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An Enigmatic PeVatron in an Area around HII Region G35.6$-$0.5

The LHAASO Collaboration, Zhen Cao, F. Aharonian, Axikegu, Y. X. Bai, Y. W. Bao, D. Bastieri, X. J. Bi, Y. J. Bi, W. Bian, A. V. Bukevich, Q. Cao, W. Y. Cao, Zhe Cao, J. Chang, J. F. Chang, A. M. Chen, B. Q. Chen, E. S. Chen, H. X. Chen, Liang Chen, Lin Chen, Long Chen, M. J. Chen, M. L. Chen, Q. H. Chen, S. Chen, S. H. Chen, S. Z. Chen, T. L. Chen, Y. Chen, N. Cheng, Y. D. Cheng, M. C. Chu, M. Y. Cui, S. W. Cui, X. H. Cui, Y. D. Cui, B. Z. Dai, H. L. Dai, Z. G. Dai, Danzengluobu, X. Q. Dong, K. K. Duan, J. H. Fan, Y. Z. Fan, J. Fang, J. H. Fang, K. Fang, C. F. Feng, H. Feng, L. Feng, S. H. Feng, X. T. Feng, Y. Feng, Y. L. Feng, S. Gabici, B. Gao, C. D. Gao, Q. Gao, W. Gao, W. K. Gao, M. M. Ge, T. T. Ge, L. S. Geng, G. Giacinti, G. H. Gong, Q. B. Gou, M. H. Gu, F. L. Guo, J. Guo, X. L. Guo, Y. Q. Guo, Y. Y. Guo, Y. A. Han, O. A. Hannuksela, M. Hasan, H. H. He, H. N. He, J. Y. He, Y. He, Y. K. Hor, B. W. Hou, C. Hou, X. Hou, H. B. Hu, Q. Hu, S. C. Hu, C. Huang, D. H. Huang, T. Q. Huang, W. J. Huang, X. T. Huang, X. Y. Huang, Y. Huang, Y. Y. Huang, X. L. Ji, H. Y. Jia, K. Jia, H. B. Jiang, K. Jiang, X. W. Jiang, Z. J. Jiang, M. Jin, M. M. Kang, I. Karpikov, D. Khangulyan, D. Kuleshov, K. Kurinov, B. B. Li, C. M. Li, Cheng Li, Cong Li, D. Li, F. Li, H. B. Li, H. C. Li, Jian Li, Jie Li, K. Li, S. D. Li, W. L. Li, W. L. Li, X. R. Li, Xin Li, Y. Z. Li, Zhe Li, Zhuo