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Fermi J1544-0639/ASASSN-17gs/AT2017egv was identified as a gamma-ray/opticaltransient on May 15, 2017. Subsequent multiwavelength observations suggest thatthis source may belong to the new class of radio-weak BL Lacs. We studied theX-ray spectral properties and short-term variability of Fermi J1544-0639 toconstrain the X-ray continuum emission mechanism of this peculiar source. Wepresent the analysis of an XMM-Newton observation, 56 ks in length, performedon February 21, 2018. The source exhibits strong X-ray variability, both influx and spectral shape, on timescales of ~10 ks, with a harder-when-brighterbehaviour typical of BL Lacs. The X-ray spectrum is nicely described by avariable broken power law, with a break energy of around 2.7 keV consistentwith radiative cooling due to Comptonization of broad-line region photons. Wefind evidence for a `soft excess', nicely described by a blackbody with atemperature of ~0.2 keV, consistent with being produced by bulk Comptonizationalong the jet.

An XMM-Newton look at the strongly variable radio-weak BL Lac Fermi J1544-0639