AT2020mot: TDE in E+A Galaxy

• AT2020mot is a tidal disruption event characterized by a UV/optical flare in a quiescent E+A galaxy, with standard thermal properties and unique nuclear diagnostics.
• The event’s photometric evolution, blackbody fits, and infrared echoes reveal insights into sub-parsec dust structures around a supermassive black hole.
• A record optical polarisation of 25% suggests a shock-powered, aspherical scattering geometry, challenging traditional reprocessing models in TDEs.

Searching arXiv for AT2020mot and directly relevant papers. AT2020mot is a tidal disruption event (TDE) in which a star was disrupted by a supermassive black hole (SMBH), producing a UV/optical flare in a quiescent host galaxy. It is notable for combining otherwise typical thermal TDE properties with two uncommon diagnostics of the nuclear environment: an infrared/dust-echo signature consistent with sub-parsec dust structures, and an exceptionally large optical polarisation episode. In the literature summarized here, AT2020mot is described as a H+He TDE in an E+A galaxy at $z=0.07$, with no X-ray detection during the flare and a radio non-detection, while its optical/UV continuum, rise time, and bolometric properties remain broadly consistent with the wider optical TDE population (Newsome et al., 2023, Floris et al., 10 Sep 2025).

1. Discovery, classification, and host system

AT2020mot was discovered on 14 June 2020 and classified as a H+He TDE from broad He II $\lambda 4686$ and Balmer emission atop a blue continuum. Its host is an E+A (post-starburst) galaxy at $z=0.07$, a host class frequently associated with optical TDEs. The event is explicitly characterized as a typical UV/optical TDE in a quiescent host, with no radio or X-ray signatures during the flare (Floris et al., 10 Sep 2025).

This combination of properties places AT2020mot in the now well-established class of optically selected, thermal, non-jetted TDEs. A common misconception is that unusually strong polarimetric behavior necessarily implies a jet-dominated or persistently active galactic nucleus (AGN)-like system. For AT2020mot, the available evidence points in the opposite direction: the host is described as quiescent, the event lacks X-ray detection during the flare, and radio follow-up yielded a VLA non-detection at 15 GHz with a $27\,\mu{\rm Jy}$ upper limit (Floris et al., 10 Sep 2025).

2. Photometric evolution and thermal continuum

The photometric data set includes ZTF $g$ and $r$ light curves and Swift/UVOT observations in UVW2, UVM2, UVW1, $U$, $B$, and $V$. These data were host-subtracted, shifted to the rest frame, and corrected for Galactic extinction; UVOT magnitudes were converted to AB magnitudes. The reported optical peak is $t_{\rm peak}={\rm MJD}\ 59057.87$, and the rise time from asymmetric Gaussian fits is $\lambda 4686$0 d (Floris et al., 10 Sep 2025).

Peak blackbody fits based on host-subtracted Swift/UVOT and ZTF data give

$\lambda 4686$1

and

$\lambda 4686$2

Using

$\lambda 4686$3

a corresponding photospheric radius of approximately

$\lambda 4686$4

is derived from the reported $\lambda 4686$5 and $\lambda 4686$6, although this radius is not tabulated explicitly in the cited work (Floris et al., 10 Sep 2025).

In population context, these thermal parameters are not unusual. The comparison sample median excluding AT2020mot is $\lambda 4686$7 K with $\lambda 4686$8 K, $\lambda 4686$9 with $z=0.07$0, and $z=0.07$1 d with $z=0.07$2 d, placing AT2020mot comfortably within the broader distribution of optical TDE thermodynamic and temporal properties (Floris et al., 10 Sep 2025).

3. Sub-parsec dust echoes and circumnuclear structure

AT2020mot shows an $z=0.07$3-band excess and re-brightening along the decline of the light curve, interpreted as two consecutive dust echoes from the TDE. The near-infrared light curve is modeled following van Velzen et al. (2016), and the observations are described as consistent with concentric rings of thin dust within $z=0.07$4 parsecs of a $z=0.07$5 SMBH. These are among the smallest scales at which dust has been inferred near SMBHs, and the inferred dust covering factors are of order $z=0.07$6, much lower than those found for dusty tori of AGN (Newsome et al., 2023).

In this framework, the infrared response is treated as dust reprocessing of the UV/optical flare. The relevant formalism is expressed as a convolution,

$z=0.07$7

where $z=0.07$8 encodes the geometry-dependent time-delay kernel. The covering factor is written either as an instantaneous luminosity ratio or as an energy ratio,

$z=0.07$9

The published summary does not provide the explicit best-fit ring radii, widths, or kernel form in the material reproduced here, but it does identify a two-ring, sub-parsec interpretation as the principal explanation of the observed infrared behavior (Newsome et al., 2023).

The broader significance is that AT2020mot demonstrates how a TDE can function as a transient illumination source for circumnuclear reverberation studies. A plausible implication is that the nuclear dust distribution in some quiescent galaxies is sparse, geometrically limited, and inaccessible without a flare-driven echo experiment.

