Hercules X-1 (Her X-1), also known as 4U1656+35, is a moderately strong X-ray binary source first studied by the Uhurusatellite.
It is composed of a neutron star accreting matter from a normal star (HZ Her) probably due to Roche lobe overflow.
[edit]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.
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. From observations, a twisted accretion disk, in retrogradeprecession, modulates the X-rays illuminating HZ Her and Earth.[2]
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 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 effects to be associated with a highly magnetized plasma near the surface of a neutron star.
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.
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.[4] The original discovery of this periodically pulsating binary X-ray source occurred in November 1971.[5]
The position of Her X-1 was uncertain due to the failure of the Uhuru star aspect sensors, having been reported[5] at J1950 RA17h05mDec 34°52' using Uhuru and at 16h56.7mDec 35°35'[6] using OSO 7. However, there is only one weak X-ray source (2U 1735+43) within 10° of Her X-1.[7] But four radio sources: (1) RA16h56m50.75sDec 35°14'33±3" of a double point source separated by 17±2" and a stellar image 13±3" from the centroid, (2) RA16h57m10.65sDec 35°21'35±3" within 6±3" of the stellar image, (3) RA16h57m35.72sDec 35°15'19±3" with no star visible on the Palomar Sky Survey print, and (4) RA16h58m39.17sDec 35°10'53±3" were found near the overlap of Uhuru and OSO 7 positions.[8] 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.[8] 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[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.
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.
