---
title: 'AT 2022fpx: Dust-Reddened TDE in a Weak AGN'
url: https://www.emergentmind.com/topics/at-2022fpx
type: topic
---

# AT 2022fpx: Dust-Reddened TDE in a Weak AGN

Searching arXiv for AT 2022fpx and related studies.
AT 2022fpx, also known as ATLAS22kjn, ZTF22aadesap, and Gaia22cwy, is a nuclear transient in the galaxy SDSS J153103.70+532419.3 at \(z=0.073\)–0.0735 whose observed properties place it at the intersection of several rare classes of supermassive-black-hole transients. It has been described as a tidal disruption event (TDE)-like flare that simultaneously shows Bowen-fluorescence signatures, extreme coronal line emission, a long-lived slowly declining optical/UV outburst, delayed soft X-ray brightening, and variable low-level optical polarization [2403.04877]. A later reanalysis emphasized its persistently red optical color, luminous mid-infrared echo, and evidence for weak pre-existing AGN activity, and argued that the source is most plausibly a heavily dust-reddened TDE in a weak AGN, while leaving a turn-on AGN interpretation as a remaining alternative [2507.04834].

## 1. Discovery and observational setting

AT 2022fpx was discovered by ATLAS on 2022-03-31 (MJD 59669) in the orange filter at magnitude 18.54. It is spatially coincident with the nucleus of SDSS J153103.70+532419.3, and Gaia pre-flare data showed the nucleus stable at \(g\sim20.1\) mag. Follow-up spectroscopy classified it as a TDE candidate at redshift \(z=0.073\); a later analysis adopted a median redshift \(z=0.0735\) from six follow-up spectra [2403.04877].

The transient reached peak optical brightness in late July 2022. One study placed the ZTF \(g,r\) peak at MJD 59784 with \(r\approx17.6\) mag and \(g\approx17.8\) mag, roughly 115 days after discovery, whereas a later phenomenological light-curve fit placed \(t_{\rm peak}\sim59795\) [2403.04877]. This difference reflects model-dependent peak definitions rather than a disagreement about the overall temporal sequence.

From the outset, AT 2022fpx stood out among nuclear flares because its optical/UV decay was unusually slow, its spectra combined high-ionization coronal lines with Balmer and helium emission, and its X-rays were absent near optical maximum but emerged hundreds of days later. These characteristics made it a candidate link between optical TDEs with delayed X-rays, Bowen-fluorescence flares, and extreme coronal line emitters.

## 2. Optical and ultraviolet evolution

The photometric evolution is complex. The combined ZTF and Swift/UVOT data show a precursor or flattening about 70 days before the main optical maximum, and the later study identified a sharp ATLAS \(o\)-band spike at MJD 59669.5 with \(m_o=18.97\pm0.08\), followed by a return toward baseline for about a month before the main rise [2403.04877]. Excluding the precursor, the 2025 analysis fit the \(g\)-band rise and decline with a Gaussian-like rise joined to a \(t^{-5/3}\) decay, obtaining \(\sigma=41.5\pm0.7\) d, \(\tau=296\pm9\) d, \(t_{1/2,\mathrm{rise}}=48.9\pm0.8\) d, and \(t_{1/2,\mathrm{decline}}=152.7\pm4.6\) d in the rest frame [2507.04834].

Near peak, a UVOT+ZTF blackbody fit gave
\[
T_{\rm peak}=16600\pm850\ \mathrm{K},\qquad
L_{\rm bol,peak}=(5.9\pm1.1)\times10^{43}\ \mathrm{erg\,s^{-1}},
\]
while power-law fits \(F_\nu\propto \nu^{-\alpha_{\rm UV/opt}}\) generally described the UV/optical SED better, with \(\alpha_{\rm UV/opt}\) evolving from about 1 at early times to about 0 during the decay [2403.04877]. The optical/UV decline was well fit by exponentials, \(F(t)\propto\exp(-t/b)\), with \(b=181\pm17\) d in ZTF \(r\), \(155\pm12\) d in ZTF \(g\), \(248\pm30\) d in UVW1, \(202\pm20\) d in UVW2, and \(213\pm18\) d in UVM2 [2403.04877]. These timescales are more than twice typical optical-TDE decay times and resemble Bowen-fluorescence flares such as AT 2021loi.

A mild optical/UV bump occurred around \(250\)–300 days after peak, contemporaneous with the onset of X-ray brightening [2403.04877]. The later paper further emphasized that after an initial decline, the optical and UV emission flattened into a slower decline or plateau around MJD \(60050\) [2507.04834].

