Optical Variability and Evidence for a Changing-Look Event in the Galaxy Mrk 6 (IC 450)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript presents long-term photometric monitoring (2016–2026) of the Seyfert galaxy Mrk 6 in B, V, and Rc bands, complemented by several recent spectroscopic observations. The dataset is valuable, and the long temporal baseline is particularly important for studying variability in active galactic nuclei.

However, I would like to draw attention to a major issue that significantly affects the scientific interpretation and overall impact of the paper.

Major Comment: Alert for a Changing-Look Event !!!

From the presented spectra, it is evident that on 9 January 2026 Mrk 6 lost the broad component of the Hβ emission line. This is a very important result! This is a clear indication that the source has undergone a changing-look (CL) transition, i.e., a change in its spectral type due to the disappearance (or strong suppression) of the broad-line region emission (see for example Komossa et al. 2026, AdSpR. doi:10.1016/j.asr.2025.04.058.)

This is a very impotant and highly significant result, as changing-look AGN are rare and provide crucial insight into accretion physics, BLR structure, and AGN variability mechanisms. Surprisingly, this phenomenon is neither explicitly mentioned nor analyzed in the manuscript.

The absence of discussion on this point represents a major shortcoming. The detected spectral change is arguably the most important result of the dataset, and it should form the central focus of the paper, rather than being treated as a secondary or implicit feature.

Specifically, the authors should:

• Clearly identify and state that Mrk 6 has entered a changing-look phase.
• Compare the spectra before and after the transition, quantifying the disappearance of the broad Hβ component.
• Discuss possible physical interpretations (e.g., changes in accretion rate, obscuration, or BLR response), see for example :
  Sniegowska et al. 2020, A&A doi:10.1051/0004-6361/202038575,  MacLeod et al. 2019, ApJ doi:10.3847/1538-4357/ab05e2, Wang & Bon 2020A&A doi:10.1051/0004-6361/202039368, Popovic et al. A&A 2023 doi:10.1051/0004-6361/202345949,  Oknyansky et al. 2019MNRAS, ...)

• If possible, re-evaluate the derived BLR radii and lag interpretations in light of this transition, since the BLR structure is clearly evolving.

Given the importance of this result, I strongly recommend that the authors revise the title and overall framing of the manuscript to reflect the detection of a changing-look event. The current title (focused on photometric variability and lag estimates) does not adequately capture the significance of the observed spectral transformation.

This work has the important potential to serve as an alert  or first report of a changing-look transition in Mrk 6, which would be of considerable interest to the AGN community, and I propose that the result should be published in the shortest possible time!

The manuscript is based on valuable observational data and has the potential to make an important contribution. However, in its current form it misses the key physical result. I recommend fast revision and publication, with a refocus of the paper toward the alert of changing-look nature of Mrk 6.

Author Response

Authors’ general response:

We thank the referee for the valuable suggestions and constructive comments, which helped significantly improve the manuscript. We have revised the relevant sections in accordance with your comments, and we have highlighted the changes in red to ensure clarity and alignment with the overall results of the paper.

However, I would like to draw attention to a major issue that significantly affects the scientific interpretation and overall impact of the paper.

Major Comment: Alert for a Changing-Look Event !!!

From the presented spectra, it is evident that on 9 January 2026 Mrk 6 lost the broad component of the Hβ emission line. This is a very important result! This is a clear indication that the source has undergone a changing-look (CL) transition, i.e., a change in its spectral type due to the disappearance (or strong suppression) of the broad-line region emission (see for example Komossa et al. 2026, AdSpR. doi:10.1016/j.asr.2025.04.058.)

This is a very impotant and highly significant result, as changing-look AGN are rare and provide crucial insight into accretion physics, BLR structure, and AGN variability mechanisms. Surprisingly, this phenomenon is neither explicitly mentioned nor analyzed in the manuscript.

The absence of discussion on this point represents a major shortcoming. The detected spectral change is arguably the most important result of the dataset, and it should form the central focus of the paper, rather than being treated as a secondary or implicit feature.

Specifically, the authors should:

Clearly identify and state that Mrk 6 has entered a changing-look phase.

Compare the spectra before and after the transition, quantifying the disappearance of the broad Hβ component. Discuss possible physical interpretations (e.g., changes in accretion rate, obscuration, or BLR response), see for example :
Sniegowska et al. 2020, A&A doi:10.1051/0004-6361/202038575,  

MacLeod et al. 2019, ApJ doi:10.3847/1538-4357/ab05e2, 

Wang & Bon 2020A&A doi:10.1051/0004-6361/202039368, 

Popovic et al. A&A 2023 doi:10.1051/0004-6361/202345949,  

Oknyansky et al. 2019MNRAS, ...)

If possible, re-evaluate the derived BLR radii and lag interpretations in light of this transition, since the BLR structure is clearly evolving.

Given the importance of this result, I strongly recommend that the authors revise the title and overall framing of the manuscript to reflect the detection of a changing-look event. The current title (focused on photometric variability and lag estimates) does not adequately capture the significance of the observed spectral transformation.

Reply:

 

As shown in Figure~6, where the spectra are normalized to the [O,III] $\lambda5007$ flux, the broad H$\beta$ component reaches a minimum in January 2026 and then partially recovers by mid-February. Thus, the observed variability is non-monotonic and is primarily manifested as changes in the amplitude of the broad H$\beta$ component, without a significant change in the spectral type (the object remains close to type~1.5).

Such behavior may resemble that of changing-look AGN. However, the absence of a complete disappearance of the H$\beta$ line and the presence of partial recovery suggest that this event is more appropriately described as CL-like variability rather than a classical changing-look transition. In terms of the classification by Komossa et al.~(2026), it is most consistent with category~V (recurrent brightenings and dimmings on timescales from weeks to years), although the observed event occurs on a relatively short timescale.

