# Host Galaxy Line Diagnostics for the Candidate Tidal Disruption Events
XMMSL1 J111527.3$+$180638 and PTF09axc

Anne Inkenhaag,1,2 Peter G. Jonker,1,2 Giacomo Cannizzaro,1,2 Daniel Mata
Sánchez,3 Richard D. Saxton4
1Department of Astrophysics/IMAPP, Radboud University Nijmegen, P.O. Box 9010,
6500 GL Nijmegen, The Netherlands
2SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA,
Utrecht, The Netherlands
3Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy,
The University of Manchester, M13 9PL, UK
4Telespazio for ESA, XMM-Newton SOC, ESAC, Apartado 78, 28691 Villanueva de la
Cañada, Madrid, Spain E-mail<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We present results of our analysis of spectra of the host galaxies of the
candidate Tidal Disruption Events (TDEs) XMMSL1 J111527.3$+$180638 and
PTF09axc to determine the nature of these transients. We subtract the
starlight component from the host galaxy spectra to determine the origin of
the nuclear emission lines. Using a Baldwin–Phillips–Terlevich (BPT) diagram
we conclude that the host galaxy of XMMSL1 J111527.3$+$180638 is classified as
a Seyfert galaxy, suggesting this transient is likely to be caused by
(extreme) variability in the active galactic nucleus. We find that the host of
PTF09axc falls in the ’star-forming’ region of the BPT-diagram, implying that
the transient is a strong TDE candidate. For both galaxies we find a WISE-
colour difference of $W1-W2<0.8$, which means there is no indication of a
dusty torus and therefore an active galactic nucleus, seemingly contradicting
our BPT finding for the host of XMMSL1 J111527.3$+$180638\. We discuss
possible reasons for the discrepant results obtained through the two methods.

###### keywords:

transients: tidal disruption events – galaxies: individual: NGC 3599 –
galaxies: active – galaxies: nuclei – black hole physics

††pubyear: 2021††pagerange: Host Galaxy Line Diagnostics for the Candidate
Tidal Disruption Events XMMSL1 J111527.3$+$180638 and PTF09axc–A

##  1 Introduction

Two-body relaxation processes in the nucleus of a galaxy make stars wander in
energy and momentum space. This can bring the pericentre of a star’s orbit
within its tidal radius (or Roche limit) given the supermassive black hole
(SMBH) in the centre of the galaxy, causing the difference in gravitational
pull between the part of the star nearest and furthest from the SMBH, also
known as the tidal force, to overcome the self-gravity of the star. This
results in the star being pulled apart in a tidal disruption event (TDE)
(Hills, 1975; Rees, 1988; Phinney, 1989).

Part of the stellar material of the disrupted star will stay bound to the SMBH
and accrete on to it, creating a luminous flare that is visible across the
electromagnetic spectrum (Rees, 1988; Lodato & Rossi, 2011). Over the last two
decades, dozens of TDEs have been classified from among transient nuclear
flares detected in X-ray, optical or UV (see Van Velzen et al. 2020; Saxton et
al. 2021 for a review). Optically detected TDEs often go undetected in X-rays
(Gezari et al., 2012), and vice versa. Although, there are events that have
been found to emit in optical and X-rays (e.g., ASASSN-14li, ASASSN-15oi, AT
2019dsg, AT 2018fyk; Holoien et al. 2016; Gezari et al. 2017; Cannizzaro et
al. 2021; Wevers et al. 2019, respectively).

A list of properties need to be satisfied for both optical/UV and X-ray
selected TDE candidates to be confirmed. This list is based on observational
characteristics shared by the known population of TDEs, and it is refined over
time (see Zabludoff et al. 2021 for a review). Key observables for optical/UV
TDEs include broad He and/or H lines and blue continuum emission (see Van
Velzen et al. 2020 for a review). A short rise to peak, steady decline and a
soft X-ray spectrum are among the key observables for X-ray selected TDEs (see
Saxton et al. 2021 for a review). There are multiple competing models
explaining the optical-UV emission mechanism: outflows (photon-driven, Strubbe
& Quataert 2009, line-driven, Miller 2015 or circularisation-driven, Metzger &
Stone 2016), reprocessing of accretion disc emission by material in the debris
stream at larger radii (Guillochon et al., 2014) or shocks in the self-
intersecting debris stream (Piran et al., 2015; Bonnerot et al., 2017). The
lack of agreement about the importance of, for instance, the self-intersection
shock, the rate of circularisation of the stellar debris, and the accretion
radiation efficiency makes that there is no single theoretical prediction that
can serve as a guideline to classify an event as a TDE. While some of the
observed properties can be explained by the theoretical models under
consideration, there might be TDEs that do not fit in the sample of previously
classified TDEs, for instance if they occupy a different part of the parameter
space such as penetration factor ($\beta$), SMBH mass or spin, or stellar
mass. Therefore, we need to keep a critical but open mind about which
transients we classify as TDEs.

