Hercules X-1 (Her X-1), also known as 4U1656+35, is a moderately strong

X-ray binary X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the ''donor'' (usually a relatively normal star), to the other component, called the ''accretor'', whi ...

source first studied by the Uhuru

satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioi ...

. It is composed of a

neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...

accreting

matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic part ...

from a normal star (HZ Her) probably due to

Roche lobe In astronomy, the Roche lobe is the region around a star in a binary system within which orbiting material is gravitationally bound to that star. It is an approximately teardrop-shaped region bounded by a critical gravitational equipotential ...

overflow.

Intermediate-mass X-ray binary (IMXB)

Her X-1 is the prototype for the massive X-ray binaries although it falls on the borderline, , 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.

Intensity

The source exhibits complex time variability, pulsing with a

period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

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 Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In oth ...

, modulates the X-rays illuminating HZ Her and Earth. The 1.24 second pulsar period associated with Her X-1 is immediately evident from the data. The sharp cut-off at ~24

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in the flat spectrum observed for Her X-1 in this exposure provided the first reported evidence for

radiative transfer Radiative transfer is the physical phenomenon of energy transfer in the form of electromagnetic radiation. The propagation of radiation through a medium is affected by absorption, emission, and scattering processes. The equation of radiative trans ...

effects to be associated with a highly magnetized plasma near the surface of a neutron star.

Discovery of Her X-1

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

Location

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' using OSO 7. However, there is only one weak X-ray source (2U 1735+43) within 10° of Her X-1. But four radio sources: (1) RA 16^h56^m50.75^s 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 16^h57^m10.65^s Dec 35°21'35±3" within 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. 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.

References

{{Stars of Hercules Neutron stars Astronomical X-ray sources X-ray astronomy Hercules (constellation) Herculis, HZ X-ray binaries