# NGTS-7Ab: An ultra-short period brown dwarf transiting a tidally-locked and active M dwarf

## TL;DR

This paper reports the discovery of NGTS-7Ab, the shortest period transiting brown dwarf around an active M dwarf, highlighting tidal interactions, magnetic activity, and the system's hierarchical triple nature.

## Contribution

It presents the first detection of an ultra-short period brown dwarf transiting an active M dwarf, with detailed analysis of tidal synchronization and system dynamics.

## Key findings

- Shortest period transiting brown dwarf around a star to date
- Host star is magnetically active with multiple flares
- System likely to experience orbital decay within 5-10 Myr

## Abstract

We present the discovery of NGTS-7Ab, a high mass brown dwarf transiting an M dwarf with a period of 16.2 hours, discovered as part of the Next Generation Transit Survey (NGTS). This is the shortest period transiting brown dwarf around a main or pre-main sequence star to date. The M star host (NGTS-7A) has an age of roughly 55 Myr and is in a state of spin-orbit synchronisation, which we attribute to tidal interaction with the brown dwarf acting to spin up the star. The host star is magnetically active and shows multiple flares across the NGTS and follow up lightcurves, which we use to probe the flare-starspot phase relation. The host star also has an M star companion at a separation of 1.13 arcseconds with very similar proper motion and systemic velocity, suggesting the NGTS-7 system is a hierarchical triple. The combination of tidal synchronisation and magnetic braking is expected to drive ongoing decay of the brown dwarf orbit, with a remaining lifetime of only 5-10 Myr.