TDEs are often detected in otherwise inactive galaxies. However, Kennedy et
al. (2016) suggest the TDE rate in AGNs could be enhanced with a factor up to
10 due to the interaction of stars with the disc around the AGN. Detecting
TDEs in galaxies hosting an active galactic nucleus (AGN) is difficult, due to
the inherent difficulty in distinguishing them from regular AGN activity,
although some TDEs have been discovered in low-luminosity AGNs (e.g.,
ASASSN-14li and AT2019qiz; Holoien et al. 2016; Nicholl et al. 2020,
respectively) and even higher luminosity AGNs (e.g., PS16dtm and SDSS
J015957.64$+$003310.5; Blanchard et al. 2017; Merloni et al. 2015,
respectively). When confronted with AGNs at the same redshift, TDEs are
typically brighter (Auchettl et al., 2018), but more extreme AGN variability
can be as luminous as a TDE flare (e.g., Cannizzaro et al., 2020). This
emphasizes the difficulty in distinguishing between TDEs and AGN flares.
Besides this, the interaction between the TDE debris stream and the AGN disc
and the effect on the emitted luminosity are currently not well understood
(although see Chan et al. 2019, 2020 for modelling). Current theoretical
models of the interaction of the stream originating in the destruction of a
star and a pre-existing AGN disc are uncertain as they sample a restricted
section of the parameter space and do not run long enough to study the
accretion of an important fraction of the TDE debris. Finding a TDE candidate
in a quiescent galaxy means there is one less alternative explanation for the
transient.

A galaxy is classified as an AGN/non-quiescent galaxy if one or more of the
following properties is observed (Van Velzen et al., 2020): i) The luminosity
from the nucleus of the galaxy varied significantly with time before the main
flare/transient event e.g., in the optical or X-ray luminosity, ii) The WISE-
colours indicate the presence of a dusty torus, $W1-W2~{}\geq~{}0.8$ (Stern et
al., 2012), iii) The ratio between the equivalent widths (EWs) of specific
emission lines in the optical – restframe – part of the nuclear host spectrum
show that the source falls in the AGN region of the Baldwin–Phillips–Terlevich
(BPT) diagram. The ratios of EWs of emission lines reflect the physical
conditions under which these lines were formed. These conditions are different
for the different options considered for their formations (e.g., AGN, star-
forming regions, LINER-like shocks) (Baldwin et al., 1981; Cid Fernandes et
al., 2010). The AGN-region in the BPT-diagram is the region where the
ionization-mechanism is dominated by the – UV / X-ray – ionizing radiation
from the AGN (Baldwin et al., 1981).

In this work we investigate the host galaxy of two TDE candidates; the X-ray
discovered TDE candidate XMMSL1 J111527.3$+$180638 in the galaxy NGC 3599 and
the optically discovered candidate PTF09axc in the galaxy SDSS
J145313.07$+$221432.2. The nature of the observed flare – AGN activity or TDE
– has been subject of discussion in the literature for both of these
candidates (e.g., Saxton et al. 2015 for XMMSL1 J111527.3$+$180638 and Arcavi
et al. 2014; Jonker et al. 2020 for PTF09axc).

We aim to classify the host galaxies of two TDE candidates by determining the
position of the nuclear emission region on BPT-diagrams. We also look at the
WISE-colours of the host galaxies and we use the existing $L_{X}~{}\propto$ [O
iii] correlation observed in AGNs (Heckman et al., 2005) to compare the
observed [O iii] $\lambda$5007 luminosity to what is predicted on the basis of
the correlation. We finally compare the luminosity expressed in units of the
Eddington luminosity of NGC 3599 with that of other low-luminosity AGN host
galaxies of TDEs.

##  2 Data

###  2.1 XMMSL1 J111527.3+180638

XMMSL1 J111527.3$+$180638 – XMMJ1115 from now on – was first reported as a
candidate TDE based on an X-ray flare seen in XMM-Newton slew data on 2003
November 22 (Esquej et al., 2007). Its associated host galaxy is NGC 3599, at
redshift $z=0.0028$, $d_{\text{L}}=19.86$ Mpc taken from the Cosmicflows-3
Distance Catalogue (Tully et al., 2016). Flux calibrated optical spectra were
obtained from Esquej et al. (2008), originally Caldwell et al. (2003). This
data set consists of two spectra: a blue spectrum with wavelengths 3500–5500 Å
taken on 1998 May 20 and a red spectrum with wavelengths 5500–7500 Å taken on
2000 February 5, both taken at the F.L. Whipple Observatory (Mount Hopkins
near Amado, Arizona, USA) with the FAST (FAst Spectrograph for the Tillinghast
Telescope) instrument located on the 1.5-m Tillinghast telescope. Both spectra
were taken before the reported X-ray flare (see Caldwell et al. 2003 for full
observational details and the data reduction procedure).

