From the corner office

Time domain astronomy (TDA) is, both at once, one of the oldest areas of astronomy and one of the newest. Historically, novae through the use of “speed and peak luminosity’ relation were seen as a way to measure distances, particularly to what we now call as other galaxies. However, the period-luminosity relation for Cepheids turned out to be superior [Leavitt & Pickering 1908] and paved the way to settle the size of the Galaxy [Shapley 1918] and determine the distance to our neighboring galaxy, M31 [Hubble 1925]. Fritz Zwicky is properly credited with starting the field of observational supernovae research which, unlike the study of variable stars, required hurly-burly action on part of astronomers at short notice (let alone, subsequent follow up activity). Zwicky’s achievements in this field, the progression from a simple telescope atop Robinson Laboratory (to search for supernovae in the Virgo cluster) at Caltech to the 48-inch Palomar telescope is well known. What is far less known is Zwicky’s excellent grasp of the foundation of time domain astronomy methodology – pioneering the use of wide-field telescopes, automation, advanced detectors, real-time detection, massively multiplexed spectroscopy (via large objective gratings) event classification (which we now call as Machine Learning – not very different from the Morphological method) what it takes to undertake time domain astronomy. A serious student of the TDA methodology would benefit from reading F. Zwicky’s article in Supernovae and Supernova Remnants [Space Science Library, 45, 1 (1974)].

Shri Kulkarni
ZTF Principal Investigator, Professor of Astronomy and Planetary Sciences, Caltech

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From the engine room

On December 1, 2020, ZTF-II commenced its new public survey, covering the northern sky every two nights in g and r passbands. This builds on the thousand-and-one-night legacy of ZTF-I and its three night cadence public survey, which has produced such an exciting myriad of discoveries. The first three months of ZTF-II have been drier than average which, on the plus side, has meant more clear nights than expected, and almost 34 million alerts have been issued from observations taken. The ZTF Data System infrastructure at IPAC is being upgraded to continue to support the data pipelines and the ever growing collection of ZTF images, catalogs, and alerts but also to provide new functionality.

The public ZTF Forced Photometry service was launched at the 2021 Winter AAS meeting (see our how-to guide for operational details). It is hosted at IPAC and provides forced photometry at a requested sky location from all available difference images, including those originating from public survey observations up to and including the most recent night. ZTF-II also sees more frequent data releases, moving from a six month cycle to a two month cycle over the first half of 2021 – data releases in March (DR5) and June (DR6) ensure a smooth transition to the new schedule consistent with our data publication policy. Finally, the 30-day light curve history in the ZTF alert packet is being upgraded to a 30-day forced photometry light curve in the next few months.

Matthew Graham
ZTF Project Scientist, Research Professor of Astronomy, Caltech

ZTF Public Data Release 6

The Zwicky Transient Facility (ZTF) and IPAC at the California Institute of Technology announce the sixth ZTF Public Data Release. This release builds upon the fifth data release to include products from (i) an additional 7 months of survey operations from the public portion of the survey, giving a total observation span of March 2018 - January 2021, and (ii) data acquired under private survey time during the first ~18.4 months of the survey, spanning March 2018 - September 2019. The private surveys include observational programs awarded by Caltech and performed by the ZTF collaboration.

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Science Highlights

White dwarf living on the edge

Stein, R., Velzen, S.v., Kowalski, M. et al. Nat Astron (2021)

For only the second time, astronomers, led by a team at our ZTF partner DESY have linked an elusive particle called a high-energy neutrino to an object outside our galaxy, first detected by ZTF. Using ground- and space-based facilities, they traced the neutrino to a black hole tearing apart a star, a rare cataclysmic occurrence called a tidal disruption event. The study is published in Nature Astronomy.

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Bright stellar outbursts preceding interacting supernovae

Nora L. Strotjohann et al 2021 ApJ 907 99

We used the abundant amount of ZTF data to search for stellar outbursts that precede interacting SNe and discovered powerful eruptions prior to 18 Type IIn SNe and prior to the Type Ibn SN2019uo. This implies that about 25% of the IIn progenitor stars undergo months-long outbursts brighter than -13th magnitude immediately before their explosion. The high luminosity and long duration of the outbursts came as a surprise: The brightest eruptions release 1049 erg over 100 days, about 10% of the radiative energy observed in a typical SN. To produce as energetic outbursts via interaction, a star would have to accelerate one solar mass of material to a velocity of 1000 km/s which then slams into an even more massive shell of matter to convert the entire kinetic energy to radiation. Such eruptions might be triggered by turbulent nuclear burning shortly before the core collapse and could therefore offer hints about the unobservable processes in the stellar core that ultimately lead to the SN explosion.


