Recent X-ray observations of the symbiotic star AG Peg: do they signify colliding stellar winds?

Abstract

We present an analysis of recent X-ray observations of the symbiotic star AG Peg. The X-ray emission of AG Peg as observed with Swift in 2015 shows considerable variability on time scale of days as variability on shorter time scales might be present as well. Analysis of the X-ray spectra obtained in 2013 and 2015 confirms that AG Peg is an X-ray source of class β of the X-ray sources amongst the symbiotic stars. The X-ray emission of AG Peg as observed with ROSAT (1993 June) might well originate from colliding stellar winds (CSW) in binary system. On the other hand, the characteristics of the X-ray emission of AG Peg in 2013 and 2015 (Swift ) are hard to accommodate in the framework of the CSW picture. Analysis of the light curves in 2015 shows that the power spectrum of the X-ray variability in AG Peg resembles that of the flicker noise (or flickering) being typical for accretion processes in astronomical objects. This is a sign that CSWs did not play a key role for the X-ray emission from AG Peg in 2013-2015 and a different mechanism (probably accretion) is also getting into play.

An XMM–Newton observation of the symbiotic star AG Peg: the X-ray emission after the end of its 2015 outburst

“…Finally, it is worth noting that the X-ray emission of AG Peg in 2017 returned to its level before the outburst of this object in 2015. The 2017 observed flux in the 0.3 -3 keV energy range of 6.21 × 10 −13 ergs cm −2 s −1 is about a factor of 2 lower than the average flux in 2015 and comparable to the value of the pre-outburst observations in 2013 (see table 1 Zhekov & Tomov 2016).…”

“…We note that the ROSAT spectrum considered in this study is the same one as in Zhekov & Tomov (2016): its data reduction and spectral extraction are described in section 2 therein. It is worth recalling that the ROSAT spectrum is not of that high quality as the EPIC spectra are: it has only ∼ 400 source counts (to compare with ∼ 5000 source counts in the pn and MOS spectra; see Section.…”

“…We note that a direct comparison between the CSW model and observed X-ray emission of AG Peg was presented in Zhekov & Tomov (2016). Here, we will adopt the same approach and we will briefly recall below the basic characteristics of that CSW model and some changes of its as well.…”

“…For the CSW spectral models of AG Peg, we adopted the same stellar wind and binary parameters as in Zhekov & Tomov (2016). Namely, the mass-loss rates and wind velocities are based on Mürset et al (1995) and Schmutz (1996): M H S = 2 × 10 −7 M ⊙ yr −1 , V H S = 1000 km s −1 , M CS = 4 × 10 −7 M ⊙ yr −1 , V CS = 20 km s −1 (the notations HS and CS stand for the hot star and cool star, respectively).…”

New outburst of the symbiotic nova AG Pegasi after 165 yr

“…The authors ascribed the origin of the strong and variable X-rays to shocks in the variable ejecta. Analysing the same Swift/XRT observations, Zhekov & Tomov (2016) came to the conclusion that the characteristics of the X-ray emission from the 2015 outburst could not result from colliding stellar winds in a binary system as was indicated by previous ROSAT observations from June of 1993. Using optical spectroscopy and BV photometry, Tomov et al (2016) concluded that the 2015 outburst of AG Peg places it among the classical Z And-type symbiotic binary systems.…”