# Delayed Appearance and Evolution of Coronal Lines in the TDE AT2019qiz

P. Short1, A. Lawrence1, M. Nicholl2, M. Ward3, T. M. Reynolds4, S.
Mattila,5,20 C. Yin1, I. Arcavi6,7, A. Carnall1, P. Charalampopoulos8, M.
Gromadzki9, P. G. Jonker10,11, S. Kim12,13, G. Leloudas8, I. Mandel14,15, F.
Onori16, M. Pursiainen8, S. Schulze17, C. Villforth18, T. Wevers19
1Institute for Astronomy, University of Edinburgh, Royal Observatory,
Blackford Hill, Edinburgh EH9 3HJ, UK
2Birmingham Institute for Gravitational Wave Astronomy and School of Physics
and Astronomy, University of Birmingham, Birmingham, B15 2TT, UK
3Centre for Extragalactic Astronomy, Department of Physics, University of
Durham, South Road, Durham, DH1 3LE, UK
4Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen,
Denmark
5Tuorla Observatory, Department of Physics and Astronomy, University of Turku,
FI-20014 Turku, Finland
6The School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978,
Israel
7CIFAR Azrieli Global Scholars program, CIFAR, Toronto, Canada
8DTU Space, National Space Institute, Technical University of Denmark,
Elektrovej 327, 2800, Kgs. Lyngby, Denmark
9Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478
Warszawa, Poland
10SRON, Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA,
Leiden, The Netherlands
11Department of Astrophysics/IMAPP, Radboud University Nijmegen, P.O. Box
9010, 6500 GL, Nijmegen, The Netherlands
12Astro-engineering center (AIUC), Instituto de Astrofísica, Pontificia
Universidad Católica de Chile, Santiago, Chile
13Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
14Monash Centre for Astrophysics, School of Physics and Astronomy, Monash
University, Clayton, Victoria 3800, Australia
15ARC Center of Excellence for Gravitational Wave Discovery — OzGrav,
Australia
16INAF-Osservatorio Astronomico d’Abruzzo, via M. Maggini snc, I-64100 Teramo,
Italy
17The Oskar Klein Centre, Department of Physics, Stockholm University,
AlbaNova, SE-106 91 Stockholm, Sweden
18University of Bath, Department of Physics, Claverton Down, Bath, BA2 7AY, UK
19European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago,
Chile
20School of Sciences, European University Cyprus, Diogenes street, Engomi,
1516 Nicosia, Cyprus E-mail<EMAIL_ADDRESS>
###### Abstract

Tidal disruption events (TDEs) occur when a star gets torn apart by a
supermassive black hole as it crosses its tidal radius. We present late-time
optical and X-ray observations of the nuclear transient AT2019qiz, which
showed the typical signs of an optical-UV transient class commonly believed to
be TDEs. Optical spectra were obtained 428, 481 and 828 rest-frame days after
optical lightcurve peak, and a UV/X-ray observation coincided with the later
spectrum. The optical spectra show strong coronal emission lines, including
[Fe VII], [Fe X], [Fe XI] and [Fe XIV]. The Fe lines rise and then fall,
except [Fe XIV] which appears late and rises. We observe increasing flux of
narrow H$\alpha$ and H$\beta$ and a decrease in broad H$\alpha$ flux. The
coronal lines have FWHMs ranging from $\sim 150-300$km s-1, suggesting they
originate from a region between the broad and narrow line emitting gas.
Between the optical flare and late-time observation, the X-ray spectrum
softens dramatically. The 0.3-1 keV X-ray flux increases by a factor of $\sim
50$ while the hard X-ray flux decreases by a factor of $\sim 6$. WISE fluxes
also rose over the same period, indicating the presence of an infrared echo.
With AT2017gge, AT2019qiz is one of two examples of a spectroscopically-
confirmed optical-UV TDE showing delayed coronal line emission, supporting
speculations that Extreme Coronal Line Emitters in quiescent galaxies can be
echos of unobserved past TDEs. We argue that the coronal lines, narrow lines,
and infrared emission arise from the illumination of pre-existing material
likely related to either a previous TDE or AGN activity.

###### keywords:

transients: tidal disruption events - black hole physics - accretion,
accretion discs

††pagerange: Delayed Appearance and Evolution of Coronal Lines in the TDE
AT2019qiz–B

##  1 Introduction

Figure 1: The reduced X-Shooter and MUSE spectra. The X-Shooter spectra are
binned for ease of viewing. Emission line identifications are marked with the
grey dashed lines. The upper plot covers the X-Shooter UVB arm wavelength
range while the lower plot covers the VIS arm. The ‘emission’ features redward
of $\sim$6800Å in the MUSE (green) spectrum are residuals from telluric
corrections. The fluxes are scaled to approximately match the stellar
continuum in each case.

A tidal disruption event (TDE) occurs when a star’s orbit brings it within the
tidal radius of its host galaxy’s central supermassive black hole (SMBH). The
star is subsequently ripped apart, releasing a luminous flare as the stellar
material falls back onto the SMBH (Hills, 1975; Rees, 1988). Early TDE
candidates were discovered via soft X-ray flares (e.g. Komossa & Bade 1999;
Komossa & Greiner 1999), with luminosities of $\sim 10^{44}$ erg s-1 and
lightcurves matching the $t^{-5/3}$ decay rate expected from theory (Hills,
1975; Rees, 1988). Since then, a variety of nuclear transients and extreme
variables has been observed, but a consistent class of optical-UV transients
has emerged which are widely accepted as being TDEs (van Velzen et al., 2020;
Gezari, 2021). The key features which define this class include (i) a large
amplitude nuclear optical flare; (ii) a rise time of $\sim$days-weeks and a
decay time of $\sim$weeks-months; (iii) extremely broad and weak emission
lines from Hydrogen and Helium; (iv) a blue continuum with effective
temperature around 10-30,000K; and (v) roughly constant optical colours during
the decay. We refer to members of this class as “spectroscopically confirmed
optical-UV TDEs”. Strictly speaking it is not rigorously proved that these
events result from the disruption of a star, but most workers accept that a
TDE is the most likely explanation, and so refer to these objects simply as
“TDEs” rather than for example “candidate TDEs”. Empirically they form a well
defined class which is distinct from either known types of supernovae (SNe),
or extreme active galactic nuclei (AGN) variables such as Changing Look AGN.

Even within this consistent class, there is a detailed diversity. The
appearance or lack of Hydrogen, Helium and Bowen fluorescence emission lines
divide these optically-selected TDEs into three classes; TDE-H, TDE-He and
TDE-H+He (Arcavi et al., 2014; Leloudas et al., 2019; van Velzen et al.,
2021b). It has been shown that the strengths of these lines can vary as the
TDE evolves (e.g. Nicholl et al. 2019, Charalampopoulos et al. 2022, Onori et
al. 2022), allowing TDEs to transition between classes. In addition, other
features have been observed such as Fe II lines (Wevers et al., 2019b;
Cannizzaro et al., 2020) and double-peaked emission lines (Short et al., 2020;
Hung et al., 2020; Wevers et al., 2022). TDEs are also extremely diverse in
their X-ray to optical ratios, ranging from $\sim 10^{-4}$ to 1 (Auchettl et
al., 2017). This could indicate that some TDEs form accretion disks quickly
while others do not, or that X-rays are heavily absorbed/re-processed by an
atmosphere or outflow along the observer’s line of sight (e.g. Loeb & Ulmer
1997, Guillochon et al. 2014, Roth et al. 2016, Roth & Kasen 2018, Dai et al.
2018).

Given that optical-UV TDEs are seen to occur in systems with black hole masses
$10^{6-7}M\odot$, (Wevers et al., 2017; Nicholl et al., 2022) one would expect
a compact accretion disc to have a temperature $T\sim 10^{6}$ K, at odds with
the observed temperatures of $T\sim 2-4\times 10^{4}$ K. This may be explained
by reprocessing (see paragraph above), but is the expected EUV continuum
actually there? One test is to look for emission from highly ionised species.

Prior to the current era of optical sky surveys, Komossa et al. (2008)
discovered fading high-ionisation coronal Iron emission lines, He II and
double-peaked Balmer lines in the nearby galaxy SDSS J095209.56+214313.3
(J0952+2143) during a search for emission line galaxies in SDSS DR6. Optical
photometry showed a peak in optical brightness in 2004, with an increase
towards the NIR which was inconsistent with previous 2MASS observations. GALEX
data also showed a rise in UV flux. Wang et al. (2011) found a similar example
of coronal line emission in the galaxy SDSS J074820.67+471214.3 which also had
an optical flair, and went on to find 5 more in a systematic search for such
objects (Wang et al., 2012). These authors proposed that the most likely cause
of such coronal emission was an earlier, unobserved, TDE in a gas rich
environment. Of the five new events reported by Wang et al. 2012, three had
fading iron lines as found by (Yang et al., 2013), making these and J0952+2143
the strongest candidates for ECLEs which are caused by illumination by an
earlier TDE. The case for J0952+2143 was made stronger, when a clear infrared
echo was found Komossa et al. (2009), and when the associated optical flare
was covered more densely in LINEAR data (Palaversa et al., 2016)
retrospectively.

