X-ray lines and magnetic field of Her X-1

THE report¹ of a possible detection of an X-ray emission line with energy about 53 keV from the binary X-ray source Her X-1 created great excitement. Interpreting the line as arising from non-relativistic electron cyclotron emission at the surface of an accreting neutron star member of the binary star system, Trumper et al.¹ deduce a neutron star surface magnetic field of 4.6 × 10¹² G. If the line is real (of cosmic origin and not due to background effects), significant (not due to a statistical fluctuation) and correctly interpreted as arising from non-relativistic cyclotron emission) it could provide the first direct and quantitative measure of the magnetic field of a neutron star, and be direct support for our ideas on the formation of neutron stars and the role of magnetic fields in the radiation mechanisms of pulsars and binary X-ray sources. Furthermore, it would conclusively rule out the possibility that Her X-1 contains a rotating or pulsating white dwarf (which, for dynamical stability requires B<<G^1/2MR^-2~=10¹²G for M~=M_solar and R=R_WD~= 10⁹ cm).