Discovery of diffuse gamma-ray emission in the vicinity of G172.8+1.5: An old supernova remnant with different turbulence properties

TL;DR

This work investigates the origin of GeV gamma-ray emission near the G172.8+1.5 region, reevaluating whether it is an old SNR or an H II complex by combining 16 years of Fermi-LAT data with gas observations. The authors identify two extended gamma-ray sources, SrcA and SrcB, and perform detailed morphological and spectral analyses, finding SrcA best described by a power-law and SrcB by a curved Log-Parabola spectrum, with SrcB spatially coincident with a dense molecular cloud. Hadronic modelling shows SrcB requires a very low diffusion coefficient consistent with Bohm-like turbulence, while SrcA favors a higher, mixed diffusion regime; the LHAASO TeV emission can be explained by trapped high-energy CRs near the SNR and/or hadronic interactions in surrounding gas. YSCs and star-forming region activity provide another plausible hadronic pathway, whereas leptonic scenarios involving bremsstrahlung and pulsar-halo contributions offer alternative explanations; overall, the results highlight complex CR propagation and multiple emission channels in giant H II/SNR environments and call for future observations to distinguish origins and constrain diffusion properties.

Abstract

We report the detection of high-energy $γ$-ray emission in the vicinity of G172.8+1.5, which is debated as a Supernova remnant (SNR) or an ionized hydrogen (H$_{\rm{II}}$) region. Using 16-yr Pass 8 data from Fermi Large Area Telescope (Fermi-LAT), we found the GeV emission can be described by two extended sources with different photon spectra. Among them, the much more extended $γ$-ray source SrcA with a Power-law spectrum is spatially coincident with a giant neutral Hydrogen shell structure and several OB stars inside a huge H$_{\rm{II}}$ region. The softer Log-Parabola spectra $γ$-ray source SrcB is spatially coincident with a star-forming region with several OB stars, maser sources and IR sources. Gas observation results display a dense molecular cloud surrounding SrcB in the velocity range of [-25,-5] km s$^{-1}$. The possible origins of $γ$-ray emission are discussed, including CRs escaped from SNR shock surface and illuminated nearby MC, the young massive star clusters associated with the star-forming region and a pulsar halo generated by an invisible energetic pulsar inside the SNR. Furthermore, under the hadronic scenario, the measured diffusion coefficient in the compact SrcB region is significantly lower than that of the more extended SrcA. This suppression is likely attributed to cosmic-ray-driven instabilities, which reduce the diffusion efficiency in the vicinity of the source relative to the standard conditions in the interstellar medium (ISM). Future advanced analysis from LHAASO observation results would help distinguish the origins of $γ$-ray emission in this region and clarify the nature of this source.

Figures (7)

• Figure 1: TS maps in the vicinity of the G172 region calculated by Fermi-LAT in different energy bands. Top row panels for 300 MeV - 1 GeV energy band, bottom row panels for 1 GeV - 1 TeV energy band. The middle and right column panels show the residual map after subtracting the emission from SrcA and SrcB, respectively. The red and cyan circles show the best-fit R$_{\rm 68}$ extension and position of SrcA and SrcB, respectively. The white dashed circles indicate the position and extension of several ionized hydrogen regions 1976MNRAS.177..601C2012AJ....143...75K. The green crosses show the point-like sources listed in 4FGL-DR4 and subtracted in this work. Among them, two green crosses located outside of the SrcA region (red circle) show the point-like sources listed in 4FGL-DR4 and treated as background sources. The white diamond shows the position of several OB stars 2012AJ....143...75K. The black circle shows the location of a star-forming region G173.185+02.356 2012AA...542A...3S. The green and orange dashed circles represent the 39$\%$ contamination radius r$_{\rm 39}$ of LHAASO J0534+3533 WCDA and KM2A source 2023arXiv230517030C, respectively. The yellow contour is extracted from Effelsberg 1.4 GHz results 2010AA...520A..80L.
• Figure 2: The SEDs of SrcA (left), and SrcB (right). The black data points were derived by Fermi-LAT in the energy range of 100 MeV - 1 TeV. The black arrow indicates the 95$\%$ upper limits and the grey histogram shows the TS value for each energy bin. The red solid and dashed lines show the best-fit PL with 1 $\sigma$ statistical errors for SrcA. The red curve indicates the best-fit LogPb spectrum for SrcB. The solid black line in the right panel represents the predicted local $\gamma$-ray emission assuming that the CR spectra therein are the same as measured locally by AMS-02 2015PhRvL.115u1101A. The magenta solid lines represent the hadronic models assuming a power-law and broken power-law proton spectrum for SrcA and SrcB, respectively, details can be seen in Sect.\ref{['sec:4.2']}.
• Figure 3: Left: Integrated $^{12}$CO(J = 1-0) emission intensity (K km s$^{-1}$) toward G172 in the velocity range of [-25,-5] km s$^{-1}$. All labels are kept the same as Figure \ref{['fig:1']}. Right: $^{12}$CO(J = 1-0) spectra of gas inside SrcA, SrcB and LHAASO source regions, respectively.
• Figure 4: Hadronic modeling of multi-wavelength $\gamma$-ray spectra of SrcA (right column) and SrcB (left column), respectively. The solid, dashed, dotted and dash-dotted lines show different r$_{\rm s}$ value influence towards fitted results. The orange dashed line corresponds to the trapped component with a fixed value of 2.0. The Blue and purple butterflies are extracted from the 1LHAASO J0534+3533 WCDA and KM2A spectra results 2024ApJS..271...25C. The blue and red fit lines in the left column panels correspond to the contribution from escaped ions under different r$_{\rm s}$ and $\chi$ value. The gray fit lines in the right column panels show the total contributions from trapped and escaped ions under different $\chi$ value assumptions.
• Figure 5: Sketch of the CR propagation in different emission zones.