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Neutrinos from tidal disruption events

Neutrinos from tidal disruption events Download PDF News & Views . Published: 22 February 2021 . MULTI-MESSENGER ASTRONOMY Neutrinos from tidal disruption events . Kimitake Hayasaki ? ORCID: orcid.org/0000-0003-4799-1895 1 ? . Nature Astronomy ( 2021 ) Cite this article 1 Altmetric Metrics details Subjects . Compact astrophysical objects . High-energy astrophysics . Time-domain astronomy . Transient astrophysical phenomena . Tidal disruption events are an excellent probe for supermassive black holes in distant inactive galaxies because they emit bright multi-wavelength flares that last several months to years. AT2019dsg represents the first potential association of neutrino emission with such an explosive event. Download PDF According to the Big Bang theory, the neutrino is the second most common elementary particle in our Universe after photons 1 , 2 . Neutrinos are called ghost particles because they interact very weakly with matter, making it difficult to detect them. However, there is a silver lining: neutrinos carry direct physical information about astronomical phenomena that are otherwise obscured, allowing us to understand them more deeply. High-energy astrophysical neutrinos are produced by the interaction of relativistically accelerated cosmic rays with ambient matter or photons. While the observation of astrophysical neutrinos has increased in recent years, they are often detected without a clearly identifiable source. Only three astrophysical sources of neutrinos have been identified so far: the Sun, the 1987A supernova, and the blazar TXS 0506+056 (ref. 3 ), the last of which is still under debate. The first two associations were detected by Homestake, Kamiokande and Super-Kamiokande 4 , which are sensitive to low-energy neutrinos, while the blazar neutrino was detected by IceCube, which is sensitive to very high-energy neutrinos. Writing in Nature Astronomy , Robert Stein and collaborators 5 report that a recently detected high-energy IceCube neutrino, IceCube-191001A, is associated with the tidal disruption event (TDE) AT2019dsg. This neutrino has an energy of ~0.2 PeV and is thus the second most energetic astrophysical neutrino source ever detected, with energies above 100 TeV. A TDE occurs when a star on a Keplerian orbit gets close enough to a supermassive black hole (SMBH) to be disrupted by the SMBH’s tidal forces. Then the stellar debris falls back to the SMBH at a super-Eddington rate, showing a characteristic flare that lasts for months to years 6 . TDEs are among the brightest transient phenomena in our Universe over a wide range of wavebands from optical to X-rays and, therefore, work as excellent probes of dormant SMBHs at the centres of distant inactive galaxies. Recent multi-wavelength observations have revealed the diverse properties of TDEs 7 , 8 . TDEs are divided into two categories: thermal TDEs without a relativistic jet and non-thermal TDEs with a relativistic jet (so-called jetted TDEs). Remarkably, most thermal TDEs shine brightly only in soft-X-ray wavebands (soft-X-ray TDEs) or in optical/UV wavebands (optical/UV TDEs). However, AT2019dsg is an unusual type of TDE because it shows bright emission from optical to soft-X-ray wavebands as well as weak but observable radio emission 5 , 8 . Figure 1 depicts a hypothetical picture of a disk-outflow-jet system after tidal disruption of a star by an SMBH to explain the observed diversity. The IceCube-191001A-AT2019dsg association can help us understand the observed diversity of TDEs. Fig. 1: An illustration of the disk-outflow-jet system formed after the tidal disruption of a star, as in the case of AT2019dsg. Depending on the viewing angle, the waveband of observable thermal emission from TDEs changes from soft-X-rays to the optical/UV 15 . Here ν indicates a neutrino. AT2019dsg shows optical to X-ray variability with weak radio emission 5 , 8 . It has been proposed that the high-energy neutrino is produced from (i) the relativistic jet 9 , (ii) the disk (a super-Eddington MAD and/or RIAF) 13 , (iii) the disk corona or (iv) the wind/outflow 12 . Full size image The probability that IceCube-191001A has an astrophysical origin is estimated to be 59% from a simple energetics argument 5 . The possibility that it is of an atmospheric origin thus cannot be excluded completely. While the IceCube probability is only 59%, if the number of atmospheric neutrinos is low, the temporal and spatial association with AT2019dsg increases the probability that the two are associated. In the following, we assume that the neutrino was emitted from AT2019dsg and explore the relevant physical mechanism that could have caused it. According to the blazar neutrino analogy 3 , it is natural to consider that the TDE neutrino was produced in a relativistic jet. As a companion paper of Stein et al., Walter Winter and Cecilia Lunardini 9 propose a model in which neutrinos are generated from internal shocks in a relativistic jet by a photo-meson interaction. In their model, neutrino production is driven by back-scattered X-ray photons inside