Li, E. W. Liang, Y. F. Liang, S. J. Lin, B. Liu, C. Liu, D. Liu, D. B. Liu, H. Liu, H. D. Liu, J. Liu, J. L. Liu, M. Y. Liu, R. Y. Liu, S. M. Liu, W. Liu, Y. Liu, Y. N. Liu, Q. Luo, Y. Luo, H. K. Lv, B. Q. Ma, L. L. Ma, X. H. Ma, J. R. Mao, Z. Min, W. Mitthumsiri, H. J. Mu, Y. C. Nan, A. Neronov, K. C. Y. Ng, L. J. Ou, P. Pattarakijwanich, Z. Y. Pei, J. C. Qi, M. Y. Qi, B. Q. Qiao, J. J. Qin, A. Raza, D. Ruffolo, A. Sáiz, M. Saeed, D. Semikoz, L. Shao, O. Shchegolev, X. D. Sheng, F. W. Shu, H. C. Song, Yu. V. Stenkin, V. Stepanov, Y. Su, D. X. Sun, Q. N. Sun, X. N. Sun, Z. B. Sun, J. Takata, P. H. T. Tam, Q. W. Tang, R. Tang, Z. B. Tang, W. W. Tian, L. H. Wan, C. Wang, C. B. Wang, G. W. Wang, H. G. Wang, H. H. Wang, J. C. Wang, Kai Wang, Kai Wang, L. P. Wang, L. Y. Wang, P. H. Wang, R. Wang, W. Wang, X. G. Wang, X. Y. Wang, Y. Wang, Y. D. Wang, Y. J. Wang, Z. H. Wang, Z. X. Wang, Zhen Wang, Zheng Wang, D. M. Wei, J. J. Wei, Y. J. Wei, T. Wen, C. Y. Wu, H. R. Wu, Q. W. Wu, S. Wu, X. F. Wu, Y. S. Wu, S. Q. Xi, J. Xia, G. M. Xiang, D. X. Xiao, G. Xiao, Y. L. Xin, Y. Xing, D. R. Xiong, Z. Xiong, D. L. Xu, R. F. Xu, R. X. Xu, W. L. Xu, L. Xue, D. H. Yan, J. Z. Yan, T. Yan, C. W. Yang, C. Y. Yang, F. Yang, F. F. Yang, L. L. Yang, M. J. Yang, R. Z. Yang, W. X. Yang, Y. H. Yao, Z. G. Yao, L. Q. Yin, N. Yin, X. H. You, Z. Y. You, Y. H. Yu, Q. Yuan, H. Yue, H. D. Zeng, T. X. Zeng, W. Zeng, M. Zha, B. B. Zhang, F. Zhang, H. Zhang, H. M. Zhang, H. Y. Zhang, J. L. Zhang, Li Zhang, P. F. Zhang, P. P. Zhang, R. Zhang, S. B. Zhang, S. R. Zhang, S. S. Zhang, X. Zhang, X. P. Zhang, Y. F. Zhang, Yi Zhang, Yong Zhang, B. Zhao, J. Zhao, L. Zhao, L. Z. Zhao, S. P. Zhao, X. H. Zhao, F. Zheng, W. J. Zhong, B. Zhou, H. Zhou, J. N. Zhou, M. Zhou, P. Zhou, R. Zhou, X. X. Zhou, X. X. Zhou, B. Y. Zhu, C. G. Zhu, F. R. Zhu, H. Zhu, K. J. Zhu, Y. C. Zou, X. Zuo

