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We report on the presence of very rapid hard X-ray variability in the$\gamma$-ray binary LS I +61 303. The results were obtained by analysing NuSTARdata, which show two achromatic strong flares on ks time-scales beforeapastron. The Swift-BAT orbital X-ray light curve is also presented, and theNuSTAR data are put in the context of the system orbit. The spectrum andestimated physical conditions of the emitting region indicate that theradiation is synchrotron emission from relativistic electrons, likely producedin a shocked pulsar wind. The achromaticity suggests that losses are dominatedby escape or adiabatic cooling in a relativistic flow, and the overallbehaviour in hard X-rays can be explained by abrupt changes in the size of theemitting region and/or its motion relative to the line of sight, with Dopplerboosting potentially being a prominent effect. The rapid changes of the emittercould be the result of different situations such as quick changes in theintra-binary shock, variations in the re-accelerated shocked pulsar windoutside the binary, or strong fluctuations in the location and size of theCoriolis shock region. Although future multi-wavelength observations are neededto further constrain the physical properties of the high-energy emitter, thiswork already provides important insight into the complex dynamics and radiationprocesses in LS I +61-303.

Achromatic rapid flares in hard X-rays in the $\gamma$-ray binary LS I +61-303