Accretion disc cooling and narrow absorption lines in the tidal disruption event AT 2019dsg

Abstract

We present the results of a large multi-wavelength follow-up campaign of the Tidal Disruption Event (TDE) AT 2019dsg, focusing on low to high resolution optical spectroscopy, X-ray, and radio observations. The galaxy hosts a super massive black hole of mass $\rm (5.4\pm 3.2)\times 10^6\, M_\odot$ and careful analysis finds no evidence for the presence of an Active Galactic Nucleus, instead the TDE host galaxy shows narrow optical emission lines that likely arise from star formation activity. The transient is luminous in the X-rays, radio, UV and optical. The X-ray emission becomes undetected after ∼100 days, and the radio luminosity density starts to decay at frequencies above 5.4 GHz by ∼160 days. Optical emission line signatures of the TDE are present up to ∼200 days after the light curve peak. The medium to high resolution spectra show traces of absorption lines that we propose originate in the self-gravitating debris streams. At late times, after ∼200 days, narrow Fe lines appear in the spectra. The TDE was previously classified as N-strong, but after careful subtraction of the host galaxy’s stellar contribution, we find no evidence for these N lines in the TDE spectrum, even though O Bowen lines are detected. The observed properties of the X-ray emission are fully consistent with the detection of the inner regions of a cooling accretion disc. The optical and radio properties are consistent with this central engine seen at a low inclination (i.e., seen from the poles).

Tidal Disruption Events and High-energy Neutrinos

“…AT2019dsg was also detected at X-ray wavelengths by the X-Ray Telescope (XRT) [50], another telescope on board the Neil Gehrels Swift Observatory. It was further detected by the X-ray Multi-Mirror Mission (XMM)-Newton telescope [51], as well as Neutron star Interior Composition ExploreR (NICER) [52,53]. As can be seen in Figure 7, AT2019dsg was an initially-bright source which faded extremely rapidly over the course of observations.…”

Theoretical interpretation of the observed neutrino emission from Tidal Disruption Events

“…In Ref. [11], a late-term X-ray signal at about 115 days after discovery has been found as well, which does not fit the exponential decay trend described above; one may speculate that this could be a signature of the jetted radiation. It is also conceivable that there are macroscopic effects at work, such as jet precession, jet re-collimation, or twisting.…”

“…[10] and 3 10 7 [2] ; see also [11] for intermediate values. Considering this uncertainty, our arguments above leave two possible conclusions:…”

“…[12] in terms of the optical thickness at the radii of interest -or the cooling of the accretion disk, which shifts the peak of the X-ray spectrum out of the Swift energy window, see Ref. [11], or both. We anticipate that a small fraction (∼10%) of the X-rays isotropize in the region relevant for the neutrino production, see Fig.…”

“…In addition, we expect efficient particle acceleration in the jet, and our requirements for the acceleration efficiency are moderate. Although a relativistic jet has not been directly observed, there are some indications from optical polarimetry measurements [24], and even the radio signal may be interpreted as due to a relativistic jet in non-vanilla models [11]. In Ref.…”