Hercules X-1
Reference data for Hercules X-1: 1.24-second X-ray pulsar spin, 1.70-day binary orbit, 35-day warped disk cycle, and cyclotron magnetic field.
Provenance-Backed Scientific Measurements
Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Distance | 20200 ly | ± 1600 ly | Gaia DR3 parallax of optical companion HZ Herculis (~6.2 ± 0.5 kpc). | definitive | ESA Gaia Archive — Data Release 3 |
| Apparent Magnitude (V) | 13.8 mag | ± 0.8 mag | Optical counterpart HZ Herculis varies between V = 13.0 and 14.6 every 1.70 days due to X-ray heating. | definitive | SIMBAD Astronomical Database (CDS) |
| Binary Orbital Period | 1.700167 d | ± 0.000001 d | Coherent X-ray/radio timing or binary radial-velocity solution | definitive | SIMBAD Astronomical Database (CDS) |
| Neutron Star Mass | 1.5 M☉ | ± 0.14 M☉ | Eclipsing X-ray pulse arrival-time delays combined with optical radial velocities of HZ Herculis (~2.2 M☉). | definitive | SIMBAD Astronomical Database (CDS) |
| X-Ray Luminosity | 5200 L☉ | ± 800 L☉ | Accretion-powered X-ray output (~2 × 10³⁷ erg/s). | definitive | SIMBAD Astronomical Database (CDS) |
| Systemic Radial Velocity | -65 km/s | ± 2 km/s | Standard astrophysical catalog determination | definitive | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Hercules X-1 (HZ Herculis) — X-Ray Pulsar at RA 16h 57m 49.81s, Dec +35° 20′ 32.6″ within Hercules, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.
Astrophysical Context & Structure
Discovered in November 1971 by the Uhuru satellite, Hercules X-1 quickly became one of the most celebrated systems in X-ray astronomy because its light curve is governed by three distinct, interlocking periodicities. First, the magnetized neutron star spins once every 1.2377 seconds, funneling accreting gas onto its magnetic poles to create sharp X-ray pulses. Second, every 1.70017 days, the neutron star is eclipsed for 5.6 hours behind its 2.2-solar-mass donor star, the known variable star HZ Herculis. Third, X-ray emission switches on and off in a 35-day super-orbital cycle caused by a warped, tilted accretion disk precessing retrogradely across our line of sight.
In 1976, balloon-borne hard X-ray scintillation detectors observed a prominent absorption-like spectral feature at ~37 to 42 keV in the spectrum of Hercules X-1. Interpreted as a cyclotron resonance scattering feature from electrons spiraling in quantized Landau levels within the accretion column, this observation provided the first direct measurement of a neutron star's surface magnetic field strength—pinning it at roughly 3.5 × 10¹² gauss.
Because the hemisphere of HZ Herculis facing the neutron star is blasted by 5,200 solar luminosities of hard X-rays, its dayside photosphere is heated to an A-type effective temperature near 15,000 K while its shadowed backside remains a cool ~7,000 K F-type star. As the donor star revolves every 1.70 days, amateur observers see its optical brightness swing smoothly by 1.6 magnitudes between V = 13.0 and 14.6.
Practical Visual Observing Guide
- Optical counterpart HZ Herculis varies between V = 13.0 (at X-ray mid-phase) and V = 14.6 (during X-ray eclipse) every 40.8 hours.
- Accessible to 8-inch+ visual telescopes and a favorite target for amateur CCD light-curve photometrists studying X-ray heating.
- Located in central Hercules roughly 4.5 degrees northeast of the Great Globular Cluster (M13).
Catalog Cross-Identifications & Aliases
Common Aliases: Her X-1 · HZ Herculis · 4U 1656+35
| Catalog Authority | Identifier |
|---|---|
| SIMBAD | Her X-1 |
Related Objects, Comparisons & Guides
Cygnus X-1 (HDE 226868 X-Ray Black Hole)
Coordinates, VLBA/Gaia distance (2.22 kpc), 21.2-M☉ stellar black hole mass, 5.60-day O-supergiant binary, and observing guide for Cygnus X-1.
SS 433 (V1343 Aquilae) — Galactic Microquasar
Reference data for SS 433: 0.26c relativistic precessing jets, 13.08-day eclipsing binary orbit in Aquila, and the W50 Manatee Nebula.
V404 Cygni (GS 2023+338) — Black Hole Binary
Reference data for V404 Cygni: 9.0 solar mass black hole, VLBI parallax distance of 7,800 ly, 6.47-day orbit, and 2015 X-ray dust-ring outburst.
Apparent vs Absolute Magnitude Explained
Understand the logarithmic Pogson astronomical magnitude scale, apparent visual magnitude (m), absolute magnitude (M at 10 parsecs), and bolometric corrections.
Exoplanet Transit & Radial Velocity Methods
How astronomers detect and characterize exoplanets using photometric transit depth ((Rp/R*)²), Doppler radial velocity (M sin i), and JWST transmission spectra.
Scientific Sources & Dataset Provenance
- SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg
- ESA Gaia Archive — Data Release 3 (DR3 (2022-06-13)): Gaia Collaboration, Vallenari, A., et al. 2023, A&A, 674, A1 (Gaia Data Release 3) — European Space Agency / Gaia DPAC