History of UHECR production in Centaurus A

TL;DR

Problem: determine whether Cen A could produce the Centaurus UHECR hotspot. Approach: model particle escape and propagation from Cen A’s Giant Lobes under diffusion-advection physics, assess injection histories, and compare energy spectrum and composition with Auger data, using a $L \sim 200$ kpc lobe model and a Kolmogorov-turbulence diffusion coefficient to obtain escape times $\tau_{\rm esc}$. Findings: acceleration must have occurred within $\sim 2$–$30$ Myr ago; lighter components escape earlier while heavier nuclei require longer activity; current activity alone cannot reproduce nitrogen-like composition; extragalactic delays are subdominant. Significance: supports Cen A as a plausible nearby, episodic UHECR source tied to Middle/Giant Lobe re-energization, and informs expectations for composition-dependent anisotropy and the lack of extreme $>100$ EeV clustering from Cen A.

Abstract

The origin of the UHECR continues to puzzle, however, an excess of detection in the direction of the radio galaxy Centaurus A (Cen A) raises the possibility of this object being the first UHECR source identifiable. Cen A is known to be currently active, and also exhibits known past episodes of high activity. In this work, we investigate whether the known activity episodes in Cen A may be related to the excess events in the \textit{Centaurus region}. Analysing the energy of the events and the overall mass composition of UHECR, we report that an activity in the last $\sim30$ Myr is necessary to explain the excess of events. This period perfectly fits with the timescale where the transition regions and the Giant Lobes must be energized, as revealed by radio and $γ$ ray observations.

Figures (5)

• Figure 1: Time necessary for UHECR to escape the Giant Lobes of Cen A for different UHECR species. The age and timescales for different structures in Cen A (continuous black lines) and the advection timescale inside the lobes (black dashed line indicated by Adv) are also shown. The Middle Lobe timescale is taken as $30$ Myr.
• Figure 2: UHECR flux fraction escaping from the Giant Lobes for different injection scenarios. Proton, N, and Fe species are considered. Top panel: Injection starting at $2$ Myr (continuous lines), $30$ Myrs (dashed lines), and $100$ Myrs (dotted). Bottom panel: Injection ceasing $2$ Myr (continuous), $30$ Myrs (dashed), and $100$ Myrs (dotted) ago. The energy range where the excess in the Centaurus region is reported supergalactic_auger is shown as a gray band.
• Figure 3: Energy spectrum of Nitrogen nuclei escaping the lobes of Cen A for several temporal injections. Injection starting (left) or ceasing (right) in $2$, $30$, and $100$ Myr are shown. Two maximum rigidities, $5$ (blues) and $10$ EeV (reds), are considered. For comparison, the energy spectrum measured in a window $20^\circ$ around Cen A is also shown AbdulHalim_2024. The energy spectrum of continuous infinity injection ($\infty$) is arbitrary normalized, and corrected by the fraction of escaping flux (eq. \ref{['eq:escaping_flux']}).
• Figure 4: Evolution of the mean composition with the energy escaping the Giant Lobes for several time injections, considering a Wolf-Rayet-like composition. The mean logarithm of the composition obtained by the Pierre Auger Collaboration mass_deep_learning for the EPOS-LHC hadronic model is shown for comparison.
• Figure 5: Timescales for Cen A. Left panel: Timescales for propagation of different UHECR species (continuous colored line) in the Giant Lobes are shown altogether with timescales for different structures in Cen A (continuous black lines). Interaction timescales for the CMB and EBL are shown in dashed lines. Black dashed lines are estimations for the extragalactic time interval necessary UHECR to reach Earth, considering a rectilinear trajectory (Extragal), and the advection timescale inside the lobes (Adv). Together with the propagation time, thin continuous lines represent the propagation timescale for rectilinear and diffusive regimes. Right panel: Ratio between propagation and interaction timescales for different UHECR species. Continuous lines indicate the fraction of UHECR expected to escape from the Giant Lobes, both if generated in a Middle Lobe re-energization (NML, $\sim30$ Myr) or the current jet timescale ($\sim2$ Myr). Dashed lines indicate escape if accelerated on Middle Lobe re-energization, but not if injected during the current activity. Dotted lines indicate the fraction that remains imprisoned. The gray band represents the transition between a regime escape- or interaction-dominated.