Hercules X-1

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Her X-1
Observation data
Epoch J2000      Equinox J2000
Constellation Hercules
Right ascension 16 57 49.83[1]
Declination +35 20 32.6[1]
Apparent magnitude (V) 13.83[1]
Spectral typeDA[1]
Other designations
4U 1656+35, HZ Her
Database references

Hercules X-1 (Her X-1), also known as 4U1656+35, is a moderately strong X-ray binary source first studied by the Uhuru satellite. It is composed of a neutron star accreting matter from a normal star (HZ Her) probably due to Roche lobe overflow.

Intermediate-mass X-ray binary (IMXB)[edit]

Her X-1 is the prototype for the massive X-ray binaries although it falls on the borderline, ~2 M, between high- and low-mass X-ray binaries.[2]

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.[3]


This light curve of Her X-1 shows long term and medium term variability. Each pair of vertical lines delineate the eclipse of the compact object behind its companion star. In this case, the companion is a 2 Solar-mass star with a radius of nearly 4 times that of the Sun. 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.[2]

Uhuru observations revealed the presence of X-ray pulsations in Her X-1 (1.2 s) and confirmed that it contains a rapidly rotating neutron star. Figure adapted from figures by E. Schreier, STScI, taken from Figure 7-2a in Charles and Seward.

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.

The figure shows 15-second samples of the raw counts (per 20.48 ms) observed in a 1973 sounding-rocket-borne exposure to three of the X-ray brightest binary sources in the Milky Way galaxy: Her X-1 (1.7 days), Cyg X-3 (0.2 day), and Cyg X-1 (5.6 days).
OSO 8 spectra of Hercules X-1.

Discovery of Her X-1[edit]

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.[4] The original discovery of this periodically pulsating binary X-ray source occurred in November 1971.[5]


The position of Her X-1 was uncertain due to the failure of the Uhuru star aspect sensors, having been reported[5] at J1950 RA 17h05m Dec 34°52' using Uhuru and at 16h56.7m Dec 35°35'[6] using OSO 7. However, there is only one weak X-ray source (2U 1735+43) within 10° of Her X-1.[7] But four radio sources: (1) RA 16h56m50.75s Dec 35°14'33±3" of a double point source separated by 17±2" and a stellar image 13±3" from the centroid, (2) RA 16h57m10.65s Dec 35°21'35±3" within 6±3" of the stellar image, (3) RA 16h57m35.72s Dec 35°15'19±3" with no star visible on the Palomar Sky Survey print, and (4) RA 16h58m39.17s Dec 35°10'53±3" were found near the overlap of Uhuru and OSO 7 positions.[8] 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.[8] 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[8] 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.


  1. ^ a b c d "Her X-1". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 10 February 2018.
  2. ^ a b Priedhorsky WC, Holt SS (1987). "Long-term cycles in cosmic X-ray sources". Space Sci. Rev. 45 (3–4): 291–348. Bibcode:1987SSRv...45..291P. doi:10.1007/BF00171997. S2CID 120443194.
  3. ^ Podsiadlowski P, Rappaport S, Pfahl E (2001). "Evolutionary Binary Sequences for Low- and Intermediate-Mass X-ray Binaries". The Astrophysical Journal. 565 (2): 1107–1133. arXiv:astro-ph/0107261. Bibcode:2002ApJ...565.1107P. doi:10.1086/324686. S2CID 16381236.
  4. ^ Schreier E, Levinson R, Gursky H, Kellogg E, Tananbaum H, Giacconi R (Mar 1972). "Evidence for the Binary Nature of Centaurus X-3 from UHURU X-Ray Observations". Astrophys. J. 172 (3): L79–L89. Bibcode:1972ApJ...172L..79S. doi:10.1086/180896.
  5. ^ a b Tananbaum H, Gursky H, Kellogg EM, Levinson R, Schreier E, Giacconi R (1972). "Discovery of a Periodic Pulsating Binary X-Ray Source in Hercules from UHURU". Astrophys J. 174: L143–9. Bibcode:1972ApJ...174L.143T. doi:10.1086/180968.
  6. ^ Clark GW, Bradt HV, Lewin WH, Schnopper HW, Sprott G (1972). "X-ray Astronomy: Uhuru Dominates Madrid Meeting". Nature. 237 (5355): 369–370. Bibcode:1972Natur.237..369A. doi:10.1038/237369a0. S2CID 4279721.
  7. ^ Giacconi R, Murray S, Gursky H, Kellogg E, Schreier E, Tananbaum T (1972). "The UHURU catalog of X-ray sources". Astrophys J. 178: 281. Bibcode:1972ApJ...178..281G. doi:10.1086/151790.
  8. ^ a b c Doxsey R, Murthy GT, Rappaport S, Spencer J, Zaumen W (Aug 1972). "Radio Search for the Pulsing X-Ray Source in Hercules". Astrophys. J. 176 (8): L15–8. Bibcode:1972ApJ...176L..15D. doi:10.1086/181010.