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Study of Ultra-High-Energy Gamma-Ray Source 1LHAASO J0056+6346u and Its Possible Origins

Zhen Cao, F. Aharonian, Y. X. Bai, Y. W. Bao, D. Bastieri, X. J. Bi, Y. J. Bi, W. Bian, J. Blunier, A. V. Bukevich, C. M. Cai, Y. Y. Cai, W. Y. Cao, Zhe Cao, J. Chang, J. F. Chang, E. S. Chen, G. H. Chen, H. K. Chen, L. F. Chen, Liang Chen, Long Chen, M. J. Chen, M. L. Chen, Q. H. Chen, S. Chen, S. H. Chen, S. Z. Chen, T. L. Chen, X. B. Chen, X. J. Chen, X. P. Chen, Y. Chen, N. Cheng, Q. Y. Cheng, Y. D. Cheng, M. Y. Cui, S. W. Cui, X. H. Cui, Y. D. Cui, B. Z. Dai, H. L. Dai, Z. G. Dai, Danzengluobu, Y. X. Diao, A. J. Dong, 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, 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, K. J. Guo, X. L. Guo, Y. Q. Guo, Y. Y. Guo, R. P. Han, O. A. Hannuksela, M. Hasan, H. H. He, H. N. He, J. Y. He, X. Y. He, Y. He, S. Hernández-Cadena, B. W. Hou, C. Hou, X. Hou, H. B. Hu, S. C. Hu, C. Huang, D. H. Huang, J. J. Huang, X. L. Huang, X. T. Huang, X. Y. Huang, Y. Huang, Y. Y. Huang, A. Inventar, X. L. Ji, H. Y. Jia, K. Jia, H. B. Jiang, K. Jiang, X. W. Jiang, Z. J. Jiang, M. Jin, S. Kaci, M. M. Kang, I. Karpikov, D. Khangulyan, D. Kuleshov, K. Kurinov, Cheng Li, Cong Li, D. Li, F. Li, H. B. Li, H. C. Li, Jian Li, Jie Li, K. Li, L. Li, R. L. Li, S. D. Li, T. Y. Li, W. L. Li, X. R. Li, Xin Li, Y. 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, J. Liu, J. L. Liu, J. R. Liu, M. Y. Liu, R. Y. Liu, S. M. Liu, W. Liu, X. Liu, Y. Liu, Y. Liu, Y. N. Liu, Y. Q. Lou, Q. Luo, Y. Luo, H. K. Lv, B. Q. Ma, L. L. Ma, X. H. Ma, I. O. Maliy, J. R. Mao, Z. Min, W. Mitthumsiri, Y. Mizuno, G. B. Mou, A. Neronov, K. C. Y. Ng, M. Y. Ni, L. Nie, L. J. Ou, Z. W. Ou, P. Pattarakijwanich, Z. Y. Pei, D. Y. Peng, J. C. Qi, M. Y. Qi, J. J. Qin, D. Qu, A. Raza, C. Y. Ren, D. Ruffolo, A. Sáiz, D. Savchenko, D. Semikoz, L. Shao, O. Shchegolev, Y. Z. Shen, X. D. Sheng, Z. D. Shi, F. W. Shu, H. C. Song, Yu. V. Stenkin, V. Stepanov, Y. Su, D. X. Sun, H. Sun, J. X. Sun, Q. N. Sun, X. N. Sun, Z. B. Sun, N. H. Tabasam, J. Takata, P. H. T. Tam, H. B. Tan, Q. W. Tang, R. Tang, Z. B. Tang, W. W. Tian, C. N. Tong, L. H. Wan, C. Wang, D. H. Wang, G. W. Wang, H. G. Wang, J. C. Wang, K. Wang, Kai Wang, Kai Wang, L. P. Wang, L. Y. Wang, L. Y. Wang, R. Wang, W. Wang, X. G. Wang, X. J. Wang, X. Y. Wang, Y. Wang, Y. D. Wang, Z. H. Wang, Z. X. Wang, Zheng Wang, D. M. Wei, J. J. Wei, Y. J. Wei, T. Wen, S. S. Weng, C. Y. Wu, H. R. Wu, Q. W. Wu, S. Wu, X. F. Wu, Y. S. Wu, S. Q. Xi, J. Xia, J. J. Xia, G. M. Xiang, D. X. Xiao, G. Xiao, Y. F. Xiao, Y. L. Xin, H. D. Xing, Y. Xing, D. R. Xiong, B. N. Xu, C. Y. Xu, D. L. Xu, R. F. Xu, R. X. Xu, S. S. Xu, W. L. Xu, L. Xue, D. H. Yan, T. Yan, C. W. Yang, C. Y. Yang, F. F. Yang, L. L. Yang, M. J. Yang, R. Z. Yang, W. X. Yang, Z. H. Yang, Z. G. Yao, X. A. Ye, L. Q. Yin, N. Yin, X. H. You, Z. Y. You, Q. Yuan, H. Yue, H. D. Zeng, T. X. Zeng, W. Zeng, X. T. Zeng, M. Zha, B. B. Zhang, B. T. Zhang, C. Zhang, H. Zhang, H. M. Zhang, H. Y. Zhang, J. L. Zhang, J. Y. Zhang, Li Zhang, P. F. Zhang, R. Zhang, S. R. Zhang, S. S. Zhang, S. Y. Zhang, W. Zhang, W. Y. Zhang, X. Zhang, X. P. Zhang, Yi Zhang, Yong Zhang, Z. P. Zhang, J. Zhao, L. Zhao, L. Z. Zhao, S. P. Zhao, X. H. Zhao, Z. H. Zhao, F. Zheng, T. C. Zheng, 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

