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

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