SU Lyn, the prototype of LowAcc symbiotic stars, is undergoing a major surge in accretion

ATel #18083; U. Munari (INAF Padova), P. Valisa, G. L. Righetti, and S. Dallaporta (ANS Collaboration)
on 1 Oct 2026; 10:25 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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The S6,1 red giant SU Lyn was discovered by Mukai et al. (2016, MNRAS 461, L1) to be the counterpart of the unidentified hard X-ray source 4PBC J0642.9+5528 detected during the Swift Burst Alert Telescope (BAT) survey. In showing a large ultraviolet excess and feeble optical emission lines - which are properly observable only in high resolution Echelle spectra - it became the prototype (Munari et al. 2021, MNRAS 505, 6121) of low accretion (LowAcc) symbiotic stars (Lacc < 10 Lsun). Simple evolutionary arguments indicate that symbiotic stars (SySts) spend most of their intra-burning lifes in LowAcc phases (Munari 2019, arXiv1909-01389), and therefore systems in this state dominate the SySt population counts throughout the Galaxy.

Our continued photometric and spectroscopic monitoring of SU Lyn over the past 10 yrs shows that when it was identified by Mukai et al. in 2015-2016 the star was going through a much higher level of accretion (about 10 Solar luminosities) than displayed ever since (radiating as low as 0.5 Solar luminosities during 2019-2023; Munari et al. 2026, A&A accepted in press). When at the beginning of September 2026, after emergence from Solar conjunction, we resumed our monitoring of SU Lyn we unexpectedly found it to have returned to - and even surpassed - the accretion luminosity experienced in 2015-2016. This is a clean confirmation of the fact that the accretion levels in SySts are highly variable over long-time scales (Luna et al. 2020, ApJ 902, L14).

Echelle spectra of SU Lyn have been obtained with the Varese 0.84m telescope on 2026 Sept 12.151 and 28.135 UT at 16,000 resolving power and 4000-9310 Ang range. They show an Halpha emission vastly more intense than in the 2019-2023 period and somewhat even stronger than in 2015-2016. Similarly, [OIII] 4363, 4959, 5007 lines which were absent in 2015-2016 are now prominent in emission, as are HeI lines (very weak in 2015-2016 and completely absent in 2019-2023). This figure compares Halpha, [OIII], and HeI 5876 in 2015, 2020, 2023, and 2026. The current surge in accretion started during the recent Solar conjunction, because on our latest Asiago 1.82m Echelle spectrum from 2026 May 15, the appearance of SU Lyn was the usual one, with no detectable emission other than weak Hbeta and Halpha lines.

We have been maintaining a tight photometric monitoring of SU Lyn since its 2015 discovery primarily via ANS Collaboration telescopes ID 0310, 1507, and 1512. Our latest two sets of UBV data are:

 
     UT             U               B              V 
2026-09-28.974   10.339 +-0.010  10.325 +-0.011  8.529 +-0.009 
2026-09-29.968   10.473 +-0.030  10.407 +-0.012  8.519 +-0.012 

The U-B color currently close to 0.0 and much bluer than the +1.2 value of deep quiescence, clearly indicates the presence of a large excess in the near-UV emission driven by the presently enhanced accretion status.

We will keep monitoring SU Lyn, and observations at other wavelengths are encouraged.