Detectability of Polarized Gamma-ray Emission from Blazar Flares with COSI

TL;DR

This study assesses COSI's potential to detect polarized gamma-ray emission from blazar flares, analyzing 17 years of Fermi data to identify promising targets for polarization measurement in the MeV band.

Contribution

It provides the first detailed estimates of COSI's capability to detect blazar polarization in the MeV range, highlighting the most promising sources and observational strategies.

Findings

COSI can detect polarization in up to ~6 flares with MDP99<50% during its mission.

Brightest flares from flat-spectrum radio quasars are the most promising targets.

Shorter time intervals around flare peaks improve polarization detection prospects.

Abstract

We investigate the detectability of polarized gamma-ray emission from blazar flares with the Compton Spectrometer and Imager (COSI). Using 17 years of Fermi Large Area Telescope observations, we analyze light curves for 1413 blazars and identify a maximum of 787 sources with flaring episodes through Bayesian block analysis. For each flare, we estimate the minimum detectable polarization MDP99 in the COSI energy band (0.2-5 MeV) using instrument response functions under a range of spectral assumptions and background conditions. Under baseline background levels (1 counts/s), and assuming that blazar flare statistics in the MeV band are comparable to those observed at GeV energies, we find that COSI can realistically detect polarization in up to ~6 flares with MDP99<50% over its two-year prime mission depending on different spectral and flare identification assumptions, with only a few most powerful ones reaching MDP99<20%. These expectations are shown to improve when shorter intervals around bright peaks within long flares are considered. We provide a ranked list of the most promising targets, finding that flat-spectrum radio quasars dominate the population of polarization-detectable events. Through its continuous all-sky monitoring in the largely unexplored MeV band, COSI will open a new observational window on blazar variability and deliver the first direct measurements of MeV polarization, offering unique insights into jet geometry and high-energy emission processes.