The Broad-band Counterpart of the Short GRB 200522A at $z=0.5536$: A Luminous Kilonova or a Collimated Outflow with a Reverse Shock?

Abstract: We present the discovery of the radio afterglow and near-infrared (NIR) counterpart of the Swift short GRB 200522A, located at a small projected offset of $\approx 1$ kpc from the center of a young, star-forming host galaxy at $z=0.5536$. The radio and X-ray luminosities of the afterglow are consistent with those of on-axis cosmological short GRBs. The NIR counterpart, revealed by our HST observations at a rest-frame time of $\approx2.3$ days, has a luminosity of $\approx (1.3-1.7) \times 10^{42}$ erg s$^{-1}$. This is substantially lower than on-axis short GRB afterglow detections, but is a factor of $\approx 8$-$17$ more luminous than the kilonova of GW170817, and significantly more luminous than any kilonova candidate for which comparable observations exist. The combination of the counterpart's color ($i-y = -0.08\pm 0.21$; rest-frame) and luminosity cannot be explained by standard radioactive heating alone. We present two scenarios to interpret the broad-band behavior of GRB 200522A: a synchrotron forward shock with a luminous kilonova (potentially boosted by magnetar energy deposition), or forward and reverse shocks from a $\approx14^{\circ}$, relativistic ($\Gamma_0 \gtrsim 80$) jet. Models which include a combination of enhanced radioactive heating rates, low-lanthanide mass fractions, or additional sources of heating from late-time central engine activity may provide viable alternate explanations. If a stable magnetar was indeed produced in GRB 200522A, we predict that late-time radio emission will be detectable starting $\approx 0.3$-$6$ years after the burst for a deposited energy of $\approx 10^{53}$ erg. Counterparts of similar luminosity to GRB 200522A associated with gravitational wave events will be detectable with current optical searches to $\approx!250$ Mpc.

• The paper presents a detailed analysis of GRB 200522A’s multi-wavelength observations to discern the origin of its NIR excess.
• It evaluates two competing models: an exceptionally luminous kilonova requiring over 20 times the typical r-process heating and a collimated jet with reverse shock dynamics.
• The findings challenge conventional GRB models and emphasize the need for targeted follow-up observations with advanced telescopes to refine our understanding of compact object mergers.

An Analysis of the Broad-band Emission of Short GRB 200522A

The paper by Fong et al. presents a detailed investigation of the short gamma-ray burst (GRB) 200522A, characterized by diverse multi-wavelength observations. The researchers sought to understand whether the broad-band emission from this GRB arose from a luminous kilonova or a collimated outflow interacting with a reverse shock. GRB 200522A at a redshift of $z = 0.5536$ introduces compelling evidence for potential variances in astrophysical phenomena associated with GRBs and their afterglows. Here, I provide an expert overview of the findings and implications drawn from this study.

GRB 200522A offers a unique case study for examining the possible nature of short GRB counterparts. The significant detections in radio, X-ray, and near-infrared (NIR) wavelengths elucidate competing scenarios for the source of the GRB's broad-band emissions.

Two Possible Interpretations

1. Luminous Kilonova: The researchers propose that the NIR excess observed in GRB 200522A could signify a kilonova—a transient event resulting from a binary neutron star merger. The luminosity of the NIR counterpart is significantly brighter by factors of 8--17 than the kilonova associated with GW170817. The NIR emission of GRB 200522A necessitates over 20 times the r-process heating observed in typical kilonovae like that of GW170817. This finding implies a unique aspect to the radioactive heating or an overabundance of exotic ejecta properties.
2. Collimated Outflow with Reverse Shock: Alternatively, a second scenario suggests that the radio and NIR emission may result from forward and reverse shocks due to a collimated jet. This model requires a jet break to occur at about 3.5 days, implying a relatively broad jet opening angle of 14 degrees, distinctly wider than the average for short GRBs. Such a configuration could explain the observed optical and NIR flux density if the energy from the GRB's relativistic outflows dissipates in a structured manner.

Implications and Future Prospects

The discovery of such a bright NIR counterpart challenges preconceptions about short GRB emissions and encourages reassessment of the criteria used for identifying kilonova candidates. If the source indeed produced a stable magnetar, future observations could validate this scenario. The expectation of late-time synchrotron emission peaking between 0.3 to 6 years after the burst, if detected, would confirm the creation of the magnetar and bolster the kilonova hypothesis.

The paper situates GRB 200522A within the spectrum of cosmic phenomena associated with compact object mergers, and inspires speculation on a subclass of short GRBs potentially characterized by extreme energy output or rare progenitor characteristics. This research provides a foundational understanding that larger optical surveys, using instruments like the Vera Rubin Observatory, will be critical for identifying such high-luminosity events out to distances of approximately 600 Mpc.

As observational capabilities advance, resolving the exact mechanisms behind these remnants could elucidate the physics of neutron star mergers and refine estimates of their contribution to cosmic nucleosynthesis. Future studies will benefit from targeted follow-up of promising candidates with radio telescopes to search for synchrotron emission indicative of magnetar formation.

In summary, Fong et al.'s study of GRB 200522A broadens our interpretation of short GRB emissions and their progenitors, promoting the exploration of rare kilonova signatures and jet dynamics. The insights gleaned from this GRB hold implications for our understanding of cosmological transients and the evolution of elements in the universe.