Ultrafast Variability in AGN Jets: Intermittency and Lighthouse Effect
Gamma-ray flares from active galactic nuclei (AGNs) show substantial variability on ultrafast timescales (i.e., shorter than the light-crossing time of the AGN’s supermassive black hole). We propose that ultrafast variability is a by-product of the turbulent dissipation of the jet Poynting flux. Due...
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| Format: | Article |
| Language: | English |
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IOP Publishing
2023-01-01
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| Series: | The Astrophysical Journal Letters |
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| Online Access: | https://doi.org/10.3847/2041-8213/acc84d |
| _version_ | 1797695820884082688 |
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| author | Emanuele Sobacchi Tsvi Piran Luca Comisso |
| author_facet | Emanuele Sobacchi Tsvi Piran Luca Comisso |
| author_sort | Emanuele Sobacchi |
| collection | DOAJ |
| description | Gamma-ray flares from active galactic nuclei (AGNs) show substantial variability on ultrafast timescales (i.e., shorter than the light-crossing time of the AGN’s supermassive black hole). We propose that ultrafast variability is a by-product of the turbulent dissipation of the jet Poynting flux. Due to the intermittency of the turbulent cascade, the dissipation is concentrated in a set of reconnecting current sheets. Electrons energized by reconnection have a strong pitch-angle anisotropy, i.e., their velocity is nearly aligned with the guide magnetic field. Then each current sheet produces a narrow radiation beam, which dominates the emission from the whole jet when it is directed toward the observer. The ultrafast variability is set by the light-crossing time of a single current sheet, which is much shorter than the light-crossing time of the whole emission region. The predictions of our model are (i) the bolometric luminosity of ultrafast AGN flares is dominated by the inverse Compton (IC) emission as the lower-energy synchrotron emission is suppressed due to the pitch-angle anisotropy; (ii) if the observed luminosity includes a nonflaring component, the variations of the synchrotron luminosity have a small amplitude; and (iii) the synchrotron and IC emission are less variable at lower frequencies, as the cooling time of the radiating particles exceeds the light-crossing time of the current sheet. Simultaneous multiwavelength observations of ultrafast AGN flares can test these predictions. |
| first_indexed | 2024-03-12T03:16:43Z |
| format | Article |
| id | doaj.art-22ab17558ba24774bcc787c2e78a2b24 |
| institution | Directory Open Access Journal |
| issn | 2041-8205 |
| language | English |
| last_indexed | 2024-03-12T03:16:43Z |
| publishDate | 2023-01-01 |
| publisher | IOP Publishing |
| record_format | Article |
| series | The Astrophysical Journal Letters |
| spelling | doaj.art-22ab17558ba24774bcc787c2e78a2b242023-09-03T14:08:23ZengIOP PublishingThe Astrophysical Journal Letters2041-82052023-01-019462L5110.3847/2041-8213/acc84dUltrafast Variability in AGN Jets: Intermittency and Lighthouse EffectEmanuele Sobacchi0Tsvi Piran1https://orcid.org/0000-0002-7964-5420Luca Comisso2https://orcid.org/0000-0001-8822-8031Racah Institute for Physics, The Hebrew University , Jerusalem 91904, Israel ; emanuele.sobacchi@mail.huji.ac.ilRacah Institute for Physics, The Hebrew University , Jerusalem 91904, Israel ; emanuele.sobacchi@mail.huji.ac.ilDepartment of Astronomy and Columbia Astrophysics Laboratory, Columbia University , New York, NY 10027, USAGamma-ray flares from active galactic nuclei (AGNs) show substantial variability on ultrafast timescales (i.e., shorter than the light-crossing time of the AGN’s supermassive black hole). We propose that ultrafast variability is a by-product of the turbulent dissipation of the jet Poynting flux. Due to the intermittency of the turbulent cascade, the dissipation is concentrated in a set of reconnecting current sheets. Electrons energized by reconnection have a strong pitch-angle anisotropy, i.e., their velocity is nearly aligned with the guide magnetic field. Then each current sheet produces a narrow radiation beam, which dominates the emission from the whole jet when it is directed toward the observer. The ultrafast variability is set by the light-crossing time of a single current sheet, which is much shorter than the light-crossing time of the whole emission region. The predictions of our model are (i) the bolometric luminosity of ultrafast AGN flares is dominated by the inverse Compton (IC) emission as the lower-energy synchrotron emission is suppressed due to the pitch-angle anisotropy; (ii) if the observed luminosity includes a nonflaring component, the variations of the synchrotron luminosity have a small amplitude; and (iii) the synchrotron and IC emission are less variable at lower frequencies, as the cooling time of the radiating particles exceeds the light-crossing time of the current sheet. Simultaneous multiwavelength observations of ultrafast AGN flares can test these predictions.https://doi.org/10.3847/2041-8213/acc84dGalaxy jetsBlazarsGamma-rays |
| spellingShingle | Emanuele Sobacchi Tsvi Piran Luca Comisso Ultrafast Variability in AGN Jets: Intermittency and Lighthouse Effect The Astrophysical Journal Letters Galaxy jets Blazars Gamma-rays |
| title | Ultrafast Variability in AGN Jets: Intermittency and Lighthouse Effect |
| title_full | Ultrafast Variability in AGN Jets: Intermittency and Lighthouse Effect |
| title_fullStr | Ultrafast Variability in AGN Jets: Intermittency and Lighthouse Effect |
| title_full_unstemmed | Ultrafast Variability in AGN Jets: Intermittency and Lighthouse Effect |
| title_short | Ultrafast Variability in AGN Jets: Intermittency and Lighthouse Effect |
| title_sort | ultrafast variability in agn jets intermittency and lighthouse effect |
| topic | Galaxy jets Blazars Gamma-rays |
| url | https://doi.org/10.3847/2041-8213/acc84d |
| work_keys_str_mv | AT emanuelesobacchi ultrafastvariabilityinagnjetsintermittencyandlighthouseeffect AT tsvipiran ultrafastvariabilityinagnjetsintermittencyandlighthouseeffect AT lucacomisso ultrafastvariabilityinagnjetsintermittencyandlighthouseeffect |