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'', which ...

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 radioiso ...

. 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 A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

(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 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'', which ...

(IMXB) is a

binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in ...

system where one of the components is a neutron star or a

black hole A black hole is a region of spacetime where gravity is so strong that nothing, including light or other electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts that a sufficiently compact mass can def ...

. The other component is an intermediate mass star.

Intensity

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 An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other ...

. 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 keV 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 San Juan, Spanish for Saint John, may refer to: Places Argentina * San Juan Province, Argentina * San Juan, Argentina, the capital of that province * San Juan, Salta, a village in Iruya, Salta Province * San Juan (Buenos Aires Underground), ...

. 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