Cosmic Newsflash

Institutional partners

Researchers & students

Partnership publications

Community publications

Nationalities

Summer school participants

Operational nights

Science observations

Science images

Engineering Maintenance

Operational Overhead

Filter exchanges

NEA & comets

Tidal disruption events

Ultra-compact binaries

Physics of Supernovae and Relativistic Explosions

The Bright Transient Survey crew continues to collect supernovae, and it now has over 8000 classified supernovae. This fantastic influx of transients is feeding a wide variety of science and in the past three years we have published papers both on individual nearby SNe like SN2020jfo and more recently the upcoming paper by Erez et al. on SN 2023ixf. The group has also compiled some more systematic samples, like the SNe Ia-csm by Yashvi Sharma and the very comprehensive sample papers by the Superluminous supernova sub-group.
A few papers made it to high-profile journals like Nature, for example the discovery of a radio Type Ia supernova, the discovery of a jetted TDE as well as Avishay et al. paper which together with this paper led by Daniel Perley established an entirely new class of Type Icn supernovae.
Anna Ho has been leading the emerging field of fast transients and Kate Mcguire's group in Dublin and also Adam Miller's group in Chicago took care of our peculiar SNe Ia, and published a series of papers on the topic.
Some members of the team wrote up their samples and PhD theses and moved on to other teams, congratulations for example to Rachel Bruch at Weizmann Institute of Science, Israel.

Galactic & M31 science

One productive aspect of ZTF-II was the emergence of an active group finding and conducting follow-up observations of interesting cataclysmic variables (CVs). This group involved international and domestic collaborations between Caltech (Tony Rodriguez, Zach Vanderbosch, Jan van Roestel [now in Netherlands], Ben Roulston [now in New York] and Andrew Drake), U. of Washington (Paula Szkody, Eric Bellm, and Keaton Bell [now in New York]), UCLA (Michael Rich), Boston University (JJ Hermes), and U. of Warwick (Boris Gaensicke and Keith Inight). The research was aided by the use of the Fritz Marshal and the ongoing activities of the machine learning group, involving collaborations with Ashish Mahabel (Caltech), and Michael Coughlin and Brian Healy (U. of Minnesota). Among the results are the discovery of several thousand new CV candidates, including AM CVn stars, and those with magnetic white dwarfs. The use of the ZTF light curves to find orbital periods along with X-ray to optical survey data and follow-up spectroscopic and photometric data from Keck, Palomar, Apache Point, and Lick Observatories, have resulted in an array of papers providing needed information on the numbers of these systems in our Galaxy and the evolutionary paths that brought them to discovery. An upcoming catalog will allow continued future follow-up.

In addition, smaller collaborative groups have continued to expand upon the discoveries and characterizations of several different stellar populations. These populations include massive and rapidly rotating white dwarfs thought to be the products of white dwarf mergers (Ilaria Caiazzo), ultra-compact and evolved binary systems (Kevin Burdge, Tom Prince, Kareem El-Badry), white dwarfs hosting tidally-disrupted and transiting planetary debris disks (Zach Vanderbosch, JJ Hermes, Soumyadeep Bhattacharjee, Boris Gaensicke), pulsating and other variable stars within globular clusters (Chow-Choong Ngeow), the central stars of planetary nebulae (Albert Kong), young stellar objects (Lynne Hillenbrand, Michael Kuhn, Avery Wold), and M-dwarfs and dwarf carbon stars (Rocio Kiman, Ben Roulston). Several papers and catalogs are forthcoming from these efforts.

TDEs & AGNs

As with most science, the real energy and progress in the our team has been driven by early career scientists. Code development, scanning, triggering follow-up, observing and building samples has always been a collaborative effort in our group, and for ZTF-II we would particularly like to highlight the hard work of grad students Erica Hammerstein (UMD), Jean Somalwar (Caltech) and Yuhan Yao (Caltech, now Berkeley). Thanks to their efforts, we have ~50 new TDEs in ZTF-II, representing ~half of all known optical TDEs.

