Hercules X-1An X-ray binary star, the second to be established as such (after Centaurus X-3), based on Uhuru satellite observations. It exhibits a complex behavior, with regular 1.24 second X-ray pulsations, eclipses (by the binary companion star) every 1.75 days, and longer-term modulation of the X-ray intensity over a 35-day cycle. Her X-1 was the first X-ray binary to be optically identified; the low-mass optical counterpart, HZ Her, varies through the binary period from spectral type A/F to B due to X-ray heating. Its mass (about 1.5 times that of the Sun) and the 1.24-second pulsations strongly suggest the X-ray component to be a rotating neutron star. It is thus often referred to as an X-ray pulsar, with the X-ray pulsations arising most probably from channeling of the accreting gas onto the magnetic poles of the neutron star. The discovery in 1976 of a hard X-ray ‘emission line’ at 55 keV, attributed to cyclotron radiation from near the star's surface, provides support for this view and gave a direct measure of the intense polar magnetic field (about 108 tesla). More recent X-ray spectra have led to the re-interpretation of this feature as an absorption line, at 38 keV, arising from cyclotron absorption of electrons in a magnetic field of 3 × 108 tesla.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
Hercules X-1[′hər·kyə‚lēz ¦eks ′wən]
A source of x-rays that pulses with a period of 1.237 seconds, and is eclipsed for 6 of every 42 hours, associated with a variable star, designated HZ Herculis, that also has a period of 42 hours and faint 1.237-second pulsations; believed to be a binary star whose invisible member is a rotating neutron star. Abbreviated Her X-1.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.