We announce the discovery of GPX-1 b, a transiting brown dwarf with a mass of
19.7±1.6MJup and a radius of 1.47±0.10RJup, the first sub-stellar object discovered by the Galactic
Plane eXoplanet (GPX) survey. The brown dwarf transits a moderately bright (V
= 12.3 mag) fast-rotating F-type star with a projected rotational velocity
vsini∗=40±10 km/s. We use the isochrone placement algorithm to
characterize the host star, which has effective temperature 7000±200 K,
mass 1.68±0.10MSun, radius 1.56±0.10RSun
and approximate age 0.27−0.15+0.09 Gyr. GPX-1 b has an orbital period
of ∼1.75 d, and a transit depth of 0.90±0.03 %. We describe the GPX
transit detection observations, subsequent photometric and
speckle-interferometric follow-up observations, and SOPHIE spectroscopic
measurements, which allowed us to establish the presence of a sub-stellar
object around the host star. GPX-1 was observed at 30-min integrations by TESS
in Sector 18, but the data is affected by blending with a 3.4 mag brighter star
42 arcsec away. GPX-1 b is one of about two dozen transiting brown dwarfs known
to date, with a mass close to the theoretical brown dwarf/gas giant planet mass
transition boundary. Since GPX-1 is a moderately bright and fast-rotating star,
it can be followed-up by the means of Doppler tomography.Comment: 13 pages, 13 figures, accepted to MNRAS in May 202