AT2019cmw: A highly luminous, cooling featureless TDE candidate from the disruption of a high mass star in an early-type galaxy

We present optical/UV photometric and spectroscopic observations, as well as X-ray and radio follow-up, of the extraordinary event AT2019cmw. With a peak bolometric luminosity of ~$\mathrm{10^{45.6},erg,s^{-1}}$, it is one of the most luminous thermal transients ever discovered. Extensive spectroscopic follow-up post-peak showed only a featureless continuum throughout its evolution. This, combined with its nuclear location, blue colour at peak and lack of prior evidence of an AGN in its host lead us to interpret this event as a `featureless’ tidal disruption event (TDE). It displays photometric evolution atypical of most TDEs, cooling from ~30 kK to ~10 kK in the first ~300 days post-peak, with potential implications for future photometric selection of candidate TDEs. No X-ray or radio emission is detected, placing constraints on the presence of on-axis jetted emission or a visible inner-accretion disk. Modelling the optical light curve with existing theoretical prescriptions, we find that AT2019cmw may be the result of the disruption of a star in the tens of solar masses by a supermassive black hole (SMBH). Combined with a lack of detectable star formation in its host galaxy, it could imply the existence of a localised region of star formation around the SMBH. This could provide a new window to probe nuclear star formation and the shape of the initial mass function (IMF) in close proximity to SMBHs out to relatively high redshifts.

💡 Research Summary

AT2019cmw is an extraordinary nuclear transient discovered by the Zwicky Transient Facility (ZTF) and ATLAS in early 2019. Located at a redshift of z = 0.519 in an early‑type host galaxy, the event reached a peak bolometric luminosity of ≈10^45.6 erg s⁻¹, placing it among the most luminous thermal transients ever recorded. Multi‑wavelength follow‑up—including optical/UV photometry from ZTF, ATLAS, the Liverpool Telescope, and Swift/UVOT, as well as spectroscopy from several facilities—revealed a strikingly featureless continuum that persisted for the entire ∼300 day post‑peak monitoring period. No broad emission lines (e.g., He II λ4686, H α) typical of optically selected tidal disruption events (TDEs) were detected, leading the authors to classify AT2019cmw as a “featureless” or “no‑spectral‑features” TDE.

The photometric evolution is atypical for TDEs. Blackbody fits to the UV/optical data show a rapid cooling from ∼30 kK at peak to ∼10 kK over roughly 300 days, accompanied by a shrinking photospheric radius. Most known TDEs exhibit relatively constant temperatures after peak, whereas AT2019cmw’s temperature decline is reminiscent of the cooling observed in some super‑luminous supernovae but unprecedented among TDEs. The authors interpret this behavior with a “cooling envelope” model, wherein the expanding debris envelope radiates its internal energy while its effective emitting area contracts with time (R ∝ t⁻⁰·⁵). Bayesian fitting using the Redback framework yields a disrupted star mass of 20–50 M⊙ and a supermassive black hole (SMBH) mass of ∼10⁷ M⊙, implying that a high‑mass star was torn apart.

X‑ray observations with Swift/XRT and radio observations with the VLA resulted in non‑detections, providing upper limits of L_X < 10^42 erg s⁻¹ and L_radio < 10^38 erg s⁻¹. These limits rule out a bright, on‑axis relativistic jet and suggest that either an inner accretion disk is absent, heavily obscured, or radiatively inefficient. The lack of X‑ray emission also distinguishes AT2019cmw from the subset of TDEs that are X‑ray bright, reinforcing the notion that the observed UV/optical light is likely powered by reprocessing of hidden high‑energy emission in an optically thick outflow rather than direct disk radiation.

The host galaxy shows no detectable star formation (SFR < 0.01 M⊙ yr⁻¹) and is dominated by an old stellar population, yet the inferred high‑mass progenitor suggests a localized episode of recent star formation near the nucleus. This could indicate the presence of a nuclear star cluster or a brief burst of star formation that produced massive stars in the immediate vicinity of the SMBH. If such a top‑heavy initial mass function (IMF) exists near galactic nuclei, it would have profound implications for our understanding of black‑hole feeding, stellar dynamics, and the evolution of galactic cores.

From a survey perspective, AT2019cmw challenges current TDE selection criteria. Traditional optical searches rely on (i) nuclear location, (ii) blue colour, (iii) relatively stable blackbody temperatures, and (iv) the presence of broad UV/optical emission lines. AT2019cmw satisfies (i) and (ii) but violates (iii) and (iv). Its rapid colour evolution and lack of spectral features mean that many automated pipelines could miss similar events. The authors therefore advocate for incorporating time‑dependent colour changes, rapid luminosity declines, and allowance for featureless spectra into future transient classification algorithms, especially for upcoming large‑scale surveys such as LSST.

In summary, AT2019cmw represents the first well‑observed case of a highly luminous, featureless, rapidly cooling TDE that likely originates from the disruption of a massive (tens of solar masses) star by a ∼10⁷ M⊙ SMBH. The event provides a rare window into nuclear star formation, the possible top‑heavy IMF in galactic centers, and the physics of optical/UV emission in TDEs lacking X‑ray signatures. Continued monitoring of similar transients and refined selection techniques will be essential for building a statistically robust sample, which in turn will illuminate the interplay between massive stars, black holes, and the evolution of galaxy nuclei.