AT2024wpp: An Extremely Luminous Fast Ultraviolet Transient Powered by Accretion onto a Black Hole

Abstract: We present the discovery of AT 2024wpp ("Whippet"), a fast and luminous 18cow-like transient. At a redshift of z=0.0868, revealed by Keck Cosmic Web Imager spectroscopy of its faint and diffuse star-forming host, it is the fourth-nearest example of its class to date. Rapid identification of the source in the Zwicky Transient Facility data stream permitted ultraviolet-through-optical observations to be obtained prior to peak, allowing the first determination of the peak bolometric luminosity (2x10^45 erg/s), maximum photospheric radius (10^15 cm), and total radiated energy (10^51 erg) of an 18cow-like object. We present results from a comprehensive multiwavelength observing campaign, including a far-UV spectrum from the Cosmic Origins Spectrograph on the Hubble Space Telescope and deep imaging extending >100 days post-explosion from the Very Large Telescope, Hubble Space Telescope, Very Large Array, and Atacama Large Millimetre Array. We interpret the observations under a model in which a powerful rapidly-accreting central engine blows a fast (~0.15c) wind into the surrounding medium and irradiates it with X-rays. The high Doppler velocities and intense ionization within this wind prevent any identifiable features from appearing in the ejecta or in the surrounding circumstellar material, even in the far-ultraviolet. Weak H and He signatures do emerge in the spectra after 35 days in the form of double-peaked narrow lines. Each peak is individually narrow (full width ~3000 km/s) but the two components are separated by ~6600 km/s, indicating stable structures of denser material, possibly representing streams of tidal ejecta or an ablated companion star.

• The paper demonstrates that AT2024wpp is an 18cow-like transient with a record peak luminosity of 2×10^45 erg s⁻¹ reached in just 2 days.
• The study uses multiwavelength observations to reveal ultrafast photospheric expansion, persistent high temperatures, and late-time X-ray rebrightening indicative of a central engine.
• The analysis constrains progenitor properties and CSM density, challenging conventional SN models and supporting scenarios like binary mergers or tidal disruptions.

AT2024wpp: An Extremely Luminous Fast Ultraviolet Transient Powered by Accretion onto a Black Hole

Discovery and Classification

The paper presents the multiwavelength discovery and analysis of AT2024wpp, an 18cow-like Luminous Fast Blue Optical Transient (LFBOT) at $z=0.0868$. AT2024wpp is identified as the fourth-nearest member of the empirically defined LFBOT class, characterized by rapid optical evolution ($t_\mathrm{rise}\lesssim 4$ days), extreme blue color, featureless hot spectra spanning UV through optical, luminous X-ray emission, and pronounced radio/mm counterparts. The alert triggering in the Zwicky Transient Facility enabled unprecedented temporal coverage, including early ultraviolet and optical measurements prior to peak. This facilitated the first direct constraints on bolometric peak ($2\times10^{45}$ erg s$^{-1}$), maximum photospheric expansion ($10^{15}$ cm), and total radiated energy ($10^{51}$ erg) in an object of this class.

Temporal and Spectral Evolution

Photometric Behavior

AT2024wpp displays a power-law post-peak decline in both UV and optical bands, with $F_\nu\propto t^{-3}$ ($t^{-2.7}$) for ultraviolet (optical) after $t\sim10$ days. Even at late times ($t_{rf}\sim 111$ days), the color remains extremely blue, $g-i\sim-0.3$. Peak bolometric luminosity ($L_\mathrm{bol}\sim2\times10^{45}$ erg s$^{-1}$) occurs on a rise time $t_\mathrm{rise}\sim2$ days, defining AT2024wpp as both the fastest and most luminous known non-relativistic thermal transient. Total radiated energy is $\sim10^{51}$ erg, comparable to engine-powered superluminous SNe.

Blackbody Parameterization

Initial photospheric radius expansion is ultrarelativistic ($v_\mathrm{ph}\sim 0.15c$) for several days, transitioning to a long phase of monotonic contraction, reaching compact radii ($R_\mathrm{ph}\sim 10^{14}$ cm) after $\sim 100$ days. Temperature remains hot ($T_\mathrm{ph}\sim 2\times10^4$ K) throughout, and SEDs are well-modeled by a blackbody at all epochs, with minor near-IR excess after 20 days, analogous to dust echoes previously observed in AT2018cow.

High-Energy and Radio Evolution

Swift X-ray observations reveal persistent emission ($L_X\sim10^{43}$ erg s$^{-1}$ at peak), fading below detectability by $\sim 15$ days, but rebrightening after $\gtrsim 40$ days with spectral hardening. Multi-epoch VLA and ALMA radio monitoring reveals a mildly relativistic shock ($\beta_\mathrm{sh}\sim0.15-0.2c$) and an ambient medium with density profile $n_\mathrm{e}\propto r^{-3}$. The radio SED peaks maintain nearly constant frequency for $\sim 100$ days, followed by rapid fading.

