No Sign of a Magnetar Remnant Following the Kilonova-producing Long GRB 211211A ∼1.7 yr Later

In addition to a gamma-ray burst (GRB), the merger of two neutron stars (NSs) may produce a temporarily or indefinitely stable NS 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 ∼months to years after the merger (“boosted kilonova”). The nearby (z = 0.0763) long-duration GRB 211211A, which has an apparent kilonova counterpart and likely NS 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 ≈0.54–1.7 yr after the burst. We do not detect any radio emission, placing strong limits on the energy deposited into the ejecta by any remnant to ≲4.4 × 10⁵² 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 ∼4 yr 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.