A major observational peculiarity is color. The later analysis found a persistently red optical color, \(g-r\simeq0.4>0\), across most of the event, unlike the nearly constant blue colors typically used in optical TDE selection [2507.04834]. This red color became central to the subsequent debate over dust reddening, line contamination, and sample-selection bias.

## 3. Spectroscopic diagnostics: Balmer, helium, Bowen, and coronal lines

Optical spectroscopy established AT 2022fpx as an emission-line-dominated nuclear flare. A Nordic Optical Telescope spectrum obtained 126.5 days after discovery, about 41 days after optical peak, showed a blue continuum together with strong H\(\alpha\), H\(\beta\), He I, and He II emission [2403.04877]. Gaussian measurements gave H\(\alpha\) FWHM \(\approx1800\pm31\ \mathrm{km\,s^{-1}}\), H\(\beta\) FWHM \(\approx2100\pm60\ \mathrm{km\,s^{-1}}\), He II in the 4635–4685 Å blend with FWHM \(\approx2930\pm70\ \mathrm{km\,s^{-1}}\), and N III in the same blend with FWHM \(\approx2500\pm100\ \mathrm{km\,s^{-1}}\) [2403.04877]. A later seven-spectrum campaign found broad Balmer components with FWHM \(\sim1650\)–\(2500\ \mathrm{km\,s^{-1}}\), narrow components \(\sim300\)–\(500\ \mathrm{km\,s^{-1}}\), He II \(\lambda4686\) and He I \(\lambda5876\) at luminosities of a few \(10^{40}\ \mathrm{erg\,s^{-1}}\), persistent Fe II pseudo-continuum around 4400–4800 Å, and a substantial weakening of the broad Balmer lines by 2025 [2507.04834].

The 2024 study identified the 4635–4685 Å complex as clear Bowen fluorescence, including N III \(\lambda\lambda4097,4104,4640\) and He II \(\lambda4686\), and therefore placed AT 2022fpx in the growing class of Bowen-fluorescence flares [2403.04877]. The 2025 reanalysis took a more cautious view, arguing that key N III and O III features are blended with H\(\delta\), He II, or [Fe VII], so the Bowen classification could not be confirmed unambiguously [2507.04834]. This disagreement is one of the main spectroscopic subtleties of the source.

The most distinctive line diagnostics are the coronal features. AT 2022fpx shows [Fe XIV] \(\lambda5303\), [Fe X] \(\lambda6374\) or \(\lambda6375\), [Fe XI] \(\lambda7892\), [Fe VII] \(\lambda\lambda5721,6087\), and [S XII] \(\lambda7613\) [2403.04877]. In the 2024 spectrum, the [Fe XIV] \(\lambda5303\) flux exceeded [O III] \(\lambda5007\) by a factor of 1.7, comfortably satisfying the criterion that an extreme coronal line emitter has at least one coronal line more than 20% stronger than [O III] \(\lambda5007\) [2403.04877]. The later study measured coronal-line luminosities of \(\sim5\times10^{39}\) to \(3\times10^{40}\ \mathrm{erg\,s^{-1}}\) with FWHM \(300\)–\(800\ \mathrm{km\,s^{-1}}\), present from early times and fading only weakly until late epochs [2507.04834].

Taken together, the spectra resemble a hybrid of ECLEs, some TDE-in-AGN candidates, and narrow-line Seyfert 1 outbursts more than a canonical blue optical TDE or an ordinary persistent AGN spectrum.

## 4. Polarization and geometric constraints

AT 2022fpx is one of the few TDE-like transients with multi-epoch optical polarimetry. Using NOT/ALFOSC and Liverpool Telescope/MOPTOP observations, the 2024 study found low but clearly variable intrinsic continuum polarization after correction for host dilution [2403.04877]. In the \(B\) band, \(\Pi\) was about \(2.5\%\) just before peak, dropped to about \(0.4\%\) at peak, and then ranged between \(0.8\%\) and \(2.7\%\) post-peak. In the \(V\) band, one detection near peak gave \(\Pi\sim0.8\%\). In the \(R\) band there were no \(>3\sigma\) detections, with late-time \(3\sigma\) upper limits \(\Pi<0.5\%\); at 99% confidence, two marginal detections at \(\Pi\sim0.4\)–\(0.7\%\) were reported around the onset of the X-ray flare [2403.04877].

The polarization angle evolved smoothly and significantly. An early \(B\)-band observation gave \(\psi\approx62^\circ\); around peak, \(\psi\) rotated to about \(7^\circ\), a total change of roughly \(55^\circ\) over a few tens of days. The inferred \(B\)-band rotation rate was
\[
\dot{\psi}_{\rm B}\approx(1.2\pm0.3)^\circ\,\mathrm{day}^{-1},
\]
and the implied rate between the last \(B\)-band detection and a later \(V\)-band detection was \(2.6\pm0.6^\circ\,\mathrm{day}^{-1}\), consistent within \(2\sigma\) with a constant rate [2403.04877]. Several hundred days after peak, polarization became essentially undetectable.

