MWN Shock (Clustered Flares)

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Summary Table
Category Progenitor Type Energy Mechanism Emission Mechanism Counterparts References Brief Comments
LF Radio HF Radio Microwave Terahertz Optical/IR X-rays Gamma-rays Gravitational Waves Neutrinos
SNR (Magnetars) MWD Shock (Clustered Flares) Repeat Maser Synch. Yes Afterglow -- -- Maybe -- Low energy gamma-rays, sGRB if jet aligned Yes -- None

Definitions: LF Radio (3 MHz to 3 GHz); HF Radio (3 GHz to 30 GHz); Microwave (30 to 300 GHz)

Model Description

A hyper-active magnetar is proposed to produce multiple millisecond flares at different energies close to the magnetar. Such a magnetar is young with a hyper-energetic SN shell and an ultra-fast rotation period. The multiple flares interact to form a series of shocks before reaching the MWN. The FRBs arise from a synchrotron maser formed by gyrating particles at the shock front. Flares in this scenario arise from ambipolar diffusion in the magnetar core; a process which is then enhanced by the strong magnetic fields associated with the high magnetar spin. The flares will therefore be significantly more energetic than those of usual magnetars. Less active magnetars can emit FRBs by the same mechanism, but these will be non-repeating. Since repeating FRBs call for rarer magnetars, their event rate is expected to be lower.

The flares from young magnetars are consistent with the properties of the Lorimer burst. FRB 110523 is in-keeping with magnetar flares. Based on the observations of SGR 1806-20, the energy and number of particles N ~ 10^{52} in FRB 121102 are found to be consistent with magnetar ejecta, and thus it is arguably more likely to be powered by magnetic fields than rotational energy. The host galaxy of FRB 121102 supports the predicted long-duration $\gamma$-ray bursts (\acrshort{lgrb}) and hydrogen-poor \acrlong{slsn}(SLSNe I) formed in the birth of millisecond magnetars. The variable radio source associated with FRB 121102 is consistent with the giant flare theory, too: it may be emission directly from the MWN, the shock interaction between the flare and the MWN, or afterglow from an off-axis LGRB (such that only the afterglow is observed). Note that for the flare model to be consistent, this emission is expected to decay by $\sim 10 \%$ within the next few years. Constraints on the large, decreasing RM and required radio transparency for FRB 121102 is consistent with a young $\gtrsim 30-100$ year old magnetar with an expanding magnetized electron-ion nebula, akin to those associated with SLSNe. Such a nebula can also account for the observed properties of the variable counterpart associated with FRB 121102.

The flare theory has been met with various criticisms. The upper limit on RMs of giant flares is 4 orders of magnitude lower than that observed for FRB 121102. The magnetar wind therefore may not have a large enough magnetic field to account for the RM without a massive BH in its vicinity. Should this be the case, the chances of having a young magnetar near a massive BH is far lower than having a regular magnetar. The polarization percentage of pulsar emission has been observed to generally increase with decreasing observing frequency, with some pulsars also having a constant linear polarization percentage below some critical frequency. The variation of RM is also expected to increase as the distance to the pulsar increases. To be consistent with ~ 100 linear polarization and varying RM (~ 10 over 7 months) of FRB 121102, FRBs must originate some distance from the surface of magnetar. This presents a conflict: the enormous brightness temperatures of FRB emission requires strong magnetic fields close to the magnetar surface. The age of the magnetar must also fall within a ``Goldilocks Zone: for the appropriate energy budget, the magnetar cannot be too old, but to penetrate ejecta and avoid DM variation it cannot be too young.

Observational Constraints

From these models, high-energy GRBs, and possibly a coincident optical flash from the explosion are predicted. Flaring from the reverse shock could lead to additional (lower-energy) gamma-ray emission, and the interaction between the GRB and the ejecta could lead to broadband afterglow emission lasting days to weeks. The quasi-steady nebular emission of the MWN itself may be difficult to detect. X-rays are able to penetrate the ejecta, but only on 100 year timescales, and are therefore unlikely to be detected. A testable signature is that the persistent variable radio source associated with FRB 121102 is predicted to decay by ~ 10 within the next few years.

A magnetar that emits bursts at irregular intervals is a SGR. Although SGRs and FRBs share similar properties, such as: characteristic timescales, low duty factors and repetition, there is a crucial difference. SGRs are observed to be entirely thermal with frequencies above the X-ray range, whereas FRBs are observed in radio frequencies. Another possible inconsistency is that the Parkes Telescope failed to detect an FRB counterpart to the giant flare of SGR 1806-20: only one of the fifteen FRBs analyzed has a gamma-ray fluence ratio consistent with the SGR. The bursts of FRB 121102 have varied spectral characteristics, which suggests the observed fluence ratio may vary significantly between different magnetars and between bursts from the same magnetar. FRBs therefore may not be observable for all SGRs, which would explain the lack of a detectable radio counterpart in SGR 1806-20. Searches for GRBs associated with FRBs (repeating and non-repeating) with the Fermi Large Area Telescope (LAT) have not revealed any results, nor have placed any stringent constraints on the magnetar model.