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 Neil Gehrels Swift and NICER observatories and radio data from AMI. For the first time, millihertz quasi-periodic X-ray oscillations with frequencies varying between ~ 1-5 $\rm {mHz}$ were found in NICER observations and a similar feature was also detected in one Swift-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 ~ 100 times lower than found with MAXI on MJD 58558 much (~ 68 days) earlier in the outburst, suggesting that the Swift and NICER fluxes belong to the declining phase of the 2019 outburst. An additional soft component was detected in the XRT observation with the highest flux level, but at a relatively low L_X ~ $3{\times }10^{34}~(d/{\rm 6~kpc)}^2\rm {erg}~\rm {s}^{-1}$, and which we fitted with a disc component at a temperature of ~0.17 keV. The optical/UV magnitudes obtained from Swift-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....