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arxiv: 2606.27417 · v1 · pith:Q6UXV2QTnew · submitted 2026-06-25 · 🌌 astro-ph.HE

Discriminating blazar emission models with high-energy polarimetry: Multi-band predictions and detectability

Sara Capecchiacci , Ioannis Liodakis , Haocheng Zhang , Michela Negro , Jorge Otero-Santos This is my paper

Pith reviewed 2026-06-29 01:38 UTC · model grok-4.3

classification 🌌 astro-ph.HE
keywords blazarspolarimetryemission modelsleptonichadronichybridjetsgamma-rays
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The pith

Simultaneous multi-wavelength polarimetry distinguishes between leptonic, hadronic, and hybrid emission models in blazars by differences in polarization degree.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper predicts the polarization signatures expected from blazars in X-ray and gamma-ray bands under three competing models for their high-energy emission. Using a sample from the RoboPol program and spectral energy distribution fits, it calculates fluxes and polarization levels for various instruments. The central result is that observing in multiple bands at the same time allows the models to be told apart because each predicts a different degree of polarization. A sympathetic reader would care because this offers a practical way to determine the particle content and acceleration processes in relativistic jets without relying solely on spectral shapes. The work also outlines what sensitivity future polarimeters need to make such measurements on more sources.

Core claim

The paper establishes that for a statistically complete sample of gamma-ray blazars, the expected polarization degrees under leptonic, hadronic, and hybrid scenarios differ enough that simultaneous observations across soft X-ray to gamma-ray energies can discriminate between the models, after accounting for instrument minimum detectable polarization and detection probabilities in blind surveys. It derives specific sensitivity requirements for future missions to increase the number of detectable sources.

What carries the argument

Polarimetric signatures modeled for leptonic, hadronic, and hybrid emission scenarios using Bjet_MCMC spectral energy distribution fitting on a RoboPol sample of blazars.

Load-bearing premise

The polarization signatures predicted for the three emission scenarios remain distinguishable after propagation effects, instrument responses, and uncertainties from spectral energy distribution fits are taken into account.

What would settle it

A set of simultaneous multi-band polarization measurements on several blazars that show polarization degrees inconsistent with the differences predicted by the three models, or where all models predict overlapping values within measurement errors.

Figures

Figures reproduced from arXiv: 2606.27417 by Haocheng Zhang, Ioannis Liodakis, Jorge Otero-Santos, Michela Negro, Sara Capecchiacci.

Figure 1
Figure 1. Figure 1: BL Lac model used as a reference for all our LSP sources. [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Example of the shift applied to our SED fit (upper panel) [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Predicted flux and PD for the LSP sources in the 0.5- [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Kernel density distribution of the duty cycles for HSP [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: PD predictions in the 2-8 keV band and the 0.1-100 GeV [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
read the original abstract

Polarimetric properties of blazars provide key constraints on the acceleration mechanisms powering their relativistic jets, the high-energy emission processes involved, and the composition of the jet itself. We present a multi-band polarimetric study spanning from soft X-rays to gamma-ray energies, considering several bands for current and future missions (0.275 keV, 2-8 keV, 0.5-10 keV, 6-35 keV, 0.2-5 MeV, and 1-100 GeV). Our sample is drawn from the RoboPol monitoring programme, a statistically complete gamma-ray sample including low-, intermediate-, and high-synchrotron-peaked blazars. Using spectral energy distribution fitting performed with the Bjet_MCMC tool, we give predictions on the flux expected for each source in the selected energy bands. We model the polarimetric signatures under three competing emission scenarios: leptonic, hadronic, and hybrid. The detectability of each source was assessed by accounting for the instruments' minimum detectable polarisation (MDP) and by computing the probability of the detection in a blind survey. We show that simultaneous multi-wavelength observations can effectively discriminate between competing emission models due to the difference in the expected polarisation degree. Finally, we derive sensitivity requirements for future gamma-ray polarimetric missions aimed at increasing the number of detectable sources.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper uses Bjet_MCMC SED fits to a RoboPol gamma-ray blazar sample to predict multi-band fluxes (0.275 keV to 1-100 GeV) and then models the expected polarization degree under leptonic, hadronic, and hybrid emission scenarios. It claims that the resulting differences in polarization degree are large enough, relative to instrument MDP values, that simultaneous multi-wavelength polarimetry can discriminate among the three models, and it derives sensitivity requirements for future gamma-ray polarimeters to increase the number of detectable sources.

