Gravitational Redshift Detection from the Magnetic White Dwarf Harbored in RX J1712.6-2414

Abstract

Gravitational redshift is a fundamental parameter that allows us to determine the mass-to-radius ratio of compact stellar objects, such as black holes, neutron stars, and white dwarfs (WDs). In the X-ray spectra of the close binary system, RX J1712.6$-$2414, obtained from the Chandra High-Energy Transmission Grating observation, we detected significant redshifts for characteristic X-rays emitted from hydrogen-like magnesium, silicon ($\Delta E/E_{\rm rest} \sim 7 \times 10^{-4}$), and sulfur ($\Delta E/E_{\rm rest} \sim 15 \times 10^{-4}$) ions, which are over the instrumental absolute energy accuracy (${\Delta E/E_{\rm rest} \sim 3.3} \times 10^{-4}$). Considering some possible factors, such as Doppler shifts associated with the plasma flow, systemic velocity, and optical depth, we concluded that the major contributor to the observed redshift is the gravitational redshift of the WD harbored in the binary system, which is the first gravitational redshift detection from a magnetic WD. Moreover, the gravitational redshift provides us with a new method of the WD mass measurement by invoking the plasma-flow theory with strong magnetic fields in close binaries. Regardless of large uncertainty, our new method estimated the WD mass to be $M_{\rm WD}> 0.9\,M_{\odot}$.