TL;DR

This study uses LHAASO WCDA and KM2A data to characterize 1LHAASO J1857+0203u in the G35.6 region, finding an extended TeV component and a separate sub-PeV, point-like component that together imply a genuine PeVatron. The authors compare three physical origins: hadronic acceleration in the HIIR G35.6-0.5, hadronic illumination of nearby clouds by an SNR, and a leptonic evolved PWN, constrained by multi-wavelength results including CO/HI gas maps, Gaia OB-star searches, and radio/X-ray upper limits. They find the HIIR hadronic scenario can explain the GeV–PeV emission if the local gas density is sufficient and Ep,cut is around a few hundred TeV, but OB star populations are not observed; the SNR-cloud scenario is disfavored for isotropic diffusion given measured cloud masses, though anisotropic diffusion could help. A PWN origin remains plausible but requires a powering pulsar that has not yet been detected, and future observations with high-resolution gamma-ray instruments could help distinguish the competing scenarios. Overall, the work highlights the complexity of identifying Galactic PeVatrons and underscores the need for complementary multi-wavelength data to pinpoint the acceleration site and mechanism.

Abstract

Identifying Galactic PeVatrons (PeV particle accelerators) from the ultra-high-energy (UHE, >100 TeV) $γ$-ray sources plays a crucial role in revealing the origin of Galactic cosmic rays. The UHE source 1LHAASO J1857+0203u is suggested to be associated with HESS J1858+020, which may be attributed to the possible PeVatron candidate supernova remnant (SNR) G35.6$-$0.4 or HII region G35.6$-$0.5. We perform detailed analysis on the very-high-energy and UHE $γ$-ray emissions towards this region with data from the Large High Altitude Air Shower Observatory (LHAASO). 1LHAASO J1857+0203u is detected with a significance of 11.6$σ$ above 100 TeV, indicating the presence of a PeVatron. It has an extension of $\sim 0.18^\circ$ with a power-law (PL) spectral index of $\sim$2.5 in 1-25 TeV and a point-like emission with a PL spectral index of $\sim$3.2 above 25 TeV. Using the archival CO and HI data, we identify some molecular and atomic clouds that may be associated with the TeV $γ$-ray emissions. Our modelling indicates that the TeV $γ$-ray emissions are unlikely to arise from the clouds illuminated by the protons that escaped from SNR G35.6$-$0.4. In the scenario that HII region G35.6$-$0.5 could accelerate particles to the UHE band, the observed GeV-TeV $γ$-ray emission could be well explained by a hadronic model with a PL spectral index of $\sim$2.0 and cutoff energy of $\sim$450 TeV. However, an evolved pulsar wind nebula origin cannot be ruled out.

An Enigmatic PeVatron in an Area around HII Region G35.6$-$0.5

TL;DR

This study uses LHAASO WCDA and KM2A data to characterize 1LHAASO J1857+0203u in the G35.6 region, finding an extended TeV component and a separate sub-PeV, point-like component that together imply a genuine PeVatron. The authors compare three physical origins: hadronic acceleration in the HIIR G35.6-0.5, hadronic illumination of nearby clouds by an SNR, and a leptonic evolved PWN, constrained by multi-wavelength results including CO/HI gas maps, Gaia OB-star searches, and radio/X-ray upper limits. They find the HIIR hadronic scenario can explain the GeV–PeV emission if the local gas density is sufficient and Ep,cut is around a few hundred TeV, but OB star populations are not observed; the SNR-cloud scenario is disfavored for isotropic diffusion given measured cloud masses, though anisotropic diffusion could help. A PWN origin remains plausible but requires a powering pulsar that has not yet been detected, and future observations with high-resolution gamma-ray instruments could help distinguish the competing scenarios. Overall, the work highlights the complexity of identifying Galactic PeVatrons and underscores the need for complementary multi-wavelength data to pinpoint the acceleration site and mechanism.

Abstract

Identifying Galactic PeVatrons (PeV particle accelerators) from the ultra-high-energy (UHE, >100 TeV) -ray sources plays a crucial role in revealing the origin of Galactic cosmic rays. The UHE source 1LHAASO J1857+0203u is suggested to be associated with HESS J1858+020, which may be attributed to the possible PeVatron candidate supernova remnant (SNR) G35.60.4 or HII region G35.60.5. We perform detailed analysis on the very-high-energy and UHE -ray emissions towards this region with data from the Large High Altitude Air Shower Observatory (LHAASO). 1LHAASO J1857+0203u is detected with a significance of 11.6 above 100 TeV, indicating the presence of a PeVatron. It has an extension of with a power-law (PL) spectral index of 2.5 in 1-25 TeV and a point-like emission with a PL spectral index of 3.2 above 25 TeV. Using the archival CO and HI data, we identify some molecular and atomic clouds that may be associated with the TeV -ray emissions. Our modelling indicates that the TeV -ray emissions are unlikely to arise from the clouds illuminated by the protons that escaped from SNR G35.60.4. In the scenario that HII region G35.60.5 could accelerate particles to the UHE band, the observed GeV-TeV -ray emission could be well explained by a hadronic model with a PL spectral index of 2.0 and cutoff energy of 450 TeV. However, an evolved pulsar wind nebula origin cannot be ruled out.

Paper Structure

This paper contains 16 sections, 5 equations, 10 figures, 7 tables.