###  2.2 PTF09axc

PTF09axc was first reported by Arcavi et al. (2014) as part of their archival
search of the Palomar Transient Factory (PTF) data for blue transients with
$-21\leq M_{R(peak)}\leq-19$. The discovery date is 2009 June 20 and the
source is associated with the galaxy SDSS J145313.07$+$221432.2 – hereafter
SDSSJ1453 – at redshift $z=0.115$ (Arcavi et al., 2010).

We have taken two 1800 s low resolution optical spectra of the nucleus of the
host galaxy after the transient had faded (Arcavi et al., 2014) on 2019 July
14 and 15 using the Auxiliary-port CAMera (ACAM) mounted at the Cassegrain
focus of the William Herschel Telescope (WHT) located at the Roque de los
Muchachos Observatory on La Palma, Spain under program W19AN009. Using the
V400 grating, GG395A order blocking filter, and the AUXCAM CCD results in a
wavelength coverage of $3950$–$9400$ Å and resolution R$\sim 430$ for a 1
arcsec slit. We correct for instrumental broadening during the analysis of
these data.

Data reduction is done using a program written in python that uses lacosmic
(Van Dokkum, 2001) for cosmic ray cleansing, pyraf for bias and flatfield
corrections and molly, developed by T. Marsh (Marsh,
2019)111http://deneb.astro.warwick.ac.uk/phsaap/software/molly/html/INDEX.html,
for wavelength calibrations. We further use molly to flux calibrate and
average our spectra.

##  3 Analysis and Results

Figure 1: The normalised spectrum of NGC 3599 – host of XMMJ1115 – is shown in
black, the best fit for the starlight component from ppxf is over-plotted in
red, and the residuals after subtracting the best-fitting host-galaxy stellar
spectrum is shown in green and blue. The grey bands – corresponding to the
blue wavelength regions in the subtracted spectrum – are wavelengths around
relevant host galaxy emission lines or telluric absorption lines expected from
the night sky and they are therefore excluded from the ppxf fit. Left panel:
The blue part of the spectrum (3500–5500 Å) displaying the masked emission
lined from H $\beta$ $\lambda$4861 and the [O iii] $\lambda 4959,~{}5007$
doublet in grey. Right panel: The red part of the spectrum (5500–7500 Å)
displaying the masked emission lines from H $\alpha$ $\lambda$6563, the [N ii]
$\lambda$6548, 6584 doublet and the [S ii] $\lambda$6617, 6631 doublet in
grey. The region around [O i] $\lambda 6300$ is also masked, but there is no
emission line detected when trying to fit a Gaussian at this wavelength.

To obtain an accurate nuclear source classification we start by subtracting
the starlight component from the host galaxy spectrum using the Penalized
PiXel-Fitting (ppxf) method (Cappellari, 2017), used with the MILES stellar
library (Vazdekis et al., 2010). We use a degree four multiplicative Legendre
polynomial – as opposed to an additive polynomial – to correct the continuum
shape during the fit to prevent changes in the line strength of the absorption
features in the templates, to minimize the influence on the strength of any
emission line in the nuclear spectrum after subtraction. After subtraction the
continuum emission is reduced to zero which means the equivalent width – flux
in a line divided by the continuum – becomes undefined. Instead, we use the
flux of the emission lines to determine the source position in a BPT-diagram.
We use the python package lmfit to fit Gaussian curves to the stellar-host
subtracted emission – or absorption – lines to obtain the flux of the
following emission lines of interest for the BPT-diagram(s), where present: H
$\alpha$ $\lambda$6563, H $\beta$ $\lambda$4861, [O iii] $\lambda
4959,~{}5007$, [O i] $\lambda 6300$, [N ii] $\lambda$6548, 6584, [S ii]
$\lambda$6617, 6631. To reduce the number of degrees of freedom during fitting
we require the Full Width at Half Maximum (FWHM) of lines in doublets to be
the same. We also fix the wavelength separation of doublets to their
laboratory value and we fix the ratio in amplitudes for the lines in doublets
when an amplitude ratio in known ([O iii] $\lambda 5007/$[O iii] $\lambda
4959=3$ and [N ii] $\lambda 6584/$[N ii] $\lambda 6548=3$ from Osterbrock &
Ferland 2006).