The instrumental and astrophysical corrections were explicitly treated. Host-galaxy dilution was estimated from Pan-STARRS magnitudes converted to Johnson \(B,V,R\), and simulations with realistic flat-field residuals and sky backgrounds were used to verify polarization uncertainties. Strong Balmer lines contributed only 3% of the total flux in \(B\) and 8% in \(V\), so line depolarization was not expected to dominate [2403.04877].

These measurements disfavor a purely static electron-scattering photosphere. Two explanatory frameworks were proposed. In the outer-shock scenario, pericenter shocks dominate during the rise and outer shocks later add a second polarized component with a different position angle, so the vector sum produces depolarization and angle rotation. In the clumpy-torus or precessing-disk scenario, a compact scattering torus around a misaligned accretion flow yields smooth position-angle rotation through Lense–Thirring precession. The authors explicitly noted that the two pictures are not mutually exclusive.

## 5. X-ray, infrared, and continuum-SED behavior

Near optical maximum, AT 2022fpx was X-ray faint or undetected. Around MJD 59784, Swift/XRT gave a \(3\sigma\) upper limit of \(<2.5\times10^{-3}\ \mathrm{cts\,s^{-1}}\) in 0.3–1 keV, corresponding to \(<7.7\times10^{-14}\ \mathrm{erg\,s^{-1}\,cm^{-2}}\) if the later blackbody spectrum is assumed [2403.04877]. A soft X-ray flare emerged only around MJD \(60050\), about 250 days after optical peak. Joint Swift/XRT and XMM-Newton spectral fitting with `tbabs * zbb`, fixing Galactic \(N_{\rm H}=1.25\times10^{20}\ \mathrm{cm^{-2}}\) and redshift \(z=0.073\), yielded
\[
kT=95\pm5\ \mathrm{eV},
\qquad
L_{\rm bol,X}=(8.7\pm1.2)\times10^{42}\ \mathrm{erg\,s^{-1}}
\]
at the XMM epoch, with Cash statistic \(C=49.5\) for 44 degrees of freedom [2403.04877]. The temperature remained roughly constant while the flux changed, suggesting either a varying emitting area or variable obscuration.

The later X-ray analysis extended the monitoring to 75 Swift/XRT epochs plus an Einstein Probe stack. It resolved three phases: an early stacked low state with count rate \((7.7\pm2.2)\times10^{-4}\ \mathrm{ct\,s^{-1}}\), a high state at MJD 60071–60154 with \(kT=97.1^{+4.7}_{-4.5}\ \mathrm{eV}\) and \(L_X\sim10^{42.55}\ \mathrm{erg\,s^{-1}}\), and a later low state with \(kT=92.8^{+6.5}_{-6.2}\ \mathrm{eV}\) and \(L_X\sim10^{42.15}\ \mathrm{erg\,s^{-1}}\) [2507.04834]. The Einstein Probe spectrum remained very soft, with \(kT=121.1^{+14.2}_{-12.3}\ \mathrm{eV}\) and \(L_X\sim10^{42.0}\ \mathrm{erg\,s^{-1}}\) [2507.04834]. The later paper therefore characterized the X-ray evolution not merely as a delayed flare but as a \(\sim550\)-day soft X-ray plateau with little spectral evolution.

Mid-infrared observations revealed a strong dust echo. Host-subtracted WISE/NEOWISE photometry was fit with a single-temperature blackbody peaking at \(\log L_{\rm bb,MIR}\approx43.3\ \mathrm{erg\,s^{-1}}\), with dust temperature declining from \(\sim1500\) K to \(\sim900\) K and effective radius expanding from \(\sim4\times10^{16}\) cm to \(\sim2\times10^{17}\) cm [2507.04834]. This places hot dust at sub-parsec scales and implies powerful reprocessing of EUV/X-ray radiation by circumnuclear dust.

Continuum-SED modeling became a major interpretive issue. The 2025 study showed that optical-only blackbody fits for AT 2022fpx gave \(T_{\rm bb,opt}\sim7500\) K near peak, whereas optical+UV fits gave \(T_{\rm bb,opt+UV}\sim14000\) K; after adopting a conservative internal extinction \(A_V\sim1.14\) mag from the Balmer decrement, the temperatures became \(\sim13000\) K and \(\sim40000\) K, respectively [2507.04834]. Over \(2000\)–\(7000\) Å, power laws \(f_\lambda\propto\lambda^{-\alpha}\) fit better than single blackbodies, with near-peak slopes around \(\alpha\sim1.6\) in the observed SED and steeper values after extinction correction [2507.04834]. A plausible implication is that much of the radiative output lies at shorter wavelengths than the optical bands alone would suggest.

