  We report on a NIR, optical and X-ray campaign performed in 2017 with the
XMM-Newton and Swift satellites and the VLT/HAWK-I instrument on the
transitional MSP PSR J1023+0038. NIR observations were performed in
fast-photometric mode in order to detect any fast variation of the flux and
correlate them with the optical and X-ray light curves. The optical curve shows
the typical sinusoidal modulation at the orbital period (4.75hr). No flaring or
flickering is found in the optical, neither signs of transitions between active
and passive states. On the contrary, the NIR curve displays a bimodal
behaviour, showing strong flares in the first part of the curve, and an almost
flat trend in the rest. The X-ray curves show a few low/high mode transitions,
but no flaring activity. One of the low/high mode transition is found to happen
at the same time as the emission of an infrared flare. This can be interpreted
as the emission of a jet: the NIR flare could be due to the evolving spectrum
of the jet, which possesses a break frequency that moves from higher (NIR) to
lower (radio) frequencies after the launching, that has to happen at the
low/high mode transition. We also present the cross correlation function
between the optical and near infrared curves. Due to the bimodality of the NIR
curve, we divided it in two parts (flaring and quiet). While the CCF of the
quiet part is found to be flat, the one referring to the flaring part shows a
narrow peak at ~10s, which indicates a delay of the NIR emission with respect
to the optical. This lag can be interpreted as reprocessing of the optical
emission at the light cylinder radius with a stream of matter spiraling around
the system due to a phase of radio-ejection. This strongly supports a different
origin of the NIR flares observed for PSR J1023+0038 with respect to the
optical and X-ray flaring activity reported in other works on the same source.