###  3.1 XMMJ1115

Fig. 1 shows the normalised – divided by the median value – galaxy spectrum in
black, with the best fit from ppxf over-plotted in red, the grey bands
represent areas masked during the fitting procedure. The red and blue parts,
as described in Section 2.1, of the spectrum are fitted separately. The best
fitting stellar population has a redshifted radial velocity of $799\pm 2$ km
s-1 (we average the radial velocity derived from the red and the blue parts of
the spectrum). Taking into account the average rms uncertainty of 58 km s-1 on
the wavelength calibration (from Caldwell et al. 2003) the redshift we derive
is in agreement with previous measurements for this galaxy (e.g., $839\pm 5$
km -1; Cappellari et al. 2011). Subtracting the starlight component leaves us
with the nuclear emission line spectrum (blue/green in Fig. 1).

We detect emission lines of H $\alpha$, H $\beta$ and the [O iii], [N ii] and
[S ii] doublets, but we do not detect a significant emission line for [O i] in
this source. The detected emission lines are best fitted with one Gaussian
component with an average FWHM of $253\pm 9$ km s-1 (see Table 2 in the
appendix). We provide figures showing the Gaussian fits to the detected
emission lines in Fig. 7 in the appendix.

Figure 2: Baldwin–Phillips–Terlevich (BPT) diagram (Baldwin et al., 1981) for
the nucleus of NGC 3599 derived using the flux measurements of the emission
lines detected in the pre-outburst spectrum. The left panel uses the ratio
between the [O iii] $\lambda 5007$ and the H $\beta$ emission line flux and
the ratio between the [N ii] $\lambda 6584$ and the H $\alpha$ emission line
flux while the left panel used the ratio between the [O iii] $\lambda 5007$
and H $\beta$ emission line flux and the ratio between the [S ii] $\lambda
6617,~{}6731$ and H $\alpha$ emission line flux. The demarcations between
different regions caused by different ionization mechanisms are from Kewley et
al. (2001); Kauffmann et al. (2003); Kewley et al. (2006). The position of the
source in both diagrams is consistent with an AGN/Seyfert galaxy being present
in NGC 3599 prior to the flare.

Using the flux of the emission lines we calculate the position of the source
in a BPT-diagram. We use the demarcations from Kewley et al. (2001); Kauffmann
et al. (2003); Kewley et al. (2006) to indicate different ionization-mechanism
regions. The location of NGC3599 in the BPT-diagram is shown in Fig. 2 and it
is consistent with an AGN/Seyfert classification for the spectrum of the
nuclear region of the host of XMMJ1115.

###  3.2 PTF09axc

Figure 3: The normalised spectrum of SDSSJ1453 – host of PTF09axc – with the
same colour scheme as in Fig. 1. The masked emission lines in grey are H
$\alpha$ $\lambda$6563, H $\beta$ $\lambda$4861, the [O iii] $\lambda
4959,~{}5007$ doublet and the [N ii] $\lambda$6548, 6584 doublet. pPXF also
masks [O i] $\lambda 6300$ and [S ii] $\lambda$6617, 6631, but there are no
visible emission lines present in the subtracted spectrum at those
wavelengths.

We repeat the exact same data analysis procedure we employed for NGC 3599 for
the host of PTF09axc (SDSSJ1453), see Section 3.1. Blueshifting the spectrum
with $z=0.1153$ before using ppxf leaves a residual radial velocity of $9\pm
21$ km s-1, which is consistent with no residual radial velocity. Therefore,
the best-fitting stellar population has redshift of $z=0.1153\pm 0.0001$,
corresponding to radial velocity $32590\pm 30$ km s-1. For an uncertainty in
the last digit of the redshift given in Arcavi et al. (2010) (confirmed
through private communication) as small as 1, our values are completely
consistent with their redshift. This redshift corresponds to
$d_{\text{L}}=536.1$ Mpc using $\Omega_{m}=0.3$, $\Omega_{\Lambda}=0.7$ and
H${}_{0}=70$ km s-1 Mpc-1

We do not detect emission lines for [O i] and the [S ii] doublet, but we do
detect the [N ii] doublet and the H $\alpha$ and H $\beta$ emission lines. To
ensure the Gaussian functions fit to H $\alpha$ and the [N ii] doublet have
the correct centra wavelength, we have to fix the wavelength separation
between the lines. We fit a single component with an (average) FWHM=$768\pm
102$ km s-1 to the detected emission lines. We derive an upper limit to the
flux for both lines in the [O iii] doublet.

Due to the redshift of this source the [O iii] $\lambda 5007$ emission line is
redshifted to $\sim 5584$ Å which is close to the wavelength of the [O i]
$\lambda 5577$ terrestrial sky emission line. In fact, these two lines fall
within one ACAM resolution element of each other. In order to obtain as
accurate as possible an upper limit on the presence of the [O iii] $\lambda
5007$, we tried several data reduction optimisations tailored to allow as
clean a subtraction of the terrestrial sky emission line as possible. To make
sure the sky lines are perpendicular to the spectral trace we extracted a
rectified version of the 2-D spectrum. This did not significantly improve the
subtraction of the sky emission line. Next, we used the fit2d option during
the iraf apall procedure to extract the spectrum, which uses a two dimensional
function to smooth the profile to use with variance weighting or cleaning.
However, this also did not significantly improve the subtraction of the sky
emission line either. Therefore, we proceed with the spectrum obtained from
the original data reduction process, with the one difference that for
determining the upper limit to the [O iii] doublet emission line we do not
subtract the sky emission lines.

