The Most Energetic Transients: Tidal Disruptions of High-Mass Stars

Abstract

We present the class of extreme nuclear transients (ENTs), including the most energetic single transient yet discovered, Gaia18cdj. Each ENT is coincident with its host-galaxy nucleus and exhibits a smooth ($<$$10$% excess variability), luminous ($2\times$$10^{45}$ to $7\times$$10^{45}$ erg s$^{-1}$), and long-lived ($>$$150$ days) flare. ENTs are extremely rare ($\geq$$1$$\times$$10^{-3}$ Gpc$^{-3}$ yr$^{-1}$) compared to any other known class of transients. They are at least twice as energetic ($0.5\times10^{53}$ to $2.5\times10^{53}$ erg) as any other known transient, ruling out supernova origins. Instead, the high peak luminosities, long flare timescales, and immense radiated energies of the ENTs are most consistent with the tidal disruption of high-mass ($\gtrsim$$3$ M$_{\odot}$) stars by massive ($\gtrsim$$10^8$ M$_{\odot}$) supermassive black holes (SMBHs). ENTs will be visible to high redshifts ($z\sim4$ to $6$) in upcoming surveys, providing an avenue to study the high-mass end of the SMBH mass distribution, complementing recent studies of actively accreting SMBHs at high redshifts with the James Webb Space Telescope.

Figures (17)

• Figure 1: Optical and IR light curves of Gaia16aaw (left), Gaia18cdj (middle), and AT2021lwx (right). These light curves are corrected for Galactic foreground extinction, but have not had any host contribution removed. Shown are DES (circles, $griz$), ATLAS (pentagons, $co$), CRTS (squares, $V$), Gaia (hexagons, $G$), ZTF (octagons, $gr$), and WISE (diamonds, $W1W2$). Downward-facing triangles indicate 3$\sigma$ upper limits for filters of the same color. All data are in the AB magnitude system.
• Figure 2: Spectra of the ENTs Gaia16aaw, Gaia18cdj, and AT2021lwx (in red shades) as compared to other luminous transients. More luminous events are towards the top of the figure. The comparison objects are a composite AGN/QSO spectrum from SDSS vandenberk01, ASASSN-15lh (purple; dong16leloudas16), the ANTs AT2019brs frederick21, ASASSN-17jz holoien21, and ASASSN-18jd neustadt20 (orange shades), the TDE candidates PS16dtm blanchard17 and AT2019dsg vanvelzen21 (blue shades), the SLSNe SN2020qlb west23 and SN2018lzi pessi24 (violet shades), and PS1-10adi kankare17 (pink). Atmospheric telluric features are marked with an $\oplus$. Vertical dashed lines mark H, He, and Mg features in red, blue, and purple respectively. The spectra are scaled and offset as needed to enhance readability.
• Figure 3: UV/optical bolometric light curves of the ENTs Gaia16aaw, Gaia18cdj, and AT2021lwx (in red shades) as compared to other luminous transients. The comparison objects are ASASSN-15lh (purple; dong16leloudas16), the ANTs AT2019brs frederick21, ASASSN-17jz holoien21, and ASASSN-18jd neustadt20 (orange shades), the TDE candidates PS16dtm blanchard17 and AT2019dsg vanvelzen21 (blue shades), the SLSNe SN2020qlb west23 and SN2018lzi pessi24 (violet shades), and PS1-10adi kankare17 (pink). All data have had the host contribution removed and are corrected for Galactic foreground extinction.
• Figure 4: Optical absolute magnitude as compared to the characteristic timescale for a range of transient classes. Blue regions show various types of supernovae, green regions show classes of stellar mergers and/or mass transfer, and red-shaded regions show events powered by accretion onto SMBHs. The overluminous nuclear transient ASASSN-15lh is shown as a purple plus and our sample of ENTs are presented as red symbols.
• Figure 5: Host-galaxy star formation rate as compared to stellar mass. The red points are the ENTs Gaia16aaw, Gaia18cdj, and AT2021lwx. The blue points are a comparison sample of TDEs and the gold points are a comparison sample of ANTs. The background gray points are samples of local galaxies from SDSS (A) and z $\sim 1$ galaxies from COSMOS (B). In the two outer panels, we show normalized KDEs of the distributions for the various samples, excluding the limit for AT2021lwx in the ENT KDEs. The black dashed line indicates the star-forming main sequence at $z = 1$.