Ursini, F.Bassani, L.Panessa, F.Pian, E.Bruni, G.Bazzano, A.Masetti, N.Sokolovsky, K.Ubertini, P.2023-07-272023-07-272019-02-01Astronomy and Astrophysics Volume 622 1 February 2019 Article number A1160004-6361https://repositorio.unab.cl/xmlui/handle/ria/52059Indexación ScopusContext. 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. Aims. 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. Methods. We present the analysis of an XMM-Newton observation, 56 ks in length, performed on February 21, 2018. Results. 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.enBL Lacertae objects: generalQuasars: generalRadiation mechanisms: non-thermalX-rays: galaxiesArtículoAtribución 4.0 Internacional (CC BY 4.0)10.1051/0004-6361/201834662