Gaia23clr: Outburst of the symbiotic star V2905 Sgr

ATel #16257; J. Merc (Charles University), P. Velez, H. Barker (ARAS Group), S. Charbonnel, O. Garde, P. Le Du, L. Mulato, T. Petit (2SPOT team), A. Pearce (Variable Stars South)
on 27 Sep 2023; 16:16 UT
Credential Certification: Jaroslav Merc (jaroslav.merc@student.upjs.sk)

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The brightening of a transient Gaia23clr was announced as a Gaia Science Alert (GSA; Hodgkin et al., 2021; A&A, 652, A76) on September 21, 2023, indicating a 0.8 mag increase in brightness in the Gaia G filter. The Gaia data indicates that the star was in brightnening already at the end of April, however, GSA is published only after two consecutive measurements confirm a change in brightness. Consequently, this transient event was not alerted earlier.

The reported coordinates coincide with those of the known symbiotic star V2905 Sgr. Initially, this star was classified as a possible Be star by Sanduleak & Stephenson (1973; ApJ, 185, 899), but they noted the possibility of prior TiO band detections. Subsequently, Allen (1975; A&A, 40, 335) reclassified it as a likely VV Cep-type object, while Payne-Gaposchkin (1977; AJ, 82, 665) listed this target among symbiotic binaries. The symbiotic nature of V2905 Sgr was definitely confirmed by Mikołajewska et al. (1997; A&A, 327, 191) and subsequently by Pereira et al. (1998; AJ, 116, 1971). Their observations unveiled a red giant absorption spectrum and detected emission lines with higher ionization potential in their spectra, specifically [O III] and He II. Gromadzki et al. (2013; AcA, 63, 405) proposed the possibility of periodic outburst activity in the system linked to the periastron passage of the hot component and estimated the orbital period of the system to be 508 ± 9 days.

To complement the Gaia light curve of V2905 Sgr, we have acquired data from the ASAS-SN survey (Shappee et al., 2014, ApJ 788, 48; Kochanek et al., 2017, PASP, 129, 104502). These data reveal a significant brightening with an amplitude of approximately 2.4 mag in the g filter. This brightening phase commenced between April 7 and April 14, 2023, reaching its peak at the close of May or the outset of June 2023. Notably, there was a minor brightening of about 0.4 mag approximately one year prior to the main outburst. Furthermore, observations in the V filter unveiled another brightening of around 1.5 mag, that started in August 2016 and lasted for approximately 250 days.

The period of 480 ± 14 days derived from the pre-outburst ASAS-SN g data closely aligns with the presumed orbital period identified by Gromadzki et al. Interestingly, two major outbursts, as detected in the ASAS-SN data (in 2016 and 2023), recurred almost precisely after five times this period, with no other prominent brightenings observed in between. Additionally, upon consulting the long-term visual light curve available in the AAVSO database, it seems that the system has remained in an extended active phase at least since the beginning of more regular observations in 1989, extending until 2012. Subsequently, the average brightness has dimmed by at least 1.5 mag.

To investigate the ongoing outburst, we conducted low and medium-resolution spectroscopic observations using our remotely operated setups located in Australia and Chile. The data collected between September 24 and 26, 2023, are accessible in the ARAS database (Teyssier, 2019, CAOSP, 49, 217) and reveal a spectrum with very mild TiO bands, strong emission lines of H I, and fainter of He I, and Fe II. Notably, the highly ionized emission lines of [O III] and He II are absent, suggesting that the current brightening is a classical symbiotic outburst. Of particular interest, the Hα and Hβ lines exhibit a double-peaked profile.

Additionally, we are monitoring the target photometric behavior, and the latest data from September 25.56, 2023 are as follows: B = 12.08, V = 11.45, R = 10.41, and I = 9.72. The observations are available in the AAVSO database.