AT 2024wpp: Record Luminous FBOT

• AT 2024wpp is an exceptionally luminous fast blue optical transient characterized by high optical/UV luminosity and rapid evolution.
• Multiwavelength data reveal a persistently blue, featureless continuum with fast-expanding ejecta and evolving X-ray and radio signatures.
• The event supports a progenitor scenario involving a Wolf–Rayet star-black hole merger, with dense circumstellar interaction shaping its dynamics.

AT 2024wpp is an exceptionally luminous, rapidly evolving extragalactic transient belonging to the fast blue optical transient (FBOT) class, most precisely the “Cow-like” or AT 2018cow-like subclass. Discovered in 2024, AT 2024wpp set new records for peak optical and ultraviolet luminosity in the FBOT population, with compelling multiwavelength evidence for central-engine activity and a likely progenitor scenario involving the merger of a Wolf–Rayet star with a stellar-mass black hole. Its photometric, spectroscopic, polarimetric, X-ray, and radio characteristics place it at the boundary between engine-driven supernovae and tidal disruption events (TDEs), highlighting both the diversity and the unifying physics of extreme stellar explosions.

1. Discovery, Photometric and Spectroscopic Properties

AT 2024wpp was identified by the Zwicky Transient Facility (ZTF) on 2024 September 26 at $z = 0.0868$ in the outskirts of a dwarf galaxy. The transient rose to a peak absolute $r$-band magnitude $M_r\approx -21.5$ within 3–4 days and achieved a bolometric luminosity across bands of $L_{\rm pk} \approx (2-4)\times 10^{45}$ erg s$^{-1}$, radiating $>10^{51}$ erg in its first ~45 days—an order of magnitude more than AT 2018cow, the prototypical FBOT (LeBaron et al., 31 Aug 2025, Liu et al., 24 Feb 2026).

Multi-band UV/optical/infrared photometry is accurately described at all times by a single and persistently blue blackbody continuum, with color temperatures $T_{\rm bb} \gtrsim 2\times 10^4$–$3\times 10^4\,$K and blackbody radii expanding at $0.2c$–$0.3c$ up to $r$0cm. The high luminosity and blue colors were maintained for weeks, with negligible cooling and a receding photosphere (LeBaron et al., 31 Aug 2025, Pursiainen et al., 2024).

Spectroscopy from +4 to +35 days revealed a featureless, blue continuum, lacking narrow host- or hydrogen/helium-like lines in the early phase. At late times ($r$1 days), faint, broad H/He lines with both rest and blueshifted ($r$2 km s$r$3) components appeared, indicating deviations from spherical symmetry and the presence of both polar and equatorial outflows (LeBaron et al., 31 Aug 2025, Pursiainen et al., 2024). The spectra remained dominated by thermal emission, with any bound-bound features highly Doppler broadened and washed out by ionization and electron scattering.

Polarimetric observations between +6 and +14 days showed continuum polarization $r$4 across all optical bands, implying a high degree of spherical symmetry for the outflow. In combination with similar profiles for AT 2018cow, this disfavors strongly aspherical outflows during the optically thick photospheric phase (Pursiainen et al., 2024).

2. Multiwavelength Behavior: X-ray and Radio Evolution

AT 2024wpp displayed luminous and variable X-ray emission ($r$5 erg s$r$6, 0.3–10 keV) and robust radio through millimeter (0.25–203 GHz) counterparts (Nayana et al., 31 Aug 2025, Liu et al., 24 Feb 2026). The X-ray spectrum evolved from an initially soft power-law ($r$7) to extreme hardness ($r$8) around 50 days, coincident with an X-ray rebrightening and the emergence of a transient Compton “hump.” These features are interpreted as the evolving transmission of an embedded, variable high-energy source (e.g., an accreting compact object) through expanding, asymmetric, and initially Compton-thick ejecta. The X-ray light curve shows an early plateau, steep decay, and a delayed rebrightening interpreted as engine-driven fallback accretion (Nayana et al., 31 Aug 2025, Liu et al., 24 Feb 2026).

