Two-component γ-Ray Emission Spectrum and X-Ray Polarization of the Radio Galaxy Pictor A
Pictor A is a γ -ray emitting radio galaxy and has a bright hotspot called the western hotspot (WHS), located ∼4′ away from the nucleus. In this work, we present an analysis of its 16 yr Fermi Large Area Telescope (LAT) data and report the Imaging X-ray Polarimetry Explorer (IXPE) observations for t...
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2025-01-01
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| Online Access: | https://doi.org/10.3847/1538-4357/ade808 |
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| author | Jia-Xuan Li Xin-Ke Hu Ji-Shun Lian Yu-Wei Yu Wei Deng Kuan Liu Hai-Ming Zhang Liang Chen Jin Zhang |
| author_facet | Jia-Xuan Li Xin-Ke Hu Ji-Shun Lian Yu-Wei Yu Wei Deng Kuan Liu Hai-Ming Zhang Liang Chen Jin Zhang |
| author_sort | Jia-Xuan Li |
| collection | DOAJ |
| description | Pictor A is a γ -ray emitting radio galaxy and has a bright hotspot called the western hotspot (WHS), located ∼4′ away from the nucleus. In this work, we present an analysis of its 16 yr Fermi Large Area Telescope (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 _γ _,1 = 3.25 ± 0.15 to Γ _γ _,2 = 1.81 ± 0.07 at a break energy of 2.46 ± 0.09 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 _X < 6.6% for the nucleus and Π _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 _X < 56.4% for WHS is insufficient to conclusively determine the X-ray emission mechanism in this region. |
| format | Article |
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| institution | Kabale University |
| issn | 1538-4357 |
| language | English |
| publishDate | 2025-01-01 |
| publisher | IOP Publishing |
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| series | The Astrophysical Journal |
| spelling | doaj-art-1af265f08dbf47e488d9c025770ccbcb2025-08-20T03:31:54ZengIOP PublishingThe Astrophysical Journal1538-43572025-01-01988226810.3847/1538-4357/ade808Two-component γ-Ray Emission Spectrum and X-Ray Polarization of the Radio Galaxy Pictor AJia-Xuan Li0https://orcid.org/0009-0003-9471-4724Xin-Ke Hu1Ji-Shun Lian2https://orcid.org/0000-0003-2547-1469Yu-Wei Yu3https://orcid.org/0009-0000-6577-1488Wei Deng4https://orcid.org/0000-0002-9370-4079Kuan Liu5https://orcid.org/0009-0007-8686-9012Hai-Ming Zhang6https://orcid.org/0000-0001-6863-5369Liang Chen7https://orcid.org/0000-0002-1908-0536Jin Zhang8https://orcid.org/0000-0003-3554-2996School of Physics, Beijing Institute of Technology , Beijing 100081, People’s Republic of ChinaSchool of Physics, Beijing Institute of Technology , Beijing 100081, People’s Republic of ChinaSchool of Physics, Beijing Institute of Technology , Beijing 100081, People’s Republic of ChinaSchool of Physics, Beijing Institute of Technology , Beijing 100081, People’s Republic of ChinaGuangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University , Nanning 530004, People’s Republic of ChinaGuangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University , Nanning 530004, People’s Republic of ChinaGuangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University , Nanning 530004, People’s Republic of ChinaKey Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences , 80 Nandan Road, Shanghai 200030, People’s Republic of ChinaSchool of Physics, Beijing Institute of Technology , Beijing 100081, People’s Republic of ChinaPictor A is a γ -ray emitting radio galaxy and has a bright hotspot called the western hotspot (WHS), located ∼4′ away from the nucleus. In this work, we present an analysis of its 16 yr Fermi Large Area Telescope (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 _γ _,1 = 3.25 ± 0.15 to Γ _γ _,2 = 1.81 ± 0.07 at a break energy of 2.46 ± 0.09 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 _X < 6.6% for the nucleus and Π _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 _X < 56.4% for WHS is insufficient to conclusively determine the X-ray emission mechanism in this region.https://doi.org/10.3847/1538-4357/ade808Relativistic jetsRadio galaxiesNon-thermal radiation sourcesSpectropolarimetryX-ray active galactic nuclei |
| spellingShingle | Jia-Xuan Li Xin-Ke Hu Ji-Shun Lian Yu-Wei Yu Wei Deng Kuan Liu Hai-Ming Zhang Liang Chen Jin Zhang Two-component γ-Ray Emission Spectrum and X-Ray Polarization of the Radio Galaxy Pictor A The Astrophysical Journal Relativistic jets Radio galaxies Non-thermal radiation sources Spectropolarimetry X-ray active galactic nuclei |
| title | Two-component γ-Ray Emission Spectrum and X-Ray Polarization of the Radio Galaxy Pictor A |
| title_full | Two-component γ-Ray Emission Spectrum and X-Ray Polarization of the Radio Galaxy Pictor A |
| title_fullStr | Two-component γ-Ray Emission Spectrum and X-Ray Polarization of the Radio Galaxy Pictor A |
| title_full_unstemmed | Two-component γ-Ray Emission Spectrum and X-Ray Polarization of the Radio Galaxy Pictor A |
| title_short | Two-component γ-Ray Emission Spectrum and X-Ray Polarization of the Radio Galaxy Pictor A |
| title_sort | two component γ ray emission spectrum and x ray polarization of the radio galaxy pictor a |
| topic | Relativistic jets Radio galaxies Non-thermal radiation sources Spectropolarimetry X-ray active galactic nuclei |
| url | https://doi.org/10.3847/1538-4357/ade808 |
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