First AI-assisted discovery of a comet

Dmitry Duev et al.,

Astronomers from the ZTF team recently reported the first AI- assisted discovery of a comet C/2020 T2 using Tails, a novel deep-learning framework for the identification and localization of comets. Tails can sieve through ZTF image data and find comets in near real time unlike traditional methods that require observations of multiple epochs. Tails was trained on ~60000 images from the ZTF Twilight Survey and achieved 0.01% false positive rate. While Tails is trained only on ZTF data, it can be adapted to other sky surveys such as ATLAS, Pan-STARRS and the upcoming Vera Rubin Observatory’s Legacy Survey of Space and Time (LSST). The Tails’ code is open-source and can be found in the “dmitryduev/tails” repository on GitHub. This work was also featured in a Google Cloud blog post.

Following an active transitioning centaur

Bryce T. Bolin et al 2021 AJ 161 116

P/2019 LD2 is the first time we have seen a Centaur object in the process of becoming active and dynamically transferring into becoming an inner Solar System comet. Archival observations of P/2019 LD2 by ZTF going as far back as 2019 April revealed that the comet had only recently started to become active in late 2018 when it started reaching the boundary at which surface water ice on the comet begins to sublimate carrying off the dust from the comet's surface giving the comet an active and tailed appearance. In combination with observations by HST and Spitzer, we were able to determine that the characteristic dust size of the comet is ~100-micron scale and that the comet was also sublimating CO/CO2. P/2019 LD2 is Centaur, meaning, that it likely originated from the trans-Neptunian belt beyond the gas giants, but our study shows that it is in dynamic transfer of becoming an inner-Solar System comet and will likely be ejected from the Solar System into interstellar space after half a million years.

Sibling supernovae can aid precision cosmology

Rahul Biswas et al.,

In a new study submitted to the MNRAS, astronomers from the ZTF team at the OKC, Sweden describe the discovery of a pair of sibling supernovae, or supernovae found in the same galaxy. The two type Ia supernovae were discovered 200 days apart not only in the same galaxy, but in the very same pixel of the ZTF CCD camera. The authors use the difference in the observed extinction in brightness for the two siblings to estimate the properties of the dust along the line of sight in the host galaxy. Their results largely agree with those from a *statistical* analysis from over a thousand type Ia supernovae but a key advantage of using sibling systems is the lack of systematic uncertainties. As more siblings are found in large surveys such as ZTF and the upcoming Vera Rubin LSST, they can become a new tool to improve Type Ia cosmology.

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ZTF Faces

Tomas Ahumada

I was born and raised in Santiago, Chile. Over there, you can see the Southern Cross all year round, and it's probably the constellation I miss the most.

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Mathew Smith
(IN2P3, France)

So my route to Lyon has been a fairly circuitous one. I’ve studied mathematics in London, radio astronomy in South Africa and cosmic explosions in the UK.

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ZTF Science Vlog

The ZTF vlog brings you the latest ZTF results presented by the authors themselves.

The Photometric and Spectroscopic Evolution of Rapidly Evolving Extragalactic Transients in ZTF

Anna Y.Q.Ho et al.,

In our latest ZTF Science Vlog, Anna Ho shares three main take-aways from her most rec paper. Anna Ho is currently a Miller postdoctoral fellow at the UC, Berkeley.

Science with public ZTF data

We highlight scientific publications that use ZTF public data

'How-to' Guide

Retrieving single-exposure science image products from ZTF public data

This section of the public data release manual explains how to use API to retrieve file-based single-exposure science-image products that touch a fixed R.A/Dec position, fall within an observation JD range, and are likely to be of "good" (usable) quality.

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Student opportunities, summer research, schools and public outreach

Machine learning algorithm for the discovery of faint asteroids in ZTF data wins a high school student first prize at prestigious ISEF 2021

Franklin Wang was one of the thousands high school participants in this year's International Science and Engineering Fair. He was also one of the few winners. His project "The Fast and Inconspicuous: New Near Earth Asteroids Discovered Using Deep Learning and Synthetic Data Are Fainter and Move Faster than Those Previously Discovered" was awarded first prize in the Physics/Astronomy category. Franklin also received the Peggy Scripps Award for Science Communication, as the " finalist who is best able to communicate their project to the lay public, explaining both the science and its potential impact on society".

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A citizen Science project enables amateur astronomers to join chase of kilonovae

Antier S., et al., 2020, MNRAS, 492, 3904. doi:10.1093/mnras/stz3142

It is my pleasure to inform you that we have now launched the Kilonova-catcher program, which aims to bring in amateur astronomers into the search for kilonovae. Our program allows astronomy enthusiasts from around the world to use their own telescopes to survey the sky and look for counterparts to binary neutron star mergers. The main goal is to form a continuous chain of observations at very early times, before passing the torch to the larger facilities managed by the professional astronomers.

--Sarah Antier
University of Amsterdam, Netherlands

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ZTF is supported by the National Science Foundation and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, Lawrence Livermore National Laboratory, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, and Institut national de physique nucléaire et de physique des particules. Operations are conducted by COO, IPAC and University of Washington.
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.