Boring Galaxy, Surprisingly Big Booms

Title: AT2022aedm and a new class of luminous, fast-cooling transients in elliptical galaxies

Authors: M. Nicholl, S. Srivastav, M.D. Fulton, et al.

First Author’s Institution: Queens University Belfast, United Kingdom

Status: Published in The Astrophysical Journal

The elliptical galaxy looms overhead, a vast city of stars that glows like the yellow lamps on the side of the roadway. In the distance, a blinding flash: an explosion that makes even the most luminous of supernovae look like a mere firecracker. Where once there was peace and tranquility there now is unimaginable violence, the final cry of a dying star. But what kind of star was it? A white dwarf martyred on the altar of an accretion disk? A pair of neutron stars, doomed to tear each other to shreds? Or perhaps even a still-fusing star whose life is cut tragically short by the ravenous appetite of a black hole? Today’s paper addresses this mysterious event and others like it, and discusses the possible identity of the unlucky star that died on that fateful day.

Less of a Death Rattle, More of a Death Detonation

On December 30th 2022, a transient event labeled AT2022aedm was discovered by the asteroid-monitoring program ATLAS. It had an absolute magnitude of -21.5 mag compared to the -19 mag magnitude of a typical supernova (note the more negative a magnitude is, the brighter the object is). Its intensity rose and fell on timescales faster than supernovae, rising from half brightness to peak brightness in only 6.6 days compared to 10-40 days for supernovae. Similarly bright and fast-evolving explosions have been called Fast Blue Optical Transients (FBOTs) or Rapidly Evolving Transients (RETs), with the most famous being called “Cow-like” events, after the prototypical example AT2018cow. But AT2022aedm seems to be in a league of its own. It is much more luminous than typical RETs, and it shows an unusual color evolution, becoming redder as time passes. It is undetected in radio waves, unlike AT2018cow, which was bright in radio waves. AT2022aedm lacks X-ray emission as well. The light curves of AT2022aedm are compared with other types of stellar death explosions in Figure 1. The only transients comparable in luminosity, rise time, and decay time to it are AT2020bot, and the enigmatic “Dougie.”

Figure 1: Bolometric luminosities of various transients plotted over time vs AT2022aedm (shown in black points). Shown for AT2022aedm are luminosities derived from combinations of ATLAS, Pan-STARRs (griz),  UV data, and VISTA Kilo-degree Galaxy Survey (JHL). The bolometric luminosities of Dougie and AT2020bot are plotted for comparison, along with  the luminosities of AT20218cow, a Type 1a supernova (SN2011fe) and luminosities from a  general population of superluminous supernovae (SLSNe). The bolometric luminosities are marked as “pseudo” because they were technically only derived from a subset of the electromagnetic spectrum.  Adapted from Fig. 3 in the paper

Spectroscopically, AT2022aedm is also an enigma. It doesn’t have the broad emission or absorption lines prominent in supernovae. Its spectrum is quite featureless. RETs like AT2018cow do have featureless spectra right after their initial explosion, but they typically show clear hydrogen and helium emission lines as time goes on. In contrast, even 15 days after the explosion, AT2022aedm still doesn’t have these lines. The spectrum of AT2022aedm is compared to those of other transients in Figure 2. Dougie shows a similarly featureless spectrum, while AT2020bot possesses weak, unidentifiable features.

Figure 2: Spectra of various transients versus AT2022aedm at varying times since the start of their respective explosions. The spectrum for AT2022aedm is shown at both 3.5 days from the explosion and 14.9 days from the explosion, as black lines. Ibn, IIn, Icn, Ic, and SLSN represent different varieties of core-collapse supernovae. Note how AT2022aedm lacks the emission and absorption features found in the other transients.  Adapted from Fig. 4 in the paper

Gathering data from across the optical, UV, and NIR parts of the spectrum, the authors estimated the full bolometric luminosity of AT2022aedm to be a star-shattering 1045 erg/s (for comparison the luminosity of the sun is about 1033 erg/s). This blast from the past appears to have occurred in the outskirts of an elliptical galaxy, LEDA 1245338, which is extremely surprising. Transients of this luminosity almost exclusively occur in blue galaxies with conspicuous amounts of gas and significant star formation. This is because the most luminous transients come from the death of massive stars, which don’t live long enough to outlive the epochs of star formation in which they form. AT2020bot and Dougie are the main outliers in this regard, as they too occurred in elliptical galaxies. All of these unusual properties that the three transients share in common suggests they form a new class, but what does this mean?

Figure 3: The host galaxy of AT2022aedm and the location of AT2022aedm within that galaxy marked with a crosshairs. Note the host galaxy is elliptical and featureless (without any spiral arms or gas clouds) and AT2022aedm is not located in the center of the host galaxy.  Adapted from Fig. 5 in the paper

A Mysterious Addition to Star Heaven

One possibility for the cause of AT2022aedm is that it is the result of a star being ripped to pieces by a supermassive black hole in a so-called tidal disruption event (TDE). But models of TDEs have trouble fitting AT2022aedm’s light curve, and its location on the outskirts of its host galaxy (see Figure 3) challenges the idea that a supermassive black hole is the culprit. The old age of LEDA 1245338’s stellar populations makes an exploding white dwarf (Type 1a supernova) a potentially appealing progenitor, but type 1a supernovae shine primarily from the decay of radioactive nickel, cobalt and iron. This would introduce iron-group element absorption lines into AT2022aedm’s spectrum, and these are not seen. Unusual core-collapse events, such as a star imploding into a magnetar, are highly unlikely again because of the requirement of a massive star. The properties of AT2022aedm could be explained by a shock wave from the explosion of the progenitor colliding with gas ejected from the progenitor star, but this does not help explain what the progenitor actually was.

 The demise of a neutron star would not create enough light; neutron stars are simply too small. The merger of two white dwarfs of carbon-oxygen and oxygen-neon-magnesium composition would create a magnetar that could power AT2022aedm’s copious energy. But this would create lots of non-thermal emission seen in the radio and X-ray, one thing AT2022aedm conspicuously lacks.

The final possibility the authors discuss is that AT2022aedm is a merger or tidal disruption of a main sequence star by a neutron star or stellar black hole.The closest match is provided by a stellar black hole disrupting a main sequence star, but existing models for that scenario have the problem of producing too much luminosity in the UV and X-ray. The authors conclude that more investigation of stellar black hole TDEs is needed to better understand to explain AT2022aedm’s origin.

Looking for certainty in science is often a losing proposition. While we typically think of science and math as places where there is only one right answer to any given question, most of the time on the cutting edge, the fog of uncertainty clouds our view of the path ahead. Sometimes it hides even the brightest things in the universe.

Astrobite edited by Abigail Lee

Featured image credit: NASA, ESA, NSF’s NOIRLab, Mark Garlick , Mahdi Zamani

About Lynnie Saade

I'm a high energy astrophysics postdoc that uses X-rays to study the most extreme objects in the Universe, black holes and neutron stars. I have an unusual hobby of drawing comics and writing stories about personified natural phenomena. I really want to see a story with a black hole being used as an actual character, just as how they (almost) are characters with great impact on their galaxies in real life.

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