Broadband X-ray/UV/optical time-resolved spectroscopy of NGC 5548: The origin of the UV/optical variability in active galactic nuclei
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| Autores principales: | , , , |
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| Formato: | Preprint |
| Publicado: |
2024
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| _version_ | 1866910371652042752 |
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| author | Kammoun, E. Papadakis, I. E. Dovčiak, M. Panagiotou, C. |
| author_facet | Kammoun, E. Papadakis, I. E. Dovčiak, M. Panagiotou, C. |
| contents | Recently, nearby active galactic nuclei (AGN) have been subject to long X-rays/UV/optical monitoring campaigns. These campaigns reveal a strong correlation between the various UV and optical bands, with time lags increasing with wavelength. In a series of papers, we demonstrated that a scenario in which a central X-ray source illuminates the accretion disc explains the observed correlations. However, some of the monitored AGN show low/moderate X-rays-UV correlations, which could challenge this scenario. In this paper, we study the broadband X-ray/UV/optical spectral energy distributions (SEDs) of NGC 5548, one of the most intensively monitored AGN. We aim to test if the X-ray illumination model explains the broadband spectral behaviour of the source, despite the moderate X-ray-UV/optical correlation. We model the broadband time-averaged SED, from the STORM monitoring campaign of the source, using the KYNSED model which assumes an X-ray illuminated disc. We assume that the accretion process powers the X-ray corona. We also model 15 time-resolved SEDs from the same campaign to check whether this scenario can account for the observed spectral variability. The proposed model describes well the time-averaged and the time-resolved SEDs of NGC 5548. In this scenario, the corona height, the X-ray photon index, and the power transferred to the corona all vary. This explains the variability behaviour at different wavelengths. The best-fit model is obtained for a non-rotating black hole accreting at a constant rate of 5% its Eddington limit. Since each of the variable parameters affects the observed flux in a particular way, the combined variability of all parameters explains the moderate X-ray-UV/optical correlation. The X-ray illuminated disc model provides a complete description of the behaviour of NGC 5548, explaining its broadband SEDs, time-lag spectrum, and its power spectral distribution. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2403_12208 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | Broadband X-ray/UV/optical time-resolved spectroscopy of NGC 5548: The origin of the UV/optical variability in active galactic nuclei Kammoun, E. Papadakis, I. E. Dovčiak, M. Panagiotou, C. High Energy Astrophysical Phenomena Astrophysics of Galaxies Recently, nearby active galactic nuclei (AGN) have been subject to long X-rays/UV/optical monitoring campaigns. These campaigns reveal a strong correlation between the various UV and optical bands, with time lags increasing with wavelength. In a series of papers, we demonstrated that a scenario in which a central X-ray source illuminates the accretion disc explains the observed correlations. However, some of the monitored AGN show low/moderate X-rays-UV correlations, which could challenge this scenario. In this paper, we study the broadband X-ray/UV/optical spectral energy distributions (SEDs) of NGC 5548, one of the most intensively monitored AGN. We aim to test if the X-ray illumination model explains the broadband spectral behaviour of the source, despite the moderate X-ray-UV/optical correlation. We model the broadband time-averaged SED, from the STORM monitoring campaign of the source, using the KYNSED model which assumes an X-ray illuminated disc. We assume that the accretion process powers the X-ray corona. We also model 15 time-resolved SEDs from the same campaign to check whether this scenario can account for the observed spectral variability. The proposed model describes well the time-averaged and the time-resolved SEDs of NGC 5548. In this scenario, the corona height, the X-ray photon index, and the power transferred to the corona all vary. This explains the variability behaviour at different wavelengths. The best-fit model is obtained for a non-rotating black hole accreting at a constant rate of 5% its Eddington limit. Since each of the variable parameters affects the observed flux in a particular way, the combined variability of all parameters explains the moderate X-ray-UV/optical correlation. The X-ray illuminated disc model provides a complete description of the behaviour of NGC 5548, explaining its broadband SEDs, time-lag spectrum, and its power spectral distribution. |
| title | Broadband X-ray/UV/optical time-resolved spectroscopy of NGC 5548: The origin of the UV/optical variability in active galactic nuclei |
| topic | High Energy Astrophysical Phenomena Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2403.12208 |