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arxiv: 2604.06586 · v1 · submitted 2026-04-08 · 🌌 astro-ph.HE · astro-ph.CO

The host galaxies and merger environments of short and long gamma-ray bursts producing kilonovae

Hannah Skobe , Brendan O'Connor , Antonella Palmese , Lewi Westcott , Christopher J. Conselice , Katelyn Breivik This is my paper

Pith reviewed 2026-05-10 18:40 UTC · model grok-4.3

classification 🌌 astro-ph.HE astro-ph.CO
keywords hostgrbsenvironmentsformationhostslightrecentstar
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The pith

Five of nine kilonova-associated GRB hosts show tidal features from recent mergers with no morphological distinction between short and long GRB hosts, and single-Sersic fits can overestimate offsets in complex galaxies.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

Gamma-ray bursts come in short and long varieties. Short ones are linked to neutron star mergers that also produce kilonovae, bright flashes from heavy element creation. Long ones usually come from collapsing massive stars, but some recent cases also show kilonovae. The authors studied the galaxies hosting nine such events at distances where we can see details. They modeled the light from these galaxies using both simple single-component fits and more complex multi-component fits to capture different parts like bulges and disks. Five of the nine galaxies showed stretched or distorted shapes called tidal features, which are signs that the galaxies recently merged with others. This hints that some neutron star pairs might form during these chaotic mergers rather than in isolated galaxies. Surprisingly, the hosts of short and long bursts looked similar overall, including spirals, ellipticals, and interacting systems. The complex modeling also revealed that using a single simple fit can make the burst appear farther from the galaxy center than it really is when the light comes from a specific sub-part of the galaxy.

Core claim

We find that five of the nine hosts display tidal features that show they have likely undergone recent mergers, suggesting that merger-driven, dynamical formation pathways may contribute in some systems. We find no clear morphological distinction between sGRB-KN and LGRB-KN hosts as both populations span a wide range of morphologies, including ellipticals, spirals, and interacting systems with tidal features.

Load-bearing premise

The kilonova candidates are genuine associations with the GRBs (especially the long ones) and that the tidal features directly indicate recent mergers relevant to the neutron star binary formation channels rather than unrelated galaxy evolution.

Figures

Figures reproduced from arXiv: 2604.06586 by Antonella Palmese, Brendan O'Connor, Christopher J. Conselice, Hannah Skobe, Katelyn Breivik, Lewi Westcott.

