An extreme case of X-ray reflection in the symbiotic V648 Car

Abstract

V648 Car is a $\delta$-type symbiotic star known for its strong hard thermal X-ray emission, heavy local photoelectric absorption, and stochastic variability. We analyze NuSTAR observations, complemented by nearly contemporaneous Swift/XRT observations, to check for the expected presence and physical implications of Compton reflection in its X-ray spectrum. The spectra were fitted with X-ray models that accounted for thermal plasma components, different prescriptions to quantify the intrinsic absorption, the presence of the fluorescent iron line at 6.4 keV, and a reflection component. Our results reveal a strong Compton reflection contribution, accounting for approximately 37% of the total unabsorbed flux in the 3-50 keV band. The inclusion of reflection significantly improves the spectral fits and reduces the value for the maximum plasma temperature by a factor of 2 (from about 49 keV to 25 keV). If the maximum temperature is attributed to strong shock of a Keplerian flow, the inferred white dwarf mass decreases from about 20% (from $1.2$ to $0.95\,M_{\odot}$), demonstrating that neglecting reflection can lead to significant overestimates of this parameter. Reflection models yield high reflection scaling factors ($R \sim 2$), inconsistent with a simple picture of a primary X-ray source above a reflecting disk. The timing analysis reveals stochastic flickering variability with no evidence for coherent periodic modulations, thus the accreting object is likely a non-magnetic white dwarf. We propose a multiple-reflection model to explain the extraordinary strength of reflection features in V648 Car.