A nearby long gamma-ray burst from a merger of compact objects

Abstract

Gamma-ray bursts (GRBs) are flashes of high-energy radiation arising from energetic cosmic explosions. Bursts of long (greater than two seconds) duration are produced by the core-collapse of massive stars¹, and those of short (less than two seconds) duration by the merger of compact objects, such as two neutron stars². A third class of events with hybrid high-energy properties was identified³, but never conclusively linked to a stellar progenitor. The lack of bright supernovae rules out typical core-collapse explosions^4-6, but their distance scales prevent sensitive searches for direct signatures of a progenitor system. Only tentative evidence for a kilonova has been presented^7,8. Here we report observations of the exceptionally bright GRB 211211A, which classify it as a hybrid event and constrain its distance scale to only 346 megaparsecs. Our measurements indicate that its lower-energy (from ultraviolet to near-infrared) counterpart is powered by a luminous (approximately 10⁴² erg per second) kilonova possibly formed in the ejecta of a compact object merger.

Fig. 1. The field of GRB 211211A.

a, False colour image combining optical (F814W; blue) and near-infrared (F160W; red and green) HST observations of GRB 211211A, carried out with the Wide Field Camera 3 (WFC3) camera in April 2022 (approximately 4 months after the burst). Two bright galaxies (G1 at z ≈ 0.0762, and G2 at z ≈ 0.4587) and several fainter ones are visible, but no source is detected at the location of GRB 211211A. The most probable host galaxy is G1, a low-mass, late-type galaxy. The projected physical offset between the burst and the centre of the galaxy is approximately 8 kpc, one of the largest ever measured for a long burst. b,c, The same field is shown in the UV w2 filter observed by Swift at 1 h after the burst (b), and in the optical I filter acquired by the 3.6-m DOT/4K × 4K CCD imager at 10 h after the burst (c). The solid lines show the slit position used for optical spectroscopy with Gemini/GMOS-S. The bright UV counterpart rules out a high-redshift origin, whereas its rapid reddening is consistent with the onset of a kilonova.

Fig. 2. Spectral evolution of the GRB afterglow and kilonova.

a, SED combining gamma-ray (diamonds), X-ray (circles) and UVOIR (squares) data at different times, as indicated by the labels. It shows that non-thermal radiation (solid line) dominates at early times and at higher energies. At lower energies, we identify the emergence of a thermal component peaking at blue wavelengths at 5 h, and rapidly shifting toward redder colours. Error bars represent 1σ; upper limits (downward triangles) are 3σ. For plotting purposes, each epoch was rescaled by the following factors (from top to bottom): 1, 1, 10^−0.8, 10^−1.6, 10^−2.4, 10^−3.2. b–d, The bolometric luminosity (b), temperature (c) and emitting radius (d) of the thermal component are similar to AT2017gfo (grey circles). Solid lines show the best-fit power-law models to the dataset. Dashed lines in d show the predicted radius for constant expansion velocities of 0.3c and 0.6c (c, speed of light in a vacuum).

Fig. 3. A kilonova in the long GRB 211211A.

Multicolour light curves in X-ray, UV (uvm2), optical (BRI) and infrared (K) are compared to models’ predictions of a kilonova (solid line) in addition to the non-thermal emission (dashed line). The shaded area shows the range of possible fluxes reproduced by kilonova simulations with wind mass M_w between 0.01M_☉ (lower bound) and 0.1M_☉ (upper bound), and dynamical ejecta mass M_d between 0.01M_☉ (lower bound) and 0.03M_☉ (upper bound). Error bars represent 1σ; upper limits (downward triangles) are 3σ. For plotting purposes, light curves were shifted by a constant factor, as indicated by the numbers in the legend.

