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

¹ INAF-Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy
e-mail: francesco.ursini@inaf.it
² INAF-Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
³ Departamento de Ciencias Físicas, Universidad Andrés Bello, Fernández Concha 700, Las Condes, Santiago, Chile
⁴ Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA
⁵ Astro Space Center of Lebedev Physical Institute, Profsoyuznaya St. 84/32, 117997 Moscow, Russia
⁶ Sternberg Astronomical Institute, Moscow State University, Universitetskii pr. 13, 119992 Moscow, Russia

Received: 15 November 2018
Accepted: 26 December 2018

Abstract

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.