Hercules X-1

Hercules X-1 (Her X-1), also known as 4U1656+35, is a moderately strong X-ray binary source first studied by the Uhuru satellite.

From observations, a twisted accretion disk, in retrograde precession, modulates the X-rays illuminating HZ Her and Earth.

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

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.

At that time Doxsey[8] 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.

This light curve of Her X-1 shows long term and medium term variability. Each pair of vertical lines delineate the eclipse of the compact object behind its companion star. In this case, the companion is a 2 Solar-mass star with a radius of nearly 4 times that of the Sun . This eclipse shows the 1.7-day orbital period of the system.
Uhuru observations revealed the presence of X-ray pulsations in Her X-1 (1.2 s) and confirmed that it contains a rapidly rotating neutron star. Figure adapted from figures by E. Schreier, STScI, taken from Figure 7-2a in Charles and Seward.
The figure shows 15-second samples of the raw counts (per 20.48 ms) observed in a 1973 sounding-rocket-borne exposure to three of the X-ray brightest binary sources in the Milky Way galaxy: Her X-1 (1.7 days), Cyg X-3 (0.2 day), and Cyg X-1 (5.6 days).
OSO 8 spectra of Hercules X-1