The study addresses the nature of the unidentified extended UHE γ-ray source 1LHAASO J0056+6346u by analyzing 979 days of WCDA and 1389 days of KM2A data from LHAASO, applying a 3D likelihood analysis to extract morphology and spectrum. The source is extended ($r_{39}\approx0.34^\circ$ at 1–25 TeV and $r_{39}\approx0.24^\circ$ above 25 TeV) and has a differential spectrum well described by an exponential cutoff power-law with index $\alpha\approx1.97$ and cutoff $E_{\rm cut}\approx55$ TeV, with a significant preference for an extended ECPL model at high energies. Multiwavelength analysis shows no clear GeV counterpart in Fermi-LAT data and demonstrates that both leptonic IC and hadronic pp scenarios can fit the GeV–TeV SED, implying the gamma-ray data alone cannot distinguish the emission mechanism. Gas-rich regions along the line of sight, identified with MWISP CO/13CO and CGPS HI data, reveal two velocity components that could serve as target material for CR illumination, while potential accelerators include SNR G124.0+1.4 and the Cas OB7 association; a pulsar-origin PWN/pulsar halo remains a natural explanation for the observed morphology and spectrum, though definitive association requires future hard X-ray observations and more precise gas/distance measurements.

Abstract

We report a dedicated study of the newly discovered extended UHE $γ$-ray source 1LHAASO J0056+6346u. Analyzing 979 days of LHAASO-WCDA data and 1389 days of LHAASO-KM2A data, we observed a significant excess of $γ$-ray events with both WCDA and KM2A. Assuming a point power-law source with a fixed spectral index, the significance maps reveal excesses of ${\sim}12.65\,σ$, ${\sim}22.18\,σ$, and ${\sim}10.24\,σ$ in the energy ranges of 1--25 TeV, 25--100 TeV, and $> 100$ TeV, respectively. We use a 3D likelihood algorithm to derive the morphological and spectral parameters, and the source is detected with significances of $12.65\,σ$ by WCDA and $25.27\,σ$ by KM2A. The best-fit positions derived from WCDA and KM2A data are (R.A. = $13.96^\circ\pm0.09^\circ$, Decl. = $63.92^\circ\pm0.05^\circ$) and (R.A. = $14.00^\circ\pm0.05^\circ$, Decl. = $63.79^\circ\pm0.02^\circ$), respectively. The angular size ($r_{39}$) of 1LHAASO J0056+6346u is $0.34^\circ\pm0.04^\circ$ at 1--25 TeV and $0.24^\circ\pm0.02^\circ$ at $> 25$ TeV. The differential flux of this UHE $γ$-ray source can be described by an exponential cutoff power-law function: $(2.67\pm0.25) \times 10^{-15} (E/20\,\text{TeV})^{-1.97\pm0.10} e^{-E/(55.1\pm7.2)\,\text{TeV}} \,\text{TeV}^{-1}\,\text{cm}^{-2}\,\text{s}^{-1}$. To explore potential sources of $γ$-ray emission, we investigated the gas distribution around 1LHAASO J0056+6346u. 1LHAASO J0056+6346u is likely to be a TeV PWN powered by an unknown pulsar, which would naturally explain both its spatial and spectral properties. Another explanation is that this UHE $γ$-ray source might be associated with gas content illuminated by a nearby CR accelerator, possibly the SNR candidate G124.0+1.4.