In this scenario the broad low-ionisation emission lines come from re-
processing of UV/X-ray flares by outflows directly connected with the TDE in
question, which matches well the line profiles of H$\alpha$ and He II (Roth &
Kasen, 2018), while the coronal lines originate from interactions with a
clumpy interstellar medium or other pre-existing material. If produced by
photoionisation, the coronal lines imply an ionising continuum extending into
the X-rays, requiring ionising potentials of up to nearly 400eV in the case of
[Fe XIV]. Coronal lines are not uncommon in AGN (Grandi, 1978; Penston et al.,
1984; Gelbord et al., 2009), where we also expect a hard ionising continuum.

It is therefore important to test for the presence of highly ionised species
in the class of well-confirmed optical-UV TDEs, where we have high quality
optical, X-ray and spectroscopic data at the time of the actual flare. Coronal
lines have so far only been observed in one such clear optical-UV TDE to date,
AT2017gge (Onori et al., 2022). The lines appeared in this event from $\sim
200-1700$ days after the optical lightcurve peak. Another potential candidate
for a TDE showing coronal lines is the nuclear transient AT 2019avd. Malyali
et al. (2021) observed [Fe X] and [Fe XI] emission lines accompanied by a rise
in X-ray flux, and Chen et al. (2022) argue that this event could be
interpreted as a TDE. However observationally it is definitely not a member of
our well defined class of optical-UV TDEs - it has a double peaked light
curve, and relatively strong and narrow Balmer lines. There was also a
tentative detection of coronal lines in the nuclear transient PS16dtm
(Petrushevska et al 2023). Like AT 2019avd, it does not show all the classic
signs of the optical-UV TDE class, but is nonetheless argued to be a TDE by
Petrushevska et al. (2023).

As well as exhibiting coronal lines, AT2017gge was also exceptionally luminous
in the infrared (IR). This IR emission arises from an IR echo, in which dust
in the vicinity of the SMBH absorbs UV/optical emission produced by the TDE,
and re-radiates this energy in the IR. IR echoes have previously been observed
in approximately half of optically discovered TDEs, but typically the total
proportion of luminosity arising from the dust is small, implying that the
covering factor is only $\sim$1% of that observed in the UV/optical (Jiang et
al., 2021). The covering factor derived for the IR echo associated with
AT2017gge was $\sim$20% (Wang et al., 2022), much larger than typical TDEs
discovered in optical and occurring in quiescent galaxies, and indicating the
presence of a larger quantity of dust. AT2019avd also exhibited an abnormally
bright IR echo (Malyali et al., 2021; Chen et al., 2022), further suggesting a
connection between these phenomena.

Figure 2: The H$\beta$ \+ [O III] and H$\alpha$ regions for our late-time
spectra. The narrow Balmer lines have brightened considerably over time, while
the broad H$\alpha$ has risen in the Feb 2021 (green) spectrum before dimming
again in the Jan 2022 spectrum (orange).

The subject of this work is AT 2019qiz, a clear member of the optical-UV TDE
class first identified on 2019-09-19 (Forster, 2019) at a redshift of
$z=0.0153$, and reaching a peak at 2019-10-10. Gaia data (Gaia Science Alerts
Hodgkin et al. 2013) show no further activity in the optical lightcurve since
the decline from this peak. The transient was analysed in detail by Nicholl et
al. (2020) and Hung et al. (2021). Lightcurve fitting results suggested that
the flare was produced by a $\sim$1M⊙ star being disrupted by a $\sim$106M⊙
black hole. The black hole mass derived from the TDE light curve was
consistent with that derived from velocity dispersion measurements and
$M-\sigma$ relations. Early optical spectra show broad, asymmetric H and He II
lines, which Nicholl et al. (2020) argued were driven by an expanding outflow.
The lines became more symmetrical as the TDE evolved. He II was replaced by N
III $\lambda 4641$ via Bowen fluorescence (Nicholl et al., 2020; Hung et al.,
2021), which occurs when recombining He II produces an EUV photon which goes
on to excite some O III and N III states. These subsequently recombine
producing ‘Bowen fluorescence’ lines. UV spectra show high-ionisation broad
absorption lines (HiBALs) and Fe and low-ionisation broad absorption lines
(FeLoBALs) (Hung et al., 2021). Unfortunately, there are no spectra of the
host galaxy (2MASX J04463790-1013349) taken before the flare. Both studies
suggest that the host galaxy harbours a weak AGN based on Baldwin-Phillips-
Terlevich (BPT) diagrams. Early X-ray detections suggest accretion started
promptly in this event (Nicholl et al., 2020), though the high hardness ratio
may indicate that the X-ray emission originates from the pre-existing AGN
(Hung et al., 2021). However, Nicholl et al. (2020) note that the hardness
ratio varied during outburst, implying that the TDE did affect the X-ray
emission.

In this paper we present and analyse two new spectra of AT2019qiz obtained
with X-Shooter taken 428 and 828 rest-frame days after the optical lightcurve
peak, a MUSE spectrum obtained 481 rest-frame days after optical peak and
Swift XRT data taken 816 rest-frame days post optical peak, as well as re-
examining the spectra taken during the campaign of Nicholl et al. (2020). In
§2 we detail our observations and data reduction process. In §3 we present our
spectroscopic analysis, in §4 we present analysis of Swift UV and X-ray data,
and in §5 we construct an SED of the TDE in outburst. In §6 we present
analysis of NEOWISE IR data. In §7 we discuss and interpret our results and
analysis and in §8 we provide a summary and conclusion to this work.

##  2 Observations and Data Reduction

###  2.1 X-Shooter

Observations were made on 2020-12-15 and 2022-01-26 with the X-Shooter
instrument (Vernet et al., 2011) mounted on UT3 at the Very Large Telescope
(VLT) in ESO’s Paranal observatory, Chile. X-Shooter is an intermediate
resolving power spectrograph with a wavelength range spanning 3000 - 25,000Å.
The spectrograph consists of three arms; the UVB arm which covers 3000-5595Å,
the VIS arm which spans 5595-10,240Å and the NIR arm which ranges from
10,240-24,800Å. Slit widths of 1.0", 0.9" and 0.9" were used for the UVB, VIS
and NIR arms respectively, giving respective resolving powers of R=5400, 8900
and 5600. Data were obtained under programs 106.21SS.001 (PI Short) and
108.22J7.001 (PI Nicholl). We refer to these spectra throughout this paper as
the Dec 2020 and Jan 2022 spectra, respectively.

Data reduction was performed using the X-Shooter pipeline recipes and the
EsoReflex GUI environment (Freudling et al., 2013). The NIR arm was reduced in
NOD mode while the UVB and VIS arms were reduced in STARE mode which was found
to improve the signal-to-noise. Telluric corrections were performed in the VIS
arm using telluric standards observed before and after the observations at a
similar position to the target. Telluric features in the NIR arm proved
difficult to correct and, as this wavelength region contained no features of
interest, this arm was not used in the rest of our analysis. Extinction
correction was performed using the Fitzpatrick (1999) model in the
Extinction111https://extinction.readthedocs.io/en/latest/ package in Python,
with reddening values from the NASA/IPAC Infrared Science Archive (Schlafly &
Finkbeiner, 2011). As the optical continuum luminosity at this phase is
dominated by the host galaxy, the spectra were flux corrected using the
PySynphot Python package (STScI Development Team, 2013) to an aperture-matched
host r-band mag of 16.47 as determined in Nicholl et al. (2020). The reduced
spectra are displayed in Figure 1.

###  2.2 MUSE

We observed the field on 2021-02-08 with the panoramic integral-field
spectrograph MUSE (Bacon et al., 2010), mounted at UT4 at ESO’s Very Large
Telescope, in the seeing-enhanced adaptive optics wide-field mode (WFM-AO)
under clear condition and DIMM seeing $<$ 0.6′′. MUSE WFM-AO has a large field
of view covering $1^{\prime}\times 1^{\prime}$ and a high spatial sampling of
$0.2^{\prime\prime}\times 0.2^{\prime\prime}$. It covers the wavelength range
from 4650 to 9300 Å with a spectral resolving power of 2000–4000. The
observation consists of three 1100-s exposures that were dithered by
1–$2^{\prime\prime}$ and rotated by $90^{\circ}$ with respect to each other.
We retrieved the reduced science-ready datacube from the ESO archive, which
was reduced with the ESO MUSE pipeline version 2.8.4 (Weilbacher et al.,
2020). We extracted the spectrum of the host galaxy nucleus using a circular
aperture with a diameter of $1\times$FWHM(stellar PSF). The integral field of
MUSE also allows us to check for spatial extent. The host starlight is of
course very extended, but the emission line source is compact. Comparing
extractions with 1 arcsec and 2 arcsec apertures changed the [OIII] flux by
$\sim$15%. Given the other uncertainties, this gives us reasonable confidence
in both the Xshooter and MUSE line flux values.

