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Discovery of the Ultra-high energy gamma-ray source LHAASO J2108+5157

The LHAASO collaboration, Zhen Cao, F. Aharonian, Q. An, Axikegu, L. X. Bai, Y. X. Bai, Y. W. Bao, D. Bastieri, X. J. Bi, Y. J. Bi, H. Cai, J. T. Cai, Zhe Cao, J. Chang, J. F. Chang, B. M. Chen, E. S. Chen, J. Chen, Liang Chen, Liang Chen, Long Chen, M. J. Chen, M. L. Chen, Q. H. Chen, S. H. Chen, S. Z. Chen, T. L. Chen, X. L. Chen, Y. Chen, N. Cheng, Y. D. Cheng, S. W. Cui, X. H. Cui, Y. D. Cui, B. D Ettorre Piazzoli, B. Z. Dai, H. L. Dai, Z. G. Dai, Danzengluobu, D. della Volpe, X. J. Dong, K. K. Duan, J. H. Fan, Y. Z. Fan, Z. X. Fan, J. Fang, K. Fang, C. F. Feng, L. Feng, S. H. Feng, Y. L. Feng, B. Gao, C. D. Gao, L. Q. Gao, Q. Gao, W. Gao, M. M. Ge, L. S. Geng, G. H. Gong, Q. B. Gou, M. H. Gu, F. L. Guo, J. G. Guo, X. L. Guo, Y. Q. Guo, Y. Y. Guo, Y. A. Han, H. H. He, H. N. He, J. C. He, S. L. He, X. B. He, Y. He, M. Heller, Y. K. Hor, C. Hou, H. B. Hu, S. Hu, S. C. Hu, X. J. Hu, D. H. Huang, Q. L. Huang, W. H. Huang, X. T. Huang, X. Y. Huang, Z. C. Huang, F. Ji, X. L. Ji, H. Y. Jia, K. Jiang, Z. J. Jiang, C. Ji, T. Ke, D. Kuleshov, K. Levochkin, B. B. Li, Cheng Li, Cong Li, F. Li, H. B. Li, H. C. Li, H. Y. Li, J. Li, K. Li, W. L. Li, X. R. Li, Xin Li, Xin Li, Y. Li, Y. Z. Li, Zhe Li, Zhuo Li, E. W. Liang, Y. F. Liang, S. J. Lin, B. Liu, C. Liu, D. Liu, H. Liu, H. D. Liu, J. Liu, J. L. Liu, J. S. Liu, J. Y. Liu, M. Y. Liu, R. Y. Liu, S. M. Liu, W. Liu, Y. Liu, Y. N. Liu, Z. X. Liu, W. J. Long, R. Lu, H. K. Lv, B. Q. Ma, L. L. Ma, X. H. Ma, J. R. Mao, A. Masood, Z. Min, W. Mitthumsiri, T. Montaruli, Y. C. Nan, B. Y. Pang, P. Pattarakijwanich, Z. Y. Pei, M. Y. Qi, Y. Q. Qi, B. Q. Qiao, J. J. Qin, D. Ruffolo, V. Rulev, A. Sáiz, L. Shao, O. Shchegolev, X. D. Sheng, J. Y. Shi, H. C. Song, Yu. V. Stenkin, V. Stepanov, Y. Su, Q. N. Sun, X. N. Sun, Z. B. Sun, P. H. T. Tam, Z. B. Tang, W. W. Tian, B. D. Wang, C. Wang, H. Wang, H. G. Wang, J. C. Wang, J. S. Wang, L. P. Wang, L. Y. Wang, R. N. Wang, W. Wang, W. Wang, X. G. Wang, X. J. Wang, X. Y. Wang, Y. Wang, Y. D. Wang, Y. J. Wang, Y. P. Wang, Z. H. Wang, Z. . Wang, Zhen Wang, Zheng Wang, D. M. Wei, J. J. Wei, Y. J. Wei, T. Wen, C. Y. Wu, H. R. Wu, S. Wu, W. X. Wu, X. F. Wu, S. Q. Xi, J. Xia, J. J. Xia, G. M. Xiang, D. X. Xiao, G. Xiao, H. B. Xiao, G. G. Xin, Y. L. Xin, Y. Xing, D. L. Xu, R. X. Xu, L. Xue, D. H. Yan, J. Z. Yan, C. W. Yang, F. F. Yang, J. Y. Yang, L. L. Yang, M. J. Yang, R. Z. Yang, S. B. Yang, Y. H. Yao, Z. G. Yao, Y. M. Ye, L. Q. Yin, N. Yin, X. H. You, Z. Y. You, Y. H. Yu, Q. Yuan, H. D. Zeng, T. X. Zeng, W. Zeng, Z. K. Zeng, M. Zha, X. X. Zhai, B. B. Zhang, H. M. Zhang, H. Y. Zhang, J. L. Zhang, J. W. Zhang, L. X. Zhang, Li Zhang, Lu Zhang, P. F. Zhang, P. P. Zhang, R. Zhang, S. R. Zhang, S. S. Zhang, X. Zhang, X. P. Zhang, Y. F. Zhang, Y. L. Zhang, Yi Zhang, Yong Zhang, B. Zhao, J. Zhao, L. Zhao, L. Z. Zhao, S. P. Zhao, F. Zheng, Y. Zheng, B. Zhou, H. Zhou, J. N. Zhou, P. Zhou, R. Zhou, X. X. Zhou, C. G. Zhu, F. R. Zhu, H. Zhu, K. J. Zhu, X. Zuo

