Constraining broad-line regions from time lags of broad emission lines relative to radio emission

Abstract

In this paper, a new method is proposed to estimate the broad-line region sizes of UV lines $R^{\rm{uv}}_{\rm{BLR}}$. It is applied to 3C 273. First, we derive the time lags of radio emission relative to broad emission lines Ly$\alpha$ and C IV by the ZDCF method. The broad lines lag the 5, 8, 15, 22 and 37 GHz emission. The measured lags $\tau^{\rm{uv}}_{\rm{ob}}$ are of the order of years. For a given line, $\tau^{\rm{uv}}_{\rm{ob}}$ decreases as the radio frequency increases. This trend results from the radiative cooling of relativistic electrons. Both UV lines have a lag of $\tau^{\rm{uv}}_{\rm{ob}}=-2.74^{+0.06}_{-0.25}$ yr relative to the 37 GHz emission. These results are consistent with those derived from the Balmer lines in Paper I. Second, we derive the time lags of the lines Ly$\alpha$, CIV, H$\gamma$, H$\beta$ and H$\alpha$ relative to the 37 GHz emission by the FR/RSS Monte Carlo method. The measured lags are $\tau_{\rm{ob}}=-3.40^{+0.31}_{-0.05}$, $-3.40^{+0.41}_{-0.14}$, $-2.06^{+0.36}_{-0.92}$, $-3.40^{+1.15}_{-0.20}$ and $-3.56^{+0.35}_{-0.18}$ yr for the lines Ly$\alpha$, CIV, H$\gamma$, H$\beta$ and H$\alpha$, respectively. These estimated lags are consistent with those derived by the ZDCF method within the uncertainties. Based on the new method, we derive $R^{\rm{uv}}_{\rm{BLR}}=2.54^{+0.71}_{-0.35}$--$4.01^{+0.90}_{-1.16}$ and $2.54^{+0.80}_{-0.43}$--$4.01^{+0.98}_{-1.24}$ light-years for the Ly$\alpha$ and CIV lines, respectively. Considering the uncertainties, these estimated sizes are consistent with those obtained in the classical reverberation mapping for the UV lines and the Balmer lines. This indicates that their emitting regions are not separated so large as in the classical mapping of the UV and optical lines. These results seem to depart from the stratified ionization structures obtained in the classical mapping.