Hercules X-1
Prototype intermediate-mass X-ray binary with complex variability.
Hercules X-1 (Her X-1), also cataloged as 4U1656+35, is an X-ray binary of moderate strength that was initially examined by the Uhuru satellite. The system consists of a neutron star pulling material from a normal star called HZ Her, likely through Roche lobe overflow. Although Her X-1 serves as the archetype for massive X-ray binaries, its mass sits right at the boundary—roughly two solar masses—between high-mass and low-mass systems, placing it in the category of intermediate-mass X-ray binaries (IMXBs). In such a binary, one component is a neutron star or black hole, while the other is a star of intermediate mass.
The source shows intricate changes in brightness over time. It pulses every 1.24 seconds due to the neutron star’s rotation, undergoes eclipses every 1.70 days matching the binary orbit, and fluctuates on a 35-day cycle thought to come from the accretion disk’s precession. Observations suggest a twisted accretion disk, precessing in a retrograde direction, modulates the X-rays that illuminate both HZ Her and Earth. The 1.24-second pulsar period is clearly visible in the data. A sharp cutoff in the flat spectrum around 24 keV, seen during one exposure, offered the first evidence that radiative transfer effects occur in a highly magnetized plasma near a neutron star’s surface.
The discovery of Hercules X-1 was announced at the 1971–72 Winter Meeting of the High-Energy Astrophysics Division of the AAS in San Juan, following its initial detection by Uhuru in November 1971. Pinpointing its location proved tricky because Uhuru’s star aspect sensors had failed. The Uhuru team reported coordinates of RA 17h05m, Dec 34°52′ (J1950), while OSO 7 gave RA 16h56.7m, Dec 35°35′. Only one weak X-ray source, 2U 1735+43, lies within 10 degrees of Her X-1. Four radio sources were found near the overlap of the two positions: one at RA 16h56m50.75s, Dec 35°14′33″ (a double point source separated by 17 arcseconds, with a stellar image 13 arcseconds from the centroid); another at RA 16h57m10.65s, Dec 35°21′35″ (within 6 arcseconds of a stellar image); a third at RA 16h57m35.72s, Dec 35°15′19″ (no visible star on Palomar Sky Survey prints); and a fourth at RA 16h58m39.17s, Dec 35°10′53″. At the time, researchers could not identify a radio counterpart, even if its radio emission followed the 36-day X-ray cycle, though there was no strong reason to expect a correlation
- type
- X-ray binary source
- components
- neutron star and HZ Her
- pulsar period
- 1.24 seconds
- binary orbital period
- 1.70 days
- long-term period
- 35 days
- discovery announced
- 1971–72 Winter Meeting of the High-Energy Astrophysics Division AAS
- discovery date
- November 1971
Lore & Background
Hercules X-1 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. The 1.24 second pulsar period is immediately evident from the data, and the sharp cut-off at ~24 keV in the flat spectrum provided the first reported evidence for radiative transfer effects associated with a highly magnetized plasma near the surface of a neutron star.
Reader's Guide
Hercules X-1 holds significance as the prototype for massive X-ray binaries, though it straddles the boundary between high- and low-mass systems, making it a key example of an intermediate-mass X-ray binary. Its discovery by the Uhuru satellite in November 1971, announced at the 1971–72 Winter Meeting of the High-Energy Astrophysics Division AAS, marked an early milestone in X-ray astronomy. The source's complex variability—including a 1.24-second pulsar period, 1.70-day eclipses, and a 35-day modulation—has provided critical insights into neutron star rotation, binary dynamics, and accretion disk precession. The observed spectral cutoff at ~24 keV offered the first evidence of radiative transfer in a highly magnetized plasma near a neutron star surface. Initial positional uncertainty, due to Uhuru's failed star aspect sensors, led to multiple candidate radio sources, but in 1973 Bahcall and Bahcall matched HZ Herculis's light curve to Hercules X-1, fixing its position. This system remains a cornerstone for studying accretion processes and binary evolution.
Did You Know?
- Her X-1 was first studied by the Uhuru satellite and its discovery announced at the 1971–72 Winter Meeting of the High-Energy Astrophysics Division AAS.
- The source exhibits a 1.24-second pulsar period due to neutron star rotation, a 1.70-day binary eclipse period, and a 35-day variation linked to accretion disk precession.
- A sharp cut-off at ~24 keV in its spectrum provided the first reported evidence of radiative transfer effects in a highly magnetized plasma near a neutron star surface.
- In 1973, Bahcall and Bahcall determined that HZ Herculis had a light curve matching Hercules X-1's, fixing the source's position.
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