---
title: 'AT 2023clx: Fast Low-Luminosity Tidal Disruption'
url: https://www.emergentmind.com/topics/at-2023clx
type: topic
---

# AT 2023clx: Fast Low-Luminosity Tidal Disruption

AT 2023clx, also reported as ASASSN-23bd, is a tidal disruption event (TDE) in the nucleus of the nearby galaxy NGC 3799 at redshift $z = 0.01107$. It has been characterized as the closest optical or optical/UV TDE discovered to date, and discovery papers also identified it as the faintest or least-luminous member of the optical TDE population, although the reported peak luminosity depends on the adopted distance and host-extinction correction [2307.04297]. Because it combines proximity, low luminosity, dense photometric and spectroscopic follow-up, stringent early X-ray constraints, and later optical polarimetry, AT 2023clx has become a reference object for studies of low-luminosity, fast TDEs and of the origin of optical emission during debris circularization [2401.05490].

## 1. Discovery, localization, and host-galaxy environment

AT 2023clx was first detected by the All-Sky Automated Survey for SuperNovae and reported on the Transient Name Server on 2023-02-26 UT; an ASAS-SN discovery analysis gives MJD 59997.2, with a $g$-band magnitude of 16.3 at discovery and a last non-detection on MJD 59988.3 [2307.04297]. Image-subtraction analysis placed the transient at the nucleus of NGC 3799 to within $0.21 \pm 0.17$ arcsec, corresponding to $\sim 49 \pm 40$ pc, which was central to its identification as a nuclear transient rather than an off-nuclear supernova [2307.04297].

The host galaxy NGC 3799 is described as a nearby star-forming spiral galaxy and as a SB(s)b:pec system with LINER-like nuclear line ratios [2401.05490]. Pre-outburst optical and TESS light curves showed no significant variability over a decade, and the host showed no evidence of strong AGN activity in the last decade, although weak nuclear activity is consistent with its LINER classification [2401.05490]. SED-based analyses place the host on the star-forming main sequence, with reported stellar-mass estimates of $\log (M_\star/M_\odot) = 9.87 \pm 0.02$ or $M_\star \approx 6.3 \times 10^9\,M_\odot$, and one analysis found a small AGN fraction, $f_{\rm AGN}<0.2$ [2307.04297].

Distance estimates differ modestly among studies. A discovery analysis adopted a luminosity distance of 47.8 Mpc, while another gave $50.1 \pm 3.5$ Mpc, and corrected distances up to $\sim 54.6$ Mpc were also discussed [2307.04297]. This spread propagates into the reported absolute magnitude and bolometric luminosity.

## 2. Light-curve evolution, rise timescale, and bolometric output

The early light curve establishes AT 2023clx as a fast, low-luminosity TDE. Fits to the rising ASAS-SN light curve indicate that it started brightening on $\mathrm{MJD}\,59988^{+1}_{-1}$, roughly 9 days before discovery, and peaked in the $g$ band on $\mathrm{MJD}\,60000^{+3}_{-3}$ [2401.05490]. The rise was described as nearly linear in flux,
$$
f(t)=
\begin{cases}
f_0 & t < t_0 \\
f_0 + k\left(\frac{t-t_0}{1+z}\right)^a & t \ge t_0 ,
\end{cases}
$$
with $a = 1.1 \pm 0.3$ [2401.05490]. A separate study noted that the rise was poorly sampled but consistent with a power law with $n=0.64$ [2307.04297]. The rise time to peak was reported as $<15$ days and, after host-reddening correction, as $10.4 \pm 2.5$ days; one analysis described this as the fastest-rising TDE to date [2401.11773].

The post-peak decline was similarly notable. Optical and bolometric light curves were reported to follow the canonical $t^{-5/3}$ decay expected for TDE fallback [2307.04297]. Another analysis emphasized a fast 40-day bolometric decline of $\Delta L_{40}\approx -0.7$, placing AT 2023clx in the growing “Low Luminosity and Fast” class of TDEs [2401.05490].

