Unveiling the X-ray properties of the eclipsing Cataclysmic Variable UU Aqr: spatially and spectrally-resolved two-component emission

Abstract

Non-magnetic Cataclysmic Variables (CVs) show two distinct X-ray components: a hard, optically thin component and a soft, optically thick, blackbody-like component, both produced in the boundary layer between the accretion disk and the White Dwarf (WD). An additional soft component originating from a more extended region has been reported in few CVs. In a short Chandra exposure, we identified a tentative X-ray eclipse in UU Aqr, a non-magnetic CV which shows deep optical eclipses. Using observations with the Nuclear Spectroscopic Telescope Array (NuSTAR) and the XMM-Newton, we detect total eclipses in the orbital intensity profiles of this system in the hard X-ray band (3-10 keV with XMM and 3-25 keV with NuSTAR). However, the soft X-ray band (0.3-2.0 keV) shows no evidence of an eclipse. Detailed eclipse modeling, energy-resolved power spectral analysis and broadband spectral modeling indicate that the hard absorbed X-ray emission originates from a compact region near the WD, such as a boundary layer, while the soft, unabsorbed and un-eclipsed X-ray emission originates in an extended region. Neither scattering of hard X-rays nor colliding winds can account for the observed un-eclipsed soft emission. We instead propose that this component is produced by shocks within vertically extended, radiatively driven accretion-disk winds. We also provide new estimates on the emitting regions, mass and radius of the WD and the donor star using eclipse modeling.

Figures (6)

• Figure 1: Left panel: Chandra-ACIS background subtracted lightcurve of UU Aqr, covering one orbital cycle, in the 0.3--8 keV (top panel), 0.3-2.0 keV (middle panel) and 3.0-8.0 keV (bottom panel) energy bands. Right panel: NuSTAR FPMA+FPMB background subtracted lightcurve of UU Aqr in 3--25 keV (top panel) and XMM EPIC (MOS1 + MOS2 + PN) background subtracted lightcurve in 0.3--10 keV (middle panel) and XMM OM-Fast Mode using UVW1 filter (bottom panel), binned with 100 s. The NuSTAR observation covered about five orbital cycles of the source, the XMM observation covered one orbital cycle for EPIC and 2 orbital cycles for OM-Fast Mode. The cyan dashed lines mark the expected optical eclipses using the ephemeris given in baptista1994 and an orbital period of 0.163580429 d.
• Figure 2: Orbital intensity profile of UU Aqr constructed using background subtracted Chandra-ACIS lightcurves in 0.3--8.0 keV using 20 phasebins (top panel), energy-resolved background subtracted lightcurves of the XMM observation in 0.3--2.0 keV (second panel) and 3--10 keV using 32 phasebins (third panel), background subtracted NuSTAR lightcurves in 3--25 keV in 100 phasebins (fourth panel) and XMM OM-Fast UVW1 lightcurves in 100 phasebins (bottom panel). The orbital ephemeris used for folding the lightcurves is taken from the optical eclipses in baptista1994, which mentions the mid-eclipse time T$_{0}$ = JD 2446,347.26657 and orbital period P = 0.163580429 d. The vertical cyan dashed line marks the mid-eclipse center and the horizontal magenta dashed line marks the zero count-rate of the X-ray observations.
• Figure 3: Orbital intensity profiles near the eclipse using Chandra 0.3--8 keV, XMM-EPIC 3--10 keV, NuSTAR 3--25 keV, and XMM-OM UVW1 lightcurves. These eclipse profiles are fitted with an asymmetric step and ramp function defined in Equation \ref{['eclipse_eqn']}. The results of the eclipse fits are reported in Table 1.
• Figure 4: Power spectrum of UU Aqr (left panel) and UX UMa (right panel) constructed using energy resolved lightcurves. The power spectra constructed from the 3--10 keV XMM and 3--25 keV NuSTAR lightcurves for UU Aqr is fit with a power-law model described in Section 2.3. The power spectrum constructed using 0.3--2 keV XMM lightcurves of UX UMa is fit with a power-law model and the power spectrum from 3--10 keV XMM lightcurve is fit with a power-law + Gaussian model. The orange dashed lines denote the Poissonian power of 2.
• Figure 5: Left panel: Broadband X-ray spectral fits using XMM/EPIC and NuSTAR observations in 0.3--10 keV and 3--25 keV respectively, using the spectral model described in Section 2.4. Right panel: X-ray spectral fits to the Chandra/ACIS spectrum in 0.5--8.0 keV. The residuals to the spectral fits are plotted in the bottom panels. The spectral parameters of the best fit model are tabulated in Table 2.