A phenomenological model for the X-ray spectrum of nova V2491 Cygni

Abstract

The X-ray flux of nova V2491 Cyg reached a maximum some forty days after optical maximum. The X-ray spectrum at that time, obtained with the RGS of XMM-Newton, shows deep, blue-shifted absorption by ions of a wide range of ionization. We show that the deep absorption lines of the X-ray spectrum at maximum, and those observed nine days later, are well described by a phenomenological model with emission from a central blackbody and from a collisionally ionized plasma (CIE). The blackbody spectrum (BB) is absorbed by three main highly ionized expanding shells; the CIE and BB are absorbed by cold circumstellar and interstellar matter that includes dust. The outflow density does not decrease monotonically with distance. The abundances of the shells indicate that they were ejected from an O-Ne white dwarf. We show that the variations on time scales of hours in the X-ray spectrum are caused by a combination of variation in the central source and in the column density of the ionized shells. Our phenomenological model gives the best description so far of the supersoft X-ray spectrum of nova V2491 Cyg, but underpredicts, by a large factor, the optical and ultraviolet flux. The X-ray part of the spectrum must originate from a very different layer in the expanding envelope, presumably much closer to the white dwarf than the layers responsible for the optical/ultraviolet spectrum. This is confirmed by the absence of any correlation between the X-ray and UV/optical observed fluxes.

From ultraluminous X-ray sources to ultraluminous supersoft sources: NGC 55 ULX, the missing link

“…All emission components were corrected by absorption due to the foreground interstellar medium and circumstellar medium (hot model in SPEX with low temperature 0.5 eV, see e.g. Pinto et al 2012). The double blackbody model (bb + mbb) provides a better fit than the power-law model (see Table 2) in agreement with the two-component model consisting of a blackbody (for the soft component) and a multicolour accretion disk (for the hard component) of Pintore et al (2015) for Chandra and Swift observations of NGC 55 ULX.…”

Reanalysis of high-resolutionXMM-Newtondata of V2491 Cygni using models of collisionally ionized hot absorbers

“…The final exposure times and net count rates showed that there were no sporadic high background events in our data and since the nova was bright in both observations, we did not include any corrections for flares. Figure 1 shows the light curves of the first and second observations (RGS1 data) with a bin time of 100 s. Following the analysis by Ness et al (2011) and Pinto et al (2012), we selected the three different count rate regions in the light curve of the first observation as labeled in the figure (i.e., region1, region2, region3) to investigate the nature and effect of the variation seen in the light curve on the produced spectra from the given regions. The second observation did not show any major variation, thus the entire event file was used for the analysis.…”

“…There are two main models in SPEX that can be used to model ionized intrinsic absorption; one of them is the xabs (used by Pinto et al 2012) and the other is the Hot model used in this study. The xabs model of SPEX calculates the warm, photoionized absorption by a thin slab composed of different ions located between the ionizing source and the observer.…”

“…In order to compare with the absorption modeling used in Pinto et al (2012) which used the photoionized absorber model xabs, we used the absorption model Hot which is a collisionally ionized (in equilibrium) absorber model in the SPEX software package. Since a comparative study with the xabs models would require that the data are analyzed and fitted in the same manner, we followed the analysis steps and used the same modeling in Pinto et al (2012) except for the photoionized absorber models. We utilized a composite model that includes a blackbody model Bb for the continuum and a plasma emission model in collisional equilibrium Cie for the excess in the harder X-ray band together with two Hot absorber models, one Amol model for the dust absorption (Pinto et al 2010) in the line of sight and finally another Hot model with a very low temperature (∼ 1eV) that would mimic the cold gas absorption in the interstellar medium.…”

“…We will compare the results obtained with the same XMM-Newton RGS data (we use here) using photoionized warm absorber models as described in Pinto et al (2012) with the hot collisionally ionized (in equilibrium) absorber model fits for V2491 Cyg using SPEX software. We caution that we use blackbody models for the continuum (as was used in Pinto et al 2012), thus we will be missing some photospheric absorption features.…”