Multiple emission components in the Cygnus cocoon detected from Fermi-LAT observations
Abstract
Context. Star-forming regions may play an important role in the life cycle of Galactic cosmic rays (CRs), notably as home to specific acceleration mechanisms and transport conditions. Gamma-ray observations of Cygnus X have revealed the presence of an excess of hard-spectrum gamma-ray emission, possibly related to a cocoon of freshly accelerated particles.
Aims: We seek an improved description of the gamma-ray emission from the cocoon using ~13 yr of observations with the Fermi-Large Area Telescope (LAT) and use it to further constrain the processes and objects responsible for the young CR population.
Methods: We developed an emission model for a large region of interest, including a description of interstellar emission from the background population of CRs and recent models for other gamma-ray sources in the field. Thus, we performed an improved spectro-morphological characterisation of the residual emission including the cocoon.
Results: The best-fit model for the cocoon includes two main emission components: an extended component FCES G78.74+1.56, described by a 2D Gaussian of extension r68 = 4.4° ± 0.1°−0.1°+0.1° and a smooth broken power law spectrum with spectral indices 1.67 ± 0.05−0.01+0.02 and 2.12 ± 0.02−0.01+0.00 below and above 3.0 ± 0.6−0.2+0.0 GeV, respectively; and a central component FCES G80.00+0.50, traced by the distribution of ionised gas within the borders of the photo-dissociation regions and with a power law spectrum of index 2.19 ± 0.03−0.01+0.00 that is significantly different from the spectrum of FCES G78.74+1.56. An additional extended emission component FCES G78.83+3.57, located on the edge of the central cavities in Cygnus X and with a spectrum compatible with that of FCES G80.00+0.50, is likely related to the cocoon. For the two brightest components FCES G80.00+0.50 and FCES G78.74+1.56, spectra and radial-azimuthal profiles of the emission can be accounted for in a diffusion-loss framework involving one single population of non-thermal particles with a flat injection spectrum. Particles span the full extent of FCES G78.74+1.56 as a result of diffusion from a central source, and give rise to source FCES G80.00+0.50 by interacting with ionised gas in the innermost region.
Conclusions: For this simple diffusion-loss model, viable setups can be very different in terms of energetics, transport conditions, and timescales involved, and both hadronic and leptonic scenarios are possible. The solutions range from long-lasting particle acceleration, possibly in prominent star clusters such as Cyg OB2 and NGC 6910, to a more recent and short-lived release of particles within the last 10-100 kyr, likely from a supernova remnant. The observables extracted from our analysis can be used to perform detailed comparisons with advanced models of particle acceleration and transport in star-forming regions.
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- March 2023
- DOI:
- 10.1051/0004-6361/202245573
- arXiv:
- arXiv:2301.04504
- Bibcode:
- 2023A&A...671A..47A
- Keywords:
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- acceleration of particles;
- cosmic rays;
- gamma rays: ISM;
- open clusters and associations: individual: Cygnus OB2;
- open clusters and associations: individual: NGC 6910;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- Published in A&