AT 2024wpp: the most luminous fast-evolving optical transient linked to the merger explosion of a black-hole binary

Abstract

Fast blue optical transients (FBOTs) represent one of the most exotic astrophysical transients, exhibiting unusually strong emission across X-ray, optical, and radio wavelengths. Their physical origins remain highly debated, with proposed explanations ranging from stellar explosion to tidal disruption event (TDE). Here we report observations of the most luminous FBOT, AT 2024wpp whose post-peak luminosity rebrightens in X ray and becomes flattening in optical in a manner follows the decay rate characteristic of TDEs ($L_{\rm bol} \propto t^{-5/3}$). This invokes energy contribution of accretion by a central compact object, getting further corroborations from hardening of X-ray spectral index and detection of outflow inferred from the emission lines at similar phase. Detailed modeling of luminsoity evolution favors a coalesce explosion of a 34 M$_{\odot}$ Wolf-Rayet star with a 15 M$_{\odot}$ black hole (BH), demonstrating that some FBOTs may be associated with TDE of a stellar blackhole.

Figures (14)

• Figure 1: AT 2024wpp and its host galaxy at $z = 0.0862$. (a) A Sloan $gri$-band composite image of AT 2024wpp and its host, obtained on 7 Dec. 2024 using the the 10.4 m Gran Telescopio Canarias. The coordinate axes are equatorial coordinates (J2000). (b) The host-galaxy spectrum taken with GTC on 29 Oct. 2024, which has been corrected with a redshift of 0.0862 estimated using a Gaussian fit to the H$\alpha$ emission line. The inset panel shows the zoomed-in region near the H$\alpha$ line whose rest-frame wavelength is indicated by a red dashed line, together with the redshift-corrected Gaussian profile in red. (c) The ultraviolet $UVW1$ and optical $r$ light curves of AT 2024wpp and the comparison objects, including Type Ibn SN 2019kbj2023ApJ...946...30B and AT 2018cow2018ApJ...865L...3P2019MNRAS.484.1031P2021ApJ...910...42X. The light curve of each object has been corrected for reddening and transformed to absolute magnitudes. The vertical red lines represent the phases of spectral observations.
• Figure 2: Optical spectral evolution of AT 2024wpp. The spectra are rebinned to 10 Å to increase the S/N. (a) The optical spectra of AT 2024wpp spanning phases from $t\approx +6.4$ days to +65.0 days after the explosion. All spectra have been corrected for reddening and host-galaxy redshift. Spectra taken with different telescopes are denoted in different colors as indicated at the top legend. Telluric absorption lines, visible in some of the spectra, are marked with an "Earth" symbol and grey vertical bands. (b) and (c): Zoomed-in regions indicated by the purple frames in panel (a). The potential line regions are highlighted. The vertical lines mark the wavelength positions of lines indicated in the labels, corresponding to velocities from $-$8000 to $-$4000 km s$^{-1}$ with an interval of 1000 km s$^{-1}$. The phase of each spectrum is shown on the right side.
• Figure 3: The X-ray, thermal, and radio luminosity evolution of AT 2024wpp. Top: The luminosity evolution in thermal, X-ray, and radio bands as well as an overall comparison of blackbody evolution with that of AT 2018cow2021ApJ...910...42X. Open and filled inverted triangles show the upper limits of EP-FXT and radio luminosities, respectively. Power-law fits to the bolometric light curve at three epochs, $+5.8$ to $+11$ days, $+11$ to $+35$ days, and $+35$ to $+60$ days, are overplotted as a black solid line, a green dashed line, and a dark-blue dash-dotted line, respectively. Middle: Temperature evolution and the comparison with AT 2018cow. Bottom: Photospheric radius evolution. Blue dotted lines show variation of photosphere size given different assumed velocities.
• Figure 4: The WR-BH merger-driven explosion scenario invoked for AT 2024wpp. (a) The WR-BH merger scenarioMetzger2022ApJKlencki2025arXiv, in which the WR companion is disrupted and accreted onto the BH, resulting in a luminous explosion. At t$\gtrsim 30$ days post explosion, a portion of gravitationally bound equatorial outflow falls to the BHPejcha2016MNRAS, leading to the X-ray rebrightening and flattening in optical. (b) Upper panel: The red and blue lines represent the best-fit models for the bolometric and X-ray light curves, respectively. The model consists of several emission components: the bolometric luminosity includes contributions from the initial shock interaction between the low-mass ejecta and the He-rich CSM, supplemented by the reprocessing of X-rays ($L_{\rm bolo, \, ej}$; orange dot-dashed line), the interaction between the CSM and the nonpolar slow outflow ($L_{\rm bolo, \, w}$; orange dotted line), and the emission from the second BZ jet caused by fallback accretion ($L_{\rm bolo, \, fb}$; grey dotted line). The X-ray emission includes contributions from the primary ($L_{\rm X, \, BZ–1}$; grey dot-dashed line) and second ($L_{\rm X, \, BZ–2}$; grey dotted line) jets powered by the BZ mechanism. Lower panel: The fit residuals, expressed as the ratio of the observed data to the best-fit model values.
• Figure S1: UV and optical light curves of AT 2024wpp until $\sim+$70 days relative to the estimated time of first light. Different symbols represent various facilities that were used to obtain the photometry (see top legend). Data in different filters are shown with different colors and shapes, and are shifted vertically for clarity, as indicated in the left legend. The data point with a downward arrow represents the nondetection limit of the ZTF $g$ band 2024TNSTR3719....1S, obtained 0.623 days before the earliest detection by GOTO $L$ and 0.935 days by ZTF $g$ (\ref{['table:all_lc']}); we assumed the first-light time to be the average of these two epochs, $t_0=60,578.75\pm0.31$ (Sep. 25.75). Note that, in contrast to Ref.2024TNSTR3719....1S, Ref.Perley2026arXiv reported a low S/N detection of the ZTF $g$ band on MJD 60,578.437, which is close to the $t_0$ adopted in our analysis. Our conclusions are robust against variations in the adopted explosion time.