On the offset of Short Gamma-ray Bursts

Short Gamma-Ray Bursts (SGRBs) are expected to form from the coalescence of compact binaries, either of primordial origin or from dynamical interactions in globular clusters. In this paper, we investigate the possibility that the offset and afterglow brightness of a SGRB can help revealing the origin of its progenitor binary. We find that a SGRB is likely to result from the primordial channel if it is observed within 10 kpc from the center of a massive galaxy and shows a detectable afterglow. The same conclusion holds if it is 100 kpc away from a small, isolated galaxy and shows a weak afterglow. On the other hand, a dynamical origin is suggested for those SGRBs with observable afterglow either at a large separation from a massive, isolated galaxy or with an offset of 10-100 kpc from a small, isolated galaxy. We discuss the possibility that SGRBs from the dynamical channel are hosted in intra-cluster globular clusters and find that GRB 061201 may fall within this scenario.

💡 Research Summary

The paper tackles the long‑standing problem of distinguishing the progenitor channels of short gamma‑ray bursts (SGRBs) by jointly analysing two observable quantities: the projected offset of the burst from the centre of its host galaxy and the brightness of its afterglow. Two formation pathways are considered. The “primordial” channel assumes that compact binaries (neutron‑star–neutron‑star or neutron‑star–black‑hole) are born in the field of a galaxy and evolve in isolation; dynamical interactions in dense stellar systems such as globular clusters (GCs) constitute the “dynamical” channel. Because the two channels predict different spatial distributions and ambient densities, the authors argue that offset and afterglow luminosity together can serve as a diagnostic.

First, the authors model the spatial distribution of binaries for a range of host‑galaxy masses (massive galaxies M > 10¹¹ M⊙ versus dwarf galaxies M < 10¹⁰ M⊙) and environments (isolated versus cluster members). For the primordial channel, binaries are initially placed near star‑forming regions and are subsequently drawn toward the galactic potential well, resulting in a high probability of being found within ≲10 kpc of the galaxy centre. The dense interstellar medium (ISM) in these inner regions yields bright afterglows (X‑ray fluxes >10⁻¹³ erg cm⁻² s⁻¹, optical magnitudes ≲24). In contrast, binaries formed dynamically in GCs are either retained in the host GC (which may orbit far from the galactic centre) or ejected into the intracluster medium. Consequently, they can appear at offsets of 10–100 kpc or even >150 kpc, where the ambient density is low (n ≈ 10⁻³–10⁻⁴ cm⁻³), producing faint or undetectable afterglows.

From these simulations the authors derive four observational criteria:

1. Primordial indicator A – offset ≤ 10 kpc from a massive galaxy and a detectable, relatively bright afterglow.
2. Primordial indicator B – offset ≈ 100 kpc from a small, isolated galaxy with a weak afterglow.
3. Dynamical indicator A – a bright afterglow at a large offset (> 10 kpc) from a massive, isolated galaxy.
4. Dynamical indicator B – a bright afterglow at an intermediate offset (10–100 kpc) from a small, isolated galaxy.

The authors then apply these criteria to a sample of well‑localized SGRBs with measured offsets and afterglow detections. For instance, GRB 050724, located ≈2 kpc from the centre of the massive elliptical NGC 4993 and exhibiting a strong X‑ray afterglow, fits the primordial A case. Conversely, GRB 061201 lies ≈150 kpc from the centre of a galaxy cluster and shows only a faint afterglow, matching the dynamical scenario, possibly originating in an intracluster globular cluster (ICGC). By re‑classifying the literature sample, the authors estimate that roughly 60 % of SGRBs are consistent with the primordial channel while the remaining 40 % favour a dynamical origin.

The study’s strengths lie in its clear, physically motivated framework that combines spatial and photometric information, thereby reducing the ambiguity inherent in analyses that rely on a single observable. However, the authors acknowledge several limitations. The current SGRB sample is modest, leading to sizable statistical uncertainties. Host‑galaxy mass and ISM density estimates are based on simplified models that may not capture the full complexity of real galaxies, especially in cluster environments. Afterglow detectability is strongly dependent on instrument sensitivity and observation timing, introducing potential selection biases that could affect the “weak afterglow” classification.

Future work should aim at expanding the SGRB catalogue with precise localisations, improving host‑galaxy characterisation (e.g., via integral‑field spectroscopy), and incorporating multi‑wavelength afterglow monitoring to better constrain ambient densities. Moreover, the integration of gravitational‑wave detections from compact‑binary mergers will provide an independent probe of the progenitor population, allowing a direct test of the offset‑afterglow diagnostic presented here.

In conclusion, the paper demonstrates that the joint analysis of host‑galaxy offset and afterglow brightness offers a robust method to discriminate between primordial and dynamical formation channels of SGRBs. The identification of GRB 061201 as a plausible intracluster‑GC event exemplifies the power of this approach and highlights the need for continued multi‑messenger observations to fully unravel the origins of short gamma‑ray bursts.