MWN Shock (Clustered Flares)
| 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 | -- | -- | Possible bright optical | Yes but ~100 years later | Low energy gamma-rays, sGRB if jet aligned | Yes | -- | http://adsabs.harvard.edu/abs/2017ApJ...843L..26B | Polarization is predicted to be linear and constant through the bursts. |
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 and hydrogen-poor (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 ~ 10 within the next few years. 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.
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 persistent variable radio source associated with FRB 121102 is predicted to decay by ~ 10 within the next few years.
SGRs: A magnetar that emits bursts at irregular intervals is a soft gamma repeater (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.