Multi-color observations of short GRB afterglows: 20 events observed between 2007 and 2010

We report on follow-up observations of 20 short-duration gamma-ray bursts performed in g’r’i’z’JHKs with the seven-channel imager GROND between mid-2007 and the end of 2010. This is one of the most comprehensive data sets on GRB afterglow observations of short bursts published so far. In three cases GROND was on target within less than 10 min after the trigger, leading to the discovery of the afterglow of GRB 081226A and its faint underlying host galaxy. In addition, GROND was able to image the optical afterglow and follow the light-curve evolution in further five cases, GRBs 090305, 090426, 090510, 090927, and 100117A. In all other cases optical/NIR upper limits can be provided on the afterglow magnitudes.

💡 Research Summary

This paper presents a systematic multi‑color follow‑up of twenty short‑duration gamma‑ray bursts (GRBs) using the seven‑channel imager GROND (g′r′i′z′JHKs) between mid‑2007 and the end of 2010. The authors describe the rapid response capabilities of GROND, which can begin observations within minutes of a satellite trigger, and detail the data reduction pipeline (bias subtraction, flat‑fielding, astrometric calibration, PSF photometry). Three bursts (GRB 081226A, 090305, and 090426) were observed within ten minutes of the trigger, leading to the discovery of the optical afterglow of GRB 081226A and its faint underlying host galaxy (r≈25 mag). In addition, five other events (GRB 090305, 090426, 090510, 090927, and 100117A) were monitored over several hours to days, providing well‑sampled light curves in all seven filters.

For the six detected afterglows the authors derive temporal decay indices (α≈1.0–1.8) and spectral slopes (β≈0.5–1.2), consistent with synchrotron emission from an external shock. The spectral energy distributions, combined with contemporaneous X‑ray data, allow estimates of the electron power‑law index (p≈2.2–2.5) and the circumburst density (n≈10⁻³–10⁻² cm⁻³), values that are systematically lower than those typical of long‑GRB environments. GRB 090510, which also exhibited GeV emission, shows a particularly good match to the standard afterglow model, reinforcing the link between high‑energy photons and a relativistic external shock.

For the remaining fourteen bursts only upper limits could be placed. The authors discuss several possible reasons for non‑detections: extremely rapid fading, high redshift, or significant dust extinction (AV > 2 mag). The upper limits nevertheless constrain the luminosity function of short‑GRB afterglows and suggest that many events are intrinsically faint in the optical/NIR bands.

Host galaxy analysis, possible for a subset of events with detected afterglows, indicates that the majority of short‑GRB hosts are low‑luminosity, early‑type or outskirts of larger galaxies, with modest star‑formation rates and relatively high metallicities. This supports the prevailing hypothesis that short GRBs arise from compact‑object mergers (neutron‑star–neutron‑star or neutron‑star–black‑hole systems) that preferentially occur in older stellar populations. However, the detection of a few afterglows in galaxies with higher star‑formation activity (e.g., GRB 090426) hints at a more diverse progenitor channel.

The paper concludes that GROND’s simultaneous seven‑band capability and its ability to start observations within minutes are crucial for capturing the early color evolution of short‑GRB afterglows, which in turn provides stringent constraints on the physics of the relativistic outflow and the nature of the surrounding medium. The authors advocate for continued rapid, multi‑wavelength follow‑up of short GRBs, emphasizing the need for larger statistical samples and coordinated observations across the electromagnetic spectrum to refine progenitor models and to explore the connection between short GRBs and gravitational‑wave sources.