Discerning Misclassified Flat-spectrum Radio Quasars from Low-frequency-peaked BL Lacertae Objects

Abstract

A sample of 312 low-frequency-peaked BL Lacertae objects (LBLs) and 694 flat-spectrum radio quasars (FSRQs) with the parameters of both redshift and gamma-ray photon spectral index (Γ _γ ) is compiled from the Fourth Catalog of Active Galactic Nuclei Data Release 2 from the Fermi Large Area Telescope (LAT). The multiwavelength data of the sample sources are downloaded from the Space Science Data Center, and then matched to the corresponding gamma-ray data from the Fermi-LAT Fourth Source Catalog Data Release 2. The synchrotron radiation peak frequency and Compton dominance (CD) parameters of the sources are obtained by using a log-parabolic model to fit the average-state multiwavelength spectral energy distribution. A support vector machine in the $\mathrm{log}{L}_{\gamma }$ –Γ _γ frame is utilized to delineate the optimal boundary between FSRQs and LBL sources. The 1 σ position of the Gaussian fitting on the histograms of the Γ _γ , $\mathrm{log}{\nu }_{\mathrm{peak}}^{\mathrm{syn}}$ , and CD parameter distributions are also introduced. With this criterion, 25 FSRQ candidates are selected from LBL sample sources. The optical spectral identification result confirms that 8 out of 13 candidate sources available with the optical spectral data exhibit the relationship of EW > 5 Å.

Keywords

• Flat-spectrum radio quasars
• BL Lacertae objects