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Abstract We present the first deep X-ray observations of luminous fast blue optical transient (LFBOT) AT 2018cow at ∼3.7 yr since discovery, together with the reanalysis of the observation atδt∼ 220 days. X-ray emission is significantly detected at a location consistent with AT 2018cow. The very soft X-ray spectrum and sustained luminosity are distinct from the spectral and temporal behavior of the LFBOT in the first ∼100 days and would possibly signal the emergence of a new emission component, although a robust association with AT 2018cow can only be claimed atδt∼ 220 days, while atδt∼ 1350 days contamination of the host galaxy cannot be excluded. We interpret these findings in the context of the late-time panchromatic emission from AT 2018cow, which includes the detection of persistent, slowly fading UV emission withνLν≈ 1039erg s−1. Similar to previous works (and in analogy with arguments for ultraluminous X-ray sources), these late-time observations are consistent with thin disks around intermediate-mass black holes (withM•≈ 103–104M☉) accreting at sub-Eddington rates. However, differently from previous studies, we find that smaller-mass black holes withM•≈ 10–100M☉accreting at ≳the Eddington rate cannot be ruled out and provide a natural explanation for the inferred compact size (Rout≈ 40R☉) of the accretion disk years after the optical flare. Most importantly, irrespective of the accretor mass, our study lends support to the hypothesis that LFBOTs are accretion-powered phenomena and that, specifically, LFBOTs constitute electromagnetic manifestations of super-Eddington accreting systems that evolve to ≲Eddington over a ≈100-day timescale.
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This content will become publicly available on January 13, 2027
The Most Luminous Known Fast Blue Optical Transient AT 2024wpp: Unprecedented Evolution and Properties in the Ultraviolet to the Near-infrared
Abstract We present an extensive photometric and spectroscopic ultraviolet–optical–infrared campaign on the luminous fast blue optical transient (LFBOT) AT 2024wpp over the first ∼100 days. AT 2024wpp is the most luminous LFBOT discovered to date, withLpk ≈ (2–4) × 1045erg s−1(5–10 times that of the prototypical AT 2018cow). This extreme luminosity enabled the acquisition of the most detailed LFBOT UV light curve thus far. In the first ∼45 days, AT 2024wpp radiated >1051erg, surpassing AT 2018cow by an order of magnitude and requiring a power source beyond the radioactive56Ni decay of traditional supernovae. Like AT 2018cow, the UV–optical spectrum of AT 2024wpp is dominated by a persistently blue thermal continuum throughout our monitoring, with blackbody parameters at a peak ofT > 30,000 K andRBB/t ≈ 0.2c–0.3c. We find evidence for cooling until ∼10 days; thereafter,T ≳ 20,000 K is maintained. We interpret the featureless spectra as a consequence of continuous energy injection from a central source of high-energy emission that maintains high ejecta ionization. After 35 days, faint (equivalent width (EW) ≲ 10 Å) H and He spectral features with kinematically separate velocity components centered at 0 and −6400 km s−1emerge, implying spherical symmetry deviations. A near-infrared excess of emission above the optical blackbody emerges between 20 and 30 days, with a power-law spectrumFν,NIR ∝ ν−0.3at 30 days. We interpret this distinct emission component as either reprocessing of early UV emission in a dust echo or free–free emission in an extended medium above the optical photosphere. LFBOT asphericity and multiple outflow components (including mildly relativistic ejecta), together with the large radiated energy, are naturally realized by super-Eddington accretion disks around neutron stars or black holes and their outflows.
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- Award ID(s):
- 2406637
- PAR ID:
- 10687414
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- IoP
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 997
- Issue:
- 1
- ISSN:
- 2041-8205
- Page Range / eLocation ID:
- L10
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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