Reported peak photometric quantities vary across analyses:

| Study | Reported peak quantity | Basis noted in study |
|---|---|---|
| [2307.04297] | $M_g=-17.16$ mag; $L_{BB,\rm peak}=(4.56\pm0.53)\times10^{42}\,\mathrm{erg\,s^{-1}}$ | Distance 47.8 Mpc |
| [2401.05490] | $L_{\rm peak}=(5.4\pm0.4)\times10^{42}\,\mathrm{erg\,s^{-1}}$ | UV/optical peak luminosity |
| [2401.11773] | $M_g=-18.03\pm0.07$ mag; $L_{\rm pk}=(1.57\pm0.19)\times10^{43}\,\mathrm{erg\,s^{-1}}$ | Host reddening corrected, $E(B-V)=0.179$ mag |

Blackbody modeling also differs in detail among studies, but the common picture is of a blue thermal continuum with a photospheric radius of order $10^{14}$ cm. One analysis fitted the optical+UV SED with the SUPERBOL package and found an almost constant blackbody temperature of $\sim 12{,}000$ K, ranging from 11,000 to 13,000 K for months, with a peak radius of roughly $4.6\times10^{14}$ cm that decreased after peak [2307.04297]. Another reported temperatures in the range $\sim 14{,}000$–20,000 K, a break in the temperature evolution during the first $\sim 20$ days after peak, and a photospheric expansion velocity of $(4{,}060 \pm 225)\,\mathrm{km\,s^{-1}}$ [2401.11773]. The bolometric luminosity in these analyses is represented by
$$
L_{BB} = 4\pi R_{BB}^2 \sigma T_{BB}^4 .
$$

This combination of a rapid rise, low optical luminosity, and fast early decline is the basis for the classification of AT 2023clx as an extreme low-luminosity, fast TDE [2401.05490].

## 3. Spectroscopic and multiwavelength phenomenology

Optical spectroscopy showed the characteristic TDE combination of a blue continuum with broad hydrogen and helium emission. Multiple analyses reported broad Balmer lines with widths of order $15{,}000\,\mathrm{km\,s^{-1}}$, strong H$\alpha$, broad He II $\lambda 4686$, and weaker He I features at 5876 and 6678 Å [2307.04297]. The spectra lacked the low-ionization metal features expected in many supernovae, arguing against a supernova interpretation [2307.04297]. H$\alpha$ profiles were asymmetric in some epochs and were attributed possibly to outflows [2307.04297].

Several spectroscopic details are unusual even within the TDE class. One study found a flat Balmer decrement,
$$
\frac{L_{H\alpha}}{L_{H\beta}} \sim 1.58,
$$
and argued that the line emission was collisionally excited rather than produced via photoionization, in contrast to typical active galactic nuclei [2401.11773]. The same study reported a sharp, narrow emission peak at a rest wavelength of $\sim 6353$ Å, visible up to 10 days post-peak and interpreted as clumpy material preceding the bulk outflow, manifesting as a high-velocity component of H$\alpha$ at $-9584\,\mathrm{km\,s^{-1}}$ [2401.11773]. This feature was described there as the first such case seen in TDE spectra.

Ultraviolet and near-infrared follow-up further strengthened the TDE classification. UV spectroscopy showed nitrogen emission lines such as N III] and N IV], without the strong C IV or Mg II features typical of AGN, while NIR spectroscopy lacked AGN-like broad Paschen, He I, or coronal lines [2401.05490]. This multiwavelength line phenomenology supported a stellar-disruption origin rather than an AGN flare [2401.05490].

Early X-ray observations yielded stringent upper limits. One study reported no detection in any single or stacked Swift/XRT image, with a $3\sigma$ upper limit of $9.53\times10^{39}\,\mathrm{erg\,s^{-1}}$ in the 0.3–10 keV band [2307.04297]. Another gave a comparable stacked limit of $L_{0.3-10\,\mathrm{keV}}<1.0\times10^{40}\,\mathrm{erg\,s^{-1}}$ before MJD 60061, and also reported a late-time XMM-Newton detection on MJD 60095 of soft thermal X-rays with $kT\sim 0.1$ keV and $L_{0.3-2\,\mathrm{keV}}\sim 4.2\times10^{39}\,\mathrm{erg\,s^{-1}}$; more than 90% of the counts were below 2 keV [2401.05490]. The inferred X-ray blackbody radius of $\sim 1.8\times10^9$ cm was noted as much smaller than the Schwarzschild radius for the expected SMBH mass, a known issue for TDE X-ray spectra [2401.05490].

