What Is Inside the Double–Double Structure of the Radio Galaxy J0028+0035?

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have presented a well-written article about an interesting double-double AGN with intriguing indications that it might be a triple-double source (only four such sources are known).  The key scientific question is posed already in the article title: "What is Inside the Double–Double Structure of the Radio Galaxy J0028+0035?" To answer that, the authors have obtained new observation data with higher resolution.

Authors conducted new MERLIN and EVN L-band observations of this source and used archival data from JVLA to analyze its structure. The conducted observations and data reduction/calibration are well explained, with detailed steps. The obtained VLBI data are nicely presented in high-quality images.  Archival low-resolution images show two symmetric relic lobes, but high-resolution images reveal that two of the three compact inner radio sources are newer inner radio lobes. Asymmetry of these innear lobes and unclear nature of central source made a possibility J0028+0035 to be a triple-double source, a very interesting object for studying evolution of AGN activity. Nevertheless  the high-resolution study conducted by the authors, showed that the nature of the source is different from what was previously thought (speculated).

Past discoveries and information about the optical counterpart (which position corresponds to central radio source) are extensively referred to in the introduction. The milliarcsecond resolution image by EVN showed no evidence of an innermost double-lobe structure, overturning the speculation that this source is a triple-double AGN. Additionally, the authors did not find a mass-scale jet, which supports the main conclusion that the J0028+0035 source is switching from a radio-loud to a radio-quiet state.

Furthermore, a much more distant foreground AGN was also analyzed. New results show that it is most likely not a blazar as previously classified; measurements from the latest observations indicate it does not meet the blazar classification criteria.

Objections / critique:

A minor point to raise is the use of unorthodox statement forms, such as "the source may still be astrometrically well-behaved." However, similar phrases are heard at conferences, and understanding them should not be problematic for readers.

 Additionally, authors could consider to transform same of parameter (number) heavy text parts in table form, for example: J0028+0035 Discussion part with Gaia and WISE data. However, text is still easily readable.

Similarity same of observation parameters, could be considered to be put table format.

   

Overall, the article can be accepted in its current form. With clear novelty and impact: showing  the J0028+0035 nature, and overturning the idea of triple-double source structure.

 

Author Response

Please find our reply in the attached PDF document.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In general I don’t see anything significantly new in this work that takes us beyond what is in Marecki, et al, 2021, MNRAS, 501, 853 but I will leave that to the editor and comment on the potential of this paper. Apart from this criticism, my main issue with this work is that it makes no attempt to explain the timescales that have emerged from the observations. These timescales produce constraints on the physical processes that should be explored. More detail as follows.

 

-              Your observations suggest that this is likely a double-double radio source with the original jet lasting in excess of 10⁸ yrs. followed by a quiescent phase of 1.1 x 10⁷ yrs., followed by a brief jetted period of 3.6 x 10⁶  yrs.  What kinds of physical processes are associated with such timescales? Could these periods be related to black hole spin? Could the 2.45 x 10⁸ yrs. be associated with black hole spin-down at some accretion rate? We know that spin down from high spin at the Eddington limit occurs in about 10⁷ yrs. so it is conceivable that the original jet was produced while the system accreted at values that are slightly sub-Eddington, say 10% Eddington. What about the quiescent jet period? Could that be the time it took for the system to spin up again to a noticeable value? The timescale to spin a black hole up from zero spin to about 0.98 is about 10⁸ yrs. at the Eddington limit.  At 10% Eddington, the spin of the black hole could have reached close to 0.1 which is then compatible with the formation of a weak jet. Are there models suggesting a visible jet at such spin values? What is the uncertainty in the 1.1 x 10⁷ yr. timescale? For that matter, what is the uncertainty in the other timescales? The last timescale could result from a limited supply of accretion but it seems there is larger uncertainty on this last feature. Can you discuss?

 

-              Is there an image that includes the outer lobes?

 

-              What kind of environment is this source in?

 

-              You talk about the definition of a blazar as being “elusive”. Perhaps you mean “vague”?

Author Response

Please find our reply in the attached PDF document.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript presents a study of the double-double radio galaxy  (DDRG) J0028+0035. The work is compactly, but mostly well presented. With a few scattered clarifications it is suitable for publication in Symmetry.  My specific suggestions follow:

The statement at the top of page two that jets"eventually interact" with the surrounding medium is misleading, since the jets always interact at some level with their surroundings. More accurately, those interactions eventually  transfer sufficient jet energy and momentum to the ambient medium to substantially reduce its propagation speed. In the same paragraph the authors need to introduce the intermittency of the jet action, since it plays a crucial role especially in DDRGs. Also in this paragraph reference is made to spectral properties of the radio lobes as a tool to understand the history of the source.. But, nothing has been said to suggest what relevant spectral properties are at issue, nor what information they convey about the source history. A brief statement is needed.

The images in Figs 1 and 2 are in "relative" coordinates. The nominal reference positions in the two images are close, but cannot be identical. This should be clarified.

The units used for terms in equation 2 need to be specified.

At the bottom of page 8 "neither a" should be "nor a".

In the data availability statement on page 10 "data are in public domain" should read "data are in the public domain".

Author Response

Please find our reply in the attached PDF document.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

I don't see much that is new beyond Marecki et al 2021 except for the conclusion that no triple jet structure is observed. I will leave it up to the editors to decide if that is enough for publication. I would suggest the authors explore the timescales for the inner and outer jet, and the time of quiescence for DDRS as these timescales may coincide with black hole spin down/up and jets turning off/on as a result. 

Author Response

Please see the attached PDF file.

Author Response File: Author Response.pdf