On May 15th, 2017, the \emph{FERMI}/LAT gamma-ray telescope observed a transient source not present in any previous high-energy catalogue: J1544-0649. It was visible for two consecutive weeks, with a flux peak on May 21st. Subsequently observed by a \emph{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-months follow-up in radio (Effelsberg-100m) and optical (San Pedro M\'artir, 2.1m) 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 jet axis, not showing significant variability in the explored post-burst time-window. The Rx ratio, common indicator of radio loudness, gives a value at the border between the radio-loud and radio-quiet AGN 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 Radio to γ-ray band shows properties typical of a low-power BL Lac. As a whole, these results suggest that this transient could well 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.
@article{arxiv.1802.01105,
title = {FERMI transient J1544-0649: a flaring radio-weak BL Lac},
author = {G. Bruni and F. Panessa and G. Ghisellini and V. Chavushyan and H. A. Peña-Herazo and L. Hernández-García and A. Bazzano and P. Ubertini and A. Kraus},
journal= {arXiv preprint arXiv:1802.01105},
year = {2018}
}