QUIJOTE scientific results - XII. Characterization of the polarized synchrotron emission at the power spectrum level in the QUIJOTE-MFI wide survey

arXiv:2610.31469 · astro-ph.CO · Submitted 2026-10-22 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: We're looking at the paper: "QUIJOTE scientific results - XII. Characterization of the polarized synchrotron emission at the power spectrum level in the QUIJOTE-MFI wide survey".

Jocelyn: Here's the gist of the paper. The gist The analysis presents an investigation into characterizing polarized synchrotron emission at the power-spectrum level using combined data from QUIJOTE-MFI, Planck, and WMAP to estimate contamination in future CMB B-mode measurements.

Vera: Data and Methodology The study utilizes polarization data from three experiments: QUIJOTE-MFI, WMAP, and Planck to constrain the synchrotron spectral energy distribution (SED) using a broad frequency coverage for a strong lever arm <ref:2610.31469#pg6>.

Jocelyn: The analysis focuses on the multipole range 30 ≤ l < 130, employing Gaussian smoothing with a full width at half maximum of 5◦ to construct maps and compute auto-spectra and cross-power spectra using the PolSpice algorithm <ref:2610.31469#pg3>.

Vera: The data combinations considered include WM+Pl, WM+Pl+QJ, and a NULL set representing instrumental noise realizations <ref:2610.31469#pg4>.

Title and authors: Vera: So, we're looking at this paper now, "QUIJOTE scientific results - XII," which is all about characterizing polarized synchrotron emission at the power spectrum level using data from QUIJOTE-MFI. Jocelyn, what's the main takeaway you want us to know right off the bat?

Jocelyn: Well, it’s essentially about estimating how much contamination we have in future measurements of CMB B-modes by figuring out this synchrotron stuff. It uses polarization data from QUIJOTE, WMAP, and Planck to constrain the synchrotron spectral energy distribution. It’s about knowing what we need to worry about when looking for that tiny primordial signal.

Subrahmanyan: That makes sense for structure formation because if we don't nail down the foregrounds, any B-mode signal we see might just be noise or something from our own galaxy. But specifically, what kind of constraints are they giving us on the synchrotron itself?

Vera: The paper uses a broad frequency coverage across those three experiments to constrain the synchrotron spectral energy distribution. They focus on the multipole range from thirty up to one hundred thirty using a Gaussian smoothing with a full width at half maximum of five degrees to build their maps and compute the auto-spectra and cross-power spectra.

Jocelyn: They’re combining data sets, you know? They look at combinations like WM plus Planck, or WM plus Planck plus QUIJOTE, and they also have a null set representing instrumental noise realizations to check everything.

Subrahmanyan: That's smart for isolating the foreground physics from the instrument artifacts. Now, when you model these things, how are they setting up the picture of what we're actually seeing? Are they assuming a lot about the dust or synchrotron components already?

Vera: They model them using a parametric framework that assumes only polarized synchrotron and thermal dust emissions contribute to this polarization. They explore four composite models based on whether each foreground is described by one or two components, denoted as s p d q, where p is one or two for synchrotron and q is one or two for dust.

Jocelyn: And the model they actually use for their final results is called the salpha d1 model. That specific model assumes a pure power law for the synchrotron with a spatially constant spectral index and a single modified blackbody for thermal dust emission.

Subrahmanyan: A single modified blackbody for dust is a big assumption, but if that model fits the data well, it simplifies things considerably when we try to connect this back to the larger cosmological picture of how structure forms. What did they find about the synchrotron properties themselves?

Title and authors: Vera: The analysis finds that both the E-E and B-B power spectra are well described by this simple power law, specifically the salpha d1 model, with no strong evidence for needing extra parameters to capture deviations from this simple behavior.

Jocelyn: They found some specific best-fit slopes for you to look at: alpha EE is negative three point two four plus or minus zero point one two, and alpha BB is negative three point eight one plus or minus zero point three four for the range of b greater than twenty degrees <ref:2610.31469#pg1>.

Subrahmanyan: That hints at a steeper B-B spectrum relative to the E-E spectrum at about a one point six sigma level, which is interesting because it suggests some physical process is making the B-mode signal stronger than we expected from just the simple power law.

Vera: They also found the synchrotron spectral indices themselves: beta EE is negative three point two six plus or minus zero point zero five, and beta BB is negative three point two five plus or minus zero point one0 for that same cut, which aligns with earlier real-space analyses we’ve done.

Jocelyn: And the synchrotron B to E power ratio over that multipole range is about zero point two, which they say is lower than typical values for thermal dust and consistent with previous measurements.

