A team of astronomers has discovered a mysterious short-lived, or transient, astronomical event as bright as a superluminous supernova, but moving much faster, a study reports. Letters from the Astrophysical Journal published on July 12.
The universe is full of energetic transients, astronomical events that occur over a short period of time. For example, most massive stars end their lives with a spectacular explosion, called a supernova, a major type of transient. In order to understand the origin of these transient phenomena, various temporal investigations have been carried out over the last decades. As more and more transients have been discovered, researchers have begun to notice new types of transients in recent years.
To understand the nature of various transient phenomena, an international transient survey project called “Subaru Multiband Survey for Early-Stage Supernovae” (MUSSES), led by Ji-an Jiang, a former Kavli Institute for Physics and mathematics of the universe (Kavli IPMU) The project researcher (currently a postdoctoral fellow at the National Astronomical Observatory of Japan (NAOJ)) attempts to catch various fast-changing transients within a day of their occurrence, using the facility most powerful survey camera in the world, the Hyper Suprime-Cam (HSC) mounted on the Subaru 8.2m telescope.
By performing consecutive Subaru/HSC observations in December 2020, 20 rapidly changing transients were discovered, and one of them, MUSSES2020J (AT 2020afay), caught Jiang’s attention.
“MUSSES2020J was discovered with very low luminosity on December 11, 2020, and its luminosity showed significant brightening during our observation. More surprisingly, the rapidly changing light curve and the very high redshift of the transient confirmed by follow-up observations indicate that the luminosity of MUSSES2020J was about 50 times higher, while the rising phase was much shorter than those of normal supernovae, which indeed bear a great similarity to a particular transient recently discovered, AT 2018cow .We suggest calling these extreme transients Fast Blue Ultraluminous Transient (FBUT).So far, only a handful of them have been discovered, and we had never seen a bit after it appeared due to their extremely fast evolution. Thanks to the high-speed survey mode and the excellent performance of Subaru/HSC, we were able to perfectly catch this phenom astonishing ene for the first time. The first multiband lightcurve data provides unique insights into understanding the origin of these incredible transients,” said first author Jiang.
The data stimulated intensive discussions about the origins of MUSSES2020J and a few other FBUTs, led by various researchers on the team, including Kyoto University graduate student Kohki Uno, Kyoto University associate professor from Kyoto Keiichi Maeda, NAOJ Assistant Professor Takashi Moriya and Kavli IPMU Senior Scientist Ken’ichi Nomoto.
The theoretical investigation is still ongoing, but the team has so far narrowed the possibilities down to a few scenarios, most of which involve an active compact object – either a black hole or a highly magnetized neutron star – to power these extremely shiny objects. .
“There is almost no doubt that an active compact object is involved, and this is one of the main reasons why these transients are so different from normal supernovae. The remaining possibilities are an event where a star is disturbed by the tide by a massive black hole, or a massive star-collapse that is different from normal supernovae in that it likely left behind a highly active compact object like an accreting black hole The very early phase data provided for the first times for a class of FBUT suggest the existence of relativistic sub-output distinctly from a mass of slower ejecta, and this must be a key to solving the problem.We are currently checking the details of each model to robustly identify the origin of MUSSES2020J, with the strong constraint provided by this new observation,” says Maeda.
“MUSSES2020J shows a light curve similar to AT 2018cow. The light curve of AT 2018cow is well reproduced by the model of interaction between circumstellar matter and ejecta from a Pulsational Pair Instability Supernova (PPISN). The PPISN is the explosion of a very massive star that would collapse to form a black hole and eject the outer layer in a jet-like form, therefore it is possible that a similar PPISN pattern with a different amount of circumstellar matter could also explain the light curve of MUSSES2020J,” says Nomoto.
Jiang’s team will continue to search for the answer behind this newly confirmed transient type by performing transient surveys with telescopes around the world.
“Thanks to the large capacity of the Subaru Hyper Suprime-Cam, we were able to find a very rare mysterious transient. Since the main target of our observation was different, we could not do follow-up observations well. It may it may be a failed explosion event of a star, or it may be related to a massive black hole. Next time, as we know we can find these events, we can prepare follow-up observations to understand what type of mysterious event,” said Kavli IPMU Visiting Senior Scientist and Professor Mamoru Doi from the University of Tokyo.
Details of this study have been published in Letters from the Astrophysical Journal July 12, 2022.