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

The central mass in a galaxy influences the dynamics and spatial distributions of stars in the inner regions. In some rare cases a supermassive black hole (SMBH) can capture and destroy a star if it passes within the radius at which the tidal shear forces on the star from the black hole exceed the star’s self-gravity (Rees, 1988). Such tidal disruption events (TDEs) produce bright flares across the electromagnetic spectrum that are usually visible for timescales of 1–2 years, with approximately half of the debris remaining in orbits bound to the black hole, and other parts flung out on hyperbolic orbits with large velocities (e.g. Lacy et al., 1982; Rees, 1988; Evans & Kochanek, 1989; Lodato et al., 2009). The bound stellar debris may circularise and form an accretion disk (e.g. Shiokawa et al., 2015; Bonnerot & Lu, 2020; Bonnerot et al., 2016; Liptai et al., 2019; Hayasaki et al., 2016; Mummery & Balbus, 2020), producing X-ray emission from the accretion stream as the material falls back towards the black hole, and optical emission possibly from the re-processing of the X-rays in the disk (e.g. Auchettl et al., 2017; Cannizzo et al., 1990; van Velzen et al., 2020; Gezari, 2021; Strubbe & Quataert, 2009; Metzger & Stone, 2016; Roth et al., 2016). The disk circularisation time, and thus the time taken for disk formation and accretion onto the SMBH to begin, is a subject of debate (Bonnerot & Stone, 2021). Some models predict varying circularisation timescales depending on the physical properties of the star, SMBH, and system (e.g. Lu & Bonnerot, 2020; Liptai et al., 2019; Hayasaki et al., 2016; Bonnerot & Stone, 2021). X-ray observations of TDEs that trace accretion onto the SMBH have shown variable behaviour, with some TDEs showing bright X-ray emission early on (e.g. Miller et al., 2015), and others showing delayed X-ray flares (e.g. Hinkle et al., 2021). In some cases radio emission is also observed from outflowing material.

Radio observations of TDEs trace the outflows produced by the debris from the destroyed star that may be ejected from the black hole (see Alexander et al., 2020, for a review), enabling detailed insight into the launching of outflows from SMBHs, the circumnuclear density, and how such outflows might provide feedback and influence the evolution of the host galaxy. The radio properties of the TDEs observed to date are diverse, with some exhibiting high luminosity emission ($\nu L_{\nu}>10^{40}$ erg/s) that is well-described by a relativistic jet (e.g. Swift J164449.3+573451 (Sw J1644+57), Levan et al., 2011; Burrows et al., 2011; Zauderer et al., 2011; Bloom et al., 2011) and others exhibiting lower-luminosity emission ($\nu L_{\nu}<10^{40}$ erg/s) that could be described by synchrotron emission from a non-relativistic spherical or mildly collimated outflow (e.g. ASASSN-14li, van Velzen et al., 2016; Alexander et al., 2016). The time relative to the optical/X-ray flare at which the radio emission is observed could provide insight into the mechanism launching the outflow, the star that was destroyed, and the nature of its orbit around the black hole. Recently it has been suggested that delayed radio emission may be common in TDEs (Horesh et al., 2021a), based on late-time radio flares observed for the thermal TDEs ASASSN-15oi and iPTF16fnl. However, radio observations of TDEs at early times are uncommon, and the apparent lack of detected early-time radio emission could naturally result from a lack of early-time observations, as we demonstrate through early-time radio observations of the thermal TDE AT2019azh in this work.

Due to the diverse properties of the TDEs that have been observed in the radio to date, there is no consistent explanation for the type of outflows that may be produced in any single event. In some rare cases, as in Sw J1644+57, a relativistic jet is produced with energy $\sim 10^{51}$ erg (Burrows et al., 2011; Brown et al., 2017; Pasham et al., 2015; Cenko et al., 2012; Zauderer et al., 2011). These arise in TDEs that present a non-thermal X-ray spectrum, and the radio emission can be well described by a relativistic jet model in which synchrotron emission is produced as the jet shocks the circumnuclear medium (CNM) and slows, producing emission similar to a gamma-ray burst (e.g. Metzger et al., 2012; Kumar et al., 2013). In other cases, where the TDEs exhibit a thermal X-ray spectrum (as for example in ASASSN-14li and AT2019dsg), a less energetic ($E\sim 10^{46}-10^{50}$ erg), non-relativistic outflow is produced.

