A broadband X-ray study of the dwarf nova SS Cyg during quiescence and outburst

Abstract

We present a broadband X-ray study (∼0.3-50 keV) of the dwarf nova SS Cyg highlighting the changes in the accretion during two phases, the quiescence and the outburst states. The investigation was based on simultaneous observations carried out with the XMM-Newton and NuSTAR telescopes in two epochs, involving medium and high-resolution spectroscopy. Spectra were harder during quiescence (kTₕᵢᶢₕ∼22.8 keV) than outburst (kTₕᵢᶢₕ ∼ 22.8 keV), while the mass accretion rate increased by ∼35 times in outburst (1.7×10¹⁶g s⁻¹) than quiescence. The bolometric luminosity (0.01-100.0 keV) during the outburst was dominated by a blackbody emission (kTᴮᴮ ∼ 28 eV) from the optically thick boundary layer, and the inner edge of the accretion disk resides very close to the WD surface. X-rays from the accretion disk boundary layer are consistent with the white dwarf having mass 1.18⁺⁰.⁰²₋₀.₀₁ M⊙. Our study conclusively confirms the presence of the reflection hump in the 10-30 keV range for both phases, which arises when X-ray photons hit colder material and undergo Compton scattering. We estimated a similarly strong reflection amplitude during quiescence (∼ 1.25) and outburst (∼ 1.31), indicating both the WD surface and disk are contributing to reflection. The neutral Fe Kα line, which is correlated with Compton reflection, also showed similar strength (∼ 80 eV) in both phases. Finally, X-rays also revealed the presence of a partial intrinsic absorber during the outburst, possibly due to an outflowing accretion disk wind.