NuSTAR broadband X-ray observation of EF Eri following its reawakening into a high accretion state

Abstract

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 years since 1997. NuSTAR detected hard X-ray emission up to 50 keV with an X-ray flux of $1.2\times 10^{-10}$ ergs s${}^{-1}$ cm${}^{-2}$ ($3-50$ keV). Folded X-ray lightcurves exhibit a single peak with $\sim 65\%$ spin modulation throughout the $3-50$ keV band. We found no evidence of QPO signals at $\hat{I}½=0.1-100$ Hz with an upper limit on the QPO amplitude below $5\%$ ($90\%$ CL) at $\hat{I}½\sim 0.5$ Hz where the optical QPO was previously detected. Our 1-D accretion column model, called $\text{MCVSPEC}$, was fitted to the NuSTAR spectral data, yielding an accurate WD mass measurement of $M=(0.55-0.63){\mathrm{M}}_{\odot }$. $\mathtt{MCVSPEC}$ 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 $M=(0.55-0.65){\mathrm{M}}_{\odot }$ in the optical band. This demonstrates a combination of broadband X-ray spectral analysis and the $\mathtt{MCVSPEC}$ model that can be employed in our ongoing NuSTAR observation campaign of other polars to determine their WD masses accurately.

18 pages, 10 figures, 3 tables. Submitted to ApJ