There are a few possible scenarios for producing the observed properties of these non-relativistic outflows, some of which are difficult to rule out with the current set of observations. The prevalent models include a disk wind model in which the outflow is produced early on by accretion onto the SMBH, and emits synchrotron radiation as it moves through the interstellar medium around the black hole (e.g. Alexander et al., 2016). Alternatively, in a similar scenario, a mildly collimated, non-relativistic jet could be produced by the accretion onto the SMBH (e.g. van Velzen et al., 2016). In this case, the jet, emitting radio emission by an internal emission mechanism, could switch on at later times and a constant injection of energy could be observed, with the energy increasing with time (e.g. Falcke & Biermann, 1995). Another possibility is a collision-induced outflow, in which the debris from the destroyed star undergoes stream-stream collisions as it circularises into an accretion disk, with a significant amount of gas becoming unbound and ejected in an approximately spherical outflow (Lu & Bonnerot, 2020). Finally, the radio emission could also be produced by the unbound tidal debris stream, which would be ejected from the system with escape velocities $\sim 10^{4}$ km/s in a concentrated cone close to the orbital plane (Krolik et al., 2016).

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 X-ray 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.

In this work we present over two years of radio monitoring observations of the thermal TDE AT2019azh. AT2019azh was first discovered on 2019 February 22 by the All-Sky Automated Survey for Supernovae (ASASSN) (Brimacombe et al., 2019) and named ASASSN-19dj. It was detected by the Zwicky Transient Facility (ZTF) on 2019 February 12 and denoted ZTFaaazdba (van Velzen et al., 2019). The source was coincident with the nucleus of the E+A galaxy KUG 0810+227, with a redshift of $z=0.022$ (luminosity distance of 96 Mpc). Spectra obtained by the Nordic Optical Telescope Unbiased Transient Survey (NUTS) on 2019 February 22 (Heikkila et al., 2019), ePESSTO on 2019 February 25 (Barbarino et al., 2019), and the Spectral Energy Distribution Machine mounted on the Palomar 60-in telescope on 2019 February 24 and March 10 (van Velzen et al., 2019) all revealed a blue, featureless spectrum with narrow emission and Balmer absorption features associated with the host galaxy. The event was also detected in X-ray and UV with the Neils Gehrels Swift Observatory and was found to have a high X-ray blackbody temperature of $kT=0.06$ eV assuming a thermal spectrum (van Velzen et al., 2019). The combination of optical spectral properties, high blackbody temperature, location at center of host galaxy, and lack of spectroscopic AGN or supernova features led van Velzen et al. (2019) to classify the source as a tidal disruption event. The first reported radio detection of the event was by Perez-Torres et al. (2019) with the electronic Multi-Element Remotely Linked Interferometer Network (e-MERLIN) at 5 GHz on 2019 May 21 and 2019 June 11. In this work we present an earlier radio detection, on 2019 March 9.

Hinkle et al. (2021) analysed the optical, UV, and X-ray observations of AT2019azh from 30 d before to $\sim$300 d after the optical peak. During the first 200 d there was very low-level X-ray emission with a harder spectral index, and strong optical/UV emission that evolved as expected for a thermal TDE. The X-rays brightened by a factor of 30–100 approximately 250 d after discovery, and the X-ray spectrum became softer. The optical/UV flare was observed to begin on MJD 58528 (2019 Feb 14) and peaked on approximately MJD 58560 (2019 Mar 18, Hinkle et al., 2021). From a power-law fit of the optical rise observed by ASASSN, Hinkle et al. (2021) inferred that the time of first light for the event was MJD 58522 (2019 Feb 8).

Liu et al. (2019) found X-ray flaring episodes during the early times, which were temporally uncorrelated with the optical/UV emission. They deduced that the optical and X-ray data could be explained by a two-process scenario in which the early emission in UV/optical is explained by emission from debris stream-stream collisions as the bound debris is becoming circularised, and the low-level early X-ray emission is due to a low-mass accretion disk forming during this time. Liu et al. (2019) explain the late X-ray brightening as due to the major body of the disk forming after circularisation of the bound stellar debris. However, Hinkle et al. (2021) concluded that the late-time X-ray brightening was a consequence of an increase in the area of the X-ray emitting region via the blackbody radius, while the short term X-ray variability was due to changes in the X-ray temperature. In this scenario, the late-time X-ray brightening is not due to delayed accretion disk formation, but rather an expansion of the X-ray emitting region. However, Mummery (2021b) recently showed that the X-ray blackbody radius is not a good measure of length scales in a TDE system, implying that the change in blackbody radius may not be caused by a change in the disk radius. The nature of AT2019azh is thus a subject of debate, with evidence both for and against a delayed accretion scenario to explain the multiwavelength emission from the event.

