Using India’s AstroSat space observatory and ESA’s XMM-Newton spacecraft, astronomers have carried out X-ray observations of a symbiotic recurrent nova known as V3890 Sgr during its outburst in 2019.
Symbiotic stars (SySts) are in general long-period interacting binaries consisting of an evolved giant transferring mass to a hot compact object—usually a late-sequence red giant providing material to a white dwarf. Symbiotic novae (SyNe) are a type of SySt in which white dwarfs experience thermonuclear explosions on their surface. When these outburst occur in SyNe more than once, astronomers classify such objects as symbiotic recurrent novae (SyRNe).
SyRNe are extremely rare and to date, only four objects of this subclass have been detected. One of them is V3890 Sgr with an orbital period of 519.6 days and recurrence time of about 28 years. The system is located some 14,300 light years away from the Earth.
V3890 Sgr went into its third recorded outburst in August 2019 and shortly after the bursting activity started a team of astronomers led by Kulinder P. Singh of the Indian Institute of Science Education and Research in Mohali, India, decided to observe this SyRNe with AstroSat. AstroSat observations enhancement in the flux below 0.8 keV around the 8th day since the outburst discovery, almost coincident with the beginning of the super soft source (SSS) phase earlier on that day. In general, a stable intensity level with a hard X-ray spectrum has been observed during this phase of monitoring campaign. The astronomers attribute such behavior to shocks between the nova ejecta and the pre-existing stellar companion.
The subsequent spectral evolution of V3890 Sgr remained highly variable, which, according to the authors of the paper, demonstrates the importance of long-term high-cadence monitoring. Afterward, a complete vanishing of the supersoft emission occurred, followed by another extremely low flux state lasting for a day, starting 16 days after the outburst was detected. Meanwhile, it was also observed that the shock component was stable leading to increase in hardness ratio during the period of fading.
The astronomers underlined that their study of V3890 Sgr could have implications for our understanding of the rare population of symbiotic recurrent nova in general.