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

This paper presents a multi-band polarimetric study of a statistically complete blazar sample to demonstrate how simultaneous observations from soft X-rays to gamma-rays can effectively discriminate between leptonic, hadronic, and hybrid emission models based on predicted polarization signatures, while also establishing sensitivity requirements for future gamma-ray polarimetric missions.

Original authors: Sara Capecchiacci, Ioannis Liodakis, Haocheng Zhang, Michela Negro, Jorge Otero-Santos

Published 2026-06-29

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CC BY 4.0

Original authors: Sara Capecchiacci, Ioannis Liodakis, Haocheng Zhang, Michela Negro, Jorge Otero-Santos

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe is filled with giant cosmic lighthouses called blazars. These aren't just ordinary lights; they are supermassive black holes shooting out powerful beams of energy (jets) directly at Earth. For decades, astronomers have been trying to figure out exactly how these beams are powered. Is the fuel made of lightweight particles like electrons (a "leptonic" engine), heavy particles like protons (a "hadronic" engine), or a mix of both (a "hybrid" engine)?

The problem is that if you just look at how bright the light is (its intensity), all three engines look almost identical. It's like trying to tell the difference between a car, a truck, and a motorcycle just by looking at their headlights from a mile away; they all just look like bright white dots.

The New Tool: Cosmic Polarized Sunglasses
This paper proposes a new way to solve the mystery: looking at the polarization of the light. Think of polarization as the "direction" in which the light waves wiggle.

If the light waves wiggle in a neat, organized line, it's highly polarized.
If they wiggle in all chaotic directions, it's unpolarized.

The authors argue that the three different engine types produce light with very different "wiggles."

The Electron Engine (Leptonic): Predicts that the high-energy light will be messy and chaotic (low polarization).
The Proton Engine (Hadronic): Predicts the light will stay neat and organized (high polarization), just like the lower-energy light.
The Mixed Engine (Hybrid): Somewhere in between.

The Study: A Cosmic Weather Report
The researchers took a sample of 62 blazars (a mix of different types) and used a computer model to predict what these "wiggles" should look like across the entire electromagnetic spectrum, from soft X-rays to high-energy gamma rays. They acted like meteorologists, creating a forecast for what future telescopes should see.

They looked at several "cameras" (telescopes), some already flying (like IXPE) and some planned for the future (like eXTP, EXPO, and COSI).

Key Findings

Timing is Everything: Blazars are chaotic; their light direction changes rapidly. If you stare at them for too long, the "wiggles" get scrambled, and the signal disappears. The team calculated that to get a clear picture, telescopes need to take snapshots in short bursts (about 2 to 4 weeks) before the direction changes too much.
The "Sweet Spot" for Detection:
- StokeSAT (a future soft X-ray telescope) is predicted to be the most successful "detective," able to see the polarization of almost all the blazars in their sample.
- IXPE (currently flying) can see about half of the brightest blazars, but only if they are powered by protons or a mix. If they are powered purely by electrons, IXPE might miss them.
- Gamma-ray telescopes (like COSI) are currently too weak to see polarization in most of these sources, except for the very brightest ones.
The Smoking Gun: The most exciting part is that if we look at a blazar in both X-rays and Gamma-rays at the same time, we can tell the engines apart.
- If the Gamma-ray light is highly polarized (wiggling neatly), we know it's not the electron engine.
- If the Gamma-ray light is weak or messy, it points toward the electron engine.
- This simultaneous "double-check" is the only way to definitively rule out the wrong theories.

What Needs to Happen Next?
The paper concludes that to solve this mystery, we need better cameras. Specifically, future telescopes looking at high-energy gamma rays need to be about 10 to 100 times more sensitive than they are today. If we build instruments that can detect faint signals (down to a specific sensitivity level), we will finally be able to see the "wiggles" clearly enough to know exactly what kind of engine is driving these cosmic lighthouses.

In short: We have a map of what to expect, and we know that if we build better "polarized sunglasses" for our telescopes, we can finally figure out how the universe's most powerful engines work.

Technical Summary: Discriminating Blazar Emission Models with High-Energy Polarimetry

Problem Statement
The origin of high-energy emission in blazars remains a subject of debate, with competing scenarios proposing leptonic (inverse Compton scattering), hadronic (proton synchrotron and cascades), or hybrid mechanisms. While spectral energy distribution (SED) fitting often yields indistinguishable results for these models, they predict fundamentally different polarization degrees (PD) at high energies. Leptonic models generally predict low PD in the high-energy band due to depolarization during inverse Compton scattering, whereas hadronic models can maintain or even increase PD. Current and upcoming polarimetric missions (e.g., IXPE, eXTP, COSI) offer the potential to distinguish these scenarios, but there is a lack of comprehensive predictions regarding the detectability of a statistically complete blazar sample across multiple energy bands and under different emission models.

Methodology
The authors constructed a predictive framework applied to a statistically complete $\gamma$ -ray sample of 62 blazars (36 Low-Synchrotron-Peaked [LSP], 13 Intermediate-Synchrotron-Peaked [ISP], and 13 High-Synchrotron-Peaked [HSP]) drawn from the RoboPol monitoring programme.

