Two-Component gamma-ray Emission Spectrum and X-Ray Polarization of the Radio Galaxy Pictor A
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arXiv
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| Natura: | Preprint |
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2024
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| _version_ | 1866911028685570048 |
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| author | Li, Jia-Xuan Hu, Xin-Ke Lian, Ji-Shun Yu, Yu-Wei Deng, Wei Liu, Kuan Zhang, Hai-Ming Chen, Liang Zhang, Jin |
| author_facet | Li, Jia-Xuan Hu, Xin-Ke Lian, Ji-Shun Yu, Yu-Wei Deng, Wei Liu, Kuan Zhang, Hai-Ming Chen, Liang Zhang, Jin |
| contents | Pictor A is a $γ$-ray emitting radio galaxy and has a bright hotspot called WHS, located $\sim$4 arcmin away from the nucleus. In this work, we present an analysis of its 16-year Fermi-LAT data and report the Imaging X-ray Polarimetry Explorer (IXPE) observations for this source. Our analysis of the Fermi-LAT observations reveals evidence of two components in the average $γ$-ray spectrum of Pictor A, exhibiting a statistically significant hardening from $Γ_{\rm γ,1}=3.25\pm0.15$ to $Γ_{\rm γ,2}=1.81\pm0.07$ at a break energy of $2.46\pm0.09$ GeV. Notably, variability of $γ$-rays is evident in Pictor A, predominantly driven by the component below the break energy, while the component above the break energy remains stable. Furthermore, our analysis reveals that a power-law function provides an adequate fit for the high-flux-state spectrum, while a broken power-law function remains necessary to accurately model the low-flux-state spectrum. We suggest that the low-energy component originates from the nucleus, while the high-energy component primarily stems from WHS. The broadband spectral energy distributions of both nucleus and WHS can be well represented by a simple leptonic model, with both $γ$-ray components attributed to the synchrotron-self-Compton (SSC) process. Analysis of IXPE data provides upper limits on the polarization degree of $Π_{\rm X}<$6.6% for the nucleus and $Π_{\rm X}<$56.4% for the WHS within the 2--8 keV band. For the nucleus, this result aligns with X-ray emission originating from the SSC process. However, the upper limit of $Π_{\rm X}<$56.4% for WHS is insufficient to conclusively determine the X-ray emission mechanism in this region. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2407_21277 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | Two-Component gamma-ray Emission Spectrum and X-Ray Polarization of the Radio Galaxy Pictor A Li, Jia-Xuan Hu, Xin-Ke Lian, Ji-Shun Yu, Yu-Wei Deng, Wei Liu, Kuan Zhang, Hai-Ming Chen, Liang Zhang, Jin High Energy Astrophysical Phenomena Pictor A is a $γ$-ray emitting radio galaxy and has a bright hotspot called WHS, located $\sim$4 arcmin away from the nucleus. In this work, we present an analysis of its 16-year Fermi-LAT data and report the Imaging X-ray Polarimetry Explorer (IXPE) observations for this source. Our analysis of the Fermi-LAT observations reveals evidence of two components in the average $γ$-ray spectrum of Pictor A, exhibiting a statistically significant hardening from $Γ_{\rm γ,1}=3.25\pm0.15$ to $Γ_{\rm γ,2}=1.81\pm0.07$ at a break energy of $2.46\pm0.09$ GeV. Notably, variability of $γ$-rays is evident in Pictor A, predominantly driven by the component below the break energy, while the component above the break energy remains stable. Furthermore, our analysis reveals that a power-law function provides an adequate fit for the high-flux-state spectrum, while a broken power-law function remains necessary to accurately model the low-flux-state spectrum. We suggest that the low-energy component originates from the nucleus, while the high-energy component primarily stems from WHS. The broadband spectral energy distributions of both nucleus and WHS can be well represented by a simple leptonic model, with both $γ$-ray components attributed to the synchrotron-self-Compton (SSC) process. Analysis of IXPE data provides upper limits on the polarization degree of $Π_{\rm X}<$6.6% for the nucleus and $Π_{\rm X}<$56.4% for the WHS within the 2--8 keV band. For the nucleus, this result aligns with X-ray emission originating from the SSC process. However, the upper limit of $Π_{\rm X}<$56.4% for WHS is insufficient to conclusively determine the X-ray emission mechanism in this region. |
| title | Two-Component gamma-ray Emission Spectrum and X-Ray Polarization of the Radio Galaxy Pictor A |
| topic | High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2407.21277 |