Subrahmanyan: That lower ratio compared to dust is a key piece of information because it means the synchrotron emission isn't overwhelmingly masking what we might be looking for in the CMB B-modes. So, how much contamination are we actually talking about?

Vera: Using that salpha d1 model and those best-fit results, they estimate that the synchrotron emission is equivalent to r approximately three times ten to the minus three at ninety gigahertz and r approximately two times ten to the minus four at one hundred fifty gigahertz for the angular scales relevant for primordial signal detection.

Jocelyn: That translates into a forecast that any CMB B-mode experiment targeting tensor-to-scalar ratios below one point two cannot ignore the contribution of synchrotron emission in sky masks. So, it puts a floor on how good we need to be at removing this foreground.

Subrahmanyan: That means for anyone looking at primordial B-modes, they have to plan for this level of subtraction because that synchrotron contribution is still significant even when you are aiming for very low tensor-to-scalar ratios. What about spatial variations in these properties? Did they find anything unusual?

Title and authors: Vera: They found no evidence for spatial variations of the spectral index beta s across different Galactic latitude cuts, within the uncertainties they have right now. However, adding QUIJOTE-MFI data suggests a slight steepening of that B-B spectrum relative to E-E, although that significance is only about two sigma.

Jocelyn: So, they're finding hints of some variation there based on the newest data combination, but it’s not quite strong enough yet to be a solid claim. What about the dust and synchrotron correlation? Did those numbers look weird?

Subrahmanyan: The cross-correlation for both E- and B-modes falls in the range of zero point one to zero point two five over the multipole range from thirty up to three hundred which they say is consistent with what Krachmalnicoff et al. did back in two thousand eighteen <ref:2610.31469#pg1>.

Vera: Those correlation estimates match results reported by Krachmalnicoff et al., who derived theirs from S-PASS and Planck three hundred fifty-three gigahertz polarization maps. They say the small differences can be explained by the variability of that dust synchrotron correlation across the sky, so it’s not a huge surprise.

Jocelyn: So, to wrap up this paper, what's the big picture for us? What does this tell someone who only listens to this show about what they need to keep an eye on?

Vera: The main conclusion is that the inclusion of QUIJOTE-MFI data yields a marginally steeper value of beta s, though with low statistical significance, around two point four sigma for the b greater than ten degrees case. Overall, it confirms that synchrotron B to E ratios are lower than those measured for thermal dust, which are typically around zero point five.

Jocelyn: So the final word is that any analysis targeting primordial B-mode signals must include a mitigation strategy to account for this synchrotron emission in the sky masks. They’re adopting the salpha d1 model as the baseline, with srun set to zero in their main text, which keeps things simple.

Subrahmanyan: I think the most important thing here is that we have a better handle on these foregrounds when we look for B-modes. If we can reliably subtract this synchrotron component, it means any signal left over is much more likely to be what we're actually looking for in cosmology, like tensor modes from inflation.

Vera: Exactly. So the paper "QUIJOTE scientific results - XII" shows us that even when looking at these complex foregrounds, the synchrotron contribution is relatively low compared to dust, and that we have a baseline model to work from. We'll keep an eye on those hints of spatial variation as more data comes in.

The paper's summary: Vera: So, we’ve looked at how they used QUIJOTE, WMAP, and Planck data to check the synchrotron stuff across all those frequencies for this paper now.

Jocelyn: It boils down to figuring out how much of that noise is actually from the magnetic fields and electrons in our Galaxy, which can mess up our search for those faint primordial B-modes.

Subrahmanyan: From a theory standpoint, this is crucial because if we can’t model the synchrotron perfectly, any tiny signal we see could just be misinterpreted structure formation effects instead of something from inflation.

Vera: The main finding here is that they used this specific salpha d1 model to describe the power spectra of both E-modes and B-modes, and it fit the data really well.

Jocelyn: And what they found in those fits are specific values for those slopes, like alpha EE being negative three point two four plus or minus zero point one two, and alpha BB being negative three point eight one plus or minus zero point three four for angles greater than twenty degrees.

Subrahmanyan: That difference between the E-mode and B-mode slopes is what they say hints at a steeper B-B spectrum relative to the E-E spectrum at about a one point six sigma level, which is something we need to keep an eye on.

Vera: They also found that the synchrotron spectral indices themselves, beta EE and beta BB, are negative three point two six plus or minus zero point zero five and negative three point two five plus or minus zero point one0 for that same cut, which matches what they saw in earlier real-space analyses.

Jocelyn: And the B to E power ratio for synchrotron over those multipoles is about zero point two, which is lower than what you typically see from thermal dust emission, which is often around zero point five.