Figure 3: Fits to H$\alpha$ regions in (left) an early X-Shooter spectra
obtained 20191113 and (right) our Dec 2020 spectrum. In both cases the broad
H$\alpha$ feature is well fit by two broad Gaussian components, though in the
earlier spectra the second component is much broader and offset. Figure 4: The
evolution of both the broad and narrow H$\alpha$ emission line components
using spectra from Nicholl et al. 2020 and the spectra presented in this
paper. In the first three spectra there is a second broad outflow component
which we omit from our measurements. Errorbars are included but in some cases
too small to be visible.

###  2.3 Neil Gehrels Swift Observatory

A Swift (Gehrels et al., 2004) observation was made on 2022-01-13 using the
XRT instrument in photon-counting mode and the UVOT instrument with the UVW2
filter (Obs ID: 00012012043, PI Short). The XRT data was reduced and a
spectrum was generated using the online Swift XRT product builder (Evans et
al., 2009). The UVOT UVW2 flux was measured using a 5” aperture, approximately
twice the UVOT point-spread function. This matches the aperture used to
extract earlier UVOT photometry by Nicholl et al. (2020). The count rates were
obtained using the Swift uvotsource tools and converted to magnitudes using
the UVOT photometric zero points (Breeveld et al., 2011). The analysis
pipeline used software HEADAS 6.24 and UVOT calibration 20170922. We note that
the calibration files have since been updated, but we use the older
calibration for consistency with Nicholl et al. (2020). We measured UVW2 AB
magnitude of $20.58\pm 0.04$ which is consistent with the host magnitude
measured in (Nicholl et al., 2020) of $20.51\pm 0.2$, suggesting there is no
detectable contribution from the TDE.

###  2.4 NEOWISE survey

We obtained MIR data of AT 2019qiz taken as part of the NEOWISE survey
(Mainzer et al., 2011, 2014) from the public NEOWISE-R Single Exposure (L1b)
Source Table222https://irsa.ipac.caltech.edu/cgi-bin/Gator/nph-
scan?mission=irsa&submit=Select&projshort=WISE. NEOWISE observes the entire
sky at 6 month intervals, with multiple observations at each "visit". We
adopted the median value of the individual measurements at each visit as the
magnitude, removing measurements flagged as poor quality or separated from the
coordinates of AT 2019qiz by more than 2″. For the uncertainty, we adopted the
standard error of mean of the individual measurements taken at each bi-yearly
epoch of WISE observation after 3 sigma-clipping outliers. We added 0.0026 mag
and 0.0061 mag uncertainties in quadrature to the W1 and W2 measurements,
respectively, which are the RMS residuals found in the photometric calibration
during the survey period. No extinction correction was made to the WISE
magnitudes.

##  3 Spectroscopic Analysis

###  3.1 Host Black Hole Mass

Nicholl et al. (2020) follow the method of Wevers et al. (2017) and Wevers et
al. (2019a) to fit the velocity dispersion of stellar absorption lines using
PPXF (Cappellari, 2017). They used an X-Shooter spectrum obtained 140 days
after optical lightcurve peak, and measured a velocity dispersion of $70\pm
2$km s-1. We use the same method on our X-Shooter spectra and measure a value
of $72\pm 1$km s-1, consistent with that measured in Nicholl et al. (2020).
Using the relations of McConnell & Ma (2013), Gültekin et al. (2009) and
Kormendy & Ho (2013) our velocity dispersion measurement yields black hole
masses of log${}_{10}(M_{BH}/M_{\odot})$ = $5.82\pm 0.41$, $6.24\pm 0.48$ and
$6.54\pm 0.32$, respectively. Both Nicholl et al. (2020) and Hung et al.
(2021) obtain additional black hole mass estimates from UV and optical
lightcurve model fits, deriving values of log${}_{10}(M_{BH}/M_{\odot})$ =
$5.89^{+0.05}_{-0.06}$ and $6.14\pm 0.10$, respectively. These are both
consistent with the estimate from the velocity dispersion. Additionally,
Nicholl et al. (2022) re-fit the lightcurve after correcting for a calibration
error affecting UVOT photometry at the time of the AT2019qiz outburst. The
slightly increased value of log${}_{10}(M_{BH}/M_{\odot})$ = 6.22 is still
consistent with the velocity dispersion results. Given the range of estimated
values, for simplicity we assume an intermediate SMBH mass of
$M_{BH}=10^{6}M_{\odot}$ for the remainder of this analysis, but note that
this has an uncertainty of at least a factor of two.

###  3.2 Low Ionisation Lines

Figure 5: BPT diagrams showing the location of the nuclear emission in both
our spectra. Measurements from the Dec 2020 spectrum clearly suggest the
emission is due to an AGN, but the Jan 2022 emission lines more closely
resemble a star forming region. However, H$\beta$ could be overestimated if
there is an unresolved component from the TDE.

The spectra feature narrow emission lines, including [O II], [O III], [N II],
[S II], H$\alpha$ and H$\beta$, as well as broad H$\alpha$. The H$\beta$ and
H$\alpha$ regions are displayed in Figure 2. To measure the flux and widths of
each line we use the Python package lmfit333https://lmfit.github.io/lmfit-py/
to fit a single Gaussian component to the line and a polynomial component to
the local continuum in that line region. In order to separate the narrow
H$\alpha$ and [N II] emission lines from the broad H$\alpha$ feature, we fit
the entire region with 5 Gaussian components simultaneously. The broad feature
is not well fit by a single Gaussian so we fit two components to this, while
the remaining three components are fit to the narrow H$\alpha$ and [N II]
$\lambda\lambda$ 6549,6584Å lines. The [N II] $\lambda$6584Å/$\lambda$6549Å
ratio is fixed to 3 and the [N II] $\lambda$6549Å line width is fixed to match
that of the 6584Å line. The fits, shown in the right panel of Figure 3, were
performed on both X-Shooter spectra and the MUSE spectrum and provide a
satisfactory fit, hinting that the broad line may be double peaked. When
measuring H$\beta$, an absorption component is clearly visible which we fit
separately. A stellar absorption feature is likely obscured in the H$\alpha$
blend, meaning that the total H$\alpha$ flux could be underestimated. Flux
measurements of all emission lines are shown in tables LABEL:tab:fe_flx1,
LABEL:tab:fe_flx2 and LABEL:tab:fe_flx3. We note that the third spectrum seems
to have an anomalous [OIII] 5007/4959 ratio. However the ratio of peak values
is much closer to the expected value of 3. There is an additional systematic
error in deriving line fluxes that arises from the uncertain line wings in the
presence of both noise and structured stellar continuum. We estimate this
systematic error to be $\sim$ 15% of the line flux.

There are a number of changes in the emission lines from the Dec 2020 spectrum
to the Jan 2022 spectrum. The broad H$\alpha$ line increases in flux from the
Dec 2020 to Feb 2021 spectra but then drops again in the Jan 2022 spectrum.
Meanwhile, the narrow H$\alpha$, H$\beta$ and [S III] lines increase in
strength. The [O III] lines do not change significantly. We also observe the
appearance of [O I] $\lambda 6300$Å and He II $\lambda$4686Å in the Jan 2022
spectrum, neither of which were detectable in the previous spectra.

In Figure 4 we plot the evolution of both broad and narrow H$\alpha$ emission
over time. We use X-Shooter spectra from Nicholl et al. (2020) dating back to
$\sim 35$ days after optical lightcurve peak to show the complete evolution.
In the spectra from Nicholl et al. (2020) there are two broad components but
one is consistently broader and redshifted. Nicholl et al. (2020) determine
this is possibly due to an outflow so we exclude it from our measurements. An
example fit to these earlier spectra is included in Figure 3. The broad line
evolution initially follows a downward trend but increases again at some point
between $\sim 130$ days and 481 days (our Dec 2020 spectrum). It peaks at 481
days (our Feb 2021 spectrum) before declining again. The narrow line emission
appears to be increasing. This is presumably due to photons from the initial
outburst illuminating material at large radii. The fact that this is taking
place gradually over an extended time, rather than a simple spike at a time
delayed from the outburst, indicates that the material is spread over a large
range of distances and/or angles. We note that an increase in narrow line flux
was also observed in the other coronal line emitting TDE (AT2017gge; Onori et
al. 2022). In addition,Wang et al. (2011); Wang et al. (2012) see [O III]
increase after the coronal lines have faded in the Extreme Coronal Line
Emitters which they proposed to be the echoes of TDEs.

