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

We present a multifrequency study of the transient γ-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 γ-ray sky. Our analysis shows that the averaged γ-ray spectrum is well described by a power law with a photon index of 1.87 ± 0.04, while the flux above 100 MeV is (8.0 ± 0.9) × 10^-9 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 ± 0.05) × 10^-10 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 broad-band 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 multimessenger 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.