Neutrinos from tidal disruption events
NNeutrinos from tidal disruption events
Kimitake Hayasaki Department of Astronomy and Space Science, Chungbuk National University, Cheongju 361-763,Republic of Korea
Tidal disruption events are an excellent probe for supermassive black holes in distant in-active galaxies because they show bright multi-wavelength flares lasting several months toyears. AT2019dsg presents the first potential association with neutrino emission from suchan explosive event.
According to the Big Bang theory, the neutrino is the second most common elementaryparticles in our universe after photons [1,2]. Neutrinos are called ghost particle because theyso weakly interact with matter that it is difficult to detect them. However, this is an advantageon another front: neutrinos carry direct physical information about astronomical phenomena sothat we can understand them more deeply. High energy astrophysical neutrinos are produced byinteracting relativistically accelerated cosmic-rays with an ambient matter or photons. While theobservation of astrophysical neutrinos has increased in recent years, they are often detected withouta clearly identifiable source. Only three astrophysical sources of neutrinos have been identified sofar: the Sun, the 1987A supernova, and the blazar TXS 0506+056 [3], but it remains in debatefor the association with TXS 0506+056. The first two were detected by Homestake, Kamiokande,and Super-Kamiokande [4], which are sensitive to low-energy neutrinos, while the blazar neutrinowas detected by IceCube, which is sensitive to very high-energy neutrinos. In this issue of Nature1 a r X i v : . [ a s t r o - ph . H E ] F e b stronomy, Robert Stein and collaborators [5] report that a recently detected high-energy IceCubeneutrino, IceCube-191001A, is associated with the tidal disruption event (TDE) AT2019dsg. Thisneutrino has an energy of ∼ . PeV and is thus the second most energetic astrophysical neutrinosource, with energies above 100 TeV, after the blazar TX0506+056 [3].A TDE occurs when a star on a Keplerian orbit gets close enough to the supermassive blackhole (SMBH) to be disrupted by the SMBH’s tidal forces. Then the stellar debris falls back to theSMBH at a super-Eddington rate, showing a characteristic flare that lasts for months to years [6].TDEs are known as among the brightest transient phenomena in our universe over a wide range ofwavebands from optical to X-rays and, therefore, work as the excellent probes of dormant SMBHsat the centers of distant inactive galaxies. Recent multi-wavelength observations have revealed thediverse properties of TDEs [7,8]. TDEs are divided into two categories: thermal TDEs without arelativistic 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 opti-cal/UV wavebands (optical/UV TDEs). However, AT2019dsg is an unusual type of TDE becauseit shows bright emission from optical to soft-X-ray wavebands as well as weak but observable radioemission [5,8]. Fig. 1 depicts a hypothetical picture of a disk-outflow-jet system after tidal disrup-tion of a star by an SMBH to explain the observed diversity. The IceCube-191001A-AT2019dsgassociation can be a key to help us understand the observed diversity of TDEs.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 can-2ot thus be excluded completely. While the IceCube probability is only , if the number ofatmospheric neutrinos is low, the temporal and spatial association with AT2019dsg increases theprobability that the two are associated. In the following, we assume that the neutrino was emittedfrom AT2019dsg and explore the relevant physical mechanism that could have caused it. Accord-ing to the blazar neutrino analogy [3], it is natural to regard that the TDE neutrino produced froma relativistic jet. As a companion paper of Stein et al. (2021), Walter Winter & Cecilia Lunardini(2021) [9] propose a model in which neutrinos are generated from internal shocks in a relativisticjet by a photo-meson interaction. In their model, neutrino production is driven by back-scatteredX-ray photons inside the outflow, which are delivered to the plasma shell (shocked region) thattravels inside the jet. While the neutrino production rate increases as the density of supplied pho-tons increases, the production efficiency decreases as the size of the plasma shell increases. Thebalance of these two explains the ∼ days delay between the neutrino detection and the ob-served optical/UV peak of the TDE.However, there are some shortcomings in explaining the TDE neutrino by a relativistic jetmodel. ∼ TDE candidates have so far been observed, of which only three are clearly jet-ted TDEs [10]. Furthermore, no high-energy gamma-ray and hard X-ray emissions [5] has beenobserved from AT2019dsg, which would be a clear signature for the production of neutrinos in arelativistic jet. The radio emission of AT2019dsg is too weak to be supportable evidence for therelativistic jet [8]. An off-axis [11] or hidden [12] jet model could however explain some of theseinconsistencies. There are a few alternative models to produce sub-PeV neutrinos from TDEs: anaccretion disk, disk corona, and wind/outflow [12,13] (see also Fig. 1). These are mainly promis-3ng for non-jetted TDEs, for which the event rate is much higher than the jetted-TDE case. Forexample, the sub-PeV neutrinos are 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 accelerate by the second-order Fermi acceleration via disk turbulence, which isdifferent from the relativistic jet model, for which the first-order Fermi acceleration via the shockworks. Moreover, high-energy (TeV-scale) gamma-rays are not emitted by efficient pair produc-tion in the RIAF model. The main challenge for future multi-messenger studies of TDEs willbe to explore whether TDE neutrinos originate from a relativistic jet, accretion disk, disk corona,disk wind/outflow, and other sources. Clues to their production site would come from identifyingthe acceleration mechanism, cooling processes, hadronuclear and photohadronic interactions, andcascading processes, which differ depending on the given neutrino emitter model [12].The IceCube-191001A-AT2019dsg association represents the first step in study of high-energy particle emission from TDEs. Ongoing and future all-sky-survey telescopes (such asSRG/eROSITA, Vera C. Rubin Observatory Legacy Survey of Space and Time, and Einstein Probe)will increase the TDE rate up to the order of thousands per year [10]. Besides, the next-generationIceCube offers a higher sensitivity and a better angular resolution[14]. They will improve the as-sociation between astrophysical neutrinos and TDEs. The robust detections of TDE neutrinos willelucidate not only the observed diversity of TDEs but may also help constrain the as-yet-unknownneutrino’s mass and lifetime. It will be interesting to see if there is any correlation between the elec-tromagnetic radiation variability and the high-energy neutrino emission. The IceCube-191001A-AT2019dsg association marks the beginning of multi-messenger observations of TDEs.4igure 1: An illustration of the disk-outflow-jet system formed after the tidal disruption of a star,as in the case of AT2019dsg. Here νν