The fact that the Balmer line increase has not yet come to a clear maximum
means that we do not yet have a clear picture of the radial and angular
distribution of the related gas. However, we can ask whether what we see so
far is consistent with the kinematic information. We have neither a kinematic
or spatial model, but a reasonable expectation is that line widths will be of
the order of the virial velocity at the corresponding distance, so that we can
estimate distance crudely as $r=GM/v_{FWHM}^{2}$, using a black hole mass of
$M=10^{6}$ M$\odot$. Balmer line widths are $\sim$80km s-1 indicating a
distance of 2.2 light years. However at this distance, the gas may be
responding to the galaxy potential as much as the black hole, so that this
distance should be considered a lower limit. This is consistent with the fact
that Balmer line response has not yet peaked; on the other hand there clearly
is significant material already responding at delays less than two years,
indicating that the illuminated material is distributed over a large range of
radii and/or angles.

Figure 6: Comparison of Fe coronal line regions in all of our late-time
spectra. The dotted grey line marks the rest-frame wavelengths of each line.
The green spectrum is the MUSE spectrum, the orange and blue spectra are the
X-Shooter data.

In order to determine the source of the narrow line emission we plot our
measurements on BPT diagrams (Baldwin et al., 1981; Veilleux & Osterbrock,
1987; Kauffmann et al., 2003) in Figure 5. We use the Dec 2020 and Jan 2022
X-Shooter measurements as well as the 2020-02-28 spectrum from (Nicholl et
al., 2020). We do not use the Feb 2021 measurement as H$\beta$ is not resolved
in the MUSE spectrum. H$\beta$ is difficult to measure in the 2020-02-28
spectrum which gives rise to a large errorbar, however, the [N II]/H$\alpha$
and [S II]/H$\alpha$ ratios clearly place the galaxy in the AGN region rather
than star formation. In the Dec 2020 spectrum the narrow lines look to clearly
have an AGN origin, but in the Jan 2022 spectrum the emission looks more like
a star forming region. We would expect the ionising continuum for a TDE to be
similar to that for an AGN, but any changes in the ionising continuum could
cause the line ratios to change. Detailed modelling of TDE emission lines is
planned for future work.

Using the Jan 2022 spectrum we calculate an He II/H$\beta$ ratio of
0.33$\pm$0.05. Assuming both lines are produced via photoionisation, this
reflects the relative intensity of the ionising continuum at 912Å and 228Å. If
the continuum in this region is a power law then we can calculate the index
$\alpha$ using $I_{4686}/I_{4862}\propto(912/228)^{\alpha}$ (Penston &
Fosbury, 1978). With our measured ratio we calculate $\alpha=-1.3\pm 0.1$. We
use this to help construct the SED in Section 5.

###  3.3 Coronal Lines

The most interesting features in our spectra are the highly ionised Iron lines
[Fe VII] $\lambda$6087, [Fe X] $\lambda$6375, [Fe XI] $\lambda$7892 and [Fe
XIV] $\lambda$5304, as well as [Ne V] $\lambda$3426\. In Figure 6 we display
[Fe] line regions from all three of our recent spectra. The most notable
change is the appearance of [Fe XIV] in the Feb 2021 and Jan 2022 spectra
which was not detectable in the Dec 2020 observation. We also note that in the
Dec 2020 spectrum narrow features are visible either side of [Fe X] and
redward of [Fe XI]. A feature is also resolved redward of [Fe X] in the Feb
2021 spectrum but the feature is not resolved in the [Fe XI] region. To
determine whether or not these features are related we overplot the Dec 2020
lines in velocity space as shown in Figure 7. Here we can see that the red
narrow features overlap. The red feature is displaced by $\sim$260kms-1 from
the line centre, while the blue feature is displaced by $\sim$-300kms-1. That
the red feature is offset by a similar velocity in both the [Fe X] and [Fe XI]
regions suggests that these features are from the same material. We
investigated whether these features lined up with the velocity offsets of the
broad Gaussian components fit to the H$\alpha$ blend, but, as shown in Figure
7, they clearly originate from a different region.

Figure 7: Fe lines in the Dec 2020 spectrum overplotted in velocity space. In
the [Fe X], [Fe XI] and maybe [Fe VII] regions two features are visible, a
broader component centred at 0km/s and narrower line offset by $\sim$260kms-1.
There is also a feature offset $\sim$300kms-1 blueward of [Fe X]. The grey
dashed lines mark the line centres and the location of the offset components.
The orange rectangles mark the locations of broad components fit to H$\alpha$,
where the widths cover $\pm 1\sigma$. These are offset much more than the
narrow Fe features, showing they do not originate from the same region.

We estimate line fluxes by fitting Gaussian profiles as with the narrow lines
above. We fit the offset narrow emission features separately, noting that
these are typically unresolved. We also check for the presence of Fe lines in
the earlier X-Shooter spectra (Nicholl et al., 2020) dating back to $\sim$35
days after lightcurve peak. In most cases the lines are not detected in which
case we set an upper flux limit one standard deviation above the continuum
flux level. The exception to this is [Fe XI] which does seem to appear at
earlier times. In Figure 16 we plot the [Fe XI] region for all of the spectra
we used. A feature is visible at around the right wavelength in the earlier
spectra during outburst, though it is weak, broad and slightly offset so it
seems likely this is not real [Fe XI] emission. We also plot the rest of the
Fe line regions in Appendix B. The full list of lines and their flux
measurements and line widths is displayed in tables LABEL:tab:fe_flx1 and
LABEL:tab:fe_flx2. In Figure 8 we plot the evolution of each Fe emission line.
In general, the flux of each line increases between the first and second
spectra, before decreasing in the most recent spectrum, with the exception of
[Fe XIV] as it appears for the first time in the Feb 2021 spectrum and
increases in the Jan 2022 spectrum. The large errorbar in [Fe XIV] flux in the
Feb 2021 spectrum comes from the fact that the line is not well resolved by
MUSE. However, from Figure 6 there does appear to be an increase in flux in
this region compared to Dec 2020. Wang et al. (2012) also observe evolution in
the Fe line flux of their coronal line emitting objects. They carry out
spectroscopic follow-up of three of their targets, taken several years after
the first spectrum. They find that the [Fe X], [Fe XI] and [Fe XIV] flux
decreases, but that the [Fe VII] flux remains constant.

The velocity widths of the highly ionised lines range from $\sim$150 to
$\sim$300 km s-1 , with additional unresolved or barely resolved offset
components in [Fe X] and [Fe XI]. In section 7 we discuss what this implies
for the structure and origin of the illuminated gas in AT2019qiz. Here, as
with the low ionisation lines, we examine whether the kinematic information is
consistent with the flux evolution. If we once again use a “virial distance”,
we find the coronal line material in the range 50-300 light days. This is
qualitatively consistent with the coronal line response peaking well before
the low ionisation lines. Quantitatively, it is not quite in simple agreement
with observed flux peak at $\sim$500 days, and may suggest somewhat sub-virial
velocities, but a proper spatial and kinematic model would be needed before
making any stronger statement.

Figure 8: Evolution of Fe line fluxes since early outburst spectra. In cases
where we don’t detect a line we set 1$\sigma$ upper limits denoted by the
triangles. We include measurements from the Dec 2020, Feb 2022 and Jan 2021
spectra both with and without the narrow offset features. Measurements with
narrow features are marked with crosses and dashed lines. Figure 9: Our 2022
XRT spectrum (red) compared to the earlier 2019 observation (black; Nicholl et
al. 2020). Both spectra can be adequately fit by power-law or thermal disk
models (diskbb in xspec). The fit parameters are given in Table 1. The latter
models have been used here to convert counts to fluxes, and are shown in the
figure labelled by the temperatures at the inner disk edge. The softening of
the spectrum is clearly visible.

##  4 X-ray Analysis

Table 1: Parameters and fluxes from X-ray model fits

| Power-law (tbabs$\times$zpowerlaw) | Thermal disk (tbabs$\times$zashift$\times$diskbb)
---|---|---
Parametera | 2019 | 2022 | 2019 | 2022
$\Gamma$ (photon index) | $1.13\pm 0.26$ | $5.44\pm 1.12$ |  |
$k_{\rm B}T_{\rm in}$ (keV) |  |  | $2.54\pm 1.25$ | $0.13\pm 0.01$
Normalizationb | $(8.3\pm 2.1)\times 10^{-6}$ | $(4.9\pm 1.5)\times 10^{-5}$ | $(1.2\pm 1.9)\times 10^{-4}$ | $272\pm 151$
Unabsorbed flux in rest-frame (erg cm-2 s-1) |  |  |
$0.3-10$ keV | $1.1\times 10^{-13}$ | $6.9\times 10^{-13}$ | $9.4\times 10^{-14}$ | $5.2\times 10^{-13}$
$0.3-1$ keV | $1.0\times 10^{-14}$ | $6.6\times 10^{-13}$ | $7.4\times 10^{-15}$ | $5.1\times 10^{-13}$
$1-6$ keV | $5.8\times 10^{-14}$ | $2.4\times 10^{-14}$ | $6.0\times 10^{-14}$ | $9.7\times 10^{-15}$

a Redshift fixed at $z=0.0151$ and Galactic column density $n_{\rm
H}=6.6\times 10^{20}$ cm-2. Any additional intrinsic column density in the
host galaxy (modelled using ztbabs) is found to be negligible and
unconstrained in both models.
b Normalization is defined differently for power-law and disk models; see
xspec documentation for details.

