Spectral Variability Studies in Active Galactic Nuclei: Exploring Continuum and Emission Line Regions in the Age of LSST and JWST

Round 1

Reviewer 1 Report

This paper gives an in-depth look at the work that the first author and his colleagues have published on spectral variability in AGN aimed at the accretion disk, BLR, and coronal emissions. It also reviews the role of AGN as standard candles in cosmology. This review manuscript is appropriate for the Special Issue "A Multimessenger View of Supermassive Black Holes and the Quasar Main Sequence". However, before any recommendation for publication, I suggest addressing the following comments: 

1. There should have been an overview of how Seyfert 1 and 2 galaxies are categorized (based on BLR and NLR) in the introduction, as this is relevant to the BLR structure discussed about in Section 2. Moreover, it could be helpful if they could briefly describe radio-quiet (non-jetted) and radio-load AGN. The introductory paragraph on the AGN classification could be after the first paragraph in Sec. 1. Moreover, 3C 273 mentioned in this paper is classified as radio-load quasar in the literature, while other 2 AGN (IRAS 09149-6206 and NGC 3783) are radio-quiet type I (Seyfert 1) AGN, which should be noted in text.

2. The review lacked a conclusion/summary section at the end. Most of the text in the last paragraph of Sec. 1 may be part of the summary section. Similarly, "notes for the future" subsections (3.2 and 4.2) may move to the summary section.

3. The presentation of some figures needs to be improved. Font sizes are very small in Fig. 1 (right), Fig. 3 (right), Fig. 4 (bottom-right), and Fig. 5 (top-middle and right panels). In particular, hand wringing in Fig. 2 (right) is difficult to read, which should be improved by redrawing them using a computer program.

Author Response

We thank the reviewer for their comments and suggestions. We provide answers to their comments one-by-one using enumeration:

1. The revised version now provides a brief overview of the Type-1/Type-2 classification and also touches upon the RL/RQ dichotomy. As suggested by the reviewer, we have moved the introductory paragraph in Sec. 2.1 to Sec 1. We have added short notes on 3C 273, IRAS 09149-6206, and NGC 3783 to clarify their radio types.
2. We have reorganized the text to incorporate a section with concluding remarks where the text from the introduction and notes for the future have been reworded and moved.
3. The figures have been enhanced and updated in the revised version.

Reviewer 2 Report

In this review, the authors introduced their works related to AGN accretion disks, broad line regions, corona, and narrow line regions, and provided constructive insights from the era of LSST and JWST.

Major comments:

1. In section 2.3, the authors considered accretion disk continuum modeling and its contamination from BLRs, I wonder if the contamination from the host galaxy should also be considered.

2. For the method of estimating BH mass via [Si VI]/Br\gamma_{broad}, did you compare the results from [Si VI]/Br\gamma_{broad} and those from H\beta line or MgII line?

Minor comments:

1. Equation (1) "Brfl_{broad}" should be "Br \gamma _{broad}";

Author Response

We thank the reviewer for their comments and suggestions. We provide answers to their comments one-by-one using enumeration:

1. We thank the referee for bringing this omission to our attention. Our simulations do account for host-galaxy contamination and contribution from nuclear reddening, in addition to the effect from variable DCE and BLR emission lines. We have explained this in the revised version with proper references.

2. The range of BH masses reported in Figure 4 (top left panel) is taken from the AGN sample of Bentz & Katz (2015) where the masses are derived from the Hbeta-based reverberation mapping. We have now emphasized this in the revised version. Only a few sources have data for MgII and in any case, the MgII relationship is calibrated using Hbeta, so the comparison would produce very similar results.

   On the other hand, the ratio involves the flux of the forbidden line (here, [Si VI]) normalized to the flux of the nearest Hydrogen line (in this case, the BrG). The BrG line has been shown to have slightly narrower FWHMs compared to its optical counterparts (e.g., Hbeta, Landt et al. 2008) which we have referred to in the revised manuscript. On the other hand, this choice of using BrG is made to avoid wavelength-dependent AGN continuum and the torus reprocessing contribution that becomes increasingly important at wavelengths longer than 1 micron, and the stronger host-galaxy contamination in the optical.

   For other lines in the optical (e.g., [Fe VII]6087), we normalize the flux of this coronal line to Hbeta which is the nearest available Hydrogen line. Given their low redshifts, most of the sources in our sample do not have the MgII covered.

3. We have corrected the typographical error in the revised manuscript.

Reviewer 3 Report

This paper reviews recent progresses in the study of the relation between the size of the broad line region, R_BLR, and the optical luminosity, L_5100 via the reverberation mapping (RM) technique, shows how to utilize the LSST in this study and demonstrates the validity for the R_BLR – L_5100 relation to determine the quasar distances in the early universe.  I judge this paper worthy of publication as the review, but think it better to modify some parts as indicated below.

  First of all, the discourse structure is a little difficult to read smoothly.  To my understanding, the main part of this paper is the recent results of the R_BLR – L_5100 relation, but they are separately presented in $2.1 and $4.1, interrupted by the fairly long descriptions on the methodology of the reverberation mapping for future observations in $2.2.  I would like to propose that the contents of the recent progresses and the related scientific discussions in $2.1, $4.1, $2.3 and $3 are first presented, and then the methodology for future observations in $2.2 and the prospect of the contribution to the cosmology as shown in Figure 5 are to be given.

  There are some difficulties in $2.2 to understand.  1) It will be better to add brief explanations on the MS, DDF and the cadences in the top right list in Figure 3.  2) In the bottom right panel of figure 3, the color coded parameter is designated as “Ratio”.  Is this the parameter, \delta, defined in line 247 ?  If so, the smaller value means that the derived \tau better matches to the adopted \tau.  However, the sentence from line 261 seems to contradict to this.

  Minor correction:

(see Figure 4 bottom right panel, the dashed curves) on line 517 : Isn’t this “bottom left panel”?

Author Response

We thank the reviewer for their comments and suggestions. We provide answers to each of their comments one by one using enumeration:

1. We thank the reviewer for their suggestion. We have reorganized the text to outline their suggested sequence. The section “AGNs for cosmology” is now moved to Sec. 3, right after we describe the AGN accretion disk structure and BLR emission. We then discuss the recent highlights from the work with coronal lines. And then discuss the salient findings from our predictions for the accretion disk and BLR time-lag recovery expected with the LSST. We now also include a concluding section summarising our findings and notes for the future.
2. We have added brief notes on the MS, DDF, and cadences in the revised version. We thank the reviewer for bringing this to our attention this mistake. The corrected equation for the Ratio (or δ) should be tau_adopted/tau_derived. We have corrected the equation and the statements based on this for the two cases shown in Figure 5 bottom right panels.
3. We have now corrected this typographical error in the revised version of the manuscript.

Round 2

Reviewer 1 Report

The authors addressed my previous comments by including provides a brief overview of the Seyfert I/II and RL/RQ classification, adding a section on concluding remarks, and improving figures.