Constraining Turbulence within Galactic Halos with FRB 20200120E

The source of interest is FRB 20200120E (M81) DM_obs ~ 88 pc cm^-3�

• FRB source exterior to host disk, but within CGM
• MW disk DM contribution from Galactic n_e models ~ 35-40 pc cm^-3
• CGMs (host+MW) DM contribution ~ 48-53 pc cm^-3

LoS to FRB samples plasmas in CGMs and MW disk

Sashabaw Niedbalski, James Cordes, Shami Chatterjee, Kenzie Nimmo, and Stella Ocker

Scintillometry 2024 – University of Central Florida, Florida Space Institute

~ 20 kpc

Analysis Goals

Main ideas:

1. Can we constrain radio wave scattering in CGMs?
2. What can this tell us about the turbulent structure of the CGM(s)?

Context:

• Signal model for FRB emission
• Dense vs sparse shot noise
• Analysis across different scales in time and frequency domains�
• Galactic n_e models
• Estimated scattering from MW disk
• MW disk DM contribution�
• Scattering estimation tools from data
• Scintillation bandwidth – frequency domain
• Skewness function – time domain�
• Cloudlet model (Q2)
• Relates scattering time to physical structure of scattering medium

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A Tale of Two Domains

Wide in time

Narrow in freq.

Narrow in time

Wide in freq.

Ave.

Noisy

Envelopes

Need temporal structure of ~1 ns to appear at ~ 1 GHz

Narrow substructure

Narrow substructure

conserved!

Single instance

Ensemble averages

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Scintillated Amplitude Modulated Polarized Shot Noise

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• Models observed radio emission

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• ~ns shots necessary to manifest at ~GHz frequencies

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• Scattering affects envelope shape
• ~ns structure must persist

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• PBF derived from power-law structure function
• Exponential PBF rarely applicable to physical environments

Signal Model for Scattered FRB Emission

Dense noise

Sparse shot noise

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The Skewness Function = Antisymmetric part of I²-I Cross-correlation

Symmetric pulse

PBF

‘Scattered’

Pulse

FWHM = W

PBF(𝜏) = e^-1

𝛿t_max = f(W,𝜏)

𝜍(𝛿t) = 0

𝛿t_max = ln(2) 𝜏

𝜏 ~ W

5

Dense shots

Sparse shots

1000 realizations

Unscattered

Scattered

Skewness Analysis of Dense & Sparse Shots

Gaussian envelope only

Gaussian envelope only

𝛅t_max depends on shot density!

ς(𝛅t) just as likely to be + or -

ς trends + with 𝛅t

6

Three High SNR Bursts from M81 FRB (Nimmo et al., 2022)

B2

8μs

B3

8μs

B4

8μs

125 kHz

125 kHz

125 kHz

Three bursts from late Feb. to early March, 2021 (Effelsberg)

Handful of scintles per burst

single shot?

intrinsic asymmetry?

~220 μs

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A Barely-resolved Shot in Burst 3

Nearly unresolved single shot?

B3

31.25 ns

B3

31.25 ns

~60 ns wide

But ~1 ns at ~1 GHz

SAMPSN regime!

Multiple bursts?

Reproduction of Fig. 1 (Nimmo et al., 2022)

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Frequency Domain Analysis Shows Scattering as MW Disk Scintillation

B2

125 kHz

B3

125 kHz

B4

125 kHz

ACFs calculated across frequency per time-sample

Then averaged across burst (profile SNR>3, slide 7)

Rough agreement with NE 2001

Reproduction of ExData Fig. 3 (Nimmo et al., 2022)

Peaks normalized to 1

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Skewness Analysis of Bursts

Generated by averaging ς(𝛅t) of 4 brightest frequency channels per burst

B2

B3

B4

Potential peaks at ~30 μs?

But not here!

No conclusive evidence for scattering from skewness analysis

𝜏 ~ 10 μs ≣ Δν ~ 10 kHz (2 orders of magnitude lower than probed by ACF)

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Turbulent Structure in the CGMs

In absence of conclusive evidence… constrain!

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Constraining 𝜏 still informs CGM structure

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• Combine with DM estimates of individual CGMs�
• Constrain fluctuation parameter�
• Provide insights into turbulent structure
• Filling factor
• Per-cloud variance
• Across-cloud variance

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Concluding Points - Current Status

• Scintillation bandwidth in rough agreement with estimated scattering in MW disk�
• Skewness analysis shows no conclusive evidence for presence of additional scattering�
• Constraining 𝜏_CGM will provide insights into turbulent structure

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