Figure 10: The full SED of AT2019qiz in outburst on MJD $\sim$58794\. The more
recent XRT spectrum is also included to highlight the jump in soft X-ray flux.
The IR points are for the first detection on MJD 58886, 92 days after the rest
of the SED. Error bars in the photometry points are too small to be seen on
this scale.

AT2019qiz is well detected in our new Swift XRT observation. We analyse the
X-ray spectrum using xspec (Arnaud, 1996), and compare it to the stacked XRT
spectrum from the time of the optical peak (mean observation date 2019-11-07).
We consider two models: a power-law spectrum, and a thermal model consisting
of blackbody spectra summed over the annuli of an accretion disk. We initially
include both Galactic absorption, determined using the maps from HI4pi (HI4PI
Collaboration et al., 2016) and intrinsic absorption in the host galaxy.
However, for all models/epochs, we found an intrinsic column consistent with
zero, and therefore exclude this from our final fits for simplicity. In the
language of xspec, our models are therefore tbabs$\times$zpowerlaw and
tbabs$\times$zashift$\times$diskbb, where tbabs is the Tübingen-Boulder ISM
absorption model (Wilms et al., 2000). We fit the data using the Cash
statistic (Cash, 1979). The fit parameters and derived fluxes for these models
are given in Table 1.

Both models provide an adequate fit to the data, with reduced $\chi^{2}\approx
1$. The thermal disk fits are shown compared to the data in Figure 9.
Regardless of which model is the more accurate physical description, we find
an emphatic change in the spectral slope between the 2019 and 2022
observations. The new data show a much softer spectrum, with an inner disk
temperature $\sim 0.1$ keV (or a photon index $\sim 5$), compared to an almost
flat spectrum in 2019. The soft ($0.3-1$ keV) X-ray flux has increased by more
than an order of magnitude, from $\sim 10^{-14}$ erg cm-2 s-1 in 2019 to
$\gtrsim 5\times 10^{-13}$ erg cm-2 s-1 in 2022, while the hard ($1-6$ keV)
X-rays have fallen by a factor of several. We tested whether this could simply
occur as a result of a decrease in absorption, by including intrinsic
absorption in our models at the redshift of AT2019qiz (ztbabs) and fitting the
2019 data with the temperature or photon index fixed to their best-fit values
for the 2022 data. However, this resulted in poor fits, suggesting that the
underlying continuum (and not simply the degree of absorption) has changed
between 2019 and 2022.

##  5 Outburst SED

(a)

(b)

Figure 11: The estimated distance of the gas emitting various line species,
plotted against their ionisation potential. The estimated distance is based on
equating the (corrected) FWHM of the line with the expected virial velocity
for a $10^{6}$M⊙ SMBH. Based on these measurements we label narrow, coronal
and broad line regions.

(a)

(b)

Figure 12: A comparison of line ratios with other coronal line emitting
objects. The light green points are the four objects in the Wang et al. 2012
sample which were confirmed to have variable coronal lines by Komossa et al.
2008 and Yang et al. 2013. Interestingly, in Figure 12(b) AT2019qiz looks more
in keeping with suspected TDEs.

The fluxes of narrow lines at 1-2 light-years from the SMBH reflect the
ionising continuum produced 1-2 years earlier, i.e. the time of the optical
peak. This allows us to construct a full SED of the initial flare using
constraints from both the early and late-time data, filling in regions that
could not be studied in earlier works. In Figure 10 we attempt to construct
the full SED of AT2019qiz during outburst. We use ZTF and UVOT photometry
observed around MJD=58794 to coincide with the effective date of the 2019 XRT
spectrum. The data, reduced by Nicholl et al. (2020), were collected from
TDE.space444https://tde.space/ (Guillochon et al., 2017). The infrared (IR)
points are NEOWISE data (see Section 6) taken 92 days after the rest of the
SED. The $13.6-54$eV power law was determined from the He II/H$\beta$ ratio as
discussed in §3.2 and scaled to be consistent with UVOT photometry. Both the
2019 and 2022 XRT spectra are included in this plot. The 2019 spectrum shows
the X-ray flux during outburst and the 2022 spectrum shows how the increase in
soft X-ray flux leads to the appearance of the Fe lines, whose ionisation
potentials are marked on the plot. Figure 10 highlights how the photons
ionising the coronal lines fill the EUV gap between the He II ionising photons
and soft X-rays.

##  6 Infrared analysis

Figure 13: Top: Host subtracted magnitudes for NEOWISE observations of AT
2019qiz. Middle: Temperatures and radii inferred from blackbody fitting of the
host subtracted fluxes of AT 2019qiz. Bottom: Luminosities implied by the
blackbody parameters and the cumulative energy implied by those luminosities.

AT 2019qiz is well detected in both W1 and W2 bands (3.4$\mu$m and 4.6$\mu$m
respectively) for 4 epochs of NEOWISE data. The host is well detected and non-
variable in NEOWISE until the first detection of AT 2019qiz at MJD 58886, 122d
after the optical peak derived in Nicholl et al. (2020). The IR fluxes then
continue to increase until the most recent observation, 694d after the optical
peak at MJD 59458, although there is a slight decline in flux between the 2nd
and 3rd observation. We calculate the flux solely due to the transient by
subtracting the mean flux value detected from the galaxy before the first
detection, and the corresponding magnitudes are shown in the top panel of Fig.
13. The flux at 3.4$\mu$m from the UV/optical blackbody of AT 2019qiz at the
epoch of the first IR detection, calculated using the temperature and radius
values given in Nicholl et al. (2020), is $\sim$0.5% of the observed host-
subtracted W1 flux, and so negligible. Therefore, we interpret the IR
detections shown here as an IR echo from dust in the vicinity of the SMBH,
similarly to those observed in a number of other optical-UV TDEs (see e.g. van
Velzen et al., 2016; Jiang et al., 2021).

We estimated the BB parameters for the IR echo using the EMCEE python
implementation of the Markov Chain Monte Carlo (MCMC) method (Foreman-Mackey
et al., 2013) to fit the host-subtracted fluxes and derive uncertainties (as
described in Reynolds et al., 2022). The resulting parameters are shown in the
middle panel of Fig. 13. The temperature measured in the first observation,
T${}_{\text{max}}=1450^{+90}_{-80}$ K, is close to, although slightly lower
than, the sublimation temperature for astrophysical dust, which typically lies
between 1500-2000 K depending on the composition of the dust (see e.g. Waxman
& Draine, 2000; van Velzen et al., 2016; Lu et al., 2016). We note that the
inferred radii do not directly correspond to the distance from the SMBH to the
dust, and we do not necessarily expect a spherical shell, but do provide a
lower limit.

We estimate the luminosity of the IR echo at each epoch from the temperature
and radius parameters using the Stefan-Boltzmann law, and integrate the
luminosity evolution from the first to the last detection to estimate the
total radiated energy of the IR echo to date. The luminosity is highest in the
most recent observation, where the luminosity is $3.2\times 10^{42}$ erg s-1,
and the total radiated energy up until this point is $1.20\times 10^{50}$ erg.
If we calculate the covering factor by comparing the peak luminosity for the
UV/optical blackbody ($3.6\times 10^{43}$ erg s-1, Nicholl et al. (2020)) to
the peak in the IR, similarly to Jiang et al. (2021), we find a lower limit
for the covering factor of
$f_{c}=L_{\text{dust,peak}}/L_{\text{opt+UV,peak}}=0.09$, assuming the IR
fluxes are still rising. This is considerably higher than the values found for
the sample of optical TDEs in Jiang et al. (2021), where the covering factor
was typically $\sim$0.01. We can additionally consider the definition of the
covering factor given in van Velzen et al. (2021a),
$\Omega_{d}=E_{\text{bol}}/E_{\text{dust}}$, where $E_{\text{bol}}$ and
$E_{\text{dust}}$ are the total radiated energy of the TDE (integrated over
the wavelength where dust absorption is efficient) and the dust respectively.
If we estimate the total radiated energy of the TDE with the value obtained
from the UV/optical bolometric LC by Nicholl et al. (2020), $1.0\times
10^{50}$ erg, we find $\Omega_{d}=0.84$, i.e. an approximate equipartition of
the radiated energy between the IR echo and the prompt UV/optical emission.
Although the total energy radiated given in Nicholl et al. (2020) only
measures until $\sim$150d, if we conservatively assume that the TDE has
remained at the same bolometric luminosity from that point until the most
recent IR observation, the total radiated energy would only increase by
$\sim$20%, so the value of $\Omega_{d}$ will not be greatly affected.
Furthermore, the IR emission is still rising. In summary, the IR echo observed
for AT 2019qiz is exceptionally luminous for a TDE discovered in the optical.
We discuss this, and the connection to the IR echo observed for AT 2017gge, in
Sect. 7.

