2025ApJ...987...53F

We present the first NuSTAR X-ray observation of EF Eri, a well-known polar system. The NuSTAR observation was conducted in conjunction with NICER shortly after EF Eri entered a high accretion state following an unprecedented period of low activity lasting 26 yr since 1997. NuSTAR detected hard X-ray emission up to 50 keV with an X-ray flux of 1.2 × 10^–10 erg s^–1 cm^–2 (3-50 keV). Folded X-ray lightcurves exhibit a single peak with ∼65% spin modulation throughout the 3-50 keV band. We found no evidence of quasiperiodic oscillation (QPO) signals at ν = 0.1-100 Hz with an upper limit on the QPO amplitude below 5% (90% confidence limit) at ν ∼ 0.5 Hz where the optical QPO was previously detected. Our 1D accretion column model, called MCVSPEC, was fitted to the NuSTAR spectral data, yielding an accurate white dwarf (WD) mass measurement of M = (0.55-0.63) M_⊙. MCVSPEC accounts for radiative cooling by thermal bremsstrahlung and cyclotron emission, X-ray reflection off the WD surface, and a previously constrained range of the accretion column area. The derived WD mass range is in excellent agreement with the previous measurement of M = (0.55-0.65) M_⊙ in the optical band. This demonstrates a combination of broadband X-ray spectral analysis and the MCVSPEC model that can be employed in our ongoing NuSTAR observation campaign of other polars to determine their WD masses accurately.