Deciphering the gamma-ray emission in the Cygnus region

TL;DR

The paper investigates the origin of Cygnus region gamma-ray emission, motivated by multi-TeV to PeV detections, and develops a joint lepto-hadronic model that combines a past powerful supernova remnant in Cygnus OB2 with IC emission from stellar-wind termination shocks. It advances a 3D gas distribution and a two-zone diffusion framework, solving a spherical transport equation to predict spectra and energy-dependent morphology, then comparing to Fermi-LAT, HAWC, and LHAASO data via GAMERA-based emission maps. The results favor a ~50 kyr-old SN remnant as the primary PeV accelerator, capable of reaching $E_{ m max} o ext{a few PeV}$ for protons with streaming-instability amplification, while IC from OB2 winds accounts for emission below ~10 TeV; some >1 PeV photons could also be associated with Cygnus X-3 or Galactic CRs. The work highlights the importance of a realistic 3D gas model and two-zone diffusion in star-forming regions, and it suggests that feedback from past SN events plays a crucial role in shaping the galactic CR ecosystem in Cygnus.

Abstract

The Cygnus region is a vast star-forming complex harbouring a population of powerful objects, including massive star clusters and associations, Wolf-Rayet stars, pulsars, and supernova remnants. The multi-wavelength picture is far from understood, in particular the recent LHAASO detection of multi-degree scale diffuse gamma-ray emission up to PeV energies. We aim to model the broadband gamma-ray data, discriminating plausible scenarios amongst all candidate accelerators. We consider in particular relic hadronic emission from a supernova remnant expanding in a low-density environment and inverse Compton emission from stellar-wind termination shocks in the Cygnus OB2 stellar association. We first estimate the maximum particle energy from a 3D hydrodynamical simulation of the supernova remnant scenario. The transport equation is then solved numerically to determine the radial distribution of non-thermal protons and electrons. In order to compute synthetic gamma-ray spectra and emission maps, we develop a 3D model of the gas distribution. This includes, firstly, a HI component with a low-density superbubble around Cygnus OB2 and, secondly, molecular clouds lying at the edge of the superbubble and in the foreground. We find that a powerful, ~50 kyr-old supernova remnant can account for both the morphology and spectrum from 10 TeV-PeV. At PeV energies, the microquasar Cygnus X-3 and diffuse Galactic cosmic rays might also contribute to the flux. Below about 10 TeV, hadronic models are incompatible with the expected existence of a superbubble centred on Cygnus OB2. Instead, the spectrum is well fitted with inverse Compton emission from electrons accelerated at stellar-wind termination shocks in Cygnus OB2 in line with existing multi-wavelength limits.

Figures (15)

• Figure 1: Powerful objects in the Cygnus region. Stars: Wolf-Rayet stars from Rosslowe2015. Diamonds: pulsars. The distance to PSR J2032+4127 has been revised to 1.76 kpc by association with its Be star companion MT91 213. Rings: location of supernova remnants from Green2024_SNRcatalogueGreen2025_SNRcatalogue. Different methods give distances to $\gamma$-Cygni between 0.98 to 2.3 kpc. The circles labelled "10 pc" and "100 pc" are centred on Cygnus OB2, assuming a distance of 1.65 kpc to the association.
• Figure 2: Event rates for different types of core-collapse supernovae for a cluster with a total initial mass of $1.65\times 10^4 \,\mathrm{M}_\odot$, as appropriate for Cygnus OB2, computed using Hoki.
• Figure 3: Density slices from the simulation of a supernova remnant expanding in Cygnus OB2. For details, see Sect. \ref{['sec:simu']}.
• Figure 4: Top: supernova remnant properties across the forward shock surface at 2 kyr, extracted from the hydrodynamical simulation shown in Fig. \ref{['fig:SNR_density_slices']}. Bottom: integrated maximum energy at 2 kyr, for the resonant and non-resonant streaming instabilities (RSI and NRSI, respectively) and without streaming instabilities. $B_\mathrm{eq}$ was computed assuming 10% of the kinetic energy in the superbubble goes into a turbulent magnetic field.
• Figure 5: Main molecular clouds in the Cygnus region. Red: Cygnus 1.3 North at $l>80^\circ$ (Schneider Group II, 1.3 kpc) and Cygnus 1.3 South at $l<80^\circ$ (part of Schneider Group IV, $1.3$ kpc). Orange: Cygnus North Filament at $l>80^\circ$ (Schneider Group I, 1.5 kpc) and Cygnus SFR South at $l<80^\circ$ (part of Schneider Group IV, unknown distance ${>}1.4$ kpc). Pink: Cygnus South Filament (part of Schneider Group IV, unknown distance ${>}1.4$ kpc). Green: Cygnus Central Clumps (Schneider Group III, $1.6\hbox{--}1.8$ kpc). Dark blue at high latitude: Cygnus West, unknown distance (${>}1.4$ kpc). Turquoise: S106 regions (described in Schneider2007, $1.6\hbox{--}1.8$ kpc). The grey background shows foreground clouds: the North-America and Pelican nebulae on the left, the North-West clouds, and the Rift lane extending to the right in the direction to the Galactic Centre.