Instead, we derive an upper limit for the flux in the [O iii] $\lambda 5007$
line by fitting two Gaussians, one to the $5577$ Å sky line and one to the [O
iii] $\lambda$5007 line. We shift the spectrum back to the rest frame
wavelength of the host galaxy, this includes the $5577$ Å sky line. We fix the
central wavelength of the Gaussian function designed to describe this sky line
to the expected value after blueshifting this line to the galaxy rest frame.
In addition, we fix its FWHM to the spectral resolution of ACAM, leaving only
the amplitude as a free parameter for this Gaussian during the fit. For the
Gaussian designed to determine the upper limit to the [O iii] $\lambda 5007$
line, we fix the central wavelength to where we expect it to appear. We fix
the FWHM of this emission line to the average value measured in the other
lines. This leaves only the amplitude as a free parameter in the fit for this
Gaussian. We fit the [O iii] $\lambda$4959 line simultaneously, with the same
FWHM as [O iii] $\lambda$5007 and the wavelength separation between the lines
of the doublet set to the laboratory value, leaving only the amplitude free
during the fit. As the upper limit on the flux of the Gaussian-shaped emission
line is determined from a one sided Gaussian probability distribution, the
2$\sigma$ upper limit corresponds to the 95 per cent confidence level. We use
this upper limit to derive the position of the source in a BPT-diagram (see
Fig. 4). The source falls in the region of the BPT-diagram associated with
star-forming and H II galaxies.

Figure 4: Baldwin–Phillips–Terlevich (BPT) diagram (Baldwin et al., 1981)
using the flux measurements for the emission lines detected in the post-
outburst spectrum of SDSSJ1453. It uses the ratio between the [O iii] $\lambda
5007$ and H $\beta$ emission line flux and the ratio between the [N ii]
$\lambda 6584$ and H $\alpha$ emission line flux. The position of the source
is consistent with an H II/star forming region or a composite region within
the error bars.

##  4 Discussion

In this paper we investigate the nature of two candidate TDE events, XMMJ1115
and PTF09axc, by classifying the nuclear regions of their host galaxies, NGC
3599 and SDSSJ1453, respectively. We apply two methods: optical emission lines
ratios to assess the ionization-mechanism and the infrared (IR) colours as
determined by the WISE-satellite (Wright et al., 2010) to investigate if dust,
as often found in a dusty torus in an AGN, is present. The dusty torus of an
AGN will yield $W1-W2\geq 0.8$ (Stern et al., 2012), where $W1$ and $W2$
indicate the WISE-bands at 3.4 and 4.6 $\mu$m, respectively. For SDSSJ1453, we
apply a third method using the empirical relation between $L_{\text{X}}$(3–20
keV) and $L_{\text{[\text{O\,{iii}}}]}$ from Heckman et al. (2005) to assess
if the observed X-ray luminosity is consistent with that from an AGN assuming
the [O iii] emission is caused by the AGNs narrow line region.

The position of NGC 3599 on the BPT-diagram suggests it is a Seyfert galaxy,
and there have been more papers suggesting the host galaxy is not a quiescent
galaxy, see e.g., Saxton et al. (2015). Saxton et al. (2015) show that the
galaxy was luminous in X-rays 18 months before the peak flux was measured,
showing it to be bright on much longer timescales than shown by TDEs known at
that point in time. They also argue that even if one of the two measurements
was taken during rise-time and one during decay, the rise-time and the plateau
phase together would still be significantly longer than seen in previous TDE
candidates at that time. Since then, however, longer lived TDE-candidates have
been observed (e.g., Lin et al. 2017) which means the measurements by Saxton
et al. (2015) can no longer be considered unusual behaviour for a TDE. With
our current understanding of X-ray TDE light curves we can therefore not make
a definitive distinction between a TDE or AGN-activity for the flare XMMJ1115.
However, our work does strengthen the evidence that the nucleus of NGC 3599
hosts a low-luminosity AGN.