Study of Ultra-High-Energy Gamma-Ray Source 1LHAASO J0056+6346u and Its Possible Origins

TL;DR

The study addresses the nature of the unidentified extended UHE γ-ray source 1LHAASO J0056+6346u by analyzing 979 days of WCDA and 1389 days of KM2A data from LHAASO, applying a 3D likelihood analysis to extract morphology and spectrum. The source is extended ( at 1–25 TeV and above 25 TeV) and has a differential spectrum well described by an exponential cutoff power-law with index and cutoff TeV, with a significant preference for an extended ECPL model at high energies. Multiwavelength analysis shows no clear GeV counterpart in Fermi-LAT data and demonstrates that both leptonic IC and hadronic pp scenarios can fit the GeV–TeV SED, implying the gamma-ray data alone cannot distinguish the emission mechanism. Gas-rich regions along the line of sight, identified with MWISP CO/13CO and CGPS HI data, reveal two velocity components that could serve as target material for CR illumination, while potential accelerators include SNR G124.0+1.4 and the Cas OB7 association; a pulsar-origin PWN/pulsar halo remains a natural explanation for the observed morphology and spectrum, though definitive association requires future hard X-ray observations and more precise gas/distance measurements.

Abstract

We report a dedicated study of the newly discovered extended UHE -ray source 1LHAASO J0056+6346u. Analyzing 979 days of LHAASO-WCDA data and 1389 days of LHAASO-KM2A data, we observed a significant excess of -ray events with both WCDA and KM2A. Assuming a point power-law source with a fixed spectral index, the significance maps reveal excesses of , , and in the energy ranges of 1--25 TeV, 25--100 TeV, and TeV, respectively. We use a 3D likelihood algorithm to derive the morphological and spectral parameters, and the source is detected with significances of by WCDA and by KM2A. The best-fit positions derived from WCDA and KM2A data are (R.A. = , Decl. = ) and (R.A. = , Decl. = ), respectively. The angular size () of 1LHAASO J0056+6346u is at 1--25 TeV and at TeV. The differential flux of this UHE -ray source can be described by an exponential cutoff power-law function: . To explore potential sources of -ray emission, we investigated the gas distribution around 1LHAASO J0056+6346u. 1LHAASO J0056+6346u is likely to be a TeV PWN powered by an unknown pulsar, which would naturally explain both its spatial and spectral properties. Another explanation is that this UHE -ray source might be associated with gas content illuminated by a nearby CR accelerator, possibly the SNR candidate G124.0+1.4.

Paper Structure

This paper contains 11 sections, 4 equations, 8 figures, 2 tables.