On the back of this large sample, there were a number of important papers to come out of our group during ZTF II. Erica Hammerstein led a comprehensive analysis of our ZTF-I TDEs, and identified the new spectroscopic class of “featureless TDEs”. Yuhan Yao led a combined analysis of ZTF-I and ZTF-II TDEs, producing the first large sample estimates of the TDE luminosity function and volumetric rates. There was also close collaboration with the MMA working group, including the detection of three different possible neutrino-TDE associations led by Robert Stein for AT2019dsg, Simeon Reusch (DESY) for AT2019fdr, and Sjoert van Velzen (Leiden) for AT2019aalc. Igor Andreoni (UMD), as part of his search for rapid transients, identified the first optically-discovered jetted TDE. And Matthew Graham (Caltech) identified candidate optical counterparts to binary black hole mergers detected by LIGO. There is still much to learn about TDEs, and plenty of work is still ongoing. Most recently, there have been a couple of ZTF TDEs that exhibited repeated flares. A paper about it should shortly be on arxiv! We don'tdoubt that ZTF TDEs will continue to surprise us in the future.

Solar System Science

The Solar System Working Group is a small team, but we can still look back and find several accomplishments during ZTF II. Our group published papers detailing the use of state-of-the-art software techniques for solar system science, including a new method tocharacterize the rotational light curves of asteroids, and the use of deep learning for comet discovery. Our understanding of the Solar System benefits from the study of extreme objects, and our team reported on kilometer-sized asteroids that appear to have "super-fast" periods, comet P/2020 HS, one of the weakest comets in the inner-solar system, and asteroid Ayló'chaxnim, which is the first asteroid known with an orbit that is fully interior to the orbit of Venus. With ZTF II, we discovered 107 near-Earth objects, 23 cometary outbursts, and also found comet C/2022 E3, the photogenic comet that seemingly broke the Internet as the "Green Comet" not seen for tens of thousands of years. That's a comet we'll never forget!

Multi-messenger Astrophysics

ZTF-II was an exciting time for the multi-messenger astronomy working group, across our three flagship programs. The advent of Fritz, with its host of novel MMA features, has been a particular game-changer for our team. The ZTF neutrino program continued apace, and led to the identification of two separate TDEs as probable neutrino sources (see https://www.nature.com/articles/s41550-020-01295-8, https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.221101). The quest to find short gamma-ray burst (GRB) afterglows led to the unexpected discovery of a supernova counterpart, confirming that some short GRB have a collapsar origin. More recently, we found a supernova associated with the nearby, bright long GRB 230812B (Srinivasaragavan et al. 2023, in prep.). Our search for kilonova during O3 led to constraints on the kilonova luminosity function and properties of two neutron star–black hole mergers. We also developed a novel pipeline for serendipitous discovery of kilonova-like rapid transients, ZTFReST. ZTFRest has found many rare transients, including GRB and orphan afterglows, and even a jetted TDE. Our successes have been supported by the highly collaborative nature of our working group, with active participation from teams in Germany, India, Sweden, the UK, and across the US. This diverse group, with varied experiences, backgrounds and interests, have enabled new discoveries, and impactful papers. While ZTF-II may be ending, we are confident these relationships will endure and continue to lead to fruitful future cooperation!

Machine Learning

Machine learning (ML) has enabled powerful inference for astronomical data at scales and speeds surpassing human limitations. During ZTF II, we achieved significant advancements in the development of machine learning algorithms to study the time variable sky. We have successfully developed a workflow for training machine learning algorithms to make classification predictions for ZTF sources. Furthermore, we have built a deep learning model to automatically identify BTS sources, saving scanners’ time and expediting follow-up. We have harnessed the state-of-the-art transformer architecture to yield 95% accuracy in quasar vs galaxy vs star selection based on ZTF variability and photometric colors. The in-class anomaly detection pipeline is complete and is being run on SCoPe+ fields, with the best candidates selected for spectroscopy. To support multimessenger science, we have completed the development of the first autonomous planning agent for following up kilonovae, with an accuracy close to human astronomers. We have also created the most accurate observing scenarios to date, aiding in the preparation of multi-messenger searches with ZTF. The Quarter Century Sky project has streamlined its GUI with Django-React, optimized data storage and querying using PostgreSQL and Redis, implemented security measures, and enhanced functionality for bulk data uploads and querying. Finally, our engagement with the wider community includes the launch of ZARTH, the Pokemon GO like android game for transients, which has been played by people from close to 50 countries.

