near the surface of a neutron star.

Image:Rocket graph.gif

Image:Herx1 spectra.gif

The actual announcement of the discovery of Hercules X-1 by Uhuru occurred at the 1971–72 Winter Meeting of the High-Energy Astrophysics Division AAS held in San Juan.{{ cite journal |vauthors=Schreier E, Levinson R, Gursky H, Kellogg E, Tananbaum H, Giacconi R |title=Evidence for the Binary Nature of Centaurus X-3 from UHURU X-Ray Observations |journal=Astrophys. J. |volume=172 |issue=3 |date=Mar 1972 |pages=L79–L89 |doi=10.1086/180896 |bibcode=1972ApJ...172L..79S|doi-access=free }} The original discovery of this periodically pulsating binary X-ray source occurred in November 1971.{{ cite journal |vauthors=Tananbaum H, Gursky H, Kellogg EM, Levinson R, Schreier E, Giacconi R |title=Discovery of a Periodic Pulsating Binary X-Ray Source in Hercules from UHURU |journal=Astrophys J |date=1972 |volume=174 |pages=L143–9 |doi=10.1086/180968 |bibcode=1972ApJ...174L.143T|doi-access=free }}

The position of Her X-1 was uncertain due to the failure of the Uhuru star aspect sensors, having been reported at J1950 RA 17^h05^m Dec 34°52' using Uhuru and at 16^h56.7^m Dec 35°35'{{ cite journal |vauthors=Clark GW, Bradt HV, Lewin WH, Schnopper HW, Sprott G |date=1972 |journal=Nature |volume=237 |issue=5355 |pages=369–370 |doi=10.1038/237369a0 |title=X-ray Astronomy: Uhuru Dominates Madrid Meeting |bibcode = 1972Natur.237..369A |s2cid=4279721 |doi-access=free }} using OSO 7. However, there is only one weak X-ray source (2U 1735+43) within 10° of Her X-1.{{cite journal |vauthors=Giacconi R, Murray S, Gursky H, Kellogg E, Schreier E, Tananbaum T |title=The UHURU catalog of X-ray sources |journal=Astrophys J|date=1972 |volume=178 |page=281|doi=10.1086/151790 |bibcode=1972ApJ...178..281G|doi-access=free }} But four radio sources: (1) RA 16^h56^m50.75^s Dec 35°14'33 ± 3" of a double point source separated by {{val|17|2}}" and a stellar image {{val|13|3}}" from the centroid, (2) RA 16^h57^m10.65^s Dec 35°21'35 ± 3" within {{val|6|3}}" of the stellar image, (3) RA 16^h57^m35.72^s Dec 35°15'19 ± 3" with no star visible on the Palomar Sky Survey print, and (4) RA 16^h58^m39.17^s Dec 35°10'53 ± 3" were found near the overlap of Uhuru and OSO 7 positions.{{ cite journal |vauthors=Doxsey R, Murthy GT, Rappaport S, Spencer J, Zaumen W |title=Radio Search for the Pulsing X-Ray Source in Hercules |journal=Astrophys. J. |date=Aug 1972 |volume=176 |issue=8 |pages=L15–8 |bibcode=1972ApJ...176L..15D |doi=10.1086/181010 |doi-access=free }} At that time the search could not discover the radio counterpart of Her X-1 if its radio emission were analogous to its 36-day periodic X-ray behavior, although there was no compelling astrophysical reason for the two fluxes to be correlated. The four sources above were observed during several eclipse phases of the X-ray star. No radio eclipses were detected that corresponded. At that time Doxsey specified that (1) repeated radio searches, especially during the high X-ray luminosity state of Her X-1, should be made and (2) there was a clear need for a better position determination for Her X-1.

In 1973, Bahcall and Bahcall determined that HZ Herculis had a light curve that matched Hercules X-1's, fixing Hercules X-1's position.

{{Reflist}}

{{Stars of Hercules}}

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Category:Neutron stars

Category:Astronomical X-ray sources

Category:X-ray astronomy

Category:Hercules (constellation)

Herculis, HZ

Category:X-ray binaries