No Sign of a Magnetar Remnant Following the Kilonova-Producing Long GRB 211211A $\sim 1.7~$Years Later

TL;DR

GRB 211211A, a nearby long-duration burst with kilonova-like emission, offers a prime test for the magnetar-remnant scenario in NS mergers. The authors use deep VLA 6 GHz limits at ~0.5–1.7 years post-burst and revised light-curve models for magnetar-boosted kilonova and kilonova afterglow to bound the energy deposited into the ejecta, $E_{ m ej,max}$, and the afterglow parameter space ($\alpha$–$n_0$). They find $E_{ m ej,max} \\lesssim 4.4\times10^{52}$ erg with median afterglow/kilonova parameters, and $\\lesssim 6.1\times10^{51}$ erg under fiducial microphysics, effectively ruling out an indefinitely stable magnetar but not a temporarily stable one in all configurations considered. The results also constrain the potential kilonova afterglow by the $(\\alpha, n_0)$ relation, and demonstrate how future radio facilities (ngVLA, SKA, DSA-2000) will improve sensitivity to magnetar-boosted kilonova and distinguish NS-merger scenarios from collapsars for GRB 211211A and similar events.

Abstract

In addition to a $γ$-ray burst (GRB), the merger of two neutron stars may produce a temporarily or indefinitely stable neutron star remnant with a strong magnetic field (a "magnetar"). As this magnetar remnant spins down, it can deposit its rotational energy into the surrounding kilonova ejecta, producing synchrotron emission that peaks in the radio bands $\sim$months-years after the merger ("boosted kilonova"). The nearby ($z=0.0763$) long-duration GRB 211211A, which has an apparent kilonova counterpart and likely neutron star merger progenitor, may have produced such a remnant. We observed the location of GRB 211211A at 6 GHz with the NSF's Karl G. Jansky Very Large Array (VLA) spanning $\approx 0.54$-$1.7~$years after the burst. We do not detect any radio emission, placing strong limits on the energy deposited into the ejecta by any remnant to $\lesssim 4.4 \times 10^{52}~{\rm erg}$. Due to the proximity of the event, we are also able to place limits on a kilonova afterglow that did not receive any additional energy deposition, though it is possible such emission will be suppressed until $\sim 4~{\rm years}$ after the burst, when the kilonova is expected to overtake the forward shock of the GRB. Future observations with the VLA and next-generation radio facilities will be able to further constrain the magnetar-boosted kilonova and kilonova afterglow scenarios, as well as directly constrain models in the scenario that GRB 211211A was instead produced by a collapsar.

Figures (4)

