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

F. Ursini, L. Bassani, F. Panessa, E. Pian, G. Bruni, A. Bazzano, N. Masetti, K. Sokolovsky, P. Ubertini

Research output: Contribution to journalArticle


Context. 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.

Original languageEnglish
Article numberA116
JournalAstronomy and Astrophysics
Publication statusPublished - 6 Feb 2019


  • BL Lacertae objects: general
  • Quasars: general
  • Radiation mechanisms: non-thermal
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Ursini, F., Bassani, L., Panessa, F., Pian, E., Bruni, G., Bazzano, A., Masetti, N., Sokolovsky, K., & Ubertini, P. (2019). An XMM-Newton look at the strongly variable radio-weak BL Lac Fermi J1544-0639. Astronomy and Astrophysics, 622, [A116].