In this work we present 13 epochs of radio observations of AT2019azh taken with the Very Large Array (VLA) and MeerKAT beginning 2019 March 9 (before the optical peak) and spanning until 2021 June 5. These radio observations enable further insight into the nature of the TDE, including the disk formation and launching of the outflow. The paper is outlined as follows: in Section 2 we describe the radio observations and data processing. In Section 3 we present the radio spectral observations and synchrotron emission fits. In Section 4 we describe the modelling of the radio emission to predict physical properties of the outflow. In Section 5 we present a more detailed accretion-disk model of the multiwavelength observations. In Section 6 we discuss the implications of these results, the possible nature of the outflow in AT2019azh, and relate the outflow properties to those of other TDEs. Finally in Section 7 we provide a summary of our results and concluding remarks.

The 2.25, 5, and 9 GHz VLA lightcurves for AT2019azh are plotted in Figure 1, as well as a comparison of the 5 GHz lightcurve with other thermal TDE lightcurves. The radio emission from AT2019azh rose relatively slowly at all radio wavelengths until approximately 625 d post optical discovery, at which time the higher frequency ($>4$ GHz) emission started to decrease while the 2 GHz emission remained relatively constant. Such a slow rise in the radio relative to the optical peak, which occurred around the time of our first radio detection, places AT2019azh in the slow-rising thermal TDE population (Figure 1). In contrast, some thermal TDEs have been observed to begin fading in the radio soon after the optical peak (e.g. Alexander et al., 2016; Horesh et al., 2021b).

The 5 GHz luminosity of AT2019azh increases approximately linearly with time, similar to that of the relativistic event Sw J1644+57. However, Sw J1644+57 rose to a peak within $\sim$100 d (Eftekhari et al., 2018), whereas AT2019azh took over $\sim$600 d. We note that AT2019azh was detected in the radio significantly earlier relative to the optical peak than the other thermal TDEs, and a similar slow rise cannot be ruled out for ASSASN-14li, CNSS J0019+00, or XMMSL1 J0740-85. The rise observed for AT2019azh is significantly different than those of ASASSN-15oi, which had early radio non-detections (Horesh et al., 2021b)), and AT2019dsg, which rose to a peak over $<350$ d with $L\propto t^{2.5}$ (Stein et al., 2021).

The luminosity of AT2019azh is now sharply decreasing, and similar to the fading rates of AT2019dsg, ASASSN-14li, CNSS J0019+00, and ASASSN-15oi (Figure 1).

We model the radio emission from the outflow using the standard synchrotron emission model outlined in Barniol Duran et al. (2013), in which the ambient electrons are accelerated into a power-law distribution by the blastwave from the outflow, $N(\gamma)\propto\gamma^{-p}$, where $\gamma$ is the electron Lorentz factor ($\gamma\geq\gamma_{\rm m}$, where $\gamma_{\rm m}$ is the minimum Lorentz factor) and $p$ is the synchrotron energy index. We assume equipartition between the electron and magnetic field energy densities in order to derive the equipartition energy and radius. Although the emitting region may not be in equipartition, we can derive estimates of the physical system parameters by parameterising the deviation from equipartition and accounting for its effect. This approach allows us to estimate key physical quantities such as the ambient electron density, magnetic field strength, mass of the emitting region, and velocity of the ejecta. We assume the fraction of the total energy in the magnetic field is 0.1$\%$, $\epsilon_{B}=10^{-3}$, based on observations of other TDEs and supernovae (e.g. Eftekhari et al., 2018; Horesh et al., 2013). We assume that 10$\%$ of the total energy is carried by the electrons, $\epsilon_{e}=0.1$, given that electrons are typically accelerated much less efficiently than protons in astrophysical accelerators (e.g. Morlino & Caprioli, 2012). We find no evidence for a relativistic outflow (see Section 6.2), and assume the outflow is non-relativistic (i.e. the bulk Lorentz factor, $\Gamma=1$), and that the peak of the radio spectrum is associated with the synchrotron self-absorption frequency (i.e. $\nu_{\rm a}$ = $\nu_{\rm p}$). We model two different geometries, one where the emitting region is approximately spherical (with geometric factors¹¹1The geometric factors, defined in Barniol Duran et al. (2013), are given by $f_{\mathrm{A}}=A/(\pi R^{2}/\Gamma^{2})$ and $f_{\mathrm{v}}=V/(\pi R^{3}/\Gamma^{4})$, for an outflow with area, $A$, volume, $V$, and distance from the origin of the outflow, $R$. $f_{\mathrm{A}}=1$ and $f_{\mathrm{V}}=4/3$), and one where the emitting region is conical (with geometric factors $f_{\mathrm{A}}=0.13$ and $f_{\mathrm{V}}=1.15$), corresponding to a mildly collimated outflow with a half-opening angle of 30 degrees.

