Swift X-ray and ultraviolet followup observations of the new enhanced activity phase of SU Lyn

ATel #18086; N. Masetti (INAF-OAS, Bologna), U. Munari (INAF Padova)
on 2 Oct 2026; 16:12 UT
Credential Certification: Nicola Masetti (nicola.masetti@inaf.it)

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Following the announcement in ATel #18083 that the symbiotic star SU Lyn has entered a new phase of high optical activity likely due to enhanced accretion onto the white dwarf hosted in the system, we observed the source in X-rays and ultraviolet (UV) with the Neil Gehrels Swift satellite, starting at 02:17 UT of 2026 October 02.

The field was acquired in UV using UVOT with filters UVW1, UVM2 and UVW2 for 256 s, 254 s and 254 s, respectively; preliminary analysis shows that the source was well detected, with Vega system magnitudes UVM2 = 11.39+-0.04 and UVW2 = 11.25+-0.04, whereas an upper limit of <11.01 mag (i.e. a lower limit in brightness) was obtained in the UVW1 band due to coincidence loss in the detector produced by the source intensity; a more detailed analysis using the method of Page et al. (2013, MNRAS, 436, 1684) is underway.

We note that the average UVM2 countrate (152+-3 counts/s) in the present UVOT pointing is higher than that (138+-13 counts/s) reported by Lopes de Oliveira et al. (2018, ApJ, 864, 46) for the observation of November 2015 in which SU Lyn was detected in a high activity phase. We therefore support the optical results of ATel #18083 which reported an accretion surge for this object.

Integrating the present UV emission of SU Lyn over the 1700-3000 Angstroms using the present UVOT magnitudes and assuming the above distance and a color excess of E(B-V) = 0.1 mag (see again Mukai et al. 2016, MNRAS, 461, L1), we find a lower limit to the radiated UV luminosity of 2.7e34 erg/s, equivalent to about 7 Solar luminosities.

No X-rays were instead observed from the source: a simultaneous XRT pointing of about 780 s did not reveal any emission in the 0.3-10 keV band down to a 3-sigma limit of ~1e-2 counts/s. Assuming a collisionally ionized plasma X-ray spectrum with kT~17 keV and adopting a distance to the source of 640 pc (Mukai et al. 2016, MNRAS, 461, L1), the XRT observation implies a flux upper limit of <9e-13 erg/cm2/s and a luminosity limit of <4e31 erg/s.

We thank the Swift Team for the quick approval, scheduling and acquisition of the observation presented in this communication.