Figure 1
Figure 1. Figure 1: Mosaic of HST/F160W image cutouts (HST/F814W for GRB 050709; JWST/F150W for GRB 230307A) of each host galaxy in our sample. The red circles indicate the approximate location of the transients, with the exception of GRB 230307A being at ∼ 30′′ offset outside the scope of the cutout. 2017; Levan et al. 2017; see [PITH_FULL_IMAGE:figures/full_fig_p007_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: The CDF for the observed physical (left panel) and host-normalized (right panel) offsets of our sample. We calculate the host-normalized offset by taking the ratio of physical offset, R, to the r50 of the galaxy. The CDF shows sGRBs (orange; Gompertz et al. 2020, Fong et al. 2022, Nugent et al. 2022, O’Connor et al. 2022), LGRBs (grey; Blanchard et al. 2016), sGRB-KNe (blue), and LGRB-KNe (magenta). We use… view at source ↗
Figure 3
Figure 3. Figure 3: Morphology classification based on concentration versus asymmetry of our host sample – sGRB-KNe (blue) and LGRB-KNe (magenta) – compared to a sGRB (orange; Palmese et al. 2017) and LGRB (grey; Lyman et al. 2017) host population. The boundaries are defined by Eq. 2 of Bershady et al. (2000). The bulk of the host sample (∼ 77%) is classified as late-type galaxies, with the exception of GRB 150101B and GRB 17… view at source ↗
Figure 4
Figure 4. Figure 4: A Gini-M20 comparison between our sample with the observed galaxies in Pan-STARRS (left panels) and the simulated galaxies from IllustrisTNG (right panels) from Rodriguez-Gomez et al. (2018). Each host in our sample has a unique marker. The color for our sample indicates the concentration value from MORFOMETRYKA while the color for the populations is the median concentration values for 2D bins calculated w… view at source ↗
Figure 5
Figure 5. Figure 5: The CDF of stellar mass (left panel) and sSFR (right panel) for sGRBs (orange; O’Connor et al. 2020, Fong et al. 2022, Nugent et al. 2022), LGRBs (grey; Svensson et al. 2010, Perley et al. 2013, Vergani et al. 2015, Wang & Dai 2014, Niino et al. 2017), sGRBEEs (dark orange; O’Connor et al. 2020, Fong et al. 2022, Nugent et al. 2022), sGRB-KNe (blue), and LGRB-KNe (magenta) with a redshift cut at z < 0.6. T… view at source ↗
Figure 6
Figure 6. Figure 6: A comparison of the stellar masses against the sSFR (left panel) and the redshift, z, (right panel) for sGRBs (orange; O’Connor et al. 2020, Fong et al. 2022, Nugent et al. 2022), LGRBs (grey; Svensson et al. 2010, Perley et al. 2013, Vergani et al. 2015, Wang & Dai 2014, Niino et al. 2017), sGRBEEs (dark orange; O’Connor et al. 2020, Fong et al. 2022, Nugent et al. 2022), sGRB-KNe (blue), and LGRB-KNe (ma… view at source ↗
Figure 7
Figure 7. Figure 7: A comparison of the host-normalized offsets given a single (solid) or multi-S´ersic model. We show the smallest (dotted) and largest (dashed) r50 for the multi-S´ersic models. The single S´ersic profile overestimates the host-normalized offset. ture from a galaxy merger. Given the component that best represents the environment of the GRB, the cal￾culated host-normalized offset is dependent on the r50 of sa… view at source ↗
Figure 8
Figure 8. Figure 8: Ratio of 0.3 − 10 keV X-ray flux at 11 hours, FX,11, to the 15 − 150 keV gamma-ray fluence, ϕγ, versus the projected physical offset from the GRB host galaxy. We highlight the events in our sample, divided by their gamma￾ray duration into sGRB-KNe (blue) and LGRB-KNe (ma￾genta), and compare them to a population of short and long GRBs (orange and gray, respectively). We note that some events in our sample (… view at source ↗
Figure 9
Figure 9. Figure 9: A comparison of the median Mej and Vej for the blue (left), purple (middle), and red (right) components of the GRB associated kilonova. Likely galaxy mergers or galaxies with visible tidal features are colored purple and non-merging galaxies are in green. The ejecta masses and velocities are gathered from Rastinejad et al. (2025). The density of the surrounding local environment (the circumburst environmen… view at source ↗
Figure 10
Figure 10. Figure 10: GALFIT results. Left panel: original data; Middle panel: GALFIT model of best-fitting S´ersic light profiles; Right panel: the residual image of GALFIT after image subtraction. The S´ersic profile components are marked with dotted lines with the approximate location of transient indicated by the red circle. GRB 230307A falls outside the cutout and is located in the direction of the red arrow [PITH_FULL_I… view at source ↗
Figure 10
Figure 10. Figure 10: Continued [PITH_FULL_IMAGE:figures/full_fig_p030_10.png] view at source ↗
Figure 10
Figure 10. Figure 10: Continued [PITH_FULL_IMAGE:figures/full_fig_p031_10.png] view at source ↗
Figure 10
Figure 10. Figure 10: Continued [PITH_FULL_IMAGE:figures/full_fig_p032_10.png] view at source ↗
Figure 10
Figure 10. Figure 10: Continued [PITH_FULL_IMAGE:figures/full_fig_p033_10.png] view at source ↗
Figure 10
Figure 10. Figure 10: Continued [PITH_FULL_IMAGE:figures/full_fig_p034_10.png] view at source ↗
Figure 10
Figure 10. Figure 10: Continued [PITH_FULL_IMAGE:figures/full_fig_p035_10.png] view at source ↗
read the original abstract

Gamma-ray bursts (GRBs) have traditionally been classified by their prompt emission duration and spectral hardness, with short GRBs (sGRB; $\lesssim2 \ \rm{s}$) originating from compact object mergers and long GRBs (LGRB; $\gtrsim2 \ \rm{s}$) from massive star core-collapse. Recent kilonova (KN) associations with long-duration GRBs have challenged this standard picture. We analyze the host galaxies of nine GRBs with associated kilonova candidates at $z<0.6$, including five sGRB-KNe and four LGRB-KNe. Using both parametric and non-parametric modeling of the host light distributions, we investigate the progenitor environments of these events and test whether their hosts show evidence for recent galaxy interactions that could favor dynamical formation channels or isolated pathways following merger-driven star formation episodes for neutron star binaries. We find that five of the nine hosts display tidal features that show they have likely undergone recent mergers, suggesting that merger-driven, dynamical formation pathways may contribute in some systems. We find no clear morphological distinction between sGRB-KN and LGRB-KN hosts as both populations span a wide range of morphologies, including ellipticals, spirals, and interacting systems with tidal features. Multi-S\'ersic modeling of the host light profiles further shows that host-normalized offsets inferred from single-S\'ersic fits can be overestimated when the transient is associated with a specific subcomponent of a complex host light profile. These results highlight the importance of decomposing host morphology into physically relevant components when interpreting GRB environments and galactocentric offsets.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The paper analyzes the host galaxies of nine GRBs at z<0.6 with kilonova candidates (five short-duration and four long-duration) using both parametric multi-Sersic and non-parametric modeling of the light profiles. It reports that five of the nine hosts exhibit tidal features indicative of recent mergers, finds no clear morphological distinction between the sGRB-KN and LGRB-KN hosts (both spanning ellipticals, spirals, and interacting systems), and shows that single-Sersic fits can overestimate host-normalized offsets when the transient is tied to a subcomponent of a complex host.

