Seven years ago, an international collaboration of astronomers installed a state-of-the-art camera on a robotic telescope at the Palomar Observatory near San Diego. Today, this collaboration, named the "
Zwicky Transient Facility" (ZTF), announces that it has made over 10,000 detections of cosmic supernovae, by far the largest census ever conducted. The goal is to measure the history of the Universe's expansion with ever-increasing precision.
Artist's view of a supernova - M. Weiss
"
There are billions of stars in the Universe, and about every second, one of them explodes. ZTF detects hundreds of these explosions every night, and a handful of them are later confirmed as supernovae. By systematically doing this for seven years, we have obtained the most complete record of confirmed supernovae to date," explains Christoffer Fremling, an astronomer at the California Institute of Technology (Caltech), who leads the BTS (Bright Transient Survey) program of ZTF, dedicated to the search for supernovae.
The Bright Transient Survey is currently the primary source of discovery of these cosmic flashes—called "transient events" in the world. ZTF shares a stream of transient event detections with the entire astronomical community for further study through spectral analysis. This analysis reveals the distance, type, and other physical properties of a transient event.
Most events in the BTS sample are classified into one of the two most common types: Type Ia supernova, when a white dwarf steals matter from a neighboring star until it explodes, or Type II supernova, when a massive star dies and collapses under its own gravity. Thanks to these data, astronomers are now better equipped to investigate the workings of dark energy and understand the processes at play in the death of stars.
"
With ZTF, we have now collected more Type Ia supernovae than all the surveys of the past thirty years had provided," notes Mickaël Rigault, head of the ZTF group in France and the "Cosmology" program of the collaboration.
About 70% of the supernovae in the BTS study were detected and classified by the ZTF team thanks to the complementary work of two telescopes at the Palomar Observatory near San Diego. The first is the 1.2-meter (4-foot) diameter Samuel Oschin Telescope. It scans the entire visible sky every two nights using a 60-million-pixel wide-field camera mounted on its focus. To detect new astronomical events, astronomers subtract images of the same portion of the sky from subsequent scans.
The supernova SN 1994D (the bright white dot at the bottom left of the image), in the outer part of the disk of the spiral galaxy NGC 4526.
In the next phase of the discovery process, members of the ZTF team study the subtracted images and trigger additional observations for the most promising candidates with the neighboring 1.5-meter (5-foot) telescope that houses ZTF's spectrograph called SEDm. "
We combine the brightness information provided by ZTF's camera with SEDM data to correctly identify the origin and type of a transient event, a process astronomers call transient event classification," explains Yu-Jing Qin, a postdoctoral researcher at Caltech.
To do this, the IP2I group (CNRS/Claude Bernard University) led by Mickaël Rigault has set up the first fully automated pipeline for reducing spectroscopic data from SEDm. "
We thus obtain a spectrum analyzable by scientists less than 5 minutes after the end of the observation," specifies Mickaël Rigault.
Since 2012, astronomers have been tracking transient event discoveries on a public platform called the "
Transient Name Server" (TNS). The global community uses this platform to announce detections and classifications of transient events.
By sharing their discoveries, astronomers create a virtual global observatory where unclassified transient events can be classified by other teams of astronomers with spectroscopic facilities. Thanks to this joint effort, the TNS now contains about 16,000 records of classified supernovae and other transient events. "
SEDm is responsible for classifying nearly half of all supernovae discovered worldwide," notes Mickaël Rigault.
Two years ago, Christopher Fremling teamed up with machine learning experts at Caltech to train computers to read SEDm spectroscopic data, classify supernovae, and automatically transmit the results to the
Transient Name Server minutes after observations. In 2023, PhD student Nabeel Rehemtulla from
Northwestern University extended the use of machine learning to the entire observation cycle. He developed the BTSbot system, which is currently used in ZTF to discover, classify, and report supernovae without human intervention.
"
Since BTSbot started operating, it has found about half of the brightest ZTF supernovae before a human. For some types of supernovae, we have automated the entire process, and BTSbot has so far achieved excellent results in over a hundred cases. This is the future of supernova studies, especially when the Vera Rubin Observatory comes online," adds Nabeel Rehemtulla.
The Vera Rubin Observatory is under construction in the Chilean mountains and, once completed, will be much more sensitive than ZTF, enabling the discovery of millions of supernovae. "
The machine learning and artificial intelligence tools we have developed for ZTF will become essential when the Vera Rubin Observatory begins operations," explains Daniel Perley, an astronomer at
John Moores University in Liverpool, UK, who developed the search and discovery procedures for BTS and manages the study's public database. "
The results of our work on ZTF and the tools we develop there will be valuable for studying the millions of supernovae that Rubin will discover," adds Mickaël Rigault.
Thanks to an additional $1.6 million in funding from the U.S. National Science Foundation (NSF), ZTF will continue to scan the night sky over the next two years.
"
In 2025 and 2026, ZTF and Vera Rubin will be able to operate in tandem, which is fantastic news," says Mansi Kasliwal, a professor of astronomy at Caltech, who will lead ZTF over the next two years. "
By combining data from both observatories, astronomers can directly tackle the physics underlying supernova explosions and discover fast and young transient events inaccessible to ZTF or Rubin alone. I am very excited for the future," concludes Mansi Kasliwal.