Consequences of narrow cyclotron emission from Hercules X–1

Abstract

A LINE feature has been observed^1–4 in the hard X-ray (E≈58 keV) spectrum of the compact X-ray source Hercules X–1. If this were interpreted as a cyclotron line emission from a strong magnetic field (B≳10¹²G), then the observations could be used to place strict limits on the total source luminosity in the cyclotron line. The salient feature of the observed line emission is that the FWHM is <12 keV (ref. 1), with a total observed luminosity in the line of approximately 1% of the total X-ray luminosity. The line seems to be pulsed in phase with the 1.24s hard X-ray pulse². If we understand, even vaguely, the physical conditions at the pole of an accreting neutron star, as in Her X–1, we would expect a lot of cyclotron emission. For a temperature of kT_e≈20 keV, as inferred from the X-ray spectral data⁵, and a strong magnetic field (B≈5×10¹² G), as inferred by flux conservation, it is easy to excite the electrons, either by collisions or by blue-shifted photo-excitation. For a neutron star with such a magnetic field and a gravitational redshift of 0.1, the fundamental harmonic of the cyclotron radiation appears at approximately 58 keV. In the data of Trumper et al.¹, there is some evidence for the second harmonic, tending to give credence to the reality of the interpretation. If the line emission is real and the hard X-ray pulse is due to the occultation of a more isotropic hard X-ray emission by a shell of gas at the neutron star magnetosphere^6,7, the calculations presented here indicate that the total source luminosity of the cyclotron line is more than an order of magnitude greater than the observed amount.