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 common main sequence star), to the other component, called the ''acc ...

source first studied by the Uhuru

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

. It is composed of a

neutron star A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...

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 pa ...

from a normal

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

(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 common main sequence star), to the other component, called the ''acc ...

(IMXB) is a

binary star A binary star or binary star system 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 as separate stars us ...

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

black hole A black hole is a massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. Th ...

. 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 * Period (punctuation) * Era, a length or span of time *Menstruation, commonly referred to as a "period" Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (o ...

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 most frequently a star. Friction, uneven irradiance, magnetohydrodynamic effects, and ...

. From observations, a twisted accretion disk, in

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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 o ...

, 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 physics, an electronvolt (symbol eV), also written electron-volt and electron volt, is the measure of an amount of kinetic energy gained by a single electron accelerating through an electric potential difference of one volt in vacuum. When us ...

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

radiative transfer Radiative transfer (also called radiation transport) 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 process ...

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 (disambiguation), Saint John, most commonly refers to: * San Juan, Puerto Rico * San Juan, Argentina * San Juan, Metro Manila, a highly urbanized city in the Philippines San Juan may also refer to: Places Arge ...

. 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 " and a stellar image " from the centroid, (2) RA 16^h57^m10.65^s Dec 35°21'35 ± 3" within " 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

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