Similar episodes of temporary weakening of broad emission lines have been reported in a number of objects (Komossa et al.2026; MacLeod et al.2019; Oknyansky et al.2019; 'Sniegowska et al.2020; Wang & Bon~2020; Popovi'c et al.2023).

Figure 6. Optical spectra of Mrk 6 (4800–5100 Å) obtained at multiple epochs between November 2025 and February 2026. The Hβ emission line exhibits notable profile variability, with the broad component significantly weakened in January 2026 and partially recovered by February 2026.

This work has the important potential to serve as an alert  or first report of a changing-look transition in Mrk 6, which would be of considerable interest to the AGN community, and I propose that the result should be published in the shortest possible time!

The manuscript is based on valuable observational data and has the potential to make an important contribution. However, in its current form it misses the key physical result. I recommend fast revision and publication, with a refocus of the paper toward the alert of changing-look nature of Mrk 6.

Reply:

We thank the referee for this insightful and constructive suggestion. We fully agree that the detection of a possible changing-look event significantly enhances the importance of our results.

Following the referee’s recommendation, we have revised the title of the manuscript to better reflect this aspect. The new title now reads:

“Optical Variability and Evidence for a Changing-Look Event in the Galaxy Mrk 6 (IC 450)”.

We believe that the revised title more accurately conveys the scientific significance of the observed spectral transformation while still retaining the context of optical variability and time-delay analysis presented in our study.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

I have read the short paper for Universe with interest. Reverberation mapping is a tried and true technique for mass estimates of nearby black holes where repeat observations are practical.

 

I do not have major concerns but several suggestions to put these new observations in context. I think the authors have already done an admirable job of this. Here are my suggestions for improvement:

 

1. Show a plot of all the photometry since the 1970s combined. This allows the reader to see the relevance of the new photometry. 

2. Compare the ZTF and new observations in one plot to show how well they agree.

3. Consider a plot of the line measurements as the results are based on these.

 

Small issue: please write out any abbreviations and acronyms upon first use. For example, in the introduction, the terms BLR, VLBA, AGN and IFU are all used without explanation. 

 

Section 4 contains my only issue that I would like to see addressed. The authors state that “We analyzed spectral data acquired on November 15 and 22 following the methodology detailed in [16]. “ Please briefly describe here what this methodology is. That would help the reader understand what the approach here is without further searches in the literature. 

These are all my suggestions and my one big comment. 

Author Response

Authors’ general response:

We thank the referee for the valuable suggestions and constructive comments, which helped significantly improve the manuscript. We have revised the relevant sections in accordance with your comments, and we have highlighted the changes in red to ensure clarity and alignment with the overall results of the paper.

1. Show a plot of all the photometry since the 1970s combined. This allows the reader to see the relevance of the new photometry. 

Reply: 

Thank you for this valuable comment. The photometric observations of Mrk 6 in our monitoring program have been conducted since 2016. For this reason, to give the reader the full context and to illustrate the significance of the new photometric measurements, we included in the figure all available photometric data obtained over the period 2016–2026.

2. Compare the ZTF and new observations in one plot to show how well they agree.

Reply:

Thank you for this valuable comment. We have included a direct comparison between the ZTF data and our new observations in Figure 2, where both datasets are shown in a single plot to illustrate the level of agreement between them.

3. Consider a plot of the line measurements as the results are based on these. Small issue: please write out any abbreviations and acronyms upon first use. For example, in the introduction, the terms BLR, VLBA, AGN and IFU are all used without explanation. Section 4 contains my only issue that I would like to see addressed. The authors state that “We analyzed spectral data acquired on November 15 and 22 following the methodology detailed in [16]. “ Please briefly describe here what this methodology is. That would help the reader understand what the approach here is without further searches in the literature.  These are all my suggestions and my one big comment. 

Reply: 

We replace the sentence “We analyzed spectral data acquired on November 15 and 22 following the method- 162 ology detailed in [16].” with more elaborated discussion: 

The study employs a Bayesian statistical approach for the approximation of observed spectra. Unlike the least squares method, this approach allows not only for finding the optimal parameters of emission lines but also for evaluating the adequacy of the spectral description model itself by calculating the 'Bayesian evidence'.

To identify the global maximum of the likelihood function and construct posterior parameter distributions, we utilized the Dynamic Nested Sampling method implemented in the dynesty [Speagle, Joshua S. DYNESTY: a dynamic nested sampling package for estimating Bayesian posteriors and evidence. Mon. Not. R. Astron. Soc., 2020, 493, 3, 3132-3158, \url{https://10.1093/mnras/staa278}] package. Non-informative priors were defined over broad ranges of physically grounded values. Optimal parameter values were determined based on the median of the distribution (50th percentile), with uncertainties estimated at the 16th and 84th percentile levels.

The total model spectrum included a power-law continuum and a set of emission lines from both the Narrow Line Region (NLR) and the Broad Line Region (BLR). The central wavelengths for the [NII], [SII], [OI], and [OIII] doublets, as well as the Balmer series lines and fixed doublet amplitude ratios, were set according to standard values. Seven model types (M0​–M6​) were tested, differing in their profiles and number of components: the NLR was modeled using Voigt (M0​, M3​), Gaussian (M1​), or asymmetric Gaussian profiles with an error function (M2​, M4​, M5​, M6​). The BLR was represented by either three components (central, 'blue', and 'red') to account for complex BLR dynamics (M0​, M1​, M2​), two components (M3​, M4​), or a single broad component (M5​, M6​). Some more details can be found in [16].

Author Response File: Author Response.pdf