Significance. If the polarization differences survive propagation of SED-fit uncertainties and instrument effects, the work supplies concrete, observationally actionable predictions that link existing multi-wavelength monitoring to high-energy polarimetry as a model discriminator for blazar jets. The use of a statistically complete sample and explicit MDP-based detectability calculations are strengths that make the forecasts directly usable for proposal planning.

major comments (2)
  1. [polarization modeling and detectability sections] The central discrimination claim (abstract and final section) requires that polarization-degree separations between the three scenarios exceed both MDP and the spread induced by Bjet_MCMC posterior uncertainties. The manuscript computes polarization only at the best-fit SED parameters rather than drawing from the MCMC chains or marginalizing over magnetic-field and particle-distribution parameters; this leaves the claimed distinguishability untested against realistic uncertainties.
  2. [results table of polarization predictions] Table of predicted polarization degrees and detection probabilities (the table that lists per-source values for the three models) reports point estimates without error bars propagated from the SED fits. Because the separation between leptonic/hadronic/hybrid values is the load-bearing quantity for the discrimination argument, the absence of these uncertainties directly affects whether the differences remain statistically significant once fit errors are folded in.
minor comments (2)
  1. Notation for the three emission scenarios is introduced without a compact reference table; adding one would help readers track which model corresponds to which polarization curve in the figures.
  2. The MDP values adopted for each instrument band are stated without citing the exact references or formulas used to compute them; a short appendix or footnote with the MDP expressions would improve reproducibility.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful and constructive review. The major comments correctly identify that our polarization predictions rely on best-fit SED parameters and that the table lacks propagated uncertainties. We address each point below and will incorporate revisions to strengthen the discrimination analysis.

read point-by-point responses

  1. Referee: [polarization modeling and detectability sections] The central discrimination claim (abstract and final section) requires that polarization-degree separations between the three scenarios exceed both MDP and the spread induced by Bjet_MCMC posterior uncertainties. The manuscript computes polarization only at the best-fit SED parameters rather than drawing from the MCMC chains or marginalizing over magnetic-field and particle-distribution parameters; this leaves the claimed distinguishability untested against realistic uncertainties.

    Authors: We agree that the current analysis uses best-fit parameters and does not fully propagate posterior uncertainties from Bjet_MCMC. This choice was made for computational practicality across the full sample. To address the concern, we will add a new subsection that samples from the MCMC posteriors for a representative subset of sources (e.g., 10-15 objects spanning the synchrotron-peak classes). Polarization degrees will be recomputed for each draw under the three emission models, yielding uncertainty ranges. These ranges will be compared directly to the inter-model separations and MDP values to test whether the discrimination remains viable. The results and any necessary qualifications will be reported in the revised text. revision: yes

  2. Referee: [results table of polarization predictions] Table of predicted polarization degrees and detection probabilities (the table that lists per-source values for the three models) reports point estimates without error bars propagated from the SED fits. Because the separation between leptonic/hadronic/hybrid values is the load-bearing quantity for the discrimination argument, the absence of these uncertainties directly affects whether the differences remain statistically significant once fit errors are folded in.

    Authors: We concur that the absence of uncertainties in the table limits the strength of the discrimination argument. In the revision we will augment the table with uncertainty estimates on the polarization degrees. For the subset of sources where full posterior sampling is performed, we will report the standard deviation across samples. For the remaining sources we will derive representative uncertainty ranges by varying key parameters (magnetic field strength, particle indices) within their posterior widths and recomputing polarization. These additions will allow direct evaluation of whether model separations exceed the combined uncertainties and MDP. revision: yes

Circularity Check

0 steps flagged

No significant circularity; polarization modeling is independent of SED fits

full rationale

The derivation uses Bjet_MCMC SED fits solely to predict fluxes in selected bands, then applies separate modeling of polarimetric signatures under leptonic, hadronic, and hybrid scenarios. No quoted equations or steps in the abstract or description show polarization degrees reducing by construction to fitted parameters, self-citations, or ansatzes imported from prior work. The central claim of distinguishability rests on differences in modeled polarization degrees after accounting for MDP and survey probabilities, which are presented as independent outputs. This is the most common honest finding for papers whose core computation chain remains externally falsifiable.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review yields no explicit list of fitted parameters or axioms; the work implicitly relies on the validity of the three emission scenarios and the accuracy of prior SED fits without providing independent evidence for either.

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