Cosmic-ray cooling by dark matter in astrophysical jets

TL;DR

This work investigates sub-GeV dark matter interacting with cosmic-ray electrons in AGN jets, focusing on Markarian 421. It introduces a multi-zone leptonic jet model (BHJet) that jointly fits jet dynamics and DM–electron scattering, explicitly accounting for uncertainties in jet parameters. By performing a 4D scan over jet parameters and the DM–electron cross section for various DM masses, the authors derive 5σ upper limits, notably achieving σ_{DM-e} ≲ 1×10^{-34} cm^2 at m_{DM}=1 MeV, outperforming some traditional methods by factors of 2–10 due to the full spectral-energy-distribution treatment and consideration of degeneracies. The approach is complementary to existing bounds and demonstrates the importance of incorporating realistic astrophysical modeling when constraining light DM with indirect probes, with potential extensions to other jets and future high-energy facilities.

Abstract

Astrophysical jets from powerful active galactic nuclei (AGN) have recently been proposed as promising probes of dark matter (DM) in the sub-GeV mass range. AGN launch relativistic jets that accelerate cosmic rays (CRs) to very high energies, which can then interact with their surroundings and produce multiwavelength (MW) emission spanning from radio frequencies to TeV $γ$ rays. If DM consists of light particles, their interactions with CRs could lead to an additional cooling mechanism that modifies the expected MW emission. In this work, we analyse the MW spectrum of Markarian 421, a well-studied AGN, using a multizone leptonic jet model that includes the interactions between CR electrons and DM particles. For the first time, we account for the uncertainties in the astrophysical jet dynamics, which have been previously neglected when constraining the CR-DM interactions. By fitting simultaneously jet parameters and DM-electrons interactions, we use the MW data from \mkn to set constraints on the DM-induced CR cooling. We obtain 5$σ$ upper limit $σ_\text{DM-e} \lesssim 1 \times 10^{-34}~\text{cm}^2$ for a DM mass of $1~{\rm MeV}$. We demonstrate that this is about a factor of 2--10 stronger than traditional approaches depending on DM mass. This improvement originates from having indeed considered the full multi-wavelength emission from the source, instead if a simplified approach. Properly accounting for degeneracies between jet dynamics and DM interactions is also key to deriving robust constraints on DM interactions.

Figures (7)

• Figure 1: The MW emitted energy flux $\nu F_{\nu}$ of a Markarian 421-like blazar for different injected jet power $L_{\rm jet}$, jet base radius $R_0$, and particle acceleration region $z_{\rm diss}$, as indicated in the legend. We assume a magnetization at $z_{\rm diss}$ of 0.02 and a power-law index of the non-thermal electrons $p=2$.
• Figure 2: The characteristic timescales of electron acceleration and losses as a function of the electron kinetic energy, as they occur in the jet segment at $z_{\rm diss}$. The solid thin black line represents the acceleration timescale, while standard astrophysical losses are given by the dot-dashed grey line (escape), the dashed grey line (IC), and the dashed red line (synchrotron). The thick solid blue line corresponds to the timescale of CR-DM collisions (see Eq. \ref{['eq:tDM-e']}) assuming $m_{\rm DM} = 1~{\rm MeV}$ and $\sigma_\text{DM-e}=10^{-34}~{\rm cm^2}$. The intersection between the acceleration and the fastest cooling process (lowest cooling timescale) gives the maximum attainable kinetic energy of the electrons $T_{e,{\rm max}}$, which is indicated by the vertical lines for the case with and without DM cooling.
• Figure 3: The spectral energy distribution of Markarian 421 with the best fit of the total predicted emission to the MW dataset ARGO-YBJ:2015qiq. The contribution of different processes and different jet regions are indicated in the legend, with the synchrotron emission (Syn) dominating the radio-to-X-ray spectrum, and the Inverse-Compton (IC) scattering explaining the $\gamma$-ray one. The bottom panel shows the residuals of the fit.
• Figure 4: Similar to Fig. \ref{['fig:astroBF']}, but for the case where the emitted spectrum is modified due to CR cooling by elastic scattering on the DM particles, and jet modelling uncertainties are accounted for in the joint fit (solid line). The best astrophysical model (dashed line, labelled as without DM) is shown for comparison. We also plot the case where the jet parameters remain the same as the astrophysical case and the DM cooling occurs with $m_{\rm DM} =$ 1 MeV and $\sigma_\text{DM-e}$$=10^{-34}\, \rm cm^2$ (dotted line, labelled as fixed jet). The green lines show the synchrotron component and the blue lines show the IC component.
• Figure 5: The upper limits of the cross section of DM-electrons interactions for different values of the mass of the light DM candidate. The light shaded areas are the constrained regions by other experiments and methods. In particular, the BBN Knapen:2017xzo, and direct detection CRESST:2015txjEmken:2018runCRESST:2017uesKouvaris:2016afsDolan:2017xbu. We also include upper bounds set by the solar reflection with thin solid black line An:2017ojc. The red lines represent the 5$\sigma$ upper limits from the joint fit to the MW emission of Markarian 421 (this work). They show different DM self-annihilation scenarios as indicated in the legend. The red shaded band quantifies the uncertainty related to the DM distribution in Markarian 421. For comparison, we show the optimistic bounds (blue dashed line) set by following a very simplistic approach, namely by constraining the timescales as explained by Eq. \ref{['eq: timescale constraints']}.