We calculate $W1-W2=-0.032\pm 0.029<0.8$, which means that, according to the
WISE-colours, this source should not be classified as an AGN. This seemingly
contradicts our findings that this source is a Seyfert galaxy given its
position in the BPT-diagram. As WISE has a low spatial resolution (namely 6.1
arcsec, in band $W1$ and 6.4 arcsec, in band $W2$), the WISE-colour will be a
combination of the starlight of the galaxy plus that of the central Seyfert
region of the galaxy. Additionally, LaMassa et al. (2019) found that not all
AGN are detected by WISE, explaining that a non-detection of a known AGN in
WISE is a possible result of different dust properties, or absence of dust,
compared to AGN that are detected by WISE, rather than absence of the AGN. We
therefore deem our result that NGC 3599 hosts a low-luminosity AGN based on
the optical emission line ratios not to be in contradiction of the WISE non-
detection. Our conclusion that the nuclear region of NGC 3599 hosts an
actively accreting Seyfert-like AGN increases the probability that the
observed flare was related to the AGN, although this does not rule out that
the XMMJ1115 event was caused by a TDE interacting with the AGN accretion disc
(Blanchard et al., 2017; Chan et al., 2019, 2020).

Table 1: X-ray luminosities, BH masses and Eddington ratios of host galaxies
of known TDEs in low-luminosity AGNs and the host galaxy of XMMJ1115.

Transient | Host galaxy |  $L_{\text{X}}$(3–20 keV) | BH mass | log($L/L_{\text{Edd}}$) |
---|---|---|---|---|---
name | name | (erg s-1) | (M⊙) |  | Reference
XMMJ1115 | NGC 3599 | 2.25${}^{+7.26}_{-1.77}\times$1041 * | 2.34$\pm 2.27\times$107 | $-4.11^{+2.15}_{-0.99}$ | a
ASASSN-14li | PGC 043234 | 1.32${}^{+3.09}_{-0.93}\times$1041 | 1.70${}^{+2.47}_{-1.02}\times$106 | $-3.19^{+0.92}_{-0.90}$ | b,c,d
– | IC3599 | 6.73${}^{+1.96}_{-1.6}\times$1040 | 7$\pm 5\times$106 | $-4.11^{+0.65}_{-0.35}$ | e, f, g
AT2019qiz | 2MASX J04463790$-$1013349 | 5.6${}^{+20.2}_{4.7}\times$1040 * | 1.15${}^{+0.85}_{-0.49}\times$106 | $-3.40^{+0.90}_{-1.0}$ | h

Note. For IC3599 no transient name is listed as this host galaxy has seen
multiple flares classified as TDEs since the early 1990s. $L_{\text{X}}$
marked with * are calculated using the relation between $L_{\text{X}}$(3–20
keV) and $L_{\text{[\text{O\,{iii}}}]}$ from Heckman et al. (2005), while the
other given values are observed $L_{\text{X}}$ converted to the 3–20 keV band
using W3PIMMS2. References: a Saxton et al. (2015), b Miller et al. (2015), c
Van Velzen et al. (2016), d Wevers et al. (2017), e Campana et al. (2015), f
Grupe et al. (2001), g Grupe et al. (2015), h Nicholl et al. (2020)

Figure 5: The Eddington ratio for X-ray luminosity of the nucleus three host
galaxies of confirmed TDEs in low-luminosity AGNs is shown in green. Our
measurement of the Eddington ratio for NGC 3599 based on the spectrum and
using the relation from Heckman et al. (2005) to calculate the X-ray
luminosity in the 3–20 keV band is shown in red. The blue range shows the
Eddington ratios found in Seyfert 1 and Seyfert 2 galaxies by Singh et al.
(2011) to represent the range in Eddington ratios for the population of
Seyfert galaxies. For the masses, and luminosities used to calculate the
Eddington ratio, see Table 1.

Assuming NGC 3599 is an AGN and using the empirical relation between
$L_{\text{X}}$(3–20 keV) and $L_{\text{[\text{O\,{iii}}}]}$ for AGNs from
Heckman et al. (2005), including the 1$\sigma$ uncertainty in this relation
and our 1$\sigma$ uncertainty on the flux measurement, we calculate that
$L_{\text{X}}$(3–20 keV)$=2.25^{+7.26}_{-1.77}\times 10^{41}$ erg s-1 for NGC
3599. We use this to compute the Eddington ratio of this galaxy in quiescence
and compare the value to the observed Eddington ratio of host galaxies of
previously confirmed TDEs in low-luminosity AGNs, ASASSN-14li, IC3599 and
AT2019qiz (Holoien et al. 2016; Campana et al. 2015; Nicholl et al. 2020,
respectively). We list the observed $L_{\text{X}}$(3–20 keV) values from the
literature in Table 1, with the X-ray luminosities converted to the 3–20 keV
energy band using W3PIMMS222https://heasarc.gsfc.nasa.gov/cgi-
bin/Tools/w3pimms/w3pimms.pl, as well as our calculated value for NGC 3599.
The Eddington ratio calculated for NGC 3599 in quiescence is consistent with
Eddington ratios found in other Seyfert galaxies within 1$\sigma$ (see e.g.,
Singh et al. 2011). It is also consistent with the Eddington ratios of the
host galaxies of known TDEs in low luminosity AGNs within 1$\sigma$, see Fig
5. There we plot the Eddington ratios for the different host galaxies as well
as the range of Eddington ratios found by Singh et al. (2011). The uncertainty
in the Eddington ratio for NGC 3599 is dominated by the uncertainty in our
flux measurements, in the Heckman et al. (2005) relation and in the black hole
mass estimate. Therefore, we favour the conclusion based on the position of
the source in the BPT-diagram. We do however note that this comparison of the
Eddington ratios also does not exclude a TDE nature of the flare XMMJ1115.