TL;DR

This paper reports the discovery of LHAASO J2108+5157, a new ultra-high-energy gamma-ray source detected by the KM2A sub-array of LHAASO with a significance of $9.6\sigma$ in 25–100 TeV and $8.5\sigma$ above 100 TeV over 308.33 live days, presenting a hard power-law spectrum $dN/dE \propto E^{-2.83}$ from $20$ to $500$ TeV and a 95% upper limit on the extension of $0.26^{\circ}$. The source is spatially coincident with a dense molecular cloud [MML2017]4607, and no obvious VHE counterpart is found, favoring a hadronic origin via CR protons interacting with ambient gas, though a leptonic pulsar-wind nebula scenario remains possible in the absence of identified pulsars. A detailed multiwavelength analysis, including Fermi-LAT, CO surveys, radio, and X-ray data, shows an extended GeV component near the region and highlights the importance of the cloud association for interpreting the UHE emission. The findings demonstrate LHAASO-KM2A's capability to probe Galactic PeVatrons and underscore the need for deeper multiwavelength follow-up to pin down the acceleration mechanism and the nature of this intriguing source.

Abstract

We report the discovery of a UHE gamma-ray source, LHAASO J2108+5157, by analyzing the LHAASO-KM2A data of 308.33 live days. Significant excess of gamma-ray induced showers is observed in both energy bands of 25-100 TeV and $\gt$100 TeV with 9.5 sigma and 8.5 sigma, respectively. This source is not significantly favored as an extensive source with the angular extension smaller than the point-spread function of KM2A. The measured energy spectrum from 20 to 200 TeV can be approximately described by a power-law function with an index of -2.83$\pm$ 0.18stat. A harder spectrum is demanded at lower energies considering the flux upper limit set by Fermi-LAT observations. The position of the gamma-ray emission is correlated with a giant molecular cloud, which favors a hadronic origin. No obvious counterparts have been found, deeper multiwavelength observations will help to shed new light on this intriguing UHE source.