SED Modeling and Flux Prediction:
- Multi-wavelength flux data were obtained from the Markarian Multiwavelength Data Center (MMDC) and the Space Science Data Center (SSDC).
- SEDs were fitted using the Bjet_MCMC tool with a one-zone Synchrotron Self-Compton (SSC) model.
- To align with current observations, the fitted SEDs underwent multiplicative scaling: a horizontal shift to match the synchrotron peak frequency from the Fermi 4LAC-DR3 catalogue and a vertical shift to match X-ray fluxes from the CAZ catalogue.
- Monochromatic fluxes were extracted at central frequencies of selected bands (0.275 keV, 2–8 keV, 0.5–10 keV, 6–35 keV, 0.2–5 MeV, and 1–100 GeV) and integrated over the full bands.
Polarization Modeling:
- Three emission scenarios were modeled for each source class: purely leptonic, purely hadronic, and hybrid.
- Reference templates were derived from specific sources: BL Lac (LSP), Mrk 501 (HSP), and BZBJ0211+1051 (ISP).
- Models incorporated semi-analytical multi-zone depolarization effects to account for inhomogeneous magnetic fields.
- Polarization models were scaled to match the median PD measured by RoboPol in the optical R-band. The authors note that while this scaling assumes a preserved PD spectral shape (valid for hadronic scenarios), it serves as an approximation for leptonic and hybrid models where non-linear depolarization effects may occur.
Detectability and Duty Cycle Analysis:
- The study assessed detectability for instruments including StokeSAT, IXPE, eXTP, EXPO, COSI, and a Fermi-like instrument.
- Exposure times were optimized (14 and 28 days) based on the stability of the Polarization Angle (PA), derived from RoboPol light curves, to minimize depolarization from PA rotations.
- The Minimum Detectable Polarization (MDP) for each instrument was used to determine if a source's predicted PD and flux would yield a detection.
- A "duty cycle" was calculated for each source-instrument pair by generating 1,000 random realizations of flux and PD (based on Gaussian and CDF distributions from observations) to estimate the probability of detection in a blind survey.

Key Results

Instrument Performance:
- StokeSAT (soft X-ray) is predicted to detect the majority of sources across all classes (LSP, ISP, HSP) regardless of the emission model.
- IXPE (2–8 keV) shows variable detectability: it can detect ~50% of HSP sources under hadronic/hybrid models but only ~33% under leptonic models. Detectability for LSP sources is highly model-dependent, with only BL Lac being detectable under leptonic/hybrid scenarios.
- eXTP and EXPO (harder X-rays) are expected to detect most HSP and ISP sources across models, with EXPO showing a drastic increase in detectable LSP sources under the hadronic model.
- COSI (MeV range) is predicted to detect polarization from only one source in the sample (1ES 1959+650, an HSP).
Model Discrimination:
- Simultaneous multi-wavelength observations are crucial for distinguishing models. The study highlights that HSP sources exhibit distinct PD predictions in X-ray (2–8 keV) versus $\gamma$ -ray (0.1–100 GeV) bands.
- Specifically, under the leptonic model, GeV PD is predicted to be significantly lower (<5%) compared to X-ray PD. Conversely, hadronic models predict comparable PD in both bands. A detection of PD > 5% in the GeV band for LSP sources would effectively rule out the leptonic model.
Sensitivity Requirements:
- To detect 25% of the source population with a PD threshold of 5% (a level chosen to differentiate models), future MeV and GeV missions require specific flux sensitivities.
- For leptonic models, sensitivities of $\sim 10^{-12}$ erg cm $^{-2}$ s $^{-1}$ (MeV) and down to $\sim 10^{-13}$ erg cm $^{-2}$ s $^{-1}$ (GeV for HSP) are required.
- Hadronic models require sensitivities of $\sim 10^{-11}$ erg cm $^{-2}$ s $^{-1}$ , while hybrid scenarios require $\sim 10^{-12}$ erg cm $^{-2}$ s $^{-1}$ .

Significance and Claims
The paper claims to provide a framework for predicting instrument performance for current and future high-energy polarimetry missions applied to a statistically complete blazar sample. The primary significance lies in demonstrating that simultaneous multi-wavelength polarimetric observations can effectively discriminate between competing emission models (leptonic, hadronic, hybrid) due to the distinct differences in expected polarization degrees.

The authors modestly conclude that while their exposure time estimates (14–28 days) are average benchmarks subject to the unpredictable variability of individual blazars, the analysis establishes necessary sensitivity requirements for next-generation instruments. They assert that instruments capable of reaching sensitivities of $\sim 10^{-11}$ erg cm $^{-2}$ s $^{-1}$ in the MeV and GeV bands, combined with simultaneous X-ray observations, would have a strong chance of measuring PD and resolving the nature of blazar jet emission. The work emphasizes that detecting a large population of sources in these energy ranges is essential for advancing the understanding of multi-messenger emission from relativistic jets.

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