Where within the 3C 84 jet are $γ$-rays produced?

Abstract

The location of $γ$-ray creation and emission within extra-galactic jets is a matter of active debate. One particularly well-suited source to pinpoint the location is the nearby, bright radio galaxy 3C 84, harbouring a powerful jet. Here we investigate the origin of $γ$-rays measured during a recent $γ$-ray flare, by analysing the linear polarisation signal of close-in-time very long baseline interferometry (VLBI) observations at centimetre and millimetre wavelengths. While 3C 84 is overall almost unpolarised, we find that close-in-time to the $γ$-ray flare peak regions at parsec-scale distances from the central engine shows a fractional linear polarisation increase. Under the physically well-motivated assumption of a causal relation between this polarisation enhancement and the $γ$-ray flare, and combined with insights from concurrent X-ray polarisation measurements, the $γ$-rays being created in this region is a physically motivated scenario, in a process consistent with synchrotron self-Compton.

Figures (4)

• Figure 1: Linear polarisation degree and $\gamma$-ray flux as a function of time. Shown here are observations taken with the KVN at 22, 43, 86, and 129 GHz, with the IRAM 30 m telescope as part of the POLAMI programme at 86 and 225 GHz, with the SMA at 225 GHz (all in colour and round markers) and Fermi-LAT in $\gamma$-rays (grey crosses connected by a dashed line). In addition, the black triangular markers denote the VLBI polarisation degree of the core and C3 at 43 GHz. The filled ones with an error bar correspond to a detection (close to the $\gamma$-ray peak, with the entirety of the flare being denoted with the light pink shaded area), the empty one to a non-detection upper limit. All values are listed in Table \ref{['tab:Epochs']}. The well-known trend for 3C 84 of higher polarisation values at higher frequencies is observed here as well. The radio observation dates span from MJD 60696 to 60767. During this time-frame some depolarisation is also observed, possibly due to multiple jet components blurring the s.d./c.i measurements.
• Figure 2: Images of 3C 84 at 43 GHz (BEAM-ME) and 15 GHz (MOJAVE). Top panel: The 43 GHz total intensity (contours) and linear polarisation (colour) images of 3C 84 for all available epochs during the time frame of interest are shown here. The contour levels were set at 1% of the total intensity maximum ($I_\textrm{max}$) per epoch. The linear polarisation flux density is displayed in units of brightness temperature. The chosen colour scale is meant to display the detections, while also masking the surrounding noise. The cyan cross and circle denote the core region and the arrows C3, as discussed in the main text, and region L (see discussion in Sect. \ref{['sec:Discussion']}). The white sticks indicate the EVPAs. At the epoch nearest to the $\gamma$-ray flare peak (15 Dec. 2024) we detect linear polarisation at a distance of $\sim1.5$ parsec (C3) from the central engine. Bottom panel: The 15 GHz images of the 3C 84 jet for the available epochs in the time frame of interest, shown in a similar manner as in the top panel, prepared by the MOJAVE collaboration (CLEAN-reconstruction). The green ellipse next to each reconstruction illustrates the restoring CLEAN-beam, which corresponds to $(0.8\times0.5)\,\textrm{mas}\,(15^\circ)$ on average. We note that the linear polarisation is similar in magnitude to the noise level, amounting to marginal/non-detections.
• Figure 3: Publicly available CLEAN-reconstruction images of 3C 84 at 43 GHz (BEAM-ME). The display is in a similar manner to the bottom panel of Fig. \ref{['fig:BU']}. The green ellipse next to each reconstruction illustrates the restoring CLEAN-beam, which corresponds to $(0.15\times0.30)\,\textrm{mas}\,(10^\circ)$ on average. We note the consistency between the RML and CLEAN reconstructions. Specifically, the core region of 3C 84 exhibits a lower linear polarisation signal, whereas the C3 region appears brighter in the November and December 2024 epochs (around the time of the flare) before also returning to a lower signal state.
• Figure 4: Total intensity flux density light curve of the core and C3 region, displayed along the $\gamma$-ray light curve. The setup of the figure is similar to Fig. \ref{['fig:SingleDish']}. While the core flux density remains stable during the $\gamma$-ray flare, the one of the C3 region is higher, following the same trend as the polarisation degree.