The 3 month γ-ray light curve of 4FGL J1510.1+5702 reveals that it is at a high flux state in an epoch of several tens of days in 2018; meanwhile, optical flux densities of GB 1508+5714 in two bands rise in the same epoch. To further investigate the relationship between these two domains of emissions, a 3 day time bin γ-ray light curve is presented, together with the zoomed-in ZTF light curves; see Figure 4. In spite of the limited statistics and no Fermi-LAT observation toward the target at the exact time of the optical flares, the time bin (i.e., centered at MJD 58288.5) with the largest TS value in the 3 day γ-ray light curve is very close to the peaking time of the optical flares. Since optical variations are likely from the jet because of the large variability amplitude, based on the simultaneous γ-ray and optical brightening, we conclude that 4FGL J1510.1+5702 is the γ-ray counterpart of GB 1508+5714. There are three other time bins in the 3 month γ-ray light curve with TS values ≥10, centered at MJD 54909, 57265, and 56540, respectively. The first two time intervals do not fall into the operation time range of iPTF/ZTF. Moreover, no iPTF/ZTF data of GB 1508+5714 around MJD 56,540 are available; see Figure 3. Theoretically, in the leptonic radiation scenario, the optical and GeV γ-ray emissions of low synchrotron peaked blazars, including FSRQs, are proposed to be from the same population of emitting electrons. It is supported by correlated optical/γ-ray flares in FSRQs (e.g., Abdo et al. 2010; Bonning et al. 2012). Meanwhile, the redder-when-brighter spectral variability behavior has been detected in the optical wavelengths of γ-ray FSRQs, which is explained by the influence of the blue and slowly varying accretion disk emission (e.g., Bonning et al. 2012; Fan et al. 2018). For GB 1508+5714, a similar trend is shown. The optical spectral color, rmag–imag, is 0.66 mag in MJD 58286, while it is 0.43 mag in MJD 58272, despite the relatively large photometric uncertainties. The optical spectral color scaled by rmag corresponding to observations in different epochs are plotted, as shown in Figure 4. Its i-band variability amplitude is larger than that in the r band. All of these facts suggest that the contribution of the jet emission becomes significant at the optical wavelengths of GB 1508+5714 when the jet activity is intense with rising γ-ray emission. Moreover, the significant γ-ray emission together with the rapid optical variation in the r band provide information of the emitting jet blob. The radius of the emitting blob is constrained by the variability timescale, 





, where 





 days for the current event. Meanwhile, assuming that the optical and γ-ray photons of GB 1508+5714 are from the same region, to avoid serious absorption on γ-rays from soft photons via the 





 process, the corresponding optical depth should not be high,
1





where 





 is the scattering Thomson cross section, 





 is the differential comoving number density of the target photon per energy, 





 is the energy of the target photon in dimensionless units, and 





 is the absorption length (Dondi & Ghisellini 1995; Begelman et al. 2008). The soft photons from the jet itself could be responsible for the absorption. Since the highest energy of the detected γ-ray photons of GB 1508+5714 is ∼8 GeV, the corresponding soft photons are those detected at a few keV (3





  erg s−1; Marcotulli et al. 2020), and the absorption length can be set the same as the radius of the emitting blob. A constraint of the Doppler factor of the jet blob, 





, is given.
Citation: Abdo et al. 2010
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