Subrahmanyan: That lower ratio between synchrotron and dust is important because it means the synchrotron isn't completely overwhelming what we’re trying to measure in the CMB polarization.

Vera: So, when they put all that together with their model, they estimate the total synchrotron emission to be equivalent to r approximately three times ten to the minus three at ninety gigahertz and two times ten to the minus four at one fifty gigahertz for those relevant angular scales.

Jocelyn: That translates directly into a forecast that any CMB B-mode experiment aiming for tensor-to-scalar ratios below one point two cannot ignore the contribution of synchrotron emission in sky masks.

Subrahmanyan: That means you have to plan for this level of subtraction because even at very low tensor-to-scalar ratios, you still need a strategy to account for this synchrotron noise.

Vera: They also found no evidence for spatial variations in the spectral index beta s across different Galactic latitude cuts, within the uncertainties they currently have.

Jocelyn: But they did find that adding those QUIJOTE-MFI data suggests a slight steepening of that B-B spectrum relative to E-E, though that significance is only about two sigma right now.

Subrahmanyan: So the paper confirms the baseline model works well, but it points toward a subtle variation in the synchrotron properties as we get more data.

Vera: And they noted that a more detailed analysis using smaller sky patches is what they plan to do next to really nail down those spatial variations.

Jocelyn: That leads us perfectly into how this fits with the other papers we’ve been looking at, like the ones on dust launch in jets or the missing ultra-faint satellites.

The paper's improvements: Tom: So we’re moving on to what the paper suggests we do next, after they finished their main analysis of the synchrotron properties using those WMAP and Planck data.

Vera: The improvements they suggest focus on refining how they handle that spatial information across different regions of the sky.

Jocelyn: They specifically mention that a more detailed analysis based on smaller sky patches is what’s coming next to really nail down those subtle hints of variation in the spectral index beta s.

Subrahmanyan: From my side, that makes sense because if there are spatial variations, it means the physics creating that synchrotron isn't uniform across our Galaxy, which could be tied to local magnetic field structures or localized star formation rates.

Vera: Exactly. They're saying the current uncertainties aren't quite tight enough to see those spatial shifts in beta s across different Galactic latitude cuts yet.

Jocelyn: They are using the QUIJOTE-MFI data specifically to test this, and while they found a slight steepening of the B-B spectrum relative to E-E, that significance is only about two sigma right now.

Subrahmanyan: Two sigma is a low bar for confirmation in cosmology, so we need more leverage on those measurements before we start building new theoretical models around those variations.

Vera: The authors are flagging that they still need improved sensitivity at the MFI frequencies or adding even lower frequency data to truly clarify that trend.

Jocelyn: That means the next step for radio astronomy is getting better sensitivity in those specific frequency bands to see if we can confirm that two sigma signal.

Subrahmanyan: If those variations are real, it could impact how we model the overall Galactic foregrounds, which is a prerequisite for any clean measurement of primordial B-modes.

Vera: The paper also sets up a path forward by deferring the very detailed analysis—the one with smaller sky patches—to future work.

Jocelyn: So they’re basically saying this paper gives us the baseline, but we need more dedicated effort to push past those current uncertainties and confirm if that B-B spectrum steepening is real.

Subrahmanyan: I think the bigger implication here is that pinning down these synchrotron parameters is a necessary first step before we can even seriously constrain those tensor modes from inflation.

Conclusion: Tom: So we’re wrapping up this discussion on "QUIJOTE scientific results - XII. Characterization of the polarized synchrotron emission at the power spectrum level in the QUIJOTE-MFI wide survey".

Vera: To recap, this paper used combined data from WMAP, Planck, and QUIJOTE to constrain how much synchrotron contamination we have in future CMB B-mode measurements by modeling it with a simple salpha d1 model.

Jocelyn: It showed that the synchrotron B to E power ratio is lower than thermal dust, which is good news for our search for primordial signals.

Subrahmanyan: If we can keep this level of foreground modeling consistent across all experiments, it gives us a much clearer path to isolating the physics of cosmic inflation.

Vera: I think the main implication is that any CMB B-mode experiment aiming for tensor-to-scalar ratios below one point two really has to build a mitigation strategy specifically designed for this synchrotron emission in the sky masks.

Jocelyn: It puts a real floor on how good we need to be at removing this foreground, which is something every next generation of CMB experiments has to plan for.

Subrahmanyan: What this study confirms is that the baseline model, srun set to zero, is robust for now, but we still have those hints of spatial variation in beta s that need more data to settle.

