AT 2022aedm and a New Class of Luminous, Fast-cooling Transients in Elliptical Galaxies

Abstract

We present the discovery and extensive follow-up of a remarkable fast-evolving optical transient, AT 2022aedm, detected by the Asteroid Terrestrial impact Last Alert Survey (ATLAS). In the ATLAS o band, AT 2022aedm exhibited a rise time of 9 ± 1 days, reaching a luminous peak with M g ≈ −22 mag. It faded by 2 mag in the g band during the next 15 days. These timescales are consistent with other rapidly evolving transients, though the luminosity is extreme. Most surprisingly, the host galaxy is a massive elliptical with negligible current star formation. Radio and X-ray observations rule out a relativistic AT 2018cow–like explosion. A spectrum in the first few days after explosion showed short-lived He ii emission resembling young core-collapse supernovae, but obvious broad supernova features never developed; later spectra showed only a fast-cooling continuum and narrow, blueshifted absorption lines, possibly arising in a wind with v ≈ 2700 km s−1. We identify two further transients in the literature (Dougie in particular, as well as AT 2020bot) that share similarities in their luminosities, timescales, color evolution, and largely featureless spectra and propose that these may constitute a new class of transients: luminous fast coolers. All three events occurred in passive galaxies at offsets of ∼4–10 kpc from the nucleus, posing a challenge for progenitor models involving massive stars or black holes. The light curves and spectra appear to be consistent with shock breakout emission, though this mechanism is usually associated with core-collapse supernovae. The encounter of a star with a stellar-mass black hole may provide a promising alternative explanation.

“Tidal Peeling Events”: Low-eccentricity Tidal Disruption of a Star by a Stellar-mass Black Hole

“…The observations of ZTF19aailpwl show a longer rise time than a typical TDE, indicating a more gradual tidal disruption than a TDE by an SMBH, e.g., produced by a microTDE with low eccentricity such as a TPE. Additionally, Nicholl et al (2023) discovered AT 2022aedm, a fast luminous transient that could possibly be explained by an encounter between a star and a stellar-mass BH, but the rise time of this event (∼9 days) is much longer than the typical timescale of TPEs according to our simulation results. Finally, the rate of microTDEs is expected to be low in AGNs, at roughly 2 Gpc −3 yr −1 (Yang et al 2022), and even lower in star clusters or stellar triple systems with BHs, although these predictions have large uncertainties.…”

Discovery of the Optical and Radio Counterpart to the Fast X-Ray Transient EP 240315a

“…However, given the continuing rapid fade, AT 2024eju was only detected in i band. The derived magnitudes and 2σ upper limits from the LT images were estimated using the Python-based Photometry Sans Frustration code 15 (Nicholl et al 2023).…”

SN 2022jli: A Type Ic Supernova with Periodic Modulation of Its Light Curve and an Unusually Long Rise

“…Photometry in griz bands was calibrated against Pan-STARRS field stars (Flewelling et al 2020), and BV-band photometry was calibrated using the APASS catalog (Henden et al 2016). LT u-band measurements were performed using the PSF package 37 (Nicholl et al 2023) and calibrated against the Sloan Digital Sky Survey (SDSS; Alam et al 2015) catalog.…”