The ratio of the flux of the emission lines in the optical spectrum of the
nuclear region of SDSSJ1453 falls in the H II/starforming region, with the
1$\sigma$ error bar extending into the composite galaxy region (see Fig. 4).
The WISE-colour difference for SDSSJ1453 is $W1-W2=0.15\pm 0.08<0.8$, which
does not indicate the presence of an AGN in this galaxy. As both the location
of the source in a BPT-diagram as well as the WISE-colours indicate an
inactive galaxy, we conclude that an AGN origin of the observed flare PTF09axc
is unlikely, whereas a TDE nature of this transient is consistent with a
quiescent galaxy.

Figure 6: The empirical relation between $L_{\text{X}}$(3–20 keV) and
$L_{\text{[\text{O\,{iii}}}]}$ for AGNs from Heckman et al. (2005) including
the 1$\sigma$, 2$\sigma$ and 3$\sigma$ uncertainty regions on the relation in
black and increasingly light shades of grey, respectively. In red is our
2$\sigma$ upper limit on $L_{\text{[\text{O\,{iii}}}]}$ combined with the
measurement for $L_{\text{X}}$(3–20 keV) from Jonker et al. (2020) to show our
upper limit of the host of PTF09axc is not consistent with the relation within
2$\sigma$, but it is within 3$\sigma$.

Using the empirical relation between $L_{\text{X}}$(3–20 keV) and
$L_{\text{[\text{O\,{iii}}}]}$ from Heckman et al. (2005) for AGNs (with a
scatter of $\sigma=0.51$ dex or a factor of $\approx 3.25$) and
$L_{\text{X}}=8\times 10^{42}$ erg s-1 from Jonker et al. (2020), the
predicted AGN luminosity from the [O iii] line would be
$L_{\text{[\text{O\,{iii}}}]}\approx 5.7\times 10^{40}$ erg s-1. Using our
2$\sigma$ upper limit for the flux in the emission line of $1.39\times
10^{-16}$ erg cm-2 s-1 and d${}_{L}=536.1$ Mpc we calculate
$L_{\text{[\text{O\,{iii}}}]}=4.78\times 10^{39}$ erg s-1, which is a factor
11.9 lower than expected for an AGN-powered emission line (see Figure 6 where
we show the relation and our upper limit on $L_{\text{[\text{O\,{iii}}}]}$).
While our observed upper limit for the $L_{\text{[\text{O\,{iii}}}]}$ is too
low compared to the luminosity predicted by the correlation from Heckman et
al. (2005), it is consistent if we take the uncertainty on this correlation
into account within 3$\sigma$. This means we cannot exclude that the PTF09axc
host SDSSJ1453 is an AGN, based on the $L_{X}~{}\propto$ [O iii] relation from
Heckman et al. (2005). However, the combined evidence provided by the upper
limit on $L_{\text{[\text{O\,{iii}}}]}$, from the position of this source on
the BPT-diagram, and the WISE-colour difference, we conclude that this galaxy
is most likely quiescent and the transient PTF09axc is most likely a TDE.

It should be noted that in all low- to medium-resolution spectroscopic ground
based observations the [O iii] $\lambda 5007$ emission line in SDSSJ1453 is
redshifted to fall close to the [O i] $\lambda 5577$ terrestrial sky line,
although in previous work (e.g., Arcavi et al. 2014) there is no mention of
this. Their $L_{\text{[\text{O\,{iii}}}]}$ is consistent with our upper limit
and not consistent with the empirical relation between $L_{\text{X}}$(3–20
keV) and $L_{\text{[\text{O\,{iii}}}]}$ and therefore supports our conclusion
about PTF09axc.

## Acknowledgements

D.M.S. acknowledges support from the ERC under the European Union’s Horizon
2020 research and innovation programme (grant agreement No. 715051; Spiders).
This work is part of the research programme Athena with project number
184.034.002, which is financed by the Dutch Research Council (NWO). We thank
Tom Marsh for the use of molly. This work uses python packages numpy, lmfit,
matplotlib, sys, astropy.

## Data Availability

All data will be made available in a reproduction package uploaded to Zenodo.

