XMM-NuSTAR Observation and Multiwavelength SED Modeling of Blazar 4FGL J1520.8-0348

Active galactic nuclei (AGNs) can power relativistic jets, which are called blazars when pointed close to our line of sight. Depending on the presence or absence of emission lines in their optical spectra, blazars are categorized into flat spectrum radio quasars (FSRQs) or BL Lacertae (BL Lacs) objects. According to the 'blazar sequence', as synchrotron peak frequency (νˢʸₚₖ) shifts to higher energies, the synchrotron peak luminosity decreases. This means that BL Lacs as luminous as FSRQs, and with synchrotron peak frequencies νˢʸₚₖ>10¹⁵ Hz, should not exist. Detected as a high-synchrotron peak (HSP; νˢʸₚₖ>10¹⁵ Hz) BL Lac, 4FGL J1520.8-0348 shows high gamma-ray luminosity (Lγ>10⁴⁶\, erg~s⁻¹), being at a high redshift of $z=$1.46. Since it is an outlier in the 'blazar sequence', the process of its jet acceleration and power may be different from bona fide BL Lacs. In this work, we constrain its spectral energy distribution (SED) by modeling the multi-wavelength data from infrared to γ-ray regime. Simultaneous X-ray data was obtained from X-ray Multi-Mirror Mission and Nuclear Spectroscopic Telescope Array to constrain the synchrotron emission and underlying electron distribution. On undertaking the SED modeling of the source, including the effect of extragalactic background light, we conclude that the source is more likely to be a 'blue FSRQ' or 'masquerading BL Lac' where the BL Lac is actually a FSRQ in disguise.