##  7 Discussion

We examine the implications of our results for several key issues - the true
TDE continuum, the structure and origin of the material surrounding the TDE,
and the relation of this object to other coronal line emitters. First however,
we discuss whether AT2019qiz was truly a tidal disruption event, as opposed to
a supernova, or an AGN outburst.

###  7.1 TDE, supernova, or AGN?

The optical spectrum of the outburst of AT2019qiz fits solidly into the
standard class of optical-UV TDEs, but its absolute magnitude at peak,
$R_{pk}\sim-19$, was somewhat towards the lower end of the range for such
objects (Nicholl et al., 2020), overlapping the range of typical supernovae.
At the same time, the low-ionisation line ratios seen in outburst (Nicholl et
al., 2020) suggest some weak AGN-like activity in the past.

Could AT2019qiz actually be a kind of supernova? The delayed appearance of
coronal lines has been seen in a handful of SNe - for example SN1988z
(Aretxaga et al., 1999), SN2005ip (Smith et al., 2009; Smith et al., 2017),
and SN1987A (Gröningsson et al., 2006); see also discussion in Komossa et al.
(2009). Such objects are clearly extremely rare - out of the thousands of
supernovae seen so far, only a handful have developed coronal lines. This
phenomenon seems to require earlier mass loss to produce significant amounts
of circumstellar material; following the outburst, shocks from the blast wave
make X-ray emission which then produces the coronal lines. The delay is due to
blast wave travel time, not light travel time, and the coronal lines can
persist for quite a long time (see references above). For the original TDE
light echo candidate, J0952+2143, Komossa et al. (2009) were able to argue
against a SN origin, because the observed [FeVII] luminosity was $1.6\times
10^{40}$ erg s-1, two orders of magnitude larger than for these rare extreme
SNe. For AT2019qiz however, the [FeVII] luminosity is $3.4\times 10^{38}$ erg
s-1, not so far from what was seen in SN2005ip, $1.5\times 10^{38}$ erg s-1
(Smith et al., 2009). Purely on luminosity grounds, we cannot rule out a
supernova origin. The outburst optical spectrum of AT2019qiz was not quite
like any of these extreme SNe - for example, SN2005ip and SN1988z show lines
to the red of H$\alpha$ such as the Ca triplet which have never been seen in a
TDE. However, supernova spectra are diverse enough that we cannot strictly
rule out a supernova origin on this basis either, if we consider AT2019qiz in
isolation. However, the similarity of AT2019qiz to other optical-UV TDEs
suggests that if we explain AT2019qiz as a supernova, we should consider this
possibility for all TDEs, including the more luminous ones. We would also need
to explain why these particular characteristics are only found in the centres
of galaxies with black holes in the range $10^{6}-10^{7}M_{\odot}$

Could the AT2019qiz outburst be due to some kind of AGN activity, rather than
a tidal disruption? In both scenarios we assume the existence of a
supermassive black hole, and an enhanced period of accretion, so that the
ionising continuum is likely much the same in either case (see section 7.2).
The key question then is the nature and origin of the surrounding material,
which we discuss in sections 7.3 and 7.4. A lack of pre-existing material
would argue for a completely dormant black hole, as opposed to a passive disk
around the black hole. However of course, it is quite possible for a stellar
disruption to take place in the presence of a passive disk.

###  7.2 Existence of a hard continuum

The coronal lines almost certainly result from photo-ionisation - the very
high gas temperatures required mean that collisional ionisation would predict
line widths of thousands rather than hundreds of km s-1. The high ionisation
coronal lines are then direct evidence of a hard EUV continuum. While He II
and Bowen line emission also require an EUV source, the coronal lines have
significantly higher ionisation potentials from $\sim$100eV up to nearly 400eV
in the case of [Fe XIV]. We also find a very soft X-ray spectrum which further
hints at the presence of an unobservable continuum. The existence of continuum
emission in this energy range is significant as it is predicted by accretion
models but is not directly observed. Typical blackbody temperatures observed
in TDEs are of order 104K, which is generally assumed to be reprocessed
emission from the true, harder continuum that peaks in the EUV. The detection
of coronal lines proves this harder continuum emission exists, and must
somehow escape the reprocessing region, perhaps because of either clumpiness,
or a non-spherical geometry. In principle, the complete set of emission lines
can constrain the shape of the EUV continuum, and the covering factor of
illuminated material, but such modelling is beyond the scope of this paper.

###  7.3 Structure of the surrounding material

The Fe line light curves in Figure 8 give us the first clues about the
geometry of the surrounding structure. The decline of the Fe lightcurve is
much slower than that of the continuum, so must be convolved with a broad
response function. This can be caused either by a spread in light travel times
due to a range of radial distances, or due to reflection from material at
different angles to our line of sight, or both. Physical models of different
reprocessing region geometries produce different response functions (e.g Perez
et al. 1992), but unfortunately our sparsely populated light curves make it
difficult to compare. The lack of a prompt response to the original outburst
tells us there is little material along our line of sight, although another
source of uncertainty is whether the coronal lines are responding to the
initial outburst or the more recent increase in soft X-ray flux.

Inconsistent ionisation parameter changes also show there is a complex
structure surrounding the SMBH. [Fe XIV] appears in the 2021 and 2022 spectra
despite not being observed previously. The other Fe lines decrease in flux
after the 2021 spectrum. We also see a significant rise in the flux of narrow
lines such as the Balmer lines, [N II] and [S III]. The changes in the
strengths of different emission lines require differing ionisation parameters,
which depends on the density of the emitting gas. The later appearance of [Fe
XIV] could imply low density material further from the SMBH (therefore with a
greater light travel time), or alternatively higher density material closer to
the SMBH, but behind the nucleus from our point of view. Even further from the
black hole, there must be material with a low ionisation parameter, thus
emitting strongly in H$\alpha$, H$\beta$ and other low ionisation lines. What
is not clear is why the narrow Balmer lines get broader in the second
spectrum. In addition the move in the BPT diagrams (Figure 5) from obvious AGN
emission to something more closely resembling a star forming region is
puzzling. Both could be a sign of an increasing contribution to the narrow
lines from the TDE emission, causing both a broadening of the narrow lines and
altering their ratios by changing the ionising continuum. The jump in broad
line flux in the Jan 2021 spectrum could be further evidence for a complex
structure. The structure surrounding the black hole may be made up of clumpy
material, in which case the obscuration of the central region may fluctuate.
If the broad emission line originates from an accretion disk then the increase
in flux could be due to there being less material obscuring our line of sight
to the disk.

The combination of large dust covering factors and coronal Fe lines observed
for both AT2019qiz and AT2017gge suggests a connection between the phenomena.
Additionally, AT 2019avd also exhibited a very high dust covering factor with
f${}_{c}\sim$0.5 (Chen et al., 2022) along with displaying coronal lines. In
particular, the coronal Fe lines could arise from Fe that was locked in dust
grains that were destroyed by the UV emission from the TDE. This would be
similar to the case of Coronal Line Forest AGNs, where it has been suggested
that the coronal lines could be produced in the inner wall of the dusty torus,
where the AGN is sublimating the dust (Rose et al., 2015). In the case of AT
2019qiz, both Hung et al. (2021) and (Nicholl et al., 2020) note that there is
likely to be a low luminosity AGN within the host galaxy of AT 2019qiz. The
AGN unification model suggests the presence of a dusty obscuring structure, a
torus, which here could be responsible for both the IR echo and potentially
the coronal lines. The luminosity of the TDE in the UV/optical and lack of
evidence for significant extinction implies that the dust producing the IR
echo is not substantially in the observer line-of-sight, which could
additionally imply an unobscured line of sight to the inner dusty regions that
Rose et al. (2015) argue is required to observe the coronal lines. The AGN
associated dust structures additionally explain the high covering factor in AT
2019qiz compared to the optical TDE sample, which are almost all in galaxies
that do not host an AGN. This is supported by comparison to TDEs and TDE
candidates that have been observed in galaxies that host AGN such as Arp 299-B
AT1 (Mattila et al., 2018; Reynolds et al., 2022), AT 2017gbl (Kool et al.,
2020) and ASASSN-15lh (Krühler et al., 2018), which also display very
energetic IR echoes, as indeed does the original TDE echo candidate J0952+2143
(Komossa et al., 2009). There is no unambiguous evidence for either AT 2017gge
or AT 2019avd hosting an AGN before undergoing their nuclear outbursts, but it
also can not be ruled out.