In the Newtonian regime, the equipartition energy, corresponding to the minimum total energy in the observed region, assuming $\nu_{\rm a}>\nu_{\rm m}$, is given by (Barniol Duran et al., 2013)

where $d$ is the distance from the observer, $z$ is the redshift, $\chi_{\rm e}=\left(\frac{p-2}{p-1}\right)\epsilon_{\rm e}\frac{m_{\rm p}}{m_{\rm e}}$ ($m_{\rm e}$ is the electron mass and $m_{\rm p}$ is the proton mass), or $\chi_{\rm e}=2$ if $\Gamma=1$ (Newtonian case), and $\xi=1+\frac{1}{\epsilon_{\rm e}}$.

The equipartition radius is given by

To infer the physical system parameters we first correct the energy and radius for the system being out of equipartition using the following assumptions

where $\epsilon=\frac{\epsilon_{B}}{\epsilon_{e}}\frac{11}{6}$, i.e. the total energy is minimised with respect to $R$ at $R_{\mathrm{eq}}$ when the energy in the magnetic field is $6/11$ the energy in the electrons, so the deviation from equipartition is parameterised by $\epsilon$ (Barniol Duran et al., 2013).

Using the corrected radius, $R$ and corrected energy, $E$, we then calculate the total number of electrons in the observed region ($N_{\mathrm{e}}$), ambient electron density ($n_{e}$), magnetic field ($B$), mass of the emitting region ($M_{\mathrm{ej}}$), and outflow velocity ($\beta=v/c$), using Equations 8–13 (Barniol Duran et al., 2013), as

where $\left(\frac{\gamma_{\rm m}}{\gamma_{\rm a}}\right)^{1-p}$ is a correction factor to account for the regime where $\nu_{\rm m}<\nu_{\rm a}$ and the extra factor of 4 arises due to the Newtonian correction; $\gamma_{\rm m}=2$, and $\gamma_{\rm a}$ is given by

The ambient electron density is then inferred via

We determine the velocity of the outflow, $\beta$, by rearranging Equation 22 from Barniol Duran et al. (2013), where the observer time, $t$ is given by

where we set the time $t$ relative to the approximate launch date of the outflow, MJD 58522; inferred based on a linear fit to the predicted radius and estimated optical time of first light (see below). The magnetic field is given by

and the approximate mass in the emitting region of the ejecta is given by

noting that this is a lower limit on the mass in the emitting region of the outflow due to the energy estimate from equipartition also being a lower limit on the energy.

The physical outflow properties as predicted by these equations are listed in Table 2 and plotted in Figure 4. All uncertainties reported correspond to the 1$\sigma$ uncertainty obtained through propagating the uncertainty of $\nu_{\mathrm{peak}}$, $F_{\mathrm{peak}}$ and $p$ obtained through the MCMC modelling of the observed spectra.

The derived radius of the outflow increases with time, following the relation $R\propto t^{0.65}$ (reduced $\chi^{2}$=1.79), or $R\propto t$ (reduced $\chi^{2}$=2.67). We thus deduce that the outflow is roughly undergoing free expansion, with the velocity remaining approximately constant at $\beta\approx 0.2\pm 0.1$ (conical) or $\beta\approx 0.1\pm 0.06$ (spherical) until the final epoch in which the velocity shows a slight decrease. We note that the powerlaw fit to $R$, indicating a decelerating outflow, is statistically preferred to the constant velocity case. 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 $\sim$constant velocity until at least the radio lightcurve peak ($t\approx 650$ d).

The derived energy of the outflow increases approximately linearly with time for all observations, but also seems to show statistically significant non-uniform fluctuations with time. The magnetic field shows a slight decrease with time and the inferred mass in the emitting region of the outflow also increases with time (based on the energy prediction).

