Crab Nebula's missing counterjet found — as a ring

Referee: §2, Fig. 1d–e: The southern ring is identified by visual inspection of public images with no quantitative morphological criterion for what constitutes a 'ring,' no measurement of statistical significance, and no assessment of coherence across wavelengths. At minimum, the author should provide a quantitative description: coordinate-based contour or intensity profile, angular extent, surface brightness contrast relative to surroundings, and a measure of how ring-like the feature is.

Authors: The referee is correct that the identification rests on visual inspection without quantitative morphometric characterization. We will add to the revised manuscript a quantitative description of the southern ring, including: (i) approximate J2000 coordinates of the ring center and extent, (ii) angular dimensions (major and minor axes), (iii) surface brightness contrast relative to the local background in the optical and IR images, and (iv) a qualitative assessment of coherence across the optical and IR wavebands. We note that the feature is visible in both the optical (Fig. 1d) and infrared (Fig. 1e) images from the Chandra composite, which provides some cross-wavelength coherence, but we agree this should be stated explicitly and quantified where possible. We will also add a contour overlay or intensity cut to the figure to make the identification reproducible. We acknowledge that a full statistical significance assessment (e.g., comparison to a null distribution of filament configurations) is beyond the scope of a Research Note, but we will state this limitation explicitly. revision: yes

Referee: §2: The inclination estimate of ~30 degrees is derived from an axis ratio of ~2, assuming the feature is intrinsically circular. This assumption is unverified and is itself a prediction of the jet interpretation rather than an independent constraint. The author should acknowledge this circularity explicitly and note that the intrinsic geometry of circum-jet rings in the cited simulations may not be circular, or should provide simulation-based priors on the expected intrinsic axis ratio.

Authors: The referee's point is well taken. The assumption that the ring is intrinsically circular is indeed a prediction of the jet interpretation rather than an independent observational constraint, and the logic is circular if presented as we have done. We will revise the text to acknowledge this explicitly. We will also note that circum-jet rings in the JJEM simulations (Akashi & Soker 2026a,b) are not perfectly circular—they are shaped by the interaction of the jet with the surrounding ejecta and can have intrinsic axis ratios deviating from unity. We will reframe the ~30 degree inclination as an order-of-magnitude estimate contingent on the intrinsic-circularity assumption, rather than a firm determination, and will note that simulation-based priors on the intrinsic axis ratio would be needed for a more robust inclination estimate. We will soften the language accordingly. revision: yes

Referee: §2–§3: The paper does not engage with alternative explanations for the southern ring feature (pulsar-wind interaction, Rayleigh-Taylor instabilities, circumstellar material interaction). Ding et al. (2026) discussed seven non-JJEM explanations for the northern jet; the author should at least briefly address whether similar processes could produce the southern ring, or explain why the ring morphology specifically favors a jet origin over these alternatives.

Authors: We agree that alternative explanations should be addressed. We will add a brief paragraph in the revised manuscript discussing whether pulsar-wind interaction, Rayleigh-Taylor instabilities, or circumstellar material interaction could produce the observed southern ring morphology. We will note the following: (1) The pulsar-wind nebula, as seen in X-rays (Fig. 1f), does not extend to the southern ring location, which argues against a direct pulsar-wind origin. (2) Rayleigh-Taylor filaments are ubiquitous in the Crab and are generally radial; the southern ring's quasi-circular morphology is not a natural outcome of RT instability alone, though we acknowledge that RT filaments are present in the region and could contribute to the observed structure. (3) We will note that Ding et al. (2026) discussed seven non-JJEM explanations for the northern jet and that similar processes could in principle operate in the south, but that the point-symmetric pairing with the northern jet axis is the primary motivation for the counterjet interpretation. We will be explicit that the jet origin is favored by morphological association rather than by exclusion of all alternatives. revision: yes

Referee: §3: The logical structure of the central argument is: JJEM simulations produce circum-jet rings; a ring is observed in the south; therefore it is a counterjet within JJEM. This is an attribution rather than a demonstration. The author should reframe the conclusion to reflect that the southern ring is consistent with JJEM counterjet predictions, rather than stating it as an identification, unless a quantitative match to a specific simulation output is provided.

Authors: We accept this point. The argument as currently framed is an attribution based on qualitative morphological similarity, not a quantitative demonstration. We will revise the conclusion to state that the southern ring is consistent with the JJEM counterjet prediction, rather than claiming a definitive identification. We will retain the language of 'attribution' where it appears, as it already conveys a degree of tentativeness, but we will modify the abstract and summary to make clear that this is a proposed interpretation supported by morphological correspondence with simulations, not a proven identification. We note that the paper already uses 'attribute' rather than 'identify' in several places, but the title and abstract use 'identifying,' which overstates the strength of the argument. We will adjust the title and abstract language to better reflect the interpretive nature of the claim. revision: yes