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) MWN Shock (Clustered Flares) Repeat Maser Synch. Yes Afterglow -- -- Possible bright optical Yes but ~100 years later Low energy gamma-rays, sGRB if jet aligned Yes -- FRB 121102 may by unlikely in this scenario.

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 may 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 magnetar wind nebula (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 than non-repeating.

Observational Constraints

High-energy GRBs from the explosion are predicted. A bright optical flash may occur with some FRBs when the blast wave strikes the wind bubble in the tail of a previous flare. The reverse shock could lead to additional lower-energy gamma-ray emission. 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. Polarization is predicted to be linear and constant through the bursts. Expected to form in active star formation regions, often inside a visible SNR, however may occasionally form far star forming regions. The event rate of repeating FRBs is expected to be lower than non-repeating. The persistent variable radio source associated with FRB 121102 is predicted to decay by ~ 10 within the next few years.

Consistency with Observations

Arecibo telescope and the Robert C. Byrd GreenBank Telescope (GBT) have spent a total of ∼20hrs observing the remnants of six GRBs (5 long and 1 short) with evidence of having a central magnetar similar to a magnetar that is assumed to powers FRB 121102 (Yunpeng Men et al. 2019). No FRBs were observed from these remnants. The probability of non-detection of FRBs akin to FRB 121102 is 8.9×10−6, which challenges the young magnetar model. The theory is not ruled out though: recent localizations of FRB 180924 (Bannister et al. 2019) and FRB 190523 (Ravi et al. 2019) suggest that the host galaxy of FRB is abnormal for repeating FRBs.

The host galaxy of FRB 121102 supports the predicted long-duration gamma-ray bursts and hydrogen-poor (SLSNe I) formed in the birth of millisecond magnetars. The variable radio source 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). Constraints on the large, decreasing RM and required radio transparency for FRB 121102 is consistent with a young 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. Based on the observations of SGR 1806-20, the energy and number of particles in FRB 121102 are found to be consistent with magnetar ejecta.

Flares are consistent with the properties of the Lorimer burst and of FRB 110523.

Soft gamma repeaters (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.