the outflow, which are delivered to the plasma shell (shocked region) that travels inside the jet. While the neutrino production rate increases as the density of supplied photons increases, the production efficiency decreases as the size of the plasma shell increases. The balance of these two explains the ~150 day delay between the neutrino detection and the observed optical/UV peak of the TDE. However, there are some shortcomings in explaining the TDE neutrino via a relativistic jet model. Around 100 TDE candidates have so far been observed, of which only three are clearly jetted TDEs 10 . Furthermore, no high-energy gamma-ray and hard X-ray emissions 5 have been observed from AT2019dsg, which would be a clear signature for the production of neutrinos in a relativistic jet, and the detected radio emission from AT2019dsg is too weak for a relativistic jet 8 . An off-axis 11 or hidden 12 jet model could, however, explain some of these inconsistencies. There are a few alternative models to produce sub-PeV neutrinos from TDEs: an accretion disk, disk corona and wind/outflow 13 , 12 (see also Fig. 1 ). These are mainly promising for non-jetted TDEs, for which the event rate is much higher than for the jetted-TDE case. For example, the sub-PeV neutrinos could be emitted from a super-Eddington magnetically arrested disk (MAD) or a radiatively inefficient accretion flow (RIAF) in the TDE context 13 . Interestingly, the disk’s protons would accelerate by the second-order Fermi acceleration via disk turbulence, which is different from the relativistic jet model, for which the first-order Fermi acceleration via the shock works. Moreover, high-energy (TeV-scale) gamma-rays are not emitted by efficient pair production in the RIAF model, which would explain the non-detection of AT2019dsg at these highest energies. The main challenge for future multi-messenger studies of TDEs will be to explore whether TDE neutrinos originate from a relativistic jet, accretion disk, disk corona, disk wind/outflow, or other sources. Clues to their production site would come from identifying the acceleration mechanism, cooling process, hadronuclear and photohadronic interactions, and cascading processes, which differ depending on the given neutrino emitter model 12 . The IceCube-191001A-AT2019dsg association represents the first step in the study of high-energy particle emission from TDEs. Ongoing and future all-sky-survey telescopes (such as SRG/e-ROSITA, the Vera C. Rubin Observatory Legacy Survey of Space of Time and the Einstein Probe) will increase the TDE detection rate up to the order of thousands per year 10 . In addition, the next-generation IceCube will offer higher sensitivity and better angular resolution 14 , further improving the localization of astrophysical neutrinos and subsequent association with TDEs or other astrophysical sources. The robust detection of TDE neutrinos will elucidate not only the observed diversity of TDEs but may also help constrain the as-yet-unknown neutrino mass and lifetime. It will be interesting to see if there is any correlation between the electromagnetic radiation variability and the high-energy neutrino emission. The IceCube-191001A-AT2019dsg association marks the beginning of multi-messenger observations for TDEs. References . 1. Weinberg, S. 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The author declares no competing interests. Rights and permissions . Reprints and Permissions About this article . Cite this article . Hayasaki, K. Neutrinos from tidal disruption events. Nat Astron (2021). https://doi.org/10.1038/s41550-021-01309-z Download citation Published : 22 February 2021 DOI : https://doi.org/10.1038/s41550-021-01309-z Download PDF Associated Content . Nature Astronomy Letter A concordance scenario for the observed neutrino from a tidal disruption event . Walter Winter . ?&? Cecilia Lunardini . Nature Astronomy Article A tidal disruption event coincident with a high-energy neutrino . Robert Stein . , Sjoert van Velzen . , Marek Kowalski . , Anna Franckowiak . , Suvi Gezari . , James C. A. Miller-Jones . , Sara Frederick . , Itai Sfaradi . , Michael F. Bietenholz . , Assaf Horesh . , Rob Fender . , Simone Garrappa . , Tomás Ahumada . , Igor Andreoni . , Justin Belicki . , Eric C. Bellm . , Markus B?ttcher . , Valery Brinnel . , Rick Burruss . , S. Bradley Cenko . , Michael W. Coughlin . , Virginia Cunningham . , Andrew Drake . , Glennys R. Farrar . , Michael Feeney . , Ryan J. Foley . , Avishay Gal-Yam . , V. Zach Golkhou . , Ariel Goobar . , Matthew J. Graham . , Erica Hammerstein . , George Helou . , Tiara Hung . , Mansi M. Kasliwal . , Charles D. Kilpatrick . , Albert K. H. Kong . , Thomas Kupfer . , Russ R. Laher . , Ashish A. Mahabal . , Frank J. Masci . , Jannis Necker . , Jakob Nordin . , Daniel A. Perley . , Mickael Rigault . , Simeon Reusch . , Hector Rodriguez . , César Rojas-Bravo . , Ben Rusholme . , David L. Shupe . , Leo P. Singer . , Jesper Sollerman . , Maayane T. Soumagnac . , Daniel Stern . , Kirsty Taggart . , Jakob van Santen . , Charlotte Ward . , Patrick Woudt . ?&? Yuhan Yao .

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