The First Short GRB Millimeter Afterglow: The Wide-Angled Jet of the Extremely Energetic SGRB 211106A

Abstract: We present the discovery of the first millimeter afterglow of a short-duration $\gamma$-ray burst (SGRB) and the first confirmed afterglow of an SGRB localized by the GUANO system on Swift. Our Atacama Large Millimeter/Sub-millimeter Array (ALMA) detection of SGRB 211106A establishes an origin in a faint host galaxy detected in Hubble Space Telescope (HST) imaging at $0.7\lesssim z\lesssim1.4$. From the lack of a detectable optical afterglow, coupled with the bright millimeter counterpart, we infer a high extinction, $A_{\rm V}\gtrsim2.6$ mag along the line of sight, making this the one of the most highly dust-extincted SGRBs known to date. The millimeter-band light curve captures the passage of the synchrotron peak from the afterglow forward shock and reveals a jet break at $t_{\rm jet}=29.2^{+4.5}_{-4.0}$~days. For a presumed redshift of $z=1$, we infer an opening angle, $\theta_{\rm jet}=(15.5\pm1.4)$~degrees, and beaming-corrected kinetic energy of $\log(E_{\rm K}/{\rm erg})=51.8\pm0.3$, making this one of the widest and most energetic SGRB jets known to date. Combining all published millimeter-band upper limits in conjunction with the energetics for a large sample of SGRBs, we find that energetic outflows in high density environments are more likely to have detectable millimeter counterparts. Concerted afterglow searches with ALMA should yield detection fractions of 24-40% on timescales of $\gtrsim2$~days at rates $\approx0.8$-1.6 per year, outpacing the historical discovery rate of SGRB centimeter-band afterglows.

• The paper demonstrates the first detection of a millimeter afterglow in a short-duration GRB using ALMA, marking a breakthrough in GRB research.
• It details multi-wavelength analyses that reveal a wide jet opening angle and beaming-corrected kinetic energy of log(EKiso)=51.8.
• The findings challenge previous models and highlight ALMA’s potential to uncover energetic outflows in high-density, dust-rich environments.

The First Millimeter Afterglow of a Short-Duration Gamma-Ray Burst: Insights from GRB 211106A

The paper by Laskar et al. presents a detailed study of the afterglow of GRB 211106A, marking the first ever detection of a millimeter afterglow from a short-duration gamma-ray burst (SGRB). Utilizing the observational capabilities of ALMA, along with data from other multi-wavelength facilities such as Swift, Chandra, XMM-Newton, and HST, the authors provide comprehensive insights into the energetics and environmental context of this SGRB, which is a notable achievement in the field of gamma-ray burst research.

Summary of Observations and Results

The discovery of the millimeter afterglow was made possible through observations conducted with the ALMA telescope approximately 12.9 days post-trigger, with detections confirming a fading source at 97.5 GHz. The observations reveal that the host galaxy of GRB 211106A is located at a redshift of approximately 0.7 to 1.4, inferred from HST imaging. The analysis indicates that the afterglow exhibits a high degree of extinction, with AV exceeding 2.6 magnitudes, likely due to the dust-rich environment of the host galaxy.

X-ray and millimeter-band light curves obtained reveal that the afterglow has one of the widest jet opening angles measured for SGRBs, highly suggestive of a late jet break occurring approximately 29.2 days post-burst. This results in a beaming-corrected kinetic energy estimate of log(EKiso)=51.8. The overall energy output is significantly larger than what is typically observed for SGRBs, which the authors attribute to the combination of high energy release and a dense circumstellar medium.

Theoretical and Practical Implications

From a theoretical standpoint, this study provides invaluable data to test and refine existing models of SGRB afterglows. In particular, the findings challenge some prevailing assumptions about jet dynamics and energy scales within high-density environments. The results support models that predict more energetic outflows and suggest that millimeter afterglows may be more prevalent in environments with higher densities than previously recognized.

In practical terms, the demonstrated sensitivity of ALMA to detect such afterglows indicates potential for future discoveries of SGRB millimeter afterglows. The authors estimate a detection rate of 24-40% for millimeter counterparts from future concerted efforts, suggesting that systematic observations with ALMA can outpace the historical detection rates of centimeter-band afterglows.

Future Directions

The paper lays out a promising path for future research, suggesting that continued advancements in observational strategies and technologies could further elucidate the physics of GRBs. Future investigations may refine the models of energy extraction and jet propagation in such highly dust-extincted environments, providing clues to the conditions that lead to SGRB production. This will likely involve collaboration between multiple facilities and advances in both observational techniques and theoretical modeling.

Moreover, with the increasing capabilities of instruments like ALMA and possibly other observatories such as the James Webb Space Telescope, subsequent studies could focus on obtaining greater time-resolution and spectral data. This will advance the understanding of the physical processes taking place in the extreme conditions near the progenitors of SGRBs.

Conclusion

The study of GRB 211106A stands out as an essential contribution to the astrophysical community's understanding of short-duration gamma-ray bursts and their afterglows. It marks a significant step forward in unraveling the complexities of these explosive events and highlights the synergy achievable between state-of-the-art observational facilities. The findings underline the evolving landscape of astrophysical research, wherein new tools and methodologies pave the way for deeper insights into the universe's most energetic phenomena.