Probing Jet Compositions with Extreme Mass Ratio Binary Black Holes

Abstract

Determining whether black hole jets are dominated by leptonic or baryonic matter remains an open question in high-energy astrophysics. We propose that extreme mass ratio binary (EMRB) black holes, where an intermediate mass secondary black hole (a "miniquasar") periodically interacts with the accretion flow of a supermassive black hole (SMBH), offer a natural laboratory to probe jet composition. In an EMRB, the miniquasar jet is launched episodically after each disk-crossing event, triggered by the onset of super-Eddington accretion. The resulting emissions exhibit temporal evolution as the jet interacts with the SMBH accretion disk. Depending on whether the jet is leptonic or hadronic in composition, the radiative signatures differ substantially. Notably, a baryonic jet produces a more pronounced gamma-ray output than a purely leptonic jet. By modeling the evolution of the multifrequency characteristic features, it is suggested that the gamma-ray-to-UV emissions may serve as a diagnostic tool capable of distinguishing between leptonic and baryonic scenarios. The resulting electromagnetic signals, when combined with multi-messenger observations, offer a powerful means to constrain the physical nature of relativistic jets from black holes.

Figures (7)

• Figure S1: A schematic illustration of an EMRB considered here (not to scale). The system comprises a primary SMBH surrounded by a thin accretion disk and a secondary intermediate mass "miniquasar" in an eccentric elliptical orbit. Periodic interactions between the miniquasar and the SMBH accretion flow produce distinct orbital phases: (a) the miniquasar approaches pericenter, prior to interacting with the SMBH's disk; (b) following the collision, the miniquasar begins accreting captured material, launching a relativistic jet that interacts with the SMBH's accretion environment; (c) as the miniquasar moves toward the apocenter, the captured material is exhausted and accretion stops. This sequence of phases (a)--(c) repeats over each orbit. For reference, the dashed circle marks the approximate outer edge of the SMBH accretion disk ($\sim$$10^{3}\,R_{\rm g}$).
• Figure S2: Orbital period as function of semi-major axis $a$ and the mass of the SMBH, in months. The sensitive frequency ranges for PTA and LISA are indicated by the blue and red regions. The selected parameters for our demonstrative case is indicated by the magenta dot.
• Figure S3: Fraction of the jet power carried by hadronic components, $\eta_{{\rm p}}$, for varying ratios between the particle Lorentz factor of proton and leptons, $\langle\gamma_{\rm p}\rangle/\langle\gamma_{\rm e}\rangle$; see Equation (\ref{['eq:eta_e']}). The shaded region corresponds to cases in which the baryonic component accounts for more than 99.9$\%$ of the total mass.
• Figure S4: Pericenter and apocenter radii as functions of the semi-major axis $a$ and the eccentricity $e$. miniquasars with orbital parameters falling within the shaded region have pericenters smaller than $10^{3}~R_{\rm{g}}$ and apocenters exceeding $10^{3}~R_{\rm{g}}$. Such orbits are expected to produce repeated interactions with the SMBH accretion disk, consistent with the configuration illustrated in Figure \ref{['fig:overview']}. The selected parameters for our demonstrative case is indicated by the magenta dot.
• Figure S5: Estimated gamma-ray-to-UV power ratio for varying ratios between the particle Lorentz factor of proton and leptons, $\langle\gamma_{\rm p}\rangle/\langle\gamma_{\rm e}\rangle$; see Equation (\ref{['eq:MeV_to_baryonic']}). The parameters $\mathcal{Q}=0.5$ and $f_{\rm IC}=0.8\mathcal{Q}$ are assumed. The shaded region corresponds to cases in which the baryonic component accounts for more than 99.9$\%$ of the total mass.