Table of Contents

  1. Introduction
  2. LHAASO Data Analysis and Results
  3. Multiwavelength observations
  4. Fermi-LAT observation
  5. $\gamma$-ray spectral fitting
  6. Study of the gas content
  7. Discussion on the origin of 1LHAASO J0056+6346u
  8. A PWN or pulsar halo from an unknown pulsar
  9. SNR candidate G124.0+1.4
  10. OB association Cas OB7
  11. Conclusions

Figures (8)

  • Figure 1: Significance maps of 1LHAASO J0056+6346u (zoomed-in view smaller than ROI) derived from the prior model, with GDE subtracted. These figures show the significance maps for the energy ranges (a) 1 -- 25 TeV, (b) 25 -- 100 TeV, and (c) above 100 TeV, respectively.
  • Figure 2: Extension and spectrum of 1LHAASO J0056+6346u. (a-b) Distribution of events as a function of square of the angular offset to 1LHAASO J0056+6346u for data (red points, the observed number of events minus the CR background), expected number of events from the source (blue region), as well as the GDE (grey region). Panel (a) displays the events detected by WCDA in the $1-25\,{\rm TeV}$ range, while panel (b) shows the events detected by KM2A with energy above $25\,{\rm TeV}$. (c) Energy dependence of source extension, with the red line representing the energy-independent model and the green line representing the power-law model. (d) The spectrum of 1LHAASO J0056+6346u, with the dotted line representing the single power-law fit and the solid line representing the ECPL fit.
  • Figure 3: Fermi-LAT residual TS map around 1LHAASO J0056+6346u. The yellow circle and red circle indicate the intrinsic extensions of 1LHAASO J0056+6346u, derived from KM2A and WCDA data, respectively. The green cross marks the position of 4FGL J0057.9+6326. The magenta triangle indicates the position of the X-ray source PBC J0057.2+6401.
  • Figure 4: (a) SED fitting results for the PP and IC models based on spectral data from WCDA (blue), KM2A (red), and Fermi-LAT (magenta) observations, with 4FGL J0057.9+6326 fluxes shown as green points for reference. (b) SED fitting results of Syn+IC models. The orange data points represent the radio flux of G124.0+1.4 and the cyan data points indicate the flux of the X-ray source PBC J0057.2+6401. For lines of the same color, solid lines represent results using the ECPL distribution, whereas dotted lines correspond to results using the BPL distribution. The gas density, background photons, or the magnetic field applied for different models are indicated in the legend. See Sec. \ref{['subsec:sed']} for further details.
  • Figure 5: Integrated intensity maps of $^{12}$CO ($J = 1 \to 0$) line. The yellow and red circles represent the intrinsic extensions of 1LHAASO J0056+6346u, derived from KM2A and WCDA data, respectively. The green cross marks the position of 4FGL J0057.9+6326. The magenta triangle marks the X-ray source PBC J0057.2+6401. (a) $^{12}$CO integrated intensity map over the velocity range $[-48, -40] \rm\ km\ s^{-1}$. The cyan dashed circle shows the Cas OB7 association. The cyan diagonal cross symbols show the positions of stars in Cas OB7. (b) $[-15, -7] \rm\ km\ s^{-1}$. The red region represents the area used for distance measurement. See Supplementary Materials. (c) $[-1, 7] \rm\ km\ s^{-1}$. The contour shows the radio emission in the Urumqi 6 cm survey Urumqi_image. The bright region in the upper left shows the radio emission from the SNR candidate G124.0+1.4. (d) $[-54, -48] \rm\ km\ s^{-1}$, which corresponds to one of the velocity ranges shown in Fig. \ref{['fig:12CO_line']}. (e) $[-12, -4] \rm\ km\ s^{-1}$, which corresponds to the other velocity range shown in Fig. \ref{['fig:12CO_line']}. The color axis uses a logarithmic scale for clarity. (f) $[-60, 10] \rm\ km\ s^{-1}$. The integrated intensity map covers the entire velocity range we investigated.
  • ...and 3 more figures