MKT J170456.2-482100: the first transient discovered by MeerKAT

We report the discovery of the first transient with MeerKAT, MKT J170456.2-482100, discovered in ThunderKAT images of the low-mass X-ray binary GX339-4. MKT J170456.2-482100 is variable in the radio, reaching a maximum flux density of 0.71± 0.11 mJy on 2019 October 12, and is undetected in 15 out of 48 ThunderKAT epochs. MKT J170456.2-482100 is coincident with the chromospherically active K-type sub-giant TYC 8332-2529-1, and ∼ 18 yr of archival optical photometry of the star shows that it varies with a period of 21.25± 0.04 d. The shape and phase of the optical light curve changes over time, and we detect both X-ray and UV emission at the position of MKT J170456.2-482100, which may indicate that TYC 8332-2529-1 has large star spots. Spectroscopic analysis shows that TYC 8332-2529-1 is in a binary, and has a line-of-sight radial velocity amplitude of 43 km s^{-1}. We also observe a spectral feature in antiphase with the K-type sub-giant, with a line-of-sight radial velocity amplitude of ∼ 12± 10 km s^{-1}, whose origins cannot currently be explained. Further observations and investigation are required to determine the nature of the MKT J170456.2-482100 system.