Fermi Transient J1544-0649: A Flaring Radio-weak BL Lac

On 2017 May 15, the FERMI/LAT gamma-ray telescope observed a transient source not present in any previous high-energy catalog: J1544-0649. It was visible for two consecutive weeks, with a flux peak on May 21. Subsequently observed by a Swift/XRT follow-up starting on May 26, the X-ray counterpart position was coincident with the optical transient ASASSN-17gs = AT2017egv, detected on May 25, with a potential host galaxy at z = 0.171. We conducted a 4-month follow-up in radio (Effelsberg-100 m) and optical (San Pedro Mártir, 2.1 m) bands, in order to build the overall Spectral Energy Distribution (SED) of this object. The radio data from 5 to 15 GHz confirmed the flat spectrum of the source, favoring a line of sight close to the jet axis, not showing significant variability in the explored post-burst time window. The Rx ratio, a common indicator of radio loudness, gives a value at the border between the radio-loud and radio-quiet active galactic nuclei (AGNs) populations. The CaII H&K break value (0.29 ± 0.05) is compatible with the range expected for the long-sought intermediate population between BL Lacs and FRI radio galaxies. An overall SED fitting from the radio to gamma-ray band shows properties typical of a low-power BL Lac. As a whole, these results suggest that this transient could be a new example of the recently discovered class of radio-weak BL Lac, showing for the first time a flare in the gamma/X-ray bands.