Magnetospheric origin of a fast radio burst constrained using scintillation (2406.11053v1)

Abstract: Fast radio bursts (FRBs) are micro-to-millisecond duration radio transients that originate mostly from extragalactic distances. The emission mechanism responsible for these high luminosity, short duration transients remains debated. The models are broadly grouped into two classes: physical processes that occur within close proximity to a central engine; and central engines that release energy which moves to large radial distances and subsequently interacts with surrounding media producing radio waves. The expected emission region sizes are notably different between these two types of models. FRB emission size constraints can therefore be used to distinguish between these competing models and inform on the physics responsible. Here we present the measurement of two mutually coherent scintillation scales in the frequency spectrum of FRB 20221022A: one originating from a scattering screen located within the Milky Way, and the second originating from a scattering screen located within its host galaxy or local environment. We use the scattering media as an astrophysical lens to constrain the size of the lateral emission region, $R_{\star\mathrm{obs}} \lesssim 3\times10^{4}$ km. We find that this is inconsistent with the expected emission sizes for the large radial distance models, and is more naturally explained with an emission process that operates within or just beyond the magnetosphere of a central compact object. Recently, FRB 20221022A was found to exhibit an S-shaped polarisation angle swing, supporting a magnetospheric emission process. The scintillation results presented in this work independently support this conclusion, while highlighting scintillation as a useful tool in our understanding of FRB emission physics and progenitors.

• The paper employs scintillation measurements to constrain the emission region of FRB 20221022A to less than 3×10⁴ km, favoring a magnetospheric origin.
• It identifies two distinct decorrelation bandwidths (6 kHz and 124 kHz) that trace scattering screens in both our Galaxy and the FRB host environment.
• The detection of an S-shaped polarization swing further reinforces the magnetospheric model, countering emission theories reliant on extensive radial distances.

Magnetospheric Origin of a Fast Radio Burst Constrained Using Scintillation

The paper of fast radio bursts (FRBs) has significantly advanced with the detection and analysis of FRB 20221022A. This paper employs scintillation as a novel lens to constrain the emission region size of an FRB, offering insights into the physical mechanisms operating near its source, potentially a magnetosphere. Through the analysis of scintillation patterns, the authors provide strong constraints on competing FRB models, emphasizing the significance of magnetospheric processes.

Summary and Numerical Results

FRBs are brief, luminous radio transients with mysterious origins, often categorized into processes occurring close to or far from a central engine. The emitted radio waves encounter inhomogeneities in the interstellar medium, resulting in diffraction-based interference patterns known as scintillation. The crucial focus of this work involves leveraging these patterns, specifically by observing two distinct scintillation scales in the frequency spectrum of FRB 20221022A. Measurements reveal a scattering screen positioned within our Galaxy and another within the host galaxy or local environment of the FRB source, providing constraints on the lateral size of the emission region.

Key numerical results include:

• Two scintillation scales with decorrelation bandwidths of 6 kHz and 124 kHz were identified at 600 MHz.
• The constraint on the observed emission region size was derived to be less than approximately $3 \times 10^4$ km, too small to be consistent with emission models where energy is broadcast over large radial distances.

Implications for FRB Models

The aforementioned constraints point towards a magnetospheric origin for FRB 20221022A. Specifically, the small emission size suggests that the FRB's source is located within or near the magnetosphere of a central compact object. This finding undermines non-magnetospheric models such as the synchrotron maser shock model, which predicts emission regions at distances of $10^7$–$10^8$ km from the central source—distances inconsistent with the determined constraints.

Additionally, the observation of an S-shaped polarization angle swing during the burst strengthens the proposition of a magnetospheric origin. This polarization characteristic is frequently observed in pulsar emissions, indicative of a beam sweeping across the line of sight, consistent with the emission arising from within a magnetosphere.

Theoretical and Practical Significance

This work presents a methodological advancement in utilizing scintillation to probe emission properties more precisely. By constraining the size of the emission region, researchers can better distinguish between different theoretical models of FRB emissions, illuminating the physical conditions near the source. The established consistency of the emission constraint with known pulsar magnetosphere parameters could reshape our comprehension of the diverse FRB population, suggesting a unified underlying mechanism for some FRBs.

Future Directions

Future investigations should continue applying scintillation analysis on a broader sample of FRBs to evaluate the prevalence of magnetospheric characteristics. Moreover, high-resolution imaging and improved models of Galactic and host galaxy scattering screens can refine the constraints on emission sizes further. Long-term monitoring might also reveal repeat bursts from non-repeating FRBs like 20221022A, providing additional data on the variability and potential lingering activity of their progenitors.

This paper compellingly demonstrates the utility of scintillation studies in astrophysics, opening a novel avenue for discerning the enigmas surrounding FRBs and their loci within the vast cosmic landscape. The insights gained augment the theoretical framework addressing the enigmatic nature of these powerful, transient radio sources.