## 4. Black-hole mass estimates and disruption scenarios

The central black-hole mass in NGC 3799 has been estimated with several methods, yielding a range concentrated around $10^6\,M_\odot$ but with non-negligible model dependence. Host-galaxy scaling in one study gave
$$
\log\left(\frac{M_{\rm BH}}{M_\odot}\right)=6.26\pm0.28,
$$
corresponding to $M_{\rm BH}\sim 10^{6.26\pm0.28}\,M_\odot$ [2307.04297]. Another host-scaling estimate found
$$
M_{\rm BH,host}=(1.6\pm1.0)\times10^6\,M_\odot,
$$
while MOSFiT light-curve modeling in the same work yielded $\log_{10} M_{\rm BH}=6.4^{+0.1}_{-0.1}$ [2401.05490]. A separate analysis using several methods reported $\log_{10}(M_{\rm BH}/M_\odot)=6.49\pm0.39$ from the $M$–$\sigma$ relation using an SDSS host spectrum, $\log_{10}(M_{\rm BH}/M_\odot)=5.71\pm0.40$ from an X-shooter spectrum, $\log_{10}(M_{\rm BH}/M_\odot)=6.62\pm0.69$ from the stellar-mass scaling, and $\log_{10}(M_{\rm BH}/M_\odot)=5.83\pm0.28$ from MOSFiT, with an adopted average $\overline{M}_{\rm BH}\approx10^{6.0}\,M_\odot$ [2401.11773].

The host-scaling relation used in these analyses follows
$$
\log\left(\frac{M_{\rm BH}}{M_\odot}\right)=\alpha+\beta\log\left(\frac{M_\star}{10^{11}M_\odot}\right),
$$
with $\alpha = 7.45 \pm 0.08$ and $\beta = 1.05 \pm 0.11$ in the formulation quoted for NGC 3799 [2307.04297].

A later Mephisto-based study obtained systematically lower MOSFiT masses, finding
$$
M_{\rm BH}\approx 10^{5.67}-10^{5.82}\,M_\odot
$$
depending on which pre-peak data were included [2408.04448]. That same analysis identified two alternative disruption scenarios: either a full disruption of a $0.1\,M_\odot$ star or a partial disruption of a $0.99\,M_\odot$ star [2408.04448]. The distinction depended on inconsistencies between the ASAS-SN and ATLAS datasets during the rising phase, making the early photometry the dominant source of ambiguity [2408.04448].

By contrast, another study argued more strongly for disruption of a very low-mass star, $\lesssim 0.1\,M_\odot$, and used the rapid rise, shallow decline, and fallback-model comparison to support that interpretation [2401.11773]. In that framework, the SMBH mass of order $10^6\,M_\odot$ ruled out an intermediate-mass black hole as the explanation for the fast rise [2401.11773].

These differing inferences are not purely numerical disagreements; they reflect the sensitivity of TDE parameter recovery to extinction corrections, pre-peak sampling, and the assumed radiative channel.

## 5. Optical emission mechanism and polarimetric constraints

The central physical question for AT 2023clx is the origin of its optical radiation and the timescale of disk formation. One interpretation emphasizes efficient circularization and prompt accretion-disk formation in the disruption of a very low-mass star, viewed through a low-density photosphere and accompanied by an outflow launched in the line of sight [2401.11773]. Another interpretation, based on composite multi-band light-curve fitting and the early lack of soft X-rays, proposes that the observed optical radiation is powered by stream-stream collision rather than prompt accretion onto a compact disk [2408.04448].

In the stream-collision picture, the self-intersection radius is at $10^{13}$–$10^{14}$ cm, the fitted radiative efficiencies are low, $\eta_{\rm fit}\sim 10^{-4}$–$10^{-4.5}$, and the lack of early X-rays follows from delayed circularization [2408.04448]. Using the orbital period of the most bound debris, $t_{\rm mb}=13$–28 days, and a collision efficiency $\mathcal{C}$, that study estimated circularization timescales of $\sim 100$ days for the low-mass full-disruption solutions and $\sim 600$ days for the near-solar-mass partial-disruption solutions, and therefore speculated that soft X-rays may emerge 100–600 days after the optical peak [2408.04448]. This is an explicitly model-dependent prediction.