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##  Appendix A Additional Material

(a)

(b)

(c)

Figure 7: Fit to the emission lines of NGC 3599 after subtracting the
starlight component of the host galaxy spectrum. The data are shown in black,
the total best fit is shown in magenta, the individual components have varying
colours. (a) shows H $\beta$ (blue) and [O iii] $\lambda 4959$ (orange) and
$\lambda 5007$ (green), (b) shows H $\alpha$ (orange) and [N ii] $\lambda
6548$ (blue) and $\lambda 6584$ (green) and (c) shows [S ii] $\lambda 6717$
(blue) and $\lambda 6731$ (orange). Error bars to the spectrum are indicated
in all frames in black, the grey shaded area corresponds to the 3$\sigma$
region of the best fit.

(a)

(b)

(c)

Figure 8: Fit of the emission lines of the hosty galaxy of PTF09axc with the
starlight component subtracted for (a) and (c) and without the background or
the starlight component subtracted for (b). The data are shown in black, the
total best fit is shown in magenta, the individual components have varying
colours. (a) shows H $\beta$ (blue), (b) shows O i $\lambda 5577$ terrestrial
skyline (orange) and [O iii] $\lambda 4959,5007$ line (green and blue
respectively) and (c) shows H $\alpha$ (blue) and [N ii] $\lambda 6548,6584$
(orange and green respectively). Error bars to the spectrum are indicated in
all frames in black, the grey shaded area corresponds to the 3$\sigma$ region
of the best fit. Table 2: Results of line fitting of the most prominent
emission lines in the host galaxy nuclear spectra of TDE candidates XMMJ1115
and PTF09axc.

| [O iii] $\lambda 4959$ | [O iii] $\lambda 5007$
---|---|---
source | WL [Å] | FWHM [km s-1] | flux [1E-16 erg cm-2 s-1] | WL [Å] | FWHM [km s-1] | flux [1E-16 erg cm-2 s-1]
XMMJ1115 | 4958.4$\pm$0.3† | 270.0$\pm$27.8∗ | 112.45$\pm$34.36 | 5006.3$\pm$0.3† | 267.5$\pm$27.5∗ | 337.35$\pm$103.07
PTF09axc | 4958.94$\pm$0.00† | 774.17$\pm$0.00∗ | 0.46 | 5006.84$\pm$0.0† | 766.77$\pm$0.0∗ | 1.39
| H $\beta$ | H $\alpha$
source | WL [Å] | FWHM [km s-1] | flux [1E-16 erg cm-2 s-1] | WL [Å] | FWHM [km s-1] | flux [1E-16 erg cm-2 s-1]
XMMJ1115 | 4860.1$\pm$0.4 | 215.9$\pm$30.2 | 87.80$\pm$43.17 | 6561.7$\pm$0.3 | 272.8$\pm$24.6 | 111.48$\pm$30.31
PTF09axc | 4861.9$\pm$2.1 | 605.3$\pm$200.1 | 2.18$\pm$1.24 | 6562.8$\pm$1.9‡ | 569.9$\pm$219.1 | 8.14$\pm$4.57
| [N ii] $\lambda 6548$ | [N ii] $\lambda 6584$
source | WL [Å] | FWHM [km s-1] | flux [1E-16 erg cm-2 s-1] | WL [Å] | FWHM [km s-1] | flux [1E-16 erg cm-2 s-1]
XMMJ1115 | 6547.5$\pm$0.3‡ | 324.0$\pm$25.1∗∗ | 31.32$\pm$6.49 | 6583.9 $\pm$0.3‡ | 322.2$\pm$25.0∗∗ | 93.95$\pm$19.48
PTF09axc | 6548.1$\pm$0.0‡ | 950.2$\pm$197.8∗∗ | 1.04$\pm$0.47 | 6583.5$\pm$1.9‡ | 945.1$\pm$196.8∗∗ | 3.13$\pm$1.41
| [S ii] $\lambda 6717$ | [S ii] $\lambda 6731$
source | WL [Å] | FWHM [km s-1] | flux [1E-16 erg cm-2 s-1] | WL [Å] | FWHM [km s-1] | flux [1E-16 erg cm-2 s-1]
XMMJ1115 | 6716.5$\pm$0.3+ | 177.1$\pm$24.3- | 23.28$\pm$9.08 | 6730.9$\pm$0.3+ | 176.8$\pm$24.2- | 24.37$\pm$9.28
PTF09axc | $\cdots$ | $\cdots$ | $\cdots$ | $\cdots$ | $\cdots$ | $\cdots$

Note. With $\cdots$ we indicate that this line could not be fitted to the
data. Different markers indicate quantities that were tied to the same value
(FWHM) or a set separation
(WL) for each of the sources. Numbers in italics were forced to the mentioned
value to obtain a 2$\sigma$ upper limit (UL) on the flux of that emission
line. Flux measurements
without error are 2$\sigma$ upper limits.