Swiftobservations of the 2015 outburst of AG Peg – from slow nova to classical symbiotic outburst

Abstract

Symbiotic stars often contain white dwarfs with quasi-steady shell burning on their surfaces. However, in most symbiotics, the origin of this burning is unclear. In symbiotic slow novae, however, it is linked to a past thermonuclear runaway. In June 2015, the symbiotic slow nova AG Peg was seen in only its second optical outburst since 1850. This recent outburst was of much shorter duration and lower amplitude than the earlier eruption, and it contained multiple peaks -like outbursts in classical symbiotic stars such as Z And. We report Swift X-ray and UV observations of AG Peg made between June 2015 and January 2016. The X-ray flux was markedly variable on a time scale of days, particularly during four days near optical maximum, when the X-rays became bright and soft. This strong X-ray variability continued for another month, after which the X-rays hardened as the optical flux declined. The UV flux was high throughout the outburst, consistent with quasi-steady shell burning on the white dwarf. Given that accretion disks around white dwarfs with shell burning do not generally produce detectable X-rays (due to Compton-cooling of the boundary layer), the X-rays probably originated via shocks in the ejecta. As the X-ray photoelectric absorption did not vary significantly, the X-ray variability may directly link to the properties of the shocked material. AG Peg's transition from a slow symbiotic nova (which drove the 1850 outburst) to a classical symbiotic star suggests that shell burning in at least some symbiotic stars is residual burning from prior novae.

New outburst of the symbiotic nova AG Pegasi after 165 yr

“…Shen & Bildsten 2007;Nomoto et al 2007). The presence of the shell burning prior to the outburst was also suggested by Ramsay et al (2016). However, the high accretion rate requires a high mass transfer ratio (i.e.…”

“…Independently, a high T h was suggested by Luna et al (2015), who detected a supersoft black-body type component in the X-ray spectrum of AG Peg with kT∼ 0.02 keV by the Swift satellite at the end of June 2015. However, Ramsay et al (2016) did not mention this result, because they couldn't find strong evidence to confirm the presence of this supersoft component (Luna 2016, private communication).…”

“…To estimate T h , Ramsay et al (2016) used only equivalent widths of He ii 4686 Å and Hβ lines, instead of their fluxes. Despite variation of the continuum level by an amplitude of ∼ 1 mag in the BV passbands over the course of the outburst, the authors obtained values of T h between ∼1.8 and ∼ 1.4 × 10 5 K, which are comparable with our quantities.…”

“…Around a maximum of the optical brightness, from June 28 to July 8, 2015, the Swift/XRT observation detected an increase in the count rate with a factor of ∼3 with respect to the previous observation on August [16][17][18][19]2013 (Nuñez & Luna 2013;Luna et al 2015). Following Swift/XRT, monitoring of the 2015 outburst indicated a markedly variable X-ray flux on a time scale of days (Ramsay et al 2015(Ramsay et al , 2016. The authors ascribed the origin of the strong and variable X-rays to shocks in the variable ejecta.…”

Flows and Shocks: Some Recent Developments in Symbiotic Star and Nova Research

“…So, does the lack of interaction between the expanded WD photosphere and its binary companion lead to lower ejection efficiency and therefore lower ejecta masses in some wide, symbiotic binaries? If so, could this difference help explain how quasi-steady shell burning can persist for up to centuries after novae in some symbiotic stars (such as AG Peg [76,103]). Regardless of the answers to these questions, the flow structure, γ-ray production, and ejection of the WD envelope seem to be strongly linked.…”