In the radio, the earliest fluxes in the millimeter/centimeter bands showed a rapid, order-of-magnitude rise during $r$917–32 days, followed by a spectral peak at 9 GHz ($M_r\approx -21.5$0 erg s$M_r\approx -21.5$1 Hz$M_r\approx -21.5$2 at 73 days) and a subsequent decay (Nayana et al., 31 Aug 2025). The radio/millimeter spectral energy distributions fit standard synchrotron self-absorption formalisms with optically thin (α ∼ -1.5) and thick (α ∼ 1–1.5) slopes. Equipartition modeling yields rapidly expanding emission regions ($M_r\approx -21.5$3 to $M_r\approx -21.5$4) and internal energies growing from $M_r\approx -21.5$5 erg to $M_r\approx -21.5$6 erg (Nayana et al., 31 Aug 2025, Liu et al., 24 Feb 2026).

The collective X-ray and radio evolution provides direct evidence for a shock propagating through a dense circumstellar medium (CSM), including a confined shell ($M_r\approx -21.5$7 cm, $M_r\approx -21.5$8 cm$M_r\approx -21.5$9) and a radial density profile $L_{\rm pk} \approx (2-4)\times 10^{45}$0 at large scales, characteristic of extreme pre-explosion mass loss (Nayana et al., 31 Aug 2025).

3. Physical Interpretation: Central Engine and Progenitor Models

AT 2024wpp’s extreme energetics and multi-component outflows require a central engine capable of sustained energy injection far exceeding that of standard core-collapse supernovae. The chronology and energetics are well explained by the delayed merger explosion of a $L_{\rm pk} \approx (2-4)\times 10^{45}$1 Wolf–Rayet (WR) star and a $L_{\rm pk} \approx (2-4)\times 10^{45}$2 black hole. In this scenario, during a common-envelope inspiral, extensive H-poor CSM is deposited. The final coalescence triggers:

• Hyper-accretion onto the BH, powering early X-ray and optical emission through a Blandford–Znajek (BZ) jet process, with $L_{\rm pk} \approx (2-4)\times 10^{45}$3 ($L_{\rm pk} \approx (2-4)\times 10^{45}$4), producing the initial X-ray plateau and high-velocity ejecta (best fit $L_{\rm pk} \approx (2-4)\times 10^{45}$5, $L_{\rm pk} \approx (2-4)\times 10^{45}$6).
• A magnetically arrested disk (MAD) transition at $L_{\rm pk} \approx (2-4)\times 10^{45}$7days induces precipitous jet power drop and an X-ray decline.
• Fallback of equatorially ejected debris ignites renewed accretion at $L_{\rm pk} \approx (2-4)\times 10^{45}$8days, with a fallback rate $L_{\rm pk} \approx (2-4)\times 10^{45}$9, fueling a second phase of X-ray and optical rebrightening and producing slower equatorial outflows ($^{-1}$0, $^{-1}$1) (Liu et al., 24 Feb 2026).

The observed late-time bolometric luminosity decay precisely follows the $^{-1}$2 power law expected for fallback-dominated TDEs, providing quantitative support for this model (Liu et al., 24 Feb 2026). The high-velocity, highly ionized, bipolar/polar outflows and delayed emergence of broad H/He lines are hallmark signatures of this engine-driven configuration, though the polarimetry at early times points to outflow sphericalization by the time the ejecta become photospheric (Pursiainen et al., 2024, LeBaron et al., 31 Aug 2025, Nayana et al., 31 Aug 2025).

