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

Abstract

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.

Figures (20)

• Figure 1: Left: PS1 $gri$ composite image of AT2019cmw's host. Right: LT IO:O $gri$ composite image taken $\sim4.8$ days post-peak in AT2019cmw's rest-frame.
• Figure 2: A combined plot of $ugriz$ photometry from LT IO:O, ZTF $gri$ forced photometry, ATLAS $co$ forced photometry, and $uvw1$, $uvm2$ and $uvw2$ UVOT photometry from Swift. An offset has been applied to all photometric bands except r in order to show each band separately. ZTF and LT $gri$ photometry, as well as ATLAS $co$ photometry has been overlaid. ZTF forced photometry $3\sigma$ upper limits have also been plotted up to 50 days before and after the first and last significant forced photometry detections respectively.
• Figure 3: Vega-magnitude plot of NEOWISE W1 (blue) and W2 (orange) photometry at the location of AT2019cmw from MJD 56948.07 to MJD 59850.88, with AT2019cmw's earliest ZTF forced photometry detection at MJD 58558.47 marked by the black vertical dashed line. Datapoints show NEOWISE photometry values found using the method outlined in Section \ref{['sec:NEOWISEPhotometry']}, with error bars corresponding to $1\sigma$ errors. Solid horizontal lines and shaded regions show ALLWISE W1 and W2 photometry values in the same colours.
• Figure 4: Spectra of AT2019cmw from Palomar P60, Palomar P200, LDT and Keck LRIS in descending order of their observation, with time in days since first ZTF forced photometry detection in the AT2019cmw's rest frame. The positions of key spectral lines from the classification scheme outlined by van_velzen_seventeen_2021 have been labelled. $\mathrm{Mg II}$ absorption and the K, H and G Fraunhofer lines of calcium have also been labelled. Late-time LRIS spectra are host galaxy dominated. Also plotted for comparison are an LDT spectrum of the featureless TDE AT2018jbv hammerstein_final_2023, an SDSS QSO composite spectrum vanden_berk_composite_2001 and two WHT ISIS spectra of the SLSN-I SN 2010gx pastorello_ultra-bright_2010. We also show a combined SOAR Goodman and Magellan-Baade FIRE spectrum of the ENT Gaia18cdj hinkle_most_2025, taken $\sim2$ years and $\sim3.5$ years post-first detection in the transient's rest-frame respectively. Spectra have been normalised to their respective mean rest-frame flux between $\mathrm{4000\,{\text{\normalfont\AA}}}$ and $\mathrm{5000\,{\text{\normalfont\AA}}}$.
• Figure 5: Pseudo-blackbody temperatures (top), radii (middle) and luminosities (bottom) of AT2019cmw and four featureless TDEs (AT2018jbv, AT2020riz, AT2020qhs and AT2020ysg) from hammerstein_final_2023, with the most luminous TDE with spectral features from their sample (AT2020ddv, classified as TDE-He) also included for comparison). Values for AT2018jbv were derived using the method described in Section \ref{['sec:Blackbody']}, with the closest to peak modelled epoch ($\sim7$ days post-peak) epoch used an an initial value. The SLSNe-I events SN2018hti and SN2019neq from chen_hydrogen-poor_2023 are also displayed. SN2018hti's data has been smoothed for better visual clarity. One data point for AT2019cmw fit using LT $griz$ and Swift UV data $\sim14.5$ days post-peak is significantly deviating from the general trend in both temperature and radius.