4. Optical polarisation and observational methodology

AT2020mot has so far exhibited the highest recorded optical polarisation in a non-jetted TDE. The reported host-corrected maximum is

$27\,\mu{\rm Jy}$0

consistent across $27\,\mu{\rm Jy}$1, $27\,\mu{\rm Jy}$2, and $27\,\mu{\rm Jy}$3 bands, whereas the uncorrected aperture-diluted maximum is approximately $27\,\mu{\rm Jy}$4. The extreme polarisation occurred during a single night, while the event appears unpolarised at approximately $27\,\mu{\rm Jy}$5 d and $27\,\mu{\rm Jy}$6 d after optical peak. Multiple late-time host-only observations with ALFOSC/NOT demonstrate that the host is unpolarised at the $27\,\mu{\rm Jy}$7 level, confirming that the large early-time signal is intrinsic to the TDE (Floris et al., 10 Sep 2025).

The polarimetric campaign used RoboPol on the 1.3 m Skinakas telescope, ALFOSC/NOT, DIPOL-1/OSN T90, and CAFOS/CAHA 2.2 m. Standard Stokes-based definitions were adopted,

$27\,\mu{\rm Jy}$8

with $27\,\mu{\rm Jy}$9 and $g$0. Host dilution was corrected assuming an unpolarised host through

$g$1

where $g$2 was estimated from pre-flare ZTF photometry. Maximum Milky Way interstellar polarisation was estimated to be $g$3 from the Serkowski law and Schlafly & Finkbeiner reddening maps, and no ISP correction was applied because it was below the statistical errors (Floris et al., 10 Sep 2025).

The late-time host-only constraints are uniformly small. Examples include $g$4 in $g$5 band at $g$6 d and $g$7 in $g$8 band at $g$9 d, with all reported late measurements remaining consistent with an unpolarised host (Floris et al., 10 Sep 2025).

5. Mass estimates, column density, and physical interpretation

Two modeling approaches were applied to infer the central and stellar parameters. In TDEMass, using the outer-shock framework with $r$0 and $r$1 as inputs, the reported values are

$r$2

and

$r$3

In MOSFiT, using a fast circularisation model with ten free parameters, the reported values are

$r$4

$r$5

with scaled impact parameter $r$6 and inferred column density $r$7 (Floris et al., 10 Sep 2025).

The elevated MOSFiT column density is explicitly noted as higher than the sample median of $r$8, suggesting a more complex environment than is typically assumed. This environmental picture is reinforced by the strong infrared flare interpreted as dust reverberation from two concentric dust rings at small radii (Newsome et al., 2023, Floris et al., 10 Sep 2025).

For the origin of the polarisation, the cited interpretation argues that electron-scattering reprocessing models usually yield $r$9 for plausible geometries, whereas AT2020mot reached $U$0 and showed variability in the polarisation angle $U$1. This combination is described as challenging pure reprocessing scenarios and as being more naturally accommodated by shock-powered models in which colliding debris streams produce strongly aspherical, time-variable scattering geometries. The absence of contemporaneous X-rays is consistent with scenarios in which shocks at large radii dominate the optical/UV output or in which X-rays are heavily suppressed or obscured (Floris et al., 10 Sep 2025).

6. Position within the TDE population and outstanding issues

Within a comparative sample of 13 TDEs with polarimetric observations, AT2020mot is an extreme outlier in optical polarisation. The sample is dominated by low or undetected optical polarisation, typically $U$2 after host correction, whereas AT2020mot reaches $U$3. By contrast, in blackbody temperature, luminosity, rise timescale, and black-hole mass range, it remains consistent with the broader sample (Floris et al., 10 Sep 2025).

This juxtaposition—ordinary global thermal properties but extraordinary polarimetric behavior—defines the main scientific importance of AT2020mot. It indicates that polarisation can be sensitive to geometric or environmental asymmetries that are not obvious in broadband photometry alone. A plausible implication is that optical TDE populations may contain brief, highly anisotropic phases that are missed without dense time-resolved polarimetry.

Several limitations remain explicit in the published discussion. Polarimetric coverage is sparse, so comparably brief high-$U$4 episodes in other TDEs could have been missed. Host-flux dilution is significant, even though the unpolarised-host assumption was validated for AT2020mot. Model-dependent quantities such as $U$5 differ between MOSFiT and TDEMass, and independent diagnostics such as Balmer decrements were not feasible with the available spectral signal-to-noise. Wavelength dependence of $U$6 may also be affected by strong emission lines, such as H$U$7 in $U$8, which can dilute continuum polarisation (Floris et al., 10 Sep 2025).

The resulting observational program advocated by the literature is correspondingly specific: time-resolved multi-band polarimetry from discovery through rise and peak, concurrent spectroscopy to correct for line depolarisation, deeper X-ray monitoring, and infrared follow-up to map the dust response. AT2020mot is therefore significant not only as an unusual event, but also as a methodological case study showing that TDEs can simultaneously probe sub-parsec dust geometry and rapidly evolving scattering structures around otherwise quiescent SMBHs (Newsome et al., 2023, Floris et al., 10 Sep 2025).