Optical polarimetry has provided the strongest direct geometric constraint. Nordic Optical Telescope observations covered five epochs from near optical peak, $\sim 6$ days after maximum, to about 35 days post-peak, with later upper limits [2508.09309]. The linear Stokes parameters were derived as
$$
q = \frac{Q_1 - Q_3}{Q_M}, \qquad
u = \frac{Q_2 - Q_4}{Q_M}, \qquad
Q_M = Q_1 + Q_2 + Q_3 + Q_4 ,
$$
with polarization degree
$$
p = \sqrt{q^2 + u^2}
$$
and polarization angle
$$
\mathrm{PA} = 0.5 \times \arctan\left(\frac{u}{q}\right).
$$

The polarimetric evolution was highly structured. The earliest observation, 6 days after peak, showed low or undetectable polarization in $B$ band, $p_0 = 0.20 \pm 0.12\%$, while $V$- and $R$-band values were at $\sim 1.6\%$ and $\sim 1.3\%$ [2508.09309]. The $B$- and $R$-band polarization degrees then increased nearly linearly until $\sim 35$ days after peak, with a linear growth rate of $0.11\%$ per day, reaching $4.80 \pm 0.65\%$ in $R$ band and $3.69 \pm 0.60\%$ in $B$ band on day 35.7 [2508.09309]. After $\sim 55$ days the polarization degree fell below 1.5% in $V$ band at the $2\sigma$ level [2508.09309]. The polarization angle was around $120^\circ$ in $V/R$ at 6 days post-peak, dropped by about $60^\circ$–$100^\circ$ over the next two epochs to $25^\circ$–$45^\circ$ at days 20–36, and then remained relatively stable [2508.09309]. The wavelength dependence also evolved from red-dominated polarization to blue-dominated polarization [2508.09309].

This variability is difficult to reconcile with simple reprocessing models, which were described as predicting minimal, slow polarization variability and generally lower polarization degree, $\lesssim 14\%$ even for favorable geometries [2508.09309]. A collisionally-induced outflow model can allow high and time-variable polarization degrees and a rise and fall in polarization, but the study noted that this scenario does not currently address polarization-angle evolution, which is the most distinctive observable in the data [2508.09309]. The close resemblance to the polarization evolution of AT 2020mot was therefore taken as strong evidence that tidal stream shocks dominate the optical outburst during accretion-disk formation [2508.09309]. This suggests that AT 2023clx probes the circularization stage directly rather than only a later reprocessing layer.

## 6. Position within the TDE population and outstanding issues

AT 2023clx occupies an important region of TDE parameter space. Discovery studies described it as the faintest or least-luminous optical TDE yet found and as the nearest event of its class, while later work emphasized its membership in the “Low Luminosity and Fast” population [2307.04297]. Its host environment is also atypical relative to the overrepresentation of post-starburst hosts in historical TDE samples: NGC 3799 is a star-forming main-sequence spiral with LINER-like nuclear emission [2307.04297].

Population-level implications have already been drawn. One luminosity-function analysis concluded that adding AT 2023clx doubled the volumetric rate at $L_g < 10^{42.5}\,\mathrm{erg\,s^{-1}}$ relative to previous ZTF-based estimates and suggested that faint TDEs may constitute up to 74% of all TDEs in the observed $g$-band peak-luminosity range $42.3 < \log L_g < 44.7$ [2307.04297]. This suggests that flux-limited surveys have likely missed a substantial nearby population of faint TDEs.

Several issues remain unsettled. The adopted host-extinction correction changes the absolute magnitude and bolometric luminosity significantly [2401.11773]. Black-hole mass estimates range from $\sim 10^{5.67}\,M_\odot$ to $\sim 10^{6.6}\,M_\odot$ depending on the method and dataset [2408.04448]. The disrupted star may have been a very low-mass star, or the light curve may permit a partial disruption of a nearly solar-mass star [2408.04448]. Most importantly, the prompt-disk and delayed-circularization pictures are not equivalent: they imply different locations of the dominant optical emitter and different expectations for late-time X-ray emergence [2401.11773]. High-cadence, multi-band early photometry, continued X-ray monitoring, and additional time-resolved polarimetry are therefore central to resolving the physical interpretation.

In this sense, AT 2023clx is not only an unusually nearby and faint TDE; it is also a stringent test case for how optical TDE emission is produced during the transition from stellar disruption to disk formation.

Source: https://www.emergentmind.com/topics/at-2023clx