• Figure 1: Radio luminosity $\nu L_{\nu}$ vs. rest frame time since gamma-ray trigger ($\delta t_{\rm rest})$. Orange triangles represent $3\sigma$ limits of our 6 GHz observations of GRB 211211A. Empty triangles represent previous 6 GHz limits of short GRBs 2016ApJ...831..141F2019ApJ...887..206Ksmf+2020ApJ...902...82S2021ApJ...923...38N2021MNRAS.500.1708R2021MNRAS.505L..41B2021ApJ...923...38N2021AA...650A.117N, whereas hatched triangles represent the deepest 3 GHz limits for GW 170817 2022ApJ...927L..17H2024GCN.36390....1E. Black lines represent single velocity shell light-curve models for a variety of ejecta energies ($E_{\rm ej} = 1,3,5,10 \times 10^{52}~$erg). We assume the afterglow-derived values for $p$, $n_0$, $\epsilon_{\rm e}$, $\epsilon_{\rm B}$, and the kilonova-derived value for $M_{\rm ej}$ (Table \ref{['tab:model']}). Given that GRB 211211A was relatively nearby, the non-detections place strong luminosity limits on any radio emission.
• Figure 2: Corner plot showing the one and two dimensional projections of the 5300 samples of the posterior distributions of the afterglow model presented in 2022Natur.612..223R. Points are colored by the maximum ejected energy ($E_{\rm ej, max}$) consistent with our radio limits for a given parameter sample. The posterior density functions for each parameter are split into 20 bins, and each bin is colored by the median $E_{\rm ej, max}$ for the samples that fall within a particular bin. The bottom row depicts the dependence on $E_{\rm ej, max}$ with each parameter. The solid (dashed) black line and red (black) point denote the solution assuming the afterglow (fiducial) parameters, with the color corresponding to $E_{\rm ej, max} \approx 4.4 \times 10^{52}~$erg ($\approx 6.1 \times 10^{51}~$erg).
• Figure 3: Left: The $\alpha$ vs. $\log_{10}(n_{0}/{\rm cm}^{-3})$ parameter space. This space is created by generating light curves for varying $\alpha$--$n_0$ pairs, while holding $E_{\rm ej} = 10^{51}~{\rm erg}$, $M_{\rm ej} = 0.01~M_\odot$, $p = 2.2$, $\epsilon_{\rm e} = 0.1$, $\epsilon_{\rm B} = 0.01$ fixed, and finding the flux density of the light curve at the time of the radio observations. Solid lines indicate the limits placed on this space for GW 170817 (yellow) and GRB 211211A (purple) based on current radio limits. The orange dashed line represents a merger at $\sim 200~$Mpc, assuming an observation at $\delta t = 2~$years (similar to our final observation of GRB 211211A at $\sim 1.7~$years) with a $3\sigma$ limit of $\lesssim 7~\mu$Jy at 6 GHz (comparable to the non-detections of 211211A and the sensitivity of the VLA for a 2 hour observation). The purple dotted line represents the limits that can be placed for an observation of 211211A at $\delta t =4~$years with a $3\sigma$ limit of $\lesssim 7~\mu$Jy at 6 GHz. Colored regions indicate the measured $n_{0}$ for 170817A (yellow) and 211211A (purple), where solid shaded regions are consistent with the current radio limits and hatched regions are ruled out by the current radio limits. Right: Light curves for the radio non-detections of GW 170817 and GRB 211211A (yellow and purple triangles, respectively). Also plotted are kilonova afterglow models for $\alpha = 2,4,9$ (solid, dashed, and dotted, respectively) and two circumburst densities ($10^{-3}~{\rm cm}^{-3}$ in orange and $10^{-1}~{\rm cm}^{-3}$ in red), roughly corresponding to the densities allowed by the $\alpha$-$n_0$ parameter space for GW 170817 and GRB 211211A.
• Figure 4: Expected radio light curves (assuming a singular bulk velocity) of two potential emission outcomes of 211211A: a collapsar scenario (purple, $E_{\rm ej} = 10^{52}$--$10^{53}~$erg, $M_{\rm ej} = 0.5$-$1.0~M_\odot$, e.g. 2023ApJ...947...55B) and a magnetar-boosted kilonova (orange, $E_{\rm ej} = 10^{52}$--$10^{53}~$erg, $M_{\rm ej} = 0.047~M_\odot$). Solid shaded regions indicate the range of light curves for the median afterglow parameters as measured by 2022Natur.612..223R. Also shown is two kilonova afterglow light curves (red, $E_{\rm ej} = 10^{51}~$erg, $M_{\rm ej} = 0.01 M_\odot$, e.g. 2019ApJ...886L..17H2022ApJ...927L..17H) assuming stratified ejecta with velocity distribution indices of $\alpha = 3$ (solid) and $\alpha = 9$ (dashed) and $n_0 = 0.1~{\rm cm}^{-3}$ (See Section \ref{['sec:KN_afterglow']} and \ref{['sec:future']}). Dark orange triangles represent the curent VLA upper limits for this burst. The solid black horizontal line indicates the $3\sigma$ sensitivity of the VLA for a 2 hour observation at 6 GHz. The vertical black dashed line indicate the expected time the ngVLA will come online, and the horizontal black dashed line indicates the expected $3\sigma$ sensitivity of the ngVLA at 8 GHz. We note that the SKA and DSA-2000 will also be able to probe the model light curves plotted here, however they will overall be less sensitive than the ngVLA.