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

Abstract

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 $0.71\pm 0.11\mathrm{mJy}$ on 2019 Oct 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 $\sim 18\mathrm{years}$ of archival optical photometry of the star shows that it varies with a period of $21.25\pm 0.04\mathrm{days}$. 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 $43\mathrm{km}{\mathrm{s}}^{-1}$. We also observe a spectral feature in anti-phase with the K-type sub-giant, with a line-of-sight radial velocity amplitude of $\sim 12\pm 10\mathrm{km}{\mathrm{s}}^{-1}$, whose origins cannot currently be explained. Further observations and investigation are required to determine the nature of the MKT J170456.2$-$482100 system.

16 pages, 12 figures, 7 tables