The Peculiar X-Ray Transient Swift J0840.7-3516: An Unusual Galactic Low-mass X-Ray Binary?

Swift J0840.7-3516 is an X-ray transient discovered with Swift/BAT in 2020 February. We investigated its X-ray properties using extensive data from Swift, MAXI, NICER, and NuSTAR. The source flux increased for ~10^3 s after the discovery, decayed rapidly over ~5 orders of magnitude in five days, and then remained almost constant over nine months. Large-amplitude short-term variations on timescales of 1-10^4 s were observed throughout the decay. In the initial flux rise, the source showed a hard power-law-shaped spectrum with a photon index of ~1.0 extending up to ~30 keV, above which an exponential cutoff was present. The photon index increased in the following rapid decay and became ~2 at the end of the decay. A spectral absorption feature at 3-4 keV was detected in the decay. It is not straightforward to explain all the observed properties by any known class of X-ray sources. We discuss the possible nature of the source, including a Galactic low-mass X-ray binary with multiple extreme properties