Millihertz X-ray variability during the 2019 outburst of black hole candidate Swift~J1357.2$-$0933

Swift J1357.2$-$0933 is a black-hole candidate X-ray transient, which
underwent its third outburst in 2019, during which several multi-wavelength
observations were carried out.~Here, we report results from the \emph{Neil
Gehrels Swift} and \emph{NICER} observatories and radio data from
\emph{AMI}.~For the first time,~millihertz quasi-periodic X-ray oscillations
with frequencies varying between ${\sim}$~1--5~$\rm{mHz}$ were found in
\emph{NICER} observations and a similar feature was also detected in one
\emph{Swift}--\textsc{XRT} dataset.~Our spectral analysis indicate that the
maximum value of the measured X-ray flux is much lower compared to the peak
values observed during the 2011 and 2017 outbursts.~This value is ${\sim}$~100
times lower than found with \emph{MAXI} on MJD~58558 much ($\sim$~68 days)
earlier in the outburst, suggesting that the \emph{Swift} and \emph{NICER}
fluxes belong to the declining phase of the 2019 outburst.~An additional soft
component was detected in the \textsc{XRT} observation with the highest flux
level, but at a relatively low $L_{\rm X}$~$\sim$~$3{\times}10^{34}~(d/{\rm
6~kpc)}^2\rm{erg}~\rm{s}^{-1}$, and which we fitted with a disc component at a
temperature of $\sim 0.17$~keV.~The optical/UV magnitudes obtained from
\emph{Swift}--\textsc{UVOT} showed a correlation with X-ray observations,
indicating X-ray reprocessing to be the plausible origin of the optical and UV
emission.~However, the source was not significantly detected in the radio
band.~There are currently a number of models that could explain this
millihertz-frequency X-ray variability; not least of which involves an X-ray
component to the curious dips that, so far, have only been observed in the
optical.