Significance. If the kilonova associations hold, the results provide direct observational support for merger-driven dynamical formation channels contributing to some neutron-star binary systems and caution against overinterpreting offsets in morphologically complex hosts. The work supplies concrete, model-based measurements on a small sample with no circular derivations, highlighting the value of decomposing host light profiles into physically motivated components for GRB environment studies.

major comments (2)
  1. [Introduction and §2] Sample definition (Introduction and §2): The claims of no clear morphological distinction between sGRB-KN and LGRB-KN hosts and the inference that tidal features trace dynamical NS-binary formation pathways rest on the four LGRB-KN candidates being genuine NS-merger events. The manuscript does not re-evaluate or quantify the robustness of these associations (despite the canonical core-collapse link for LGRBs), so even one or two misclassifications would shrink the LGRB-KN subsample and undermine both results.
  2. [§4] Tidal-feature identification (§4, results on non-parametric modeling): The statement that five hosts 'display tidal features that show they have likely undergone recent mergers' requires explicit criteria for classifying features as merger-induced rather than unrelated asymmetries; without this, the link to dynamical formation channels remains qualitative.
minor comments (3)
  1. [Figures] Figure captions (e.g., Fig. 1 and Fig. 3): Add explicit labels distinguishing sGRB-KN from LGRB-KN hosts and clarify which panels show single- versus multi-Sersic residuals.
  2. [§3.2] §3.2: The range of Sersic indices and effective radii for the multi-component fits should be tabulated for reproducibility.
  3. [Abstract and §5] Abstract and §5: The phrase 'merger-driven, dynamical formation pathways may contribute in some systems' should be tied more directly to the specific hosts showing tidal features.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for their constructive comments, which have helped us improve the clarity and rigor of the manuscript. We address each major comment below and have revised the paper where appropriate.

read point-by-point responses

  1. Referee: [Introduction and §2] Sample definition (Introduction and §2): The claims of no clear morphological distinction between sGRB-KN and LGRB-KN hosts and the inference that tidal features trace dynamical NS-binary formation pathways rest on the four LGRB-KN candidates being genuine NS-merger events. The manuscript does not re-evaluate or quantify the robustness of these associations (despite the canonical core-collapse link for LGRBs), so even one or two misclassifications would shrink the LGRB-KN subsample and undermine both results.

    Authors: We agree that the strength of our conclusions on morphological similarities and merger signatures depends on the reliability of the kilonova associations. Our study is a host-galaxy analysis that takes the associations as reported in the literature; we do not re-analyze the prompt emission, afterglow, or light-curve properties of the transients themselves. In the revised manuscript we have added a dedicated paragraph in the Introduction that summarizes existing assessments of association probabilities from the literature, explicitly notes the small sample size, and states that even one misclassification would affect the LGRB-KN subsample. This provides necessary context without expanding the scope beyond host morphology. revision: partial

  2. Referee: [§4] Tidal-feature identification (§4, results on non-parametric modeling): The statement that five hosts 'display tidal features that show they have likely undergone recent mergers' requires explicit criteria for classifying features as merger-induced rather than unrelated asymmetries; without this, the link to dynamical formation channels remains qualitative.

    Authors: We accept this criticism. The original text was insufficiently precise. In the revised §4 we now provide explicit classification criteria: visual identification of asymmetric low-surface-brightness extensions, tidal tails, shells, or bridges to companions, supported by quantitative non-parametric residuals exceeding 3σ above the smooth model and cross-checked against hydrodynamic merger simulations. We have also tempered the language to state that these features are consistent with recent mergers rather than definitively proving dynamical formation channels. revision: yes

standing simulated objections not resolved
  • Re-evaluation or quantitative robustness assessment of the kilonova associations for the long GRB candidates, as this would require new analysis of the transient data outside the scope of the present host-galaxy study.

Circularity Check

0 steps flagged

No significant circularity in observational host-galaxy analysis

full rationale

The paper reports direct measurements of host morphologies for nine GRB events selected by prior kilonova associations. It applies standard parametric (Sérsic) and non-parametric modeling to imaging data, identifies tidal features by visual and quantitative inspection, and compares morphological distributions between sGRB-KN and LGRB-KN subsamples. No equations, fitted parameters, or uniqueness theorems are introduced whose outputs are then relabeled as independent predictions. No self-citation chain supplies a load-bearing premise that is itself unverified within the present work. The central results (prevalence of tidal features, lack of morphological distinction, offset bias from single-component fits) are empirical outcomes of the applied methods rather than tautological restatements of the input sample definition. The robustness of the KN associations themselves is an external assumption, not a circular step internal to the derivation.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The analysis depends on standard galaxy morphology fitting techniques and the interpretation of tidal features as merger signatures; free parameters arise in choosing the number and form of Sersic components to model host light profiles.

free parameters (1)
  • Number and parameters of Sersic components
    The multi-Sersic modeling requires selecting the number of components and fitting their indices, effective radii, and other parameters to match the observed host light distributions.
axioms (2)
  • domain assumption Tidal features reliably indicate recent galaxy mergers
    The paper interprets observed tidal features as evidence of recent mergers without additional verification steps detailed in the abstract.
  • domain assumption Kilonova candidates are correctly associated with the GRBs
    The sample selection assumes the reported kilonova associations hold, including for long GRBs.

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Forward citations

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