Radio Observations of an Ordinary Outflow from the Tidal Disruption Event AT2019dsg

Abstract

We present detailed radio observations of the tidal disruption event (TDE) AT2019dsg, obtained with the Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA), and spanning 55−560 days post-disruption. We find that the peak brightness of the radio emission increases until ∼ 200 days and subsequently begins to decrease steadily. Using the standard equipartition analysis, including the effects of synchrotron cooling as determined by the joint VLA-ALMA spectral energy distributions, we find that the outflow powering the radio emission is in roughly free expansion with a velocity of ≈ 0.07c, while its kinetic energy increases by a factor of about 5 from 55 to 200 days and plateaus at ≈ 5 × 10 48 erg thereafter. The ambient density traced by the outflow declines as ≈ R −1.6 on a scale of ≈ (1 − 4) × 10 16 cm (≈ 6300 − 25000 R s ), followed by a steeper decline to ≈ 6 × 10 16 cm (≈ 37500 R s ). Allowing for a collimated geometry, we find that to reach even mildly relativistic velocities (Γ = 2) the outflow requires an opening angle of θ j ≈ 2 • , which is narrow even by the standards of GRB jets; a truly relativistic outflow requires an unphysically narrow jet. The outflow velocity and kinetic energy in AT2019dsg are typical of previous non-relativistic TDEs, and comparable to those from Type Ib/c supernovae, raising doubts about the claimed association with a high-energy neutrino event.

AT2019azh: an unusually long-lived, radio-bright thermal tidal disruption event

“…The index of the electron energy distribution, p, remains roughly constant at p ≈ 2.7 ± 0.2, similar to that of other thermal events (e.g. Alexander et al 2016;Cendes et al 2021;Stein et al 2021;Horesh et al 2021b) excepting three epochs at 254, 459, and 749 d post disruption at which the energy index shows a significant steepening to p ≈3, 3.7, and 3.3 respectively. We note that the slight spectral steepening at 255 d is likely not real based on an analysis of the flux density of background sources in the field for the different epochs presented in Appendix A and that the significance of the change in p is 3-σ and 2-σ respectively for the other two epochs.…”

“…Whilst it is common practice to fix p between epochs (e.g. Alexander et al 2016;Cendes et al 2021), there is evidence in the observations that the spectral slope is not constant for this event (see Figure 2), so we do not fix p in our modelling. To constrain the host flux density and spectral index (Equation 2), we first ran a MCMC fit for epoch 11 (2020 Dec 12) only, where the synchrotron spectrum is very well constrained by the observations, ensuring that F0 < 0.41 mJy and −2 < α0 < 2.…”

“…However, given the underlying assumptions of the radius calculation, that the synchrotron peak flux density and frequency were not resolved by the observations for the first few epochs, and that outflows from other thermal TDEs have all been observed to undergo approximately free-expansion at early times (e.g. Alexander et al 2016;Cendes et al 2021), in the sections that follow we operate under the assumption that the outflow was freely expanding with ∼constant velocity until at least the radio lightcurve peak (t ≈ 650 d).…”

“…A popular model to explain non-relativistic outflows from TDEs is a spherical wind driven by accretion onto the SMBH (e.g. Alexander et al 2016;Cendes et al 2021). In this scenario, the radio outflow should appear at approximately the time that high accretion luminosities are observed at X-ray wavelengths, provided there is no obscuration of the X-ray emission.…”

“…Recently there has been an increase in the number of TDEs with radio detections (e.g. Alexander et al 2016;van Velzen et al 2016;Alexander et al 2017;Cendes et al 2021;Horesh et al 2021b), with the next few years expected to bring a large number of new radio observations of these unique events due to targeted radio campaigns to follow-up optical and Xray detected events. These new observations will be crucial in characterising the mechanism behind the radio-emitting outflows that can be produced, and identifying if there is a single mechanism behind all radio outflows, or if the type of outflow differs between individual systems.…”