## 6. Host galaxy, black-hole estimates, classification, and significance

Host-galaxy inference is model-dependent. One CIGALE analysis, using SDSS, Pan-STARRS, 2MASS, and AllWISE photometry, found \(M_\star=(7.94\pm0.75)\times10^{10}\ M_\odot\), an old stellar population with average age \(4.7^{+0.5}_{-0.3}\) Gyr, a more recent starburst of age \(1.1\pm0.1\) Gyr, negligible current star formation with \(\log(\mathrm{SFR}/M_\odot\,\mathrm{yr}^{-1})=-14.1^{+0.0}_{-0.2}\), \(E(B-V)\approx0.4\) mag, and \(f_{\rm AGN}=0.0^{+0.12}\) [2403.04877]. A later CIGALE fit, built from SDSS, Pan-STARRS, DESI Legacy Survey, 2MASS, unWISE, and AllWISE photometry, instead gave \(M_*=6.89^{+2.95}_{-1.93}\times10^9\ M_\odot\) and \(f_{\rm AGN}=0.15^{+0.09}_{-0.07}\), explicitly indicating a weak AGN component [2507.04834]. This suggests that the host characterization is sensitive to the adopted photometric set and SED assumptions.

Black-hole mass estimates are similarly broad. TDE-oriented modeling in the 2024 study yielded \(\log(M_{\rm BH}/M_\odot)=7.46^{+0.00}_{-0.05}\) from MOSFiT and \(6.71^{+0.15}_{-0.18}\) from the TDEmass outer-shock prescription, while empirical scalings gave \(6.80\pm0.54\) from peak luminosity, \(7.19\pm0.44\) from radiated energy, and \(6.62\pm0.58\) from host mass, with a global mean \(\log(M_{\rm BH}/M_\odot)\approx6.95\pm0.53\) [2403.04877]. The 2025 study, using the \(M_{\rm BH}-M_*\) relation for low-mass AGN, derived \(\log(M_{\rm BH}/M_\odot)=6.24^{+0.32}_{-0.33}\) [2507.04834]. Both analyses place the system in the broad regime usually associated with TDE hosts.

The classification debate has three components. First, a supernova origin is strongly disfavored: the later paper argued that the very long rise, ultra-soft and long-lived X-rays, persistent luminous coronal lines, exceptionally bright MIR echo, and absence of late nebular SN features are all atypical of SNe IIn [2507.04834]. Second, a purely ordinary persistent AGN interpretation is also problematic, because the event shows a well-defined rise, peak, decline, delayed ultra-soft X-rays, and broad-line fading rather than stochastic AGN variability [2403.04877]. Third, a weak pre-existing AGN is plausible: the 2025 study found two low-level pre-outburst MIR flares in NEOWISE and a non-zero AGN dust component in the host SED, while the 2024 study found no strong pre-flare variability in CRTS, NEOWISE, or Gaia and no radiatively dominant AGN in its SED fit [2507.04834].

Accordingly, the 2024 study classified AT 2022fpx as a TDE-like flare that bridges long-lived Bowen-fluorescence TDEs, ECLEs, and optical TDEs with delayed X-rays [2403.04877]. The 2025 study favored a heavily dust-reddened TDE of a giant star in a weak AGN, but retained a turn-on AGN interpretation as the principal alternative [2507.04834]. The source therefore occupies an important boundary case in the TDE–AGN interface.

Its broader significance extends beyond source taxonomy. Because the observed color remained red, \(g-r\sim0.4\), AT 2022fpx would have been rejected by many optical TDE selection pipelines using a blue-color cut. The later paper showed that line contamination can redden \(g-r\) by up to \(\sim0.2\) mag, but that the continuum is still intrinsically red in the observed frame; with a conservative extinction correction of \(A_V\sim1.14\) mag, the intrinsic color becomes blue and the SED becomes consistent with TDE-like continua [2507.04834]. This suggests that color-based optical TDE samples may exclude dust-reddened or line-dominated events, especially in gas-rich or weakly active nuclei.

AT 2022fpx is therefore significant both as an individual transient and as a selection-function test case. It demonstrates that ECLE-like spectra, delayed soft X-rays, variable optical polarization, strong dust echoes, and red observed colors can coexist within a single nuclear flare, and that the boundaries between quiescent-host TDEs, TDE-in-AGN events, Bowen-fluorescence flares, and ECLEs are at least partly observational rather than categorical.

Source: https://www.emergentmind.com/topics/at-2022fpx