Observational Properties of Thermal Emission from Relativistic Jets Embedded in AGN Disks

Technical Summary: Observational Properties of Thermal Emission from Relativistic Jets Embedded in AGN Disks

Problem Statement
Active Galactic Nuclei (AGN) disks serve as environments where various astrophysical events, such as massive star core-collapse, compact object mergers, and hyper-Eddington accretion, can launch relativistic jets. While previous studies have examined specific scenarios (e.g., GRB jets or binary black hole merger remnants) within AGN disks, they often suffer from limitations such as oversimplified jet-disk interactions, neglect of cocoon structure formation, failure to account for photon trapping in opaque systems, or incomplete evaluation of radiation contributions from all material components. There is a need for a systematic investigation that simultaneously addresses jet propagation dynamics and the resulting thermal radiation production for both jets that successfully break out of the disk and those that become choked.

Methodology
The authors present a comprehensive study modeling the dynamical evolution of relativistic jets embedded in AGN disks and their associated thermal emission.

• Disk Structure: The study adopts the Sirko & Goodman (2003) model for the AGN disk, considering three vertical density profiles: uniform, isothermal (gas-pressure dominated), and polytropic (radiation-pressure dominated). The focus is on the inner disk regions ($R \le 10^5 R_g$) where optical depth is high.
• Jet-Cocoon Dynamics: Using the analytic framework of Bromberg et al. (2011), the authors calculate the evolution of the jet-head position ($z_h$), cocoon internal energy ($E_c$), and lateral radius ($r_c$). The model accounts for ram pressure balance, jet collimation, and the formation of a two-component cocoon (disk-cocoon and jet-cocoon).
• Choking Criterion: A critical condition is defined where the jet tail catches up with the head before the jet-head shock breakout occurs. If the choking time ($t_{ch}$) is less than the breakout time ($t_{bre}$), the jet is choked, and all kinetic energy is transferred to the cocoon.
• Thermal Emission Modeling: The study calculates thermal emission from three distinct components:
  1. Jet-head Shock Breakout: Modeled based on shock jump conditions, diffusion timescales, and Comptonization effects (including pair production for relativistic shocks).
  2. Disk-cocoon Emission: Modeled as an expanding radiation-dominated bubble, accounting for planar and spherical expansion phases, adiabatic cooling, and photon diffusion.
  3. Jet-cocoon Emission: Modeled considering the mixing of jet and disk materials, energy distribution per baryon, and the transition from relativistic to Newtonian expansion phases.
• Scenarios: The analysis covers both successful breakout events and choked jet scenarios, varying jet power ($L_j$), duration ($t_j$), launch radius ($R_j$), and central supermassive black hole (SMBH) mass.

Key Results

• Breakout Dynamics: The jet-head velocity at breakout varies from Newtonian to mild-relativistic regimes depending on jet power and disk density. Jets in isothermal or polytropic disks may experience reacceleration in the low-density outer regions, leading to higher breakout velocities compared to uniform disks.
• Thermal Emission Signatures:
  • Soft X-ray Flares: These are identified as the most prominent observable signatures. They typically last from $O(10^2)$ s to $O(10^5)$ s. The light curves often exhibit double-peaked structures: an initial sharp peak from the fast disk-cocoon component followed by a broader decline from the Newtonian jet-cocoon component.
  • UV and Optical Flares: These are detectable primarily for powerful jets ($L_j \gtrsim 10^{48}$ erg s$^{-1}$). They persist for several days to tens of days. Their detectability is often limited by the bright AGN background in the UV band and lower peak luminosities in the optical band.
  • Choked Jets: When jets are choked, the system produces only cocoon emission. Deeply choked jets result in significantly dimmer flares, often rendering UV and optical signals undetectable, and potentially suppressing X-ray emission entirely if the jet-cocoon material is fully dissipated.
• Multi-wavelength Correlations: The study establishes a temporal sequence where soft X-ray flares appear first, followed by UV and then optical flares. This sequencing is a potential diagnostic tool.
• Parameter Dependence: Higher jet power leads to earlier breakout, higher luminosities, and higher temperatures. Larger SMBH masses and larger disk radii generally result in longer flare durations due to the increased volume and energy of the jet-cocoon system.

Significance and Claims
The paper claims that the thermal emission from the jet-cocoon system serves as a critical electromagnetic (EM) counterpart to jet-producing events embedded in AGN disks, such as binary black hole mergers or massive star collapses.

• Diagnostic Paradigm: The distinct multi-wavelength light curves (specifically the temporal ordering of X-ray, UV, and optical flares) provide a diagnostic paradigm to distinguish genuine EM counterparts of GW events from AGN background variability.
• Choked Jet Detection: For choked jets, which do not produce non-thermal prompt emission or afterglows, these thermal flares may constitute the sole detectable EM signal, offering a unique window into jet dynamics and AGN disk properties.
• Observational Strategy: The findings highlight the necessity of multi-wavelength follow-up observations to identify candidate EM counterparts to AGN-embedded events. The study suggests that soft X-ray telescopes (e.g., Einstein Probe, Swift) are crucial for initial detection, while UV and optical facilities (e.g., ULTRASAT, LSST) are required for characterizing the longer-duration components, particularly for powerful jets.

The authors conclude that their systematic approach, which integrates jet propagation dynamics with a full treatment of thermal radiation from all system components, offers a robust framework for predicting and identifying these transient phenomena.