Radio interferometric imaging of RS Oph bipolar ejecta for the 2021 nova outburst

Abstract

The recurrent nova and symbiotic binary RS Oph erupted again in August 2021 for its eighth known outburst. We observed RS Oph 34 days after the outburst at 5 GHz with the European VLBI Network (EVN). The radio image is elongated over the east–west direction for a total extension of about 90 mas (or about 240 AU at the Gaia DR3 distance d = 2.68−0.15+0.17 kpc), and shows a bright and compact central component coincident with the Gaia astrometric position, and two lobes east and west of it, expanding perpendicular to the orbital plane. By comparing with the evolution of emission-line profiles on optical spectra, we found the leading edge of the lobes to be expanding at ∼7550 km s−1, and i = 54° as the orbital inclination of the binary. The 2021 radio structure is remarkably similar to that observed following the 2006 eruption. The obscuring role of the density enhancement on the orbital plane (DEOP) is discussed in connection to the time-dependent visibility of the receding lobe in the background to the DEOP, and the origin of the triple-peaked profiles is traced to the ring structure formed by the nova ejecta impacting the DEOP.

Filming the evolution of symbiotic novae with VLBI: the 2021 explosion of RS Oph

“…This confirms that astrophysical transients are and will remain an important area of impact for the network in the coming years, as anticipated in the VLBI20-30 EVN science roadmap [19]. The preliminary results presented here and the comparison with the optical spectroscopy information [17] indicate that this dataset will be extremely valuable in order to address open points such as the identification of the loci of shock-acceleration resulting in high energy emission, the determination of the geometrical configuration of the ejecta, and the discernment between free expansion and deceleration induced by the sweep-up of the RG wind.…”

“…However, the availability of the Gaia position and its coincidence with the central compact component allows us to interpret the data in a much clearer way. As discussed in [17], we infer the presence of a strong density enhancement on the orbital plane, which is oriented at 𝑖 = 54 • inclination. This enhancement confines the nova ejecta into two lobes, expanding in opposite directions normal to the plane, with the western lobe in foreground and the eastern lobe in background.…”

“…In terms of morphology, starting from epoch 2, emission becomes detected also on the eastern side, well separated from the core by a ∼ 20 mas gap, which we ascribe to absorption by a density enhancement on the orbital plane [17]. In the third epoch, three main regions of emission are well…”

“…In the present paper, we show some preliminary results from the EVN+e-MERLIN campaign that we carried out to follow the structural evolution. An in-depth comparison of one of our observations with constraints obtained from optical spectroscopy is presented in [17], while a more complete discussion of the results of the whole campaign will be presented in a forthcoming publication. Throughout this work, we assume a distance 𝑑 = 2.68 ± 0.17 kpc, as reported by Gaia DR3 [18], and therefore 1 mas ∼ 2.7 AU ∼ 4.0 × 10 13 cm.…”

The multi-wavelength view of shocks in the fastest nova V1674 Her

“…Synchrotron emission in novae has been identified before on the basis of high estimated of V809 Cep (Babul et al 2022), V392 Per, V357 Mus, V5855 Sgr, V5668 Sgr, V2672 Oph, V2491 Cyg, V838 Her, V1500 Cyg (Chomiuk et al 2021b), helium nova V445 Pup (Nyamai et al 2021), V906 Car (Aydi et al 2020a), V1324 Sco (Finzell et al 2018), V1723 Aql (Weston et al 2016a), 5589 Sgr (Weston et al 2016b), V959 Mon (Chomiuk et al 2014) and QU Vul (Taylor et al 1987), and via direct VLBI imag-ing of V959 Mon (Yang et al 2014), V1674 Her itself (J. Linford and M. Williams, private communication) and the embedded (red giant donor) novae RS Oph (Taylor et al 1989;O'Brien et al 2006;Rupen et al 2008;Sokoloski et al 2008;Eyres et al 2009;Munari et al 2022), V407 Cyg (Giroletti et al 2020), and V1535 Sco (Linford et al 2017). Synchrotron emission was also reported in embedded novae V745 Sco (Kantharia et al 2016) and V3890 Sgr (Nyamai et al 2023).…”