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Caltech’s Sophisticated Sky-Surveying Camera Discovers Near-Earth Asteroids and Supernovae

The latest sophisticated sky-surveying camera from Caltech started its operations at the Palomar Observatory in March 2018, the results of which are slowly emerging.

A new, composite image of the Andromeda galaxy made by combining three bands of visible light captured by ZTF. The image covers 2.9 square degrees of sky, which is one-sixteenth of ZTF’s full field of view. Andromeda, also known as Messier 31, is the nearest major galaxy to our Milky Way galaxy, lying 2.5 million light-years away. (Image credit: ZTF/D. Goldstein and R. Hurt (Caltech))

Thus far, the new instrument, which is named the Zwicky Transient Facility (ZTF), has discovered 50 small near-Earth asteroids and over 1100 supernovae. It has also detected over 1 billion stars in the Milky Way galaxy. The orbital period of one of the near-Earth asteroids spotted by ZTF, known as 2019 AQ3, is merely 165 days, the shortest known “year” for any asteroid.

It’s a cornucopia of results,” stated Shri Kulkarni, the principal investigator of ZTF and the George Ellery Hale Professor of Astronomy and Planetary Science at Caltech. In recent times, a number of new papers related to early results and technical specifications for ZTF were accepted for publication in the journal Publications of the Astronomical Society of the Pacific. “We are up and running and delivering data to the astronomical community. Astronomers are energized.”

ZTF surveys the northern skies to find anything that moves, explodes, or changes in brightness by using the 48-inch Samuel Oschin Telescope at Palomar. Since the ZTF camera covers 240 times the full moon size in a single night-sky image, it finds out the most fleeting, or short-lived, of cosmic events, which could not be captured earlier.

ZTF is surveying the whole northern sky every three nights. It’s already discovering a few supernovae a night, and we expect that rate to go up.

Shri Kulkarni, George Ellery Hale Professor of Astronomy and Planetary Science, Caltech.

The cost involved in developing and operating ZTF is nearly $24 million, of which around $11 million of the funding comes from the U.S. government through the National Science Foundation (NSF) and the remaining comes from an international collaboration of partners. Additional support is being offered by the Heising-Simons Foundation, together with Caltech itself.

The start of routine operations of ZTF marks a new era in our ability to capture the nightly and hourly changes transpiring in the universe. They are now recording real-time events from distant supernovae to nearby asteroids and are poised to discover the violent mergers and explosions generating gravitational-wave events.

Anne Kinney, Assistant Director for Mathematical and Physical Sciences, NSF.

Since the U.S. government pays for around half of ZTF, almost half of its observations are publicly shared with the astronomy community in near-real-time. Upon detecting varying (or transient) objects, an automated alert system is activated, thereby sending prompts out to astronomers, who then quickly pursue notable objects of interest using other telescopes, including the 60-inch and 200-inch Hale telescopes at Palomar. The ZTF alerts are also followed up by GROWTH, an NSF-funded program with 18 international observatories in the Northern Hemisphere.

The entire data from the ZTF camera are transmitted to IPAC through a microwave network managed by UC San Diego. IPAC is an astronomy center at Caltech processing and archiving up to 4 terabytes of data every night.

This is the first time IPAC has generated real-time alerts from a survey and the first time a survey has made public up to hundreds of thousands of alerts per night.

George Helou, ZTF Co-Investigator and Executive Director, IPAC.

Eventually, the elaborate data are also made available to astronomers across the globe through IPAC.

It takes only 10 to 20 minutes from the time a transient observation is made to the time the alert goes out. It’s like running a major newsroom. We’ve never operated at this scale before, and handling all the data is quite a feat.

Matthew Graham, ZTF Project Scientist, Caltech.

Graham specializes in “big data,” particularly how to deal with and process huge streams of astronomical data.

Discoveries from ZTF to date include not just binary stars, new supernovae, and asteroids but also two black holes observed shredding stars. Since stars travel very close to black holes, possibilities are that they are “tidally disrupted” by the black hole’s gravity and stretched into oblivion. According to Graham, he and his colleagues analyzing the tidal disruption data, headed by Suvi Gezari of the University of Maryland, were tired of referring to the technical names for the objects, which comprise of long strings of numbers. “We decided to nickname them Ned Stark and Jon Snow, after Game of Thrones characters,” he stated.

ZTF also captured two near-Earth asteroids, 2018 NW and 2018 NX, that wandered closer to Earth at distances of just 76,000 and 72,000 miles away, respectively, or roughly one-third of the distance between the moon and Earth. These discoveries were facilitated by the NSF-funded GROWTH program.

On January 4th, 2019, ZTF captured the near-Earth asteroid 2019 AQ3. “This is one of the largest asteroids with an orbit entirely within the orbit of Earth—a very rare species,” stated Quanzhi Ye, a postdoctoral scholar at IPAC who first spotted the asteroid in the ZTF data.

According to Tom Prince, one of the co-investigators of ZTF and the Ira S. Bowen Professor of Physics at Caltech, the instrument is specifically proficient in spotting new gravitational-wave sources—particularly, pairs of compact stars such as white dwarfs—that will be observed using future space-based gravitational-wave detectors.

Because we cover so much sky so often, we can find these rare exotic binary systems that contain two white dwarf stars, each about the size of Earth but about half the mass of our sun. Their orbits are predicted to become smaller and smaller because of the loss of energy due to gravitational waves.”

ZTF is also laying the foundation for the future NSF-funded Large Synoptic Survey Telescope (LSST), which will scan a volume of sky 13 times larger compared to that scanned by ZTF in each and every exposure. LSST is scheduled to commence its operations in 2022.

The same alert techniques that ZTF is developing for international networks of observatories to follow up on its findings will be applied to LSST when it joins the search.

Anne Kinney, Assistant Director for Mathematical and Physical Sciences, NSF.

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