Our observations show that in AT2019qiz, the continuum emission must be
reprocessed at a range of distances. The Fe line profiles are irregular with
disappearing offset components. The observed emission lines also have a range
of widths, with the coronal lines generally being broader than the low
ionisation narrow lines. We can use our "virial distance" estimates (see §3.2)
to examine the stratification of the material surrounding the outburst event.
Figure 11 shows estimated distance versus ionisation potential for various
observed lines, for each of the two X-Shooter epochs. We can divide material
loosely into "broad line", "coronal line", and "narrow line" regions, but even
the coronal line region seems to show ionisation stratification. To go beyond
this statement would require a full model, beyond the scope of this paper.
Furthermore, as discussed in §3.2, the distance estimates may be an
underestimate because the gas motion is responding to the galaxy potential as
well as the black hole. The key point however is that the low-ionisation
narrow lines are clearly narrower than the coronal lines.

###  7.4 Origin of the surrounding material

We have seen that the material causing the IR echo must have a large covering
factor and so seems much more likely to be connected with the nuclear
activity, rather than being random interstellar material. Assessing the amount
of material responsible for the coronal lines is not trivial, as it is rather
model independent. Detailed modelling is underway (Yin et al in preparation)
but for any reasonable SED, placing material at the relevant critical density,
and at a distance corresponding to the observed delay time, indicates that
likewise a significant covering factor is required to reproduce the line flux,
of the order 1-10%.

Furthermore, the illuminated material is too far from the black hole to have
been expelled from the 2019 outburst. We do not have a kinematic model, but
the outflow time will likely be of the order of the dynamical timescale
$t=\sqrt{R^{3}/GM}$. If we place the [Fe X] material at the light echo
distance based on the light curve peak at 500 days, the outflow time is $\sim
4000$ years. The outflow time to the Balmer lines, still rising at 800 days,
is at least $\sim 8200$ years. Even at relativistic jet-like outflow
velocities, the outflow cannot outpace the illuminating light. The material
could have been placed there by previous AGN activity, or the host could have
significant amounts of gas along the line of sight. Intriguingly, if we were
to speculate that the material is the remnant of a previous TDE, the dynamical
timescale would be consistent with the expected average TDE rate of $\sim
10^{-4}$yr-1galaxy-1. Whether or not a past TDE would leave enough material to
produce the observed line luminosities, or indeed which of these scenarios is
most likely, is left for future work to investigate.

###  7.5 Comparison to other coronal line emitters

AT2019qiz and very recently AT2017gge (Onori et al., 2022) are the only
spectroscopically confirmed optical-UV TDEs to have developed coronal lines
and it is not clear why this is the case. The Dec 2020 spectrum was obtained
as part of a study of TDE host galaxies, and was thus one of a sample of other
‘very late time’ TDE spectra. Out of the 11 objects in our sample AT2019qiz
was the only one to show signs of Fe lines, despite some of the other host
galaxies also hosting AGN. Out of our sample of late time TDE spectra,
AT2019qiz was the most recent, and it is also one of the most nearby TDEs
discovered to date. It could be that these coronal lines have been produced in
other TDEs but simply haven’t been detected. However, AT2017gge (Onori et al.,
2022) is significantly further away at z=0.0665 and the coronal lines were
still detected 1700 days after the initial outburst. It remains unclear what
is special about AT2019qiz or AT2017gge.

Coronal lines are common in AGN (Oliva et al., 1994; Gelbord et al., 2009) and
of the class of ECLEs, several are strongly suspected to be light echos from
past TDEs (Komossa et al., 2008; Wang et al., 2011; Wang et al., 2012). In
Figure 12 we compare AT2019qiz with some of these coronal line emitters from
the literature on plots of [Fe X] vs [Fe VII] and [Fe X] vs [O
III]$\lambda$5007\. The comparison sample is made up of coronal line AGN from
Gelbord et al. (2009), a nearby Seyfert 2 (the Circinus galaxy) from Oliva et
al. (1994) and the possible TDEs from Wang et al. (2012) (Note that we have
separated the four galaxies (SDSS J0952+2143, SDSS J1241+4426, SDSS J0748+4712
and SDSS J1350+2916) which had confirmed highly variable coronal lines through
follow-up spectra). In both plots there is a correlation between the line
ratios which is a sign that the lines are photoionised rather than
collisionally ionised. In Figure 12(b), the Wang et al. (2012) objects appear
to follow a different correlation to the Gelbord et al. (2009) AGN.
Interestingly, AT2019qiz is more consistent with the Wang et al. (2012)
objects than the AGN, albeit at lower luminosity than all the others. This
likely confirms the speculation of Komossa et al. (2008) and Wang et al.
(2012) that the coronal lines in many of these objects are remnants of a TDE
outburst. Based on their sample, Wang et al. (2012) estimate an event rate for
coronal line emitters of $\sim 10^{-5}$ galaxy-1 yr-1. In our sample of very
late time TDE spectra, only $\sim 10\%$ of events develop coronal lines. This
is consistent with TDE rate estimates (Donley et al., 2002; Wang & Merritt,
2004; Kesden, 2012), though we acknowledge the caveats that a) our sample size
is small and b) the spectra were obtained at different epochs relative to
initial outburst of each event.

##  8 Conclusion

AT2019qiz is the clearest example to date of a spectroscopically confirmed
optical-UV TDE developing high ionisation coronal lines in its spectra,
appearing $\sim$400 days after the main optical flare. These lines are direct
evidence for the presence of a hard EUV continuum. The lines originate from a
pre-existing, complex surrounding structure with different ionisation
potentials in different regions. Aside from AT2017gge, no other confirmed
optical-UV TDEs to date have shown coronal line emission, but the line ratios
of AT2019qiz are consistent with coronal line emitters from the literature
which are suspected to be TDE remnants. In addition, the estimated rate of
coronal line emitters is consistent with the estimated rate of TDEs and the
fraction of our late time TDE sample which develop coronal lines. The IR echo
observed from AT2019qiz, along with other coronal line emitters, is notably
more luminous than typical for optical TDEs, and suggests a connection between
these phenomena. This could be explained through the coronal lines arising
from Fe liberated from dust grains that were destroyed by the TDE flare.
Future studies should systematically search for coronal line emission in late
time TDE spectra.

## Acknowledgements

MN is supported by the European Research Council (ERC) under the European
Union’s Horizon 2020 research and innovation programme (grant agreement No.
948381) and by funding from the UK Space Agency. MJW acknowledges support of a
Leverhulme Emeritus Fellowship, EM-2021-064, during the preparation of this
paper. TMR acknowledges the financial support of the Vilho, Yrjö and Kalle
Väisälä Foundation of the Finnish academy of Science and Letters. SM
acknowledges support from the Academy of Finland project 350458. IA is a CIFAR
Azrieli Global Scholar in the Gravity and the Extreme Universe Program and
acknowledges support from that program, from the European Research Council
(ERC) under the European Union’s Horizon 2020 research and innovation program
(grant agreement number 852097), from the Israel Science Foundation (grant
number 2752/19), from the United States - Israel Binational Science Foundation
(BSF), and from the Israeli Council for Higher Education Alon Fellowship. A.
C. Carnall would like to thank the Leverhulme Trust for their support via the
Leverhulme Early Career Fellowship scheme. PC and GL are supported by a
research grant (19054) from VILLUM FONDEN. MG is supported by the EU Horizon
2020 research and innovation programme under grant agreement No 101004719. FO
acknowledges support from MIUR, PRIN 2017 (grant 20179ZF5KS) "The new frontier
of the Multi-Messenger Astrophysics: follow-up of electromagnetic transient
counterparts of gravitational wave sources" and the support of HORIZON2020:
AHEAD2020 grant agreement n.871158. For the purpose of open access, the author
has applied a Creative Commons Attribution (CC BY) licence to any Author
Accepted Manuscript version arising from this submission.

## Data Availability

All spectra will be made available on WISeREP upon acceptance of the paper.
Swift NEOWISE data are already publicly available.

