Detection of Compton scattering in the jet of 3C 84

TL;DR

The study reports the first IXPE X-ray polarization detection for 3C 84, measuring $\Pi_X = 4.18 \pm 1.31\%$ with $\psi_X = 162.8^{\circ} \pm 9.2^{\circ}$, indicating emission aligned with the jet. Through a coordinated multiwavelength campaign and joint spectral modeling with Chandra, NuSTAR, and Swift, the authors attribute the X-rays to Compton scattering by jet electrons, most plausibly Synchrotron Self-Compton downstream from the SMBH. The results argue against torus- or wind-reflected processes and, despite degeneracy in the spectral fits, demonstrate that X-ray polarimetry can decisively constrain the emission site and mechanism in radio galaxies. The findings have broad implications for understanding jet-dominated high-energy emission and the role of X-ray polarization in disentangling jet and disk contributions.

Abstract

3C 84 is the brightest cluster galaxy in the Perseus Cluster. It is among the closest radio-loud active galaxies and among the very few that can be detected from low frequency radio up to TeV $γ$-rays. Here we report on the first X-ray polarization observation of 3C~84 with the Imaging X-ray Polarimetry Explorer, for a total of 2.2 Msec that coincides with a flare in $γ$-rays. This is the longest observation for a radio-loud active galaxy that allowed us to reach unprecedented sensitivity, leading to the detection of an X-ray polarization degree of $\rmΠ_X=4.2\pm1.3\%$ ($\sim3.2σ$ confidence) at an X-ray electric vector polarization angle of $\rm ψ_X=163^{\circ}\pm9^{\circ}$, that is aligned with the radio jet direction on the sky. Optical polarization observations show fast variability about the jet axis as well. Our results strongly favor models in which X-rays are produced by Compton scattering from relativistic electrons -- specifically Synchrotron Self-Compton -- that takes places downstream, away from the supermassive black hole.

Figures (13)

• Figure 1: Left: IXPE counts image of the Perseus Cluster overlaid with surface brightness contours from Chandra, showing that many of the substructures clearly apparent in the higher angular resolution Chandra data are also visible at lower resolution in IXPE. The black X corresponds to the position of 3C 84. Right: Cleaned 15 GHz Stokes I VLBA image of the parsec-scale jet of 3C 84, observed as part of the MOJAVE program on February 21st 2025, in units of $\mathrm{Jy} \mathrm{beam}^{-1}$. Notice that the angular scale of this radio image is approximately three orders of magnitude smaller than the IXPE image. Overlaid in green is the X-ray polarization vector with the $1\sigma$ confidence interval of the polarization angle denoted by the two dashed lines. The polarization vector's direction is fully consistent with the direction of the radio jet.
• Figure 2: Polarization contours for 3C 84 using a spectropolarimetric model fit. The red star denotes the best-fit value, while the blue, orange, and green contours correspond to $1\sigma,2\sigma$ , and $3\sigma$ confidence intervals. The dashed line corresponds to the position angle of the radio jet.
• Figure 3: Multiwavelength brightness and polarization observation during the IXPE observations. The top panel shows the flux density in optical (R-band), the second panel from the top the host-corrected optical (R-band) and milli-meter radio (225.5 GHz) polarization degree (%), the third panel from the top the optical and radio polarization angle (degrees), and the bottom panel the $\gamma$-ray photon flux 3-day binned light curves. The grey shaded areas mark the IXPE exposures over the entire observation. The horizontal red shaded area marks the projected direction of the jet. The error bars correspond to the 68% (1$\sigma$) confidence interval.
• Figure 4: Example of a flare in the IXPE data, with DU3 light curve as a reference. Panel (a) shows the 2-8 keV light curve from a 1 arcmin circle centered around 3C 84. The flare is not obvious. Panels (b) and (c) show the light curve from the same time frame, but from the Perseus cluster from annular regions between 3-5 arcmin and 6-8 arcmin, respectively. The flare is much more prominent in the absence of 3C 84. Panel (d) the 3-5 arcmin light curve in 2-8 keV, outside the science window. The flare is still visible, indicating a likely nonastrophysical origin. In our subsequent analysis, we reject the flare times.
• Figure 5: IXPE DU1 light curve of 3C 84 (in gray). The flare, shaded yellow, is observed simultaneously in all IXPE DUs (orange, green, and magenta light curves in the inset), as well as the simultaneous NuSTAR observation (blue light curve in the inset). This part of the light curve coincides with the period of enhanced activity from 3C 84.