Discovery of the Ultra-high energy gamma-ray source LHAASO J2108+5157

TL;DR

This paper reports the discovery of LHAASO J2108+5157, a new ultra-high-energy gamma-ray source detected by the KM2A sub-array of LHAASO with a significance of in 25–100 TeV and above 100 TeV over 308.33 live days, presenting a hard power-law spectrum from to TeV and a 95% upper limit on the extension of . The source is spatially coincident with a dense molecular cloud [MML2017]4607, and no obvious VHE counterpart is found, favoring a hadronic origin via CR protons interacting with ambient gas, though a leptonic pulsar-wind nebula scenario remains possible in the absence of identified pulsars. A detailed multiwavelength analysis, including Fermi-LAT, CO surveys, radio, and X-ray data, shows an extended GeV component near the region and highlights the importance of the cloud association for interpreting the UHE emission. The findings demonstrate LHAASO-KM2A's capability to probe Galactic PeVatrons and underscore the need for deeper multiwavelength follow-up to pin down the acceleration mechanism and the nature of this intriguing source.

Abstract

We report the discovery of a UHE gamma-ray source, LHAASO J2108+5157, by analyzing the LHAASO-KM2A data of 308.33 live days. Significant excess of gamma-ray induced showers is observed in both energy bands of 25-100 TeV and 100 TeV with 9.5 sigma and 8.5 sigma, respectively. This source is not significantly favored as an extensive source with the angular extension smaller than the point-spread function of KM2A. The measured energy spectrum from 20 to 200 TeV can be approximately described by a power-law function with an index of -2.83 0.18stat. A harder spectrum is demanded at lower energies considering the flux upper limit set by Fermi-LAT observations. The position of the gamma-ray emission is correlated with a giant molecular cloud, which favors a hadronic origin. No obvious counterparts have been found, deeper multiwavelength observations will help to shed new light on this intriguing UHE source.

Paper Structure

This paper contains 12 sections, 2 equations, 7 figures, 2 tables.

Table of Contents

  1. INTRODUCTION
  2. LHAASO OBSERVATIONS AND RESULTS
  3. The LHAASO Detector Array
  4. Analysis Methods
  5. Results
  6. DISCUSSION
  7. Searching for counterparts at other wavelengths
  8. Scenarios for the origin of UHE emission
  9. summary
  10. Summary of LHAASO J2108+5157 Data
  11. Fermi-LAT analysis
  12. ${}^{12}\rm CO(1-0)$ radial velocity spectrum

Figures (7)

  • Figure 1: Left: significance map around LHAASO J2108+5157 as observed by KM2A for reconstructed energies from 25 TeV to 100 TeV. Right: significance map for energies above 100 TeV. The red cross and circle denote the best-fit position and the 95% position uncertainty of the LHAASO source. The white circle at bottom-right corner shows the size of PSF (containing 68% of the events).
  • Figure 2: Distribution of events as a function of the square of the angle between the event arrival direction and LHAASO J2108+5157 direction. The blue histogram is the expected event distribution by our Monte Carlo simulation assuming a point-like gamma-ray source.
  • Figure 3: The SED of LHAASO J2108+5157. The solid red line shows the best fit power-law function.
  • Figure 4: Brightness temperature distribution of ${}^{12}\rm CO(1-0)$ line survey integrated over a velocity interval between -14.3 and -9.1 $\rm km\ s^{-1}$ corresponding to a distance for the molecular gas of $\sim 3.26$ kpc 2001ApJ...547..792D. The white contours indicate 1420 MHz continue emission survey 2003AJ....125.3145T. The best-fitted location of the LHAASO J2108+5157 and the upper limit to its extension (68% containment radius) are indicated with a red cross and solid circle, respectively. The location of the young star cluster Kronberger 80 2016AA...585A.101K and open cluster candidate Kronberger 82 2006AA...447..921K are marked with magenta stars. The pink diamond represents the position of the binary RX J2107.3+5202. The light blue solid ellipse represent the molecular cloud [MML2017]4607. The cyan dashed circle shows the extent of 4FGL J2108.0+5155e (68% containment radius).
  • Figure 5: The multiwavelength SED of LHAASO J2108+5157 with hadronic and leptonic models. The red points and arrows are the LHAASO-KM2A observations. The blue triangles are the radio fluxes. The grey points and blue arrows are the Fermi-LAT spectral points and upper limits.
  • ...and 2 more figures