toplogo
Sign In

Refractive Lensing of Fast Radio Bursts by Sub-Parsec Cloudlets in the Circumgalactic Medium


Core Concepts
Refractive lensing of fast radio bursts (FRBs) by sub-parsec, cool-gas cloudlets in the circumgalactic medium (CGM) can suppress observed scintillation, providing a novel method to constrain the properties of the CGM.
Abstract

The authors investigate the refractive lensing effects of ionized, cool (T ∼104 K) gas cloudlets in the circumgalactic medium (CGM) of galaxies. They show that if the CGM comprises a mist of sub-parsec cloudlets with column densities of order 1017 cm−2, as predicted by previous models, then FRBs whose sightlines pass within a virial radius of a CGM halo will be lensed into tens of refractive images with a ∼10 ms scattering timescale.

When these lensed images are formed, they will be resolved by scintillating screens in the Milky Way interstellar medium (ISM), suppressing the observed scintillation. The authors argue that positive detections of FRB scintillation may constrain the properties of these cool-gas cloudlets, with current scintillation observations weakly disfavoring the cloudlet model. They propose that sheet-like geometries for the cool gas in the CGM can reconcile quasar absorption measurements and the lack of lensing signals observed thus far.

The authors first provide a quantitative analysis of how an additional lens that resolves the scintillation screen can suppress the observed scintillation. They then apply this framework to model the refractive lensing properties of the CGM cloudlets, deriving scaling relations for the number of lensed images and the associated scattering timescales. Based on these results, the authors argue that if the CGM is ubiquitously populated by sub-parsec cloudlets, any FRB with a sightline passing within a galactic virial radius will not exhibit significant scintillation, or will do so very weakly.

edit_icon

Customize Summary

edit_icon

Rewrite with AI

edit_icon

Generate Citations

translate_icon

Translate Source

visual_icon

Generate MindMap

visit_icon

Visit Source

Stats
The typical scattering time induced by a single CGM cloudlet lens is τc l ∼35 μs. The typical number of refractive images formed by an ensemble of CGM cloudlets along a sightline is Nim - 1 ∼20. The typical scattering time induced by an ensemble of CGM cloudlets along a sightline is τl ∼10.5 ms.
Quotes
"If the CGM comprises a mist of sub-parsec cloudlets with column densities of order 1017 cm−2, then FRBs whose sightlines pass within a virial radius of a CGM halo will be lensed into tens of refractive images with a ∼10 ms scattering timescale." "Positive detections of FRB scintillation may constrain the properties of these cool-gas cloudlets, with current scintillation observation weakly disfavoring the cloudlet model."

Deeper Inquiries

How would the lensing properties of the CGM change if the cool gas is distributed in a sheet-like geometry rather than a mist of sub-parsec cloudlets?

If the cool gas in the circumgalactic medium (CGM) is distributed in a sheet-like geometry rather than as a mist of sub-parsec cloudlets, the lensing properties would be significantly altered. In a sheet-like configuration, the effective surface density of the gas would be more uniform across a larger area, leading to a different convergence profile compared to the discrete, localized cloudlets. Increased Convergence: A sheet-like distribution could lead to a higher effective convergence (κ) because the gas would cover a larger area with a consistent column density. This could enhance the lensing effect, potentially producing more refractive images of fast radio bursts (FRBs) than the cloudlet model, where the convergence is dependent on the number of intersected cloudlets. Reduced Scattering Time: The scattering time associated with a sheet-like geometry may be shorter than that of discrete cloudlets, as the coherent scattering from a continuous medium can lead to different interference patterns. This could result in a more complex modulation of the observed FRB signals, potentially complicating the interpretation of scintillation data. Lensing Geometry: The lensing geometry would also change; instead of multiple images being formed from individual cloudlets, a sheet could create a more uniform lensing effect, possibly leading to a smoother intensity modulation pattern. This could affect the frequency scale of scintillation, making it less pronounced compared to the cloudlet model. Implications for Observations: The presence of a sheet-like structure could lead to different observational signatures, such as a more consistent suppression of scintillation across various sightlines, which could be used to constrain the properties of the cool gas in the CGM more effectively.

What other observational signatures, beyond scintillation suppression, could be used to detect or constrain the properties of the cool gas in the CGM?

Beyond scintillation suppression, several other observational signatures could be utilized to detect or constrain the properties of cool gas in the CGM: Absorption Line Studies: Observations of absorption lines in the spectra of background quasars can provide insights into the presence and properties of cool gas in the CGM. The strength and profile of these absorption lines can indicate the temperature, density, and distribution of the gas. Emission Line Observations: Emission lines from ionized gas in the CGM can be studied using spectroscopy. The presence of specific emission lines can indicate the physical conditions of the gas, such as temperature and ionization state, which can help constrain models of the CGM. Gravitational Lensing Effects: The gravitational lensing of background sources by the CGM can provide information about the mass distribution of the gas. By analyzing the lensing effects on the brightness and shape of background galaxies, researchers can infer the density and distribution of the cool gas. X-ray Emission: The hot gas in the CGM can emit X-rays, and observations from X-ray telescopes can help characterize the thermal state of the gas. The presence of cooler gas can be inferred from the thermal properties of the surrounding hot gas. Cosmic Microwave Background (CMB) Observations: The interaction of the CMB with the CGM can provide indirect evidence of the gas's properties. For instance, the Sunyaev-Zel'dovich effect can be used to study the thermal electron pressure in the CGM. Radio Continuum Emission: The presence of cosmic rays and magnetic fields in the CGM can lead to radio continuum emission. Observations of this emission can provide additional constraints on the physical conditions and structure of the CGM.

How might the lensing of FRBs by the CGM affect our ability to use them as cosmological probes, for example in measuring the cosmic baryon distribution or the intergalactic medium?

The lensing of FRBs by the CGM could have significant implications for their use as cosmological probes: Modulation of Observed Signals: If FRBs are lensed by the CGM, the resulting refractive images could lead to complex modulation patterns in the observed signals. This could complicate the interpretation of FRB data, making it challenging to extract cosmological information without accounting for the lensing effects. Bias in Measurements: The presence of lensing could introduce biases in measurements of the cosmic baryon distribution. If the lensing effects are not properly modeled, it could lead to incorrect conclusions about the amount and distribution of baryonic matter in the universe. Impact on Distance Measurements: Lensing can affect the apparent brightness and timing of FRBs, which are critical for distance measurements. If the lensing properties of the CGM are not well understood, it could lead to systematic errors in estimating the distances to FRB sources, thereby affecting their utility in cosmological studies. Constraints on the Intergalactic Medium (IGM): The lensing of FRBs can provide insights into the structure and density of the IGM. By analyzing the lensing statistics of a large sample of FRBs, researchers could infer the distribution of baryons in the IGM, helping to refine models of cosmic structure formation. Potential for New Physics: If lensing effects reveal unexpected behaviors in FRB signals, it could indicate new physics or unknown properties of the CGM and IGM. This could open new avenues for research and lead to a deeper understanding of the universe's baryonic content. In summary, while the lensing of FRBs by the CGM presents challenges for their use as cosmological probes, it also offers opportunities to gain new insights into the properties of the CGM and the broader cosmic environment.
0
star