Spectroscopic Constraints

Early-time UV/optical spectra ($t<30$ days) are featureless, lacking both emission/absorption lines at the transient or host redshift, including deep HST/COS far-UV exposures. After $t\sim35$ days, the spectra develop weak H and He emission features, all double-peaked with narrow widths ($\delta v\sim 3000$ km~s$^{-1}$) and component separation $\Delta v\sim 6600$ km~s$^{-1}$. This is inconsistent with standard homologous SN ejecta, indicating persistent, stable, high-velocity density structures, possibly tidal streams or ablated companions.

Physical Interpretation

Engine Model

Observational properties are consistent with a scenario in which a rapidly-accreting central engine (black hole) drives an ultrafast wind ($v_\mathrm{wind}\sim0.15c$), intensely irradiated by X-rays. The engine power is large enough to maintain high temperatures and ionization in the inner ejecta and CSM, with X-rays producing sufficient ionization to suppress both absorption and emission line formation. The lack of features in UV even at late times is quantitatively consistent with photoionization transparency arguments, provided X-ray luminosities and inferred densities.

Shock and Mass Loss

Early optical evolution can be explained by efficient conversion of kinetic energy to radiation via CSM interaction at large radius ($R_\mathrm{ph}\sim10^{15}$ cm) with ejecta mass $M_\mathrm{ej}\sim0.1 M_\odot$ and kinetic energy $E_\mathrm{kin}\sim 4\times10^{51}$ erg. The inferred mass-loss rate and density profile ($n_e\propto r^{-3}$) align with extreme wind scenarios or binary merger-driven mass ejection, supporting recently proposed models for binary massive star / BH mergers [Metzger+2022].

Host Galaxy and Progenitor Environment

The host is a faint star-forming spiral with subsolar metallicity, yet AT2024wpp exploded outside the brightest star-forming regions. SED fitting yields moderate age ($\sim$1.7 Gyr), stellar mass ($M_*\sim6\times10^8 M_\odot$), and SFR ($\sim 0.075\ M_\odot$ yr$^{-1}$), typical of other LFBOT hosts, excluding strict association with very massive, young stellar clusters.

Rate Implications

The occurrence rate for AT2018cow/AT2024wpp-like events is constrained to 0.9--12.5 yr$^{-1}$ Gpc$^{-3}$, $\lesssim 0.01\%$ of the core-collapse SN rate, solidifying LFBOTs as among the rarest extragalactic transients. The rate for AT2024wpp-luminosity events is even lower $<0.7$ Gpc$^{-3}$ yr$^{-1}$.

Contradictory Claims and Strong Results

• Peak luminosity ($2\times10^{45}$ erg s$^{-1}$ within 2 days) exceeds all previously documented thermal transients, challenging conventional SN/CSM interaction energetics.
• Spectral feature suppression even in far-UV is explained not as lack of dense material, but as extreme ionization and heating by a central engine—contradicting CSM interaction-driven models for most engine-powered SNe.
• Double-peaked, narrow late-time lines require persistent high-velocity structure in the ejecta/CSM, inconsistent with spherical or homologous expansion scenarios.
• Radio/mm shock speeds ($\beta_\mathrm{sh}\sim0.15$–$0.2c$) and CSM density profile steeper than steady winds suggest progenitor mass loss timescale of years, compatible with binary merger-driven outflows or tidal disruption.

Implications and Future Directions

AT2024wpp firmly establishes that a subset of LFBOTs are powered by accretion onto black holes, with the dominant transient output driven by central engine irradiation rather than isotropic SN explosion energy. The phenomenology implies that both massive-star collapse and tidal disruption/merger scenarios are physically plausible, but current constraints are insufficient to distinguish unambiguously. The observed radiative efficiency and spectral feature suppression demand high accretion rates over extended timescales and large-scale ionized outflows.

The detailed properties of the radio SED and X-ray rebrightening indicate the need for time-dependent, multi-zone shock and wind models, including non-spherical geometry, radiative transfer, and variable engine output. The double-peaked late-time emission features call for hydrodynamic modeling of asymmetric mass loss and binary interactions prior to collapse/disruption.

Continued surveys, high-cadence multiwavelength follow-up, and ultraviolet spectroscopic coverage will be critical for mapping the population properties and refining the physical models. The low volumetric rate ensures that wide-field instruments (e.g., Rubin LSST, ULTRASAT) will remain essential for LFBOT discovery. More luminous or faster events may yet exist, indicating unexplored parameter space in transient astrophysics.

Conclusion

AT2024wpp exhibits extreme radiative output on rapid timescales, a persistent and featureless high-ionization spectrum, and unprecedented multiwavelength coverage, supporting a model of accretion-powered, engine-dominated transients. Observational evidence for central engine irradiation, ultrafast outflows, and complex CSM/ejecta interaction challenge existing paradigms of SN and TDE physics and motivate continued theoretical and observational advances to elucidate the origins and mechanisms of LFBOTs.