4. AT 2024wpp in the Context of FBOTs and TDEs

AT 2024wpp stands at the luminous, energetic, and kinematic extreme of the FBOT family. Its key properties can be compared as follows:

Event	$^{-1}$3 [erg/s]	$^{-1}$4 [K]	$^{-1}$5 [c]	$^{-1}$6 [erg]	Distinctive Features
AT 2024wpp	$^{-1}$7	$^{-1}$8	$^{-1}$9	$>10^{51}$0	X-ray rebrightening, late $>10^{51}$1 falloff
AT 2018cow	$>10^{51}$2	$>10^{51}$3	$>10^{51}$4	$>10^{51}$5	Early X-ray/optical flares, polarization drop
AT 2022tsd	$>10^{51}$6	$>10^{51}$7	$>10^{51}$8	$>10^{51}$9	Numerous minute-scale optical flares

Whereas AT 2018cow and AT 2022tsd display rapidly evolving blue continua and high-velocity outflows, only AT 2024wpp shows clear late-time X-ray rebrightening, an extended $T_{\rm bb} \gtrsim 2\times 10^4$0 luminosity tail, and aspherical but predominantly spherical outflows at photospheric radii (LeBaron et al., 31 Aug 2025, Pursiainen et al., 2024, Nayana et al., 31 Aug 2025, Liu et al., 24 Feb 2026).

A defining distinction is that, despite deep searches in the window $T_{\rm bb} \gtrsim 2\times 10^4$1–$T_{\rm bb} \gtrsim 2\times 10^4$2 days, AT 2024wpp shows no evidence for the minute-scale optical flares previously observed in AT 2022tsd, with an upper limit to the flare duty cycle $T_{\rm bb} \gtrsim 2\times 10^4$3 (2$T_{\rm bb} \gtrsim 2\times 10^4$4) and flare rate $T_{\rm bb} \gtrsim 2\times 10^4$5 hr$T_{\rm bb} \gtrsim 2\times 10^4$6 for $T_{\rm bb} \gtrsim 2\times 10^4$7 erg s$T_{\rm bb} \gtrsim 2\times 10^4$8 (Ofek et al., 25 Aug 2025). This heterogeneity suggests diversity in engine properties, viewing-angle effects, or differences in envelope optical depth between AT 2018cow-like events.

5. Circumstellar Environment and Outflow Geometry

Radio modeling and late-time spectroscopy reveal that AT 2024wpp’s blastwave encountered a dense shell ($T_{\rm bb} \gtrsim 2\times 10^4$9 cm$3\times 10^4\,$0, $3\times 10^4\,$1 cm), likely created by pre-explosion or binary-driven mass loss, followed by a medium with $3\times 10^4\,$2 out to at least $3\times 10^4\,$3 cm. Such steep CSM gradients are consistently seen in other luminous radio-bright FBOTs and are interpreted as signatures of super-Eddington disk winds from compact-object progenitors or circumbinary outflows (Nayana et al., 31 Aug 2025).

The presence of two kinematic components in late-time H/He lines, the emergence of a near-infrared excess ($3\times 10^4\,$4) between 20–30 days, and polarization data together require both fast, polar and slower, equatorial outflows. The aspherical geometry is consistent with axisymmetric disk-wind models but becomes highly spherical in regions of highest optical depth (LeBaron et al., 31 Aug 2025, Pursiainen et al., 2024).

6. Outstanding Questions and Future Prospects

The case of AT 2024wpp demonstrates that FBOTs can result from engine-driven explosions that manifest classical TDE-like fallback and multi-phase accretion-driven luminosity, yet reside in massive stellar progenitors rather than galactic nuclei. The observed diversity—particularly the presence or absence of minute-scale flaring, the role of outflow geometry, and the detailed structure of the CSM—remains to be explained. Systematic multiwavelength and time-domain monitoring, as well as polarization and high-resolution spectroscopy of future events, are essential to disentangle geometrical, progenitor, and viewing-angle effects (Ofek et al., 25 Aug 2025, Pursiainen et al., 2024, Nayana et al., 31 Aug 2025, Liu et al., 24 Feb 2026). Broader samples and modeling will further clarify the relationship between FBOTs, engine-driven SNe, and stellar TDEs.