Table of Contents

  1. Introduction
  2. LHAASO data analysis and results
  3. WCDA analysis and results
  4. KM2A analysis and results
  5. Multi-wavelength studies
  6. Molecular environment
  7. Atomic clouds
  8. Search for OB stars
  9. Radio and X-ray observations
  10. Discussion
  11. HII region scenario
  12. Clouds illuminated by particles escaped from SNR
  13. PWN scenario
  14. Conclusion
  15. Gamma-ray absorption
  16. ...and 1 more sections

Figures (10)

  • Figure 1: LHAASO TS maps of a $5 \degree \times 5\degree$ region around 1LHAASO J1857$+$0203u. The white box indicates the ROI. The black dashed circles show the WCDA or KM2A components resolved in the first LHAASO catalogue. The cyan markers show the positions of potential TeV or multi-TeV counterparts detected by H.E.S.S. (box), MAGIC (diamond), and HAWC (cross). (a) WCDA TS map ($1<E_{\gamma}<25$ TeV). The black solid circles (with radii of $r_{\rm 39}$ extensions) show the WCDA components resolved in this work. (b) KM2A TS map ($E_{\gamma}>25$ TeV). The black crosses (corresponding to point-like sources) and black solid circles (with radii of $r_{\rm 39}$ extensions) show the KM2A components resolved in this work.
  • Figure 2: Differential energy spectrum of the TeV source 1LHAASO J1857$+$0203u. The blue dots and red squares represent the WCDA and KM2A data measured in this work, respectively. The combinations of statistics and systematic errors are shown. The 95% confidence upper limits are displayed with downward arrows. The grey diamonds show the H.E.S.S. flux data points hess08hess18.
  • Figure 3: TS map of a $1 \degree \times 1\degree$ region around 1LHAASO J1857$+$0203u as observed by LHAASO above 100 TeV. The blue and red crosses with $\sigma_{\rm 95}$ statistical position error circles (dashed) show the WCDA and KM2A components resolved in this work, respectively. The solid blue circle marks the $r_{\rm 39}$ extension circle of the WCDA component. The green crosses with $\sigma_{\rm 95}$ statistical position error circles (dashed) show the GeV $\gamma$-ray point-like sources (SrcA & SrcB) detected by zhang22. The magenta cross ('+') represents the position of the UHE $\gamma$-ray point-like source detected by HAWC hawc23. The yellow stars represent the PSRs located within a radius of 0.5° from the TeV source. The black contours show the TeV source HESS J1858$+$020 with 5$\sigma$, 6$\sigma$, and 7$\sigma$ significance hess18. The cyan contours indicate the GMRT 610 MHz radio emission paredes14, where the larger shell-like structure corresponds to SNR G35.6$-$0.4 and the smaller ring-shaped structure corresponds to HIIR G35.6$-$0.5. The line segment at the top-right corner shows the size of the PSF (68% containment).
  • Figure 4: Spatial distribution of the MCs. FUGIN $^{12}$CO ($J$=1--0) intensity maps integrated each 2 km s$^{-1}$ in the velocity range $+50$--$+68$ km s$^{-1}$, overlaid with black contours of GMRT 610 MHz radio emission (same as the cyan contours in Fig. \ref{['fig:tsmap']}) and white TS contours for 1LHAASO J1857$+$0203u above 100 TeV at levels of 100, 110, and 120. The yellow and red boxes marked with 'W' and 'K' in the top left panel show the regions that are used to estimate the parameters of the $+50$--$+70$ km s$^{-1}$ molecular gases. The green box in the middle left panel ($+$56--$+$58 km s$^{-1}$, corresponds to a near distance of $3.4\pm0.4$ kpc) marks the region where we search for OB stars, and the green crosses mark the positions of the OB star candidates.
  • Figure 5: HI column density map calculated in the velocity range of $+50$--$+70$ km s$^{-1}$. The crosses and circles are the same as those in Figure \ref{['fig:tsmap']}. The white box marked with 'A' shows the region used to estimate the parameters of the $+50$--$+70$ km s$^{-1}$ atomic gases.
  • ...and 5 more figures