Extended Data Fig. 1. Prompt gamma-ray phase of GRB 211211A.

a,b, The Swift background subtracted light curves of GRB 211211A are shown in two energy bands and compared with the time history of GRB 060614 (grey shaded area) rescaled at a distance of 346 Mpc. The time bin is 1 s. Error bars are 1σ. Both bursts display a first episode with hard spectrum (dominant in GRB 211211A), followed by a long-lasting tail with soft spectrum (dominant in GRB 060614). c, The inset magnifies the first 12 s, showing a weak precursor at T₀ preceding the main prompt event. The time bin is 16 ms.

Extended Data Fig. 2. GRB classification scheme.

a–d, The traditional GRB classification, based on the duration/hardness ratio diagram (a), is not unambiguous. Additional classifiers, used to break the degeneracy, are the lag–luminosity relation (b), the variability timescale (c) and the Amati relation (d). Long GRBs (circles) and short GRBs (squares) occupy different regions of these plots. Dashed lines show the boundaries of the long GRB regions (shaded areas). GRB 211211A (star symbol) belongs to the class of long soft bursts (a), but its other high-energy properties are common among short GRBs. Error bars represent 1σ; upper limits (arrows) are 3σ.

Extended Data Fig. 3. Constraints on the distance scale of GRB 211211A.

The near infrared brightness of GRB host galaxies (short GRBs: squares; long GRBs: circles) is reported as a function of their redshift. For comparison, a randomly selected sample of field galaxies from the CANDELS survey is shown in the background (octagons, with the symbol size proportional to the galaxy mass). The non-detection of an underlying galaxy in deep HST F160W imaging rules out most of the parameter space occupied by GRB hosts (hatched area). Additional constraints from the UV and X-ray afterglow rule out the case of a GRB in a distant (z > 1.5; shaded areas) faint galaxy. These observations support the physical association between GRB 211211A and the nearby galaxy at z = 0.0762 (star symbol). Upper limits (downward triangles) are 3σ.

Extended Data Fig. 4. No supernova associated with GRB 211211A.

Optical upper limits (representing 3σ CL) in the r-band (red) and i-band (blue) rule out the presence of any known supernova following GRB 211211A. Bright SNe associated with GRBs, such as SN 1998bw and SN 2006aj, would have been detected up to z = 0.8 (dashed line) and z = 0.65 (dotted line), respectively. Symbols show the peak magnitude of core-collapse supernovae from the ZTF Bright Transient Survey Sample rescaled at z = 0.3, demonstrating that most ordinary SNe were detectable up to this distance. At the distance z ≈ 0.076 of the putative host galaxy, the faint SN 2008ha (solid line) is also ruled out.

Extended Data Fig. 5. Host galaxy spectral energy distribution.

a–c, The model SED (blue line) and model photometry (blue squares) derived using Prospector is compared to the observed photometry (red circles) (a). The fit residuals (b) are displayed in the bottom panel. The inset (c) displays a Gemini/GMOS-S spectrum of the host galaxy in the vicinity of Hα, [N ii], and the [S ii] doublet, yielding z ≈ 0.0762. Error bars represent 1σ.

Extended Data Fig. 6. A comparison of models for luminous blue transients.

a, b, The UVOIR counterpart (squares) is compared with a set of models producing luminous (L_bol ≈ 10⁴² erg s⁻¹) and short-lived (<7 d) transients: a low-nickel SN from a fallback collapsar underpredicts the optical/near infrared emission (a); a magnetar-powered kilonova does not easily reproduce the timescales and colours (b). Upper limits (downward triangles) are 3σ.

Extended Data Fig. 7. GRB 211211A at high redshift.

Simulated Swift light curve in the 15–150 keV energy band (observer’s frame) of GRB 211211A assuming z = 1, an incident angle of 45°, and ~6,000 counts s⁻¹ background rate. Time bin is 1 s, error bars represent 1σ. An event similar to GRB 211211A would be detected up to redshift z ≈ 1 and beyond. At these distances, it would appear as a standard long GRB with a duration of ~20 s.