Vera: That’s right, they are deferring the more detailed analysis on smaller sky patches because they want to be sure about those variations before committing to a full modeling effort there.

Jocelyn: So it's a snapshot of what we know now, but with clear instructions on where the next big observational push needs to go.

Subrahmanyan: I just hope those future observations confirm if that two sigma deviation in the B-B spectrum is real, because if it is, it changes how we understand magnetic field evolution in our Galaxy.

Vera: Yeah, that’s what we’re looking for—a confirmed spatial variation in synchrotron properties.

Jocelyn: Next time, we'll be looking at the papers on dust launch in jets and seeing how those processes feed into the overall foreground picture.

F. Vansyngel, J. A. Rubiño-Martín, P. de Ferra, R. B. Barreiro, R. T.Íova-Santos, B.Α Casaponsa, M.Οnandez-Torreiro, D. Adak, K. Aryan, M.Κshdown, J. M. Casas, F. J. Casas, R Cepeda-Arroita, A Fasano, D Herranz, E Martínez-González, F Poidevin, R Rebolo, Β Ruiz-Granados, B Ruiz-Granados, D Tramonte, P Vielva, R A Watson

Instituto de Astrofisica de Canarias · Departamento de Astrofísica, Universidad de La Laguna · Instituto de Fí Fisica Cantabria (IFCA) · Astrophysics Group, Cavendish Laboratory, University of Cambridge · Kavli Institute for Cosmology, University of Cambridge · Universidad de Oviedo Departamento de Fí Fisica

astro-ph.CO

Submitted: 2026-10-22

Updated: 2026-10-22

Comments: 18 pages, 12 figures. Accepted for publication in MNRAS

Code: https://github.com/mpeel/fastcc

License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/

The gist: The gist The analysis presents an investigation into characterizing polarized synchrotron emission at the power-spectrum level using combined data from QUIJOTE-MFI, Planck, and WMAP to estimate

Key concepts

Synchrotron Spectral Energy Distribution (SED): This describes how the intensity of synchrotron radiation changes across different radio frequencies. The researchers used this to model the polarized emission from electrons spiraling in magnetic fields, using data from QUIJOTE-MFI, Planck, and WMAP to constrain its shape.
Power Spectrum Level: The power spectrum is a mathematical tool used to describe the statistical distribution of fluctuations (like polarization) across different angular scales. By analyzing the E-E and B-B power spectra at specific multipole ranges (30 ≤ ℓ < 130), researchers can characterize how much signal is present at various sizes.
Contamination Forecast: This involves estimating the expected level of synchrotron noise that will interfere with a search for primordial CMB B-modes. The analysis estimated this contamination to be equivalent to a specific radio intensity (r) at 90 GHz and 150 GHz, which helps determine the required sensitivity for future experiments.
Salpha d1 Model: This is the specific mathematical model used to describe the polarized foregrounds. It assumes a pure power law for synchrotron emission with a constant spectral index and a single modified blackbody component for thermal dust emission, providing a simple framework to fit the observed polarization data.

Summary

The gist The analysis presents an investigation into characterizing polarized synchrotron emission at the power-spectrum level using combined data from QUIJOTE-MFI, Planck, and WMAP to estimate contamination in future CMB B-mode measurements.

Data and Methodology

The study utilizes polarization data from three experiments: QUIJOTE-MFI, WMAP, and Planck to constrain the synchrotron spectral energy distribution (SED) using a broad frequency coverage for a strong lever arm <ref:2610.31469#pg6>. The analysis focuses on the multipole range 30 ≤ l < 130, employing Gaussian smoothing with a full width at half maximum of 5◦ to construct maps and compute auto-spectra and cross-power spectra using the PolSpice algorithm <ref:2610.31469#pg3>. The data combinations considered include WM+Pl, WM+Pl+QJ, and a NULL set representing instrumental noise realizations <ref:2610.31469#pg4>.

Foreground Modeling

The foregrounds are modeled using a parametric framework assuming only polarized synchrotron and thermal dust emissions contribute to the polarization <ref:2610.31469#pg6>. The methodology explores four composite models based on whether each foreground is described by one or two components, denoted as s p d q where p=1, 2 and q=1, 2 <ref:2610.31469#pg6>. A specific model used for the final results is the salpha d1 model <ref:2610.31469#pg6>. This model assumes a pure power law for synchrotron with a spatially constant spectral index and a single modified blackbody for thermal dust emission <ref:2610.31469#pg6>.

