Reanalysis of high-resolution XMM-Newton data of V2491 Cygni using models of collisionally ionized hot absorbers

Department of Physics, Middle East Technical University, Dumlupınar Bulvarı, 06800 Ankara, Turkey
e-mail: solen@astroa.physics.metu.edu.tr; cigdem.gamsizkan@gmail.com

Received: 20 July 2016
Accepted: 21 October 2016

Abstract

Aims. We model spectral absorption features in data of the high-resolution XMM-Newton Reflection Grating Spectrometer.

Methods. We performed X-ray spectral analysis and modeling using the SRON software SPEX version 2.05.04.

Results. We present a reanalysis of data obtained with the XMM-Newton Reflection Grating Spectrometer of the classical nova V2491 Cyg from two different pointings, 40 d and 50 d after outburst. We aim to model absorption components in the high-resolution spectra independently of the continuum model. To model the complex absorption, we used hot collisionally ionized (in equilibrium) absorber models along with interstellar absorption (of gas and dust origin separately) that we discuss in the light of observations. For an adequate approximation and to facilitate the fitting procedures, we used a blackbody model for the continuum. We find blackbody temperatures in the range 61–91 eV (slightly variable over the two observations) that yield a white dwarf (WD) mass of 1.15–1.3 M_⊙ assuming this range is the maximum temperature achieved during the H-burning phase. Our fits model the ionized absorption features simultaneously, thus calculating a global velocity shift for the absorption component in the data originating from the nova wind and/or ejecta as a result of collisionally ionized hot absorption. We derive two different hot-absorber components from our fits with blueshifts yielding 2900–3800 km s^-1 for the first (day 40) and 2600–3600 km s^-1 for the second observation 50 days after outburst that are consistent with ejecta or wind speeds. The two collisionally ionized hot-absorption components have temperatures kT₁ ≃ 1.0−3.6 keV and kT₂ ≃ 0.4−0.87 keV with rms velocities σ_v1 ~ 872 km s^-1 and σ_v2 ~ 56 km s^-1. These are consistent with shock temperatures in the X-ray wavelengths. V2491 Cyg shows signature of H-burning with underabundant carbon C/C_⊙ = 0.3−0.5, and enhanced nitrogen N/N_⊙ = 5−7 and oxygen O/O_⊙ = 16−43. The high oxygen overabundance hints at a C–O WD. We find the equivalent hydrogen column density of the hot collisionally ionized (in equilibrium) absorbers in a range (0.6–18.0) × 10²³ cm^-2 and (2.0–5.3) × 10²³ cm^-2 on days 40 and 50 after outburst, respectively. Our fits yield the most adequate ${\mathit{\chi }}_{\mathit{\nu }}^{\mathrm{2}}$ (range 1.8–2.9) currently obtained for the modeling of high-resolution X-ray data of V2491 Cyg. An additional photoionized absorber (third intrinsic absorber component) originating in the shell or the ejecta improves the model fits with ${\mathit{\chi }}_{\mathit{\nu }}^{\mathrm{2}}$ in a range 1.7–2.5, but shows only (1–0.1)% of the absorption by the collisionally ionized hot gas. Our analysis reveals a second blackbody component on day 50 with an effective temperature of 120–131 eV and an effective radius of about 10% of the WD, which may indicate the onset of magnetic accretion.

Conclusions. Our work on data of the XMM-Newton Reflection Grating Spectrometer yields two main hot collisionally ionized (in equilibrium) absorber components with different temperatures, rms velocities, and equivalent column densities originating in the shocked fast-moving ejecta and/or the wind. These two components may be the reason for the complex absorption spectra of V2491 Cyg, as a result of high- and low-density regions with different turbulent conditions and temperatures in the ejecta and/or the wind, indicating the inhomogeneity (e.g., oxygen-dense regions) and mixed morphology of the outflow.