We report on the results of optical , near-infrared ( NIR ) and mid-infrared observations of the black hole X-ray binary candidate ( BHB ) MAXI J1535–571 during its 2017/2018 outburst .
During the first part of the outburst ( MJD 58004-58012 ) , the source shows an optical-NIR spectrum that is consistent with an optically thin synchrotron power-law from a jet .
After MJD 58015 , however , the source faded considerably , the drop in flux being much more evident at lower frequencies .
Before the fading , we measure a de-reddened flux density of \gtrsim 100 mJy in the mid-infrared , making MAXI J1535–571 one of the brightest mid-infrared BHBs known so far .
A significant softening of the X-ray spectrum is evident contemporaneous with the infrared fade .
We interpret it as due to the suppression of the jet emission , similar to the accretion-ejection coupling seen in other BHBs .
However , MAXI J1535–571 did not transition smoothly to the soft state , instead showing X-ray hardness deviations , associated with infrared flaring .
We also present the first mid-IR variability study of a BHB on minute timescales , with a fractional rms variability of the light curves of \sim 15 –22 % , which is similar to that expected from the internal shock jet model , and much higher than the optical fractional rms ( \lesssim 7 % ) .
These results represent an excellent case of multi-wavelength jet spectral-timing and demonstrate how rich , multi-wavelength time-resolved data of X-ray binaries over accretion state transitions can help refining models of the disk-jet connection and jet launching in these systems .