Results on Synchrotron Properties

The analysis finds that the E-E and B-B power spectra are well described by a simple power-law model (salpha d1), with no strong evidence for additional parameters required to capture deviations from this simple behavior <ref:2610.31469#pg6>. The best-fit slopes are found to be αEE = −3.24 ± 0.12 and αBB = −3.81 ± 0.34 for b > 20◦, hinting toward a steepening of the BB spectrum relative to EE at the ∼ 1.6sigma level <ref:2610.31469#pg6>. The synchrotron spectral indices are found to be βEEs = −3.26 ± 0.05 and βBBs = −3.25 ± 0.10 for the same cut, in agreement with earlier real-space analyses <ref:2610.31469#pg6>. The synchrotron B/E power ratio is approximately 0.2 over the considered multipole range, lower than typical values for thermal dust and consistent with previous measurements <ref:2610.31469#pg6>.

Contamination Forecast

Using the salpha d1 model and best-fit results, synchrotron emission is estimated to be equivalent to r ∼ 3 × 10−3 at 90 GHz and to r ∼ 2 × 10−4 at 150 GHz at the angular scales relevant for primordial signal detection <ref:2610.31469#pg6>. This indicates that any CMB B-mode experiment targeting tensor-to-scalar ratios below r = 10−2 cannot neglect the contribution of synchrotron emission in the sky masks <ref:2610.31469#pg6>.

Spatial Variations and Future Work

The analysis finds no evidence for spatial variations of the spectral index βs across different Galactic latitude cuts, within current uncertainties <ref:2610.31469#pg6>. The inclusion of QUIJOTE-MFI data suggests a steepening of the BB spectrum relative to EE, although the significance remains at the ∼ 2sigma level <ref:2610.31469#pg6>. Future analyses with improved sensitivity at MFI frequencies or adding lower frequency data will help clarify this trend <ref:2610.31469#pg6>. The paper notes that a more detailed analysis, based on smaller sky patches, is deferred to future work <ref:2610.31469#pg6>.

Dust-Synchrotron Correlation

The cross-correlation for both E- and B-modes lies in the range 0.1–0.25 over 30 ≤ l ≤ 300, consistent with Krachmalnicoff et al. (2018) <ref:2610.31469#pg6>. The correlation estimates are consistent with results reported by Krachmalnicoff et al. (2018), derived from S-PASS and Planck 353 GHz polarization maps <ref:2610.31469#pg6>. The small differences can be explained by the variability of the dustsynchrotron correlation across the sky <ref:2610.31469#pg6>.

Conclusion

The inclusion of QUIJOTE-MFI data yields a marginally steeper value of βs, although with low statistical significance (at the ∼ 2.4sigma level for the b > 10◦ case) <ref:2610.31469#pg6>. The overall results indicate that synchrotron B/E ratios are lower than those measured for thermal dust, which are typically of the order of 0.5 <ref:2610.31469#pg6>. Any analysis targeting primordial B-mode signals must include a mitigation strategy to account for synchrotron emission <ref:2610.31469#pg6>. The paper concludes by adopting srun = 0 as the baseline model in the main text <ref:2610.31469#pg6>.

Improvements for AI systems

Enhanced Foreground Contamination Estimation: The improved AI system can accurately estimate synchrotron contamination in future CMB B-mode measurements by calculating an average power in considered sky masks equivalent to r approximately 0.003 at 90 GHz and r approximately 0.0002 at 150 GHz for relevant angular scales, as stated in the abstract.
Spectral Index Constraint Refinement: The system can determine that the best-fit synchrotron spectral index is found to be beta s = -3.26 +/- 0.05 and beta BB s = -3.25 +/- 0.10 for the b > 20 degree case, which are consistent with previous analyses based on real-space statistics using QUIJOTE-MFI data in combination with Planck and WMAP (de la Hoz et al. 2023; Adak et al. 2026).
Model Selection Robustness: The AI can perform model selection using Bayes factors to determine that the salpha d1 model is preferred over the s1d1 model, indicating that only one component for the synchrotron and the dust is necessary to explain the data in terms of power spectrum level.
Synchrotron B-to-E Ratio Calibration: The system can derive a synchrotron B-to-E power spectrum ratio of approximately 0.2, consistent with previous analyses based on Planck and WMAP data (Martire et al. 2022) and on QUIJOTE–MFI observations (Rubiño-Martín et al. 2023), which are significantly lower than those reported for thermal dust emission, typically of the order of 0.5.
Contamination Forecasting: The improved system can forecast that synchrotron emission is, on average, equivalent to r approximately 3 x 10-3 at 90 GHz and r approximately 2 x 10-4 at 150 GHz for angular scales relevant for primordial signal detection.