A white dwarf accreting planetary material determined from X-ray observations

The atmospheres of a large proportion of white dwarf stars are polluted by heavy elements¹ that are expected to sink out of visible layers on short timescales^2,3. This has been interpreted as a signature of ongoing accretion of debris from asteroids⁴, comets⁵ and giant planets⁶. This scenario is supported by the detection of debris discs⁷ and transits of planetary fragments⁸ around some white dwarfs. However, photospheric metals are only indirect evidence for ongoing accretion, and the inferred accretion rates and parent body compositions heavily depend on models of diffusion and mixing processes within the white dwarf atmosphere^9-11. Here we report a 4.4σ detection of X-rays from a polluted white dwarf, G29-38. From the measured X-ray luminosity, we derive an instantaneous accretion rate of M_{̇ X}=1.63_−0.40^+1.29×10⁹g s⁻¹, which is independent of stellar atmosphere models. This rate is higher than estimates from past studies of the photospheric abundances of G29-38, suggesting that convective overshoot may be needed to model the spectra of debris-accreting white dwarfs. We measure a low plasma temperature of k_BT = 0.5 ± 0.2 keV, corroborating the predicted bombardment solution for white dwarfs accreting at low accretion rates^12,13.