Unveiling Multimessenger Emission from Hidden Cores of Microquasars

TL;DR

This work deploys the Astrophysical Multimessenger Emission Simulator (AMES) to model broadband, multimessenger emission from microquasars with three distinct physical scenarios, motivated by recent >100 TeV and PeV photon detections. By solving coupled transport equations for photons and all relevant particles and incorporating external seed photons, absorption, and particle escape, the authors demonstrate that the observed >0.1 TeV spectra can arise from either pγ or pp interactions depending on the emission region. They apply the framework to Cygnus X-1 and Cygnus X-3, showing that Case A–C can fit the data with different dominant processes and predicting distinct high-energy spectral features and variability. Importantly, muon and pion cooling suppresses neutrino fluxes, suggesting that detecting neutrinos from these sources will be challenging for current facilities, highlighting the need for next-generation detectors and coordinated multimessenger observations.

Abstract

Microquasars are radio-emitting X-ray binaries accompanied by relativistic jets. They are established sources of 100~TeV gamma rays and are considered promising candidates for cosmic-ray acceleration. Motivated by recent detections of $\sim 100~$TeV photons from Cygnus~X-1 and $\sim~$PeV photons from Cygnus~X-3 by the Large High Altitude Air Shower Observatory (LHAASO), we employ the Astrophysical Multimessenger Emission Simulator (AMES) to model their multimessenger emission considering compact outflow regions as cosmic-ray accelerators, spanning from radio to ultra-high-energy gamma rays. Our results show that the observed $>$TeV gamma rays can originate from either $pγ$ or $pp$ interactions, depending on the location and physical conditions of the emission region, while also reproducing the lower-energy spectra. The different configurations yield unique, observationally testable predictions. In the $0.1-10$~TeV energy range, where current observations provide only upper limits, they predict either a deep dip, a mild suppression, or a power-law spectrum. Additionally, models involving AU-scale blob regions predict strong variability, while those invoking more extended and static external zones show more stable behavior. We also provide a possible qualitative explanation for the distinct modulation patterns across different energy bands, which relies primarily on changes in the Doppler factor and external $γγ$ absorption. Finally, our neutrino predictions, which properly account for muon and pion cooling effects, reveal a significantly suppressed flux, indicating that detecting these sources may be more challenging than previously anticipated.

Figures (10)

• Figure 1: A schematic picture of the geometry, assuming that neutrinos and photons are produced within the jet blob.
• Figure 2: Schematic pictures of the three physical configurations modeled in this work. Panel (a): Scenario A consists of an inner blob, originating from either the corona or an internal shock, and a jet blob, produced by either an internal or termination shock. Both blobs lie along the same direction but are located at different heights and move with distinct velocities. Panel (b): Scenario B features a primary jet emission region surrounded by the dense stellar wind environment of the companion star. In the static external emission zone, charged particles escaping from the jet interact with the stellar wind. Panel (c): Scenario C considers the contribution from the extended jet. Within the large-scale, static external shell, particles that have escaped from the jet blob interact with the surrounding parsec-scale environment.
• Figure 3: Optical depths for $\gamma\gamma$ annihilation (blue), $p\gamma$ interactions (red), and $pp$ interactions (purple) in the observer frame. The first, second, and third rows correspond to Scenarios A, B, and C, respectively. Different line styles correspond to different emission zones, as indicated in the legend.
• Figure 4: Photon (red) and neutrino (black) spectra obtained from Scenario A. The contributions to the photon and neutrino fluxes are indicated by dashed lines for the jet blob and dotted lines for the inner blob. The solid lines represent the total fluxes, summing the inner and jet blob contributions. Fainter red solid lines show photon spectra calculated without the external absorption (EA; including free-free absorption and external $\gamma\gamma$ annihilation). Additionally, the dotted orange lines represent the blackbody radiation from the donor star, while the solid pink lines indicate the de-absorbed X-ray emission from the accretion disk. The bottom row provides a zoomed-in view of the top row. The first, second, and third columns correspond to model fits for the hard state of Cygnus X-1, the soft state of Cygnus X-1, and Cygnus X-3, respectively. Note that the observational data at different energy ranges are not simultaneous.
• Figure 5: Similar to Fig. \ref{['fig:spectra_pgamma']}, but for Scenario B. The contributions to the photon (red) and neutrino (black) fluxes are indicated by dotted lines for the jet blob and dashed lines for the stellar wind region.