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##  Appendix A Emission Line Fit Results

Table 2: Results from Gaussian fits to emission lines in the Dec 2020 spectrum, after correction for instrumental resolution, which is approximately 56 km s-1 for blue arm lines, $\lambda<$ 5600 Å, and 34 km s-1 for red arm lines, $\lambda>$ 5600 Å. As discussed in the text, we estimate that line fluxes have an addition $\sim$ 15% systematic error. Line | Centre | Flux | FWHM
---|---|---|---
| Å | 10-16erg s-1cm-2 | km $s^{-1}$
[Ne V]$\lambda$3426 | 3426.27$\pm$0.12 | 5.85$\pm$1.16 | 106$\pm$26
[O II]$\lambda$3727 | 3726.26$\pm$0.15 | 4.46$\pm$0.65 | 144$\pm$16
[O II]$\lambda$3729 | 3729.48$\pm$0.12 | 5.57$\pm$0.68 | 143$\pm$16
[Ne III]$\lambda$3869 | 3869.29$\pm$0.05 | 4.99$\pm$0.44 | 90$\pm$10
H$\beta$ | 4862.15$\pm$0.09 | 2.44$\pm$0.61 | 65$\pm$16
[O III]$\lambda$4959 | 4959.77$\pm$0.03 | 10.06$\pm$0.41 | 84$\pm$4
[O III]$\lambda$5007 | 5007.59$\pm$0.01 | 35.17$\pm$0.62 | 96$\pm$2
[Fe VII]$\lambda$6087 | 6088.25$\pm$0.28 | 3.5$\pm$0.72 | 143$\pm$33
[Fe X]$\lambda$6375 | 6374.63$\pm$0.27 | 12.57$\pm$1.18 | 330$\pm$38
[Fe X] off | 6380.72$\pm$0.16 | 2.78$\pm$0.63 | 86$\pm$20
[N II]$\lambda$6549 | 6549.74$\pm$0.19 | 2.78$\pm$0.2 | 100$\pm$7
H$\alpha$ n | 6563.83$\pm$0.03 | 10.69$\pm$0.57 | 77$\pm$4
H$\alpha$ b | b1 6553.83$\pm$0.46 | 154.43$\pm$7.35 | b1 346$\pm$15
| b2 6571.93$\pm$0.49 |  | b2 313$\pm$20
[N II]$\lambda$6583 | 6584.85$\pm$0.06 | 8.35$\pm$0.61 | 100$\pm$7
[S II]$\lambda$6718 | 6717.66$\pm$0.14 | 2.88$\pm$0.35 | 248$\pm$29
[S II]$\lambda$6732 | 6732.08$\pm$0.18 | 2.23$\pm$0.33 | 246$\pm$0
[Fe XI]$\lambda$7892 | 7891.36$\pm$0.22 | 9.42$\pm$0.73 | 267$\pm$21
[Fe XI] off | 7899.4$\pm$0.08 | 2.2$\pm$0.28 | 43$\pm$8
[S III]$\lambda$9071 | 9070.98$\pm$0.35 | 3.01$\pm$0.69 | 152$\pm$32
[S III]$\lambda$9533 | 9532.54$\pm$0.09 | 5.83$\pm$0.44 | 80$\pm$7
Table 3: Results from Gaussian fits to emission lines in the Feb 2021 spectrum, after correction to instrumental resolution, which varies from 75 to 150 km s-1. As discussed in the text, we estimate that line fluxes have an addition $\sim$ 15% systematic error. Line | Centre | Flux | FWHM
---|---|---|---
| Å | 10-16erg s-1cm-2 | km $s^{-1}$
[O III]$\lambda$4959 | 4956.88$\pm$0.04 | 16.83$\pm$0.46 | 102$\pm$5
[O III]$\lambda$5007 | 5004.93$\pm$0.02 | 49.64$\pm$0.69 | 109$\pm$3
[Fe XIV]$\lambda$5304 | 5304.02$\pm$1.86 | 4.26$\pm$3.73 | 297$\pm$145
[Fe VII]$\lambda$6087 | 6088.21$\pm$0.16 | 5.08$\pm$0.49 | 125$\pm$19
[Fe X]$\lambda$6375 | 6374.91$\pm$0.19 | 15.38$\pm$1.04 | 300$\pm$27
[N II]$\lambda$6549 | 6550.94$\pm$0.31 | 5.17$\pm$0.47 | 180$\pm$2
H$\alpha$ n | 6564.36$\pm$0.04 | 16.13$\pm$0.83 | <120$\pm$5
H$\alpha$ b | b1 6555.83$\pm$0.47 | 228$\pm$10.0 | b1 405$\pm$15
| b2 6573.58$\pm$0.33 |  | b2 255$\pm$18
[N II]$\lambda$6583 | 6585.32$\pm$0.11 | 15.52$\pm$1.4 | 179$\pm$2
[S II]$\lambda$6718 | 6715.1$\pm$0.25 | 3.11$\pm$0.55 | 73$\pm$18
[S II]$\lambda$6732 | 6729.4$\pm$0.2 | 3.81$\pm$0.59 | 73$\pm$18
[Fe XI]$\lambda$7892 | 7892.44$\pm$0.2 | 11.39$\pm$0.84 | 240$\pm$20

Table 4: Results from Gaussian fits to emission lines in the Jan 2022 spectrum, after correction for instrumental resolution, which is approximately 56 km s-1 for blue arm lines, $\lambda<$ 5600 Å, and 34 km s-1 for red arm lines, $\lambda>$ 5600 Å. As discussed in the text, we estimate that line fluxes have an addition $\sim$ 15% systematic error. Line | Centre | Flux | FWHM
---|---|---|---
| Å | 10-16erg s-1cm-2 | km $s^{-1}$
[Ne V]$\lambda$3426 | 3426.26$\pm$0.06 | 4.57$\pm$0.59 | 68$\pm$13
[O II]$\lambda$3727 | 3729.61$\pm$0.08 | 4.46$\pm$0.44 | 112$\pm$10
[O II]$\lambda$3729 | 3726.72$\pm$0.08 | 4.12$\pm$0.43 | 112$\pm$10
[Ne III]$\lambda$3869 | 3869.25$\pm$0.05 | 5.65$\pm$0.38 | 116$\pm$9
[He II]$\lambda$4686 | 4686.62$\pm$0.09 | 2.82$\pm$0.36 | 118$\pm$16
H$\beta$ | 4862.24$\pm$0.03 | 8.61$\pm$0.86 | 90$\pm$6
[O III]$\lambda$4959 | 4959.41$\pm$0.04 | 8.21$\pm$0.46 | 70$\pm$5
[O III]$\lambda$5007 | 5007.3$\pm$0.02 | 31.8$\pm$0.69 | 86$\pm$2
[Fe XIV]$\lambda$5304 | 5303.71$\pm$0.29 | 4.76$\pm$0.87 | 214$\pm$38
[Fe VII]$\lambda$6087 | 6087.45$\pm$0.31 | 2.33$\pm$0.57 | 107$\pm$37
[O I]$\lambda$6300 | 6301.4$\pm$0.09 | 3.98$\pm$0.5 | 79$\pm$11
[Fe X]$\lambda$6375 | 6374.86$\pm$0.18 | 14.14$\pm$0.85 | 328$\pm$21
[N II]$\lambda$6549 | 6549.67$\pm$0.1 | 4.03$\pm$0.17 | 94$\pm$4
H$\alpha$ n | 6564.02$\pm$0.01 | 45.37$\pm$0.56 | 94$\pm$1
H$\alpha$ b | b1 6556.15$\pm$1.45 | 85.44$\pm$15.04 | b1 358$\pm$32
| b2 6572.48$\pm$2.38 |  | b2 363$\pm$60
[N II]$\lambda$6583 | 6584.83$\pm$0.03 | 12.08$\pm$0.52 | 93$\pm$4
[S II]$\lambda$6718 | 6717.84$\pm$0.12 | 3.08$\pm$0.31 | 106$\pm$10
[S II]$\lambda$6732 | 6732.23$\pm$0.14 | 2.72$\pm$0.3 | 105$\pm$10
[O I]$\lambda$7321 | 7321.32$\pm$0.18 | 2.2$\pm$0.37 | 107$\pm$19
[Fe XI]$\lambda$7892 | 7892.85$\pm$0.11 | 8.67$\pm$0.5 | 171$\pm$11
[S III]$\lambda$9071 | 9070.46$\pm$0.12 | 5.58$\pm$0.5 | 119$\pm$10
[S III]$\lambda$9533 | 9532.54$\pm$0.05 | 12.41$\pm$0.5 | 95$\pm$4

##  Appendix B Evolution of Iron Coronal lines

In this section we plot coronal line regions from 8 epochs of X-Shooter and
MUSE spectra. All plots display a wavelength region ranging from -1500kms-1 to
1500kms-1 centred on the relevant line. In each case the flux is normalised to
the median value of that region.

Figure 14: Evolution of the [FeVII] $\lambda$ 6087 coronal line region.

.

Figure 15: Evolution of the [Fe X] $\lambda$ 6375 coronal line region.

.

Figure 16: Evolution of the [Fe XI] $\lambda$ 7892 coronal line region.

.

Figure 17: Evolution of the [Fe XIV] $\lambda$ 5304 coronal line region.

.