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We report on the X-ray properties of the new transient Swift J0840.7$-$3516,discovered with Swift/BAT in 2020 February, using extensive data of Swift,MAXI, NICER, and NuSTAR. The source flux increased for $\sim 10^3$ s after thediscovery, decayed rapidly over $\sim$ 5 orders of magnitude in 5 days, andthen remained almost constant over 9 months. Large-amplitude short-termvariations on time scales of 1--$10^4$ s were observed throughout the decay. Inthe initial flux rise, the source showed a hard power-law shaped spectrum witha photon index of $\sim 1.0$ extending up to $\sim 30$ keV, above which anexponential cutoff was present. The photon index increased in the followingrapid decay and became $\sim 2$ at the end of the decay. A spectral absorptionfeature at 3--4 keV was detected in the decay. It is not straightforward toexplain all the observed properties by any known class of X-ray sources. Wediscuss the possible nature of the source, including a Galactic low mass X-raybinary with multiple extreme properties and a tidal disruption event by asupermassive black hole or a Galactic neutron star.

The Peculiar X-ray Transient Swift J0840.7-3516: an Unusual Low Mass X-ray Binary or a Tidal Disruption Event?