The strange case of the transient HBL blazar 4FGL J1544.3-0649

We present a multifrequency study of the transient $\gamma$-ray source 4FGL J1544.3-0649, a blazar that exhibited a remarkable behaviour raising from the state of an anonymous mid-intensity radio source, never detected at high energies, to that of one of the brightest extreme blazars in the X-ray and $\gamma$-ray sky. Our analysis shows that the averaged $\gamma$-ray spectrum is well described by a powerlaw with a photon index of $1.87\pm0.04$, while the flux above 100 MeV is $(8.0\pm0.9)\times10^{-9}\:{\rm photon\: cm^{-2}\: s^{-1}}$, which increases during the active state of the source. The X-ray flux and spectral slope are both highly variable, with the highest 2-10 keV flux reaching $(1.28\pm0.05)\times10^{-10}\:{\rm erg\:cm^{-2}\:s^{-1}}$. On several observations the X-ray spectrum hardened to the point implying as SED peak moving to energies larger than 10 keV. As in many extreme blazars the broadband spectral energy distribution can be described by a homogeneous one-zone synchrotron-self-Compton leptonic model. We briefly discuss the potential implications for high-energy multi-messenger astrophysics in case the dual behaviour shown by 4FGL J1544.3-0649 does not represent an isolated case, but rather a manifestation of a so far unnoticed relatively common phenomenon.