A simple linear fit (assuming constant velocity) to the predicted radius gives an outflow launch date of MJD 58435$\pm$10 (2018 Nov 13, concial) or MJD 58432$\pm$10 (2018 Nov 10, spherical), approximately 120 d before the first radio detection on MJD 58551 (2019 Mar 09). The optical/UV flare was observed to begin on MJD 58528 (2019 Feb 14), $\sim 90$ d after the estimated outflow launch date, and peaked on approximately MJD 58560 (2019 Mar 18 Liu et al., 2019; Hinkle et al., 2021). From a power-law fit of the optical rise observed by ASASSN, Hinkle et al. (2021) inferred that the time of first light for the event was MJD 58522 (2019 Feb 8), indicating the radio outflow was likely launched later than MJD 58433 with an initial velocity higher than 0.1 $c$. Thus in this work we assume an outflow launch date of MJD 58522; coincident with the optical time of first light.

To assess the possibility that the radio outflow from AT2019azh was produced by accretion onto the SMBH, in this section we model the accretion disk emission based on the optical, UV, and X-ray properties of the event. The evolving disk density profiles are calculated using the method developed in Mummery & Balbus (2020) and Mummery (2021a), to which the reader should refer for detailed information. We assume that the black hole environment is initially completely devoid of material, before feeding disk material into a ring according to the following relationship:

where $\delta$ is the Dirac delta function and $\Delta t$ ensures the feeding rate is finite as $t\rightarrow 0$, and was taken equal to one code time step. The feeding radius was taken to be $r_{0}=50R_{g}$, appropriate for a TDE around a low-mass black hole like AT2019azh, and the disk evolution was started at a time corresponding to the first observed optical emission of the source. This model assumes that the matter is fed into the disk at the rate at which disrupted stellar material returns to pericentre (the so-called ‘fall-back’ rate $\dot{M}_{\rm fb}$, Rees, 1988). The material fed into the disk then evolves according to the equations of disk angular momentum conservation and disk mass conservation. Energy conservation then allows the disk temperature to be calculated at each radius and time. We assume that each disk radius emits like a colour-corrected blackbody (using the Done et al. (2012) colour-correction model), and ray-trace the resulting disk emission profile. We include all relevant relativistic effects, such as Doppler and gravitational redshift, and gravitational lensing. The evolution of the X-ray and UV light curves of a thermal TDE are therefore determined by three fitting parameters: the black hole mass $M$, the total accreted mass $M_{\rm acc}$ (a normalisation on the source term, Eq. 14), and the viscous timescale of the evolving disk $t_{\rm visc}$. We compute $M_{\rm acc}$ in the following manner

where $\dot{M}(r_{I},t)$ is the inner-most stable circular orbit (ISCO) mass accretion rate.

Given the simplifications applied in the modelling (the black hole spin is fixed to zero, and the observer inclination angle is fixed to $\theta_{\rm obs}=30^{\circ}$) the best fit parameter values and their associated uncertainties should be treated with some caution, although Mummery & Balbus (2020) found only a moderate change (factor of $\sim 1.5$) in best fit black hole mass over a wide range of black hole spins.

Finally, we determine the best fit system parameters by simultaneously minimizing the sum of the squared differences between the model and the different UV and X-ray light curves of AT2019azh. As in previous works, we anticipate large formal values of the reduced $\chi^{2}$. These large values result as a consequence of short time-scale fluctuations present in the well-sampled TDE light curves, and are to be expected in any theoretical model using a smooth functional form for the time-dependence of the turbulent stress tensor. Our standard approach implicitly averages over rapid turbulent variations. Short time-scale fluctuations are likely to be highly correlated so that accurately assessing the statistical significance of the fit is not straightforward. We have therefore used $\chi^{2}$ minimisation as a sensible guide towards finding a best fit.

The observed and modelled optical, UV, and X-ray lightcurves for AT2019azh are plotted in Figure 6 for two scenarios: early accretion and delayed accretion. The observed optical and UV data are well fit by both the early and delayed accretion models, with little difference between the two. The observed X-ray lightcurve is significantly better fit by the delayed accretion model at early times, whilst the late-time X-ray lightcurve is well-fit by either model. We note that in the case of significant X-ray obscuration at early times the early accretion model may also be viable.

Our findings reveal a likely non-relativistic outflow with constant (or gradually decreasing) velocity and continuous kinetic energy increase during the radio rise (up to 666 d), and constant energy post-radio peak (from 666–849 d). We infer that the outflow ranges from radii of $\sim 3\times 10^{16}$ cm–$2\times 10^{17}$ cm with energies of $\sim 3\times 10^{47}$ erg–$1\times 10^{51}$ erg. These energy and radii correspond to a magnetic field of $\sim 0.05$ G and ambient electron density of $\sim 50$–3000 cm^-3. The observed energy and radius increased with time until the peak radio luminosity was reached.

The optical and X-ray observations, particularly the late-time X-ray rise, have been explained with either a delayed-accretion disk formation scenario (Liu et al., 2019) or due to an increase in the X-ray emitting region (Hinkle et al., 2021). We modelled two disk emission scenarios in Section 5, and found that the observed UV/optical behaviour of the event was well-fit by either model (Figure 6). The X-ray emission is better fit by a delayed accretion scenario except in the case of significant X-ray obscuration at early times.

We followed the radio evolution of the tidal disruption event AT2019azh for 850 days post-disruption. The radio emission rose slowly to a peak over 650 d, and is now decaying following the expected decay rate for the Sedov-Taylor solution. We modelled the radio emission as a spherical (or conical), non-relativistic outflow and infer energies of $3.5\times 10^{47}$–$2.7\times 10^{50}$ erg ($9\times 10^{47}$–$1\times 10^{51}$ erg), radii of $2\times 10^{16}$–$2\times 10^{17}$ cm ($5\times 10^{16}$–$5\times 10^{17}$ cm), and circumnuclear density of 70–1100 cm^-3 (15–1000 cm^-3), for a velocity of $\approx 0.1\,c$. We detected radio emission approximately 10 d before the optical peak, which is the first radio detection of a thermal TDE at such early times, and well before the X-ray emission indicated any signs of significant accretion. This early time radio detection is in contrast to the suggestion that late-time radio flares or delayed radio emission is common in TDEs (Horesh et al., 2021a). Such an early radio detection could rule out the accretion-driven wind outflow model at early times, but through detailed disk modelling we found that both a scenario with delayed accretion or one with early accretion but significant X-ray obscuration could explain the optical, UV, and X-ray properties of the event.

Interestingly, we observed the electron energy index determined from the optically thin part of the synchrotron emission to fluctuate between $p\approx 2.6$ to $p=3-3.5$ from 400 d post-disruption on timescales of months. We deduce that these fluctuations could be due to either inhomogenous emitting regions in a spherical or conical outflow or fluctuations in the energy injection rate of a conical outflow. We rule out the possibility of a relativistic jet producing the outflow in this event since it would require an opening angle of the jet that is smaller than expected for a jet from an SMBH. We found that the mean speed of the outflow to $t=183$ d was $\sim 0.1c$, thus ruling out a long-lived relativistic outflow. We also found a possible increase in deceleration, suggesting a transition to a Sedov-like evolution after $t\sim 600$ d.

We deduce that the unbound debris stream is unlikely to explain the radio properties of this event due to the high energy inferred through an equipartition analysis of the synchrotron emission. We propose the outflow could have originated from an accretion-driven wind or sub-relativistic jet, or a collision induced outflow from stream-stream intersections of the tidal debris. Further detailed modelling of the multiwavelength properties of this event is required to differentiate between outflow models and explain some of the unique properties observed. Future observations of the radio decay of AT2019azh will enable more robust constraints on the CNM density and further insight into the nature of the outflow.

The authors thank K. Alexander for helpful discussions, W. Lu for providing helpful comments on an earlier version of this manuscript, and N. Blagorodnova, S. Kulkarni, A. de Witt, B. Cenko, S. Gezari, R. Fender, P. Woudt, and M. Bottcher for their contributions. This work was supported by the Australian government through the Australian Research Council’s Discovery Projects funding scheme (DP200102471). The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant $\#$12540303 (PI: Graham). The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. This work was carried out in part using facilities and data processing pipelines developed at the Inter-University Institute for Data Intensive Astronomy (IDIA). IDIA is a partnership of the Universities of Cape Town, of the Western Cape and of Pretoria.

The spectral fitting and equipartition modelling software written for the purpose of this work is publicly available on Github at https://github.com/adellej/tde_spectra_fit. The observations presented in Table 1 will be published online in machine-readable format at url to be entered on publication.

This research made use of Matplotlib, a community-developed Python package (Hunter, 2007), NASA’s Astrophysics Data System Bibliographic Services, the Common Astronomy Software Application package CASA (McMullin et al., 2007), the $w$-stacking CLEAN imager (Offringa et al., 2014), and the Python packages cmasher (van der Velden, 2020), and emcee (Foreman-Mackey et al., 2013).