By Igor Andreoni

The person behind the scenes that I'd like to mention as instrumental to the discovery of AT2022cmc is Roger Smith, the detector and electronics lead at Caltech Optical Observatories. Roger and his team designed and built the ZTF camera. Importantly, they took care of it numerous times when it needed attention during the course of the survey. In January 2022, small crystalline particles called "zeolites" were found that were contaminating the camera. Their presence presented a potentially fatal risk to the electronics. Roger and other engineers brought ZTF back to Caltech campus, dismounted the camera to remove all the zeolite particles, ran additional tests, and mounted it back on the telescope at Palomar in record time. Their working schedule was relentless because in astronomy every single night matters, as this story demonstrated. On the night after ZTF started observing again, the one-of-a-kind fast transient called AT2022cmc was detected by ZTF. AT2022cmc is the first relativistic tidal disruption event identified by an optical survey, in which a luminous beam is launched when a star was pulled apart by a massive black hole's gravity, and is also the furthest of such events discovered to date. If ZTF started observing again even a couple of days later, we would have likely missed this exceptional cosmic drama. Thank you, Roger!

By Erik Kool

In order to make sense of what ultimately would be the first radio-detected thermonuclear supernova, I had to call in a lot of help. So there are many people without whose help this study would not have been as interesting as it turned out to be. But I think it’s fair to say it may not even have existed in any shape or form at all without second author Joel Johansson. The project was shelved for quite some time because we didn’t really know what we were looking at, and it took Joel’s conviction that it was in fact a thermonuclear supernova (of a kind not observed before) to get the ball rolling properly. To me, this study has been a great example of how powerful it can be to seek out and work with people with different scientific backgrounds and expertise, rather than go at it alone.

By Ilaria Caiazzo

I would like to give a shout-out to the work of Daniel Perley in developing the reduction pipeline “LPipe" for the LRIS spectrograph. Having a reliable pipeline for reducing spectroscopic data is crucial and Dan’s pipeline has been an invaluable asset for the ZTF group at Caltech, as it runs fast and smoothly, enabling an almost real-time reduction during the night. LRIS spectroscopy was the decisive dataset in the discovery of the double-faced white dwarf, as it revealed that one face of the white dwarf is covered in hydrogen, while the opposite face is covered in helium. When I discovered this strange behavior, I needed more spectra to confirm that the modulation was stable and to obtain phase-resolved data with a higher signal-to-noise ratio. In the meantime, however, a new detector was installed in the red arm of LRIS, and therefore the pipeline needed to be updated. I reached out to Dan and he swiftly modified the pipeline, so I was able to analyze the spectra within a couple of weeks. The work of developing a reduction pipeline is not always properly recognized, but reliable and fast pipelines are extremely important for maximizing the scientific return of an instrument, and Dan’s is one of the best I have had the fortune to use.

By Tomas Ahumada

Gamma-ray bursts (GRBs) are detected almost every day, but it is extremely difficult to pinpoint their optical counterpart. By August 2020, we have used ZTF several times in the hopes of finding the elusive afterglow. At 11 pm PT time, Igor Andreoni realized a short GRB was detected by Fermi, and he interrupted ZTF schedule and triggered observations on the GRB error region. Multiple people around the globe started scanning candidates in real-time and Ana Sagues-Carracedo led our first GCN, with help from folks in Germany and India. Once we secured the afterglow, Shreya Anand coordinated key observations with collaborators, while managing the resources we already had available like LCO and P200.

This burst was controversial as it showed long GRB properties while lasting only a 0.65 sec in the rest-frame. Leo Singer was instrumental in leading a Gemini Director's Discretionary Time proposal to elucidate the nature of the burst. I remember how we stayed up late in the midst of the pandemic, coding and writing over the phone, checking references, and making plots. It was one of the highlights of my PhD, incredibly insightful and exciting at the same time. Leo guided me through the process, sometimes calling Mansi or Brad to check on SN parameters or configurations of the telescope, and later introducing me to new analysis techniques and even better, bugging Michael Coughlin and theorists like Geoff Ryan to help me get the numbers right.

After long and late hours, coordination with 50+ authors and data from 10+ facilities, we submitted our paper to Nature Astronomy along with Bin-Bin Zhang's work on the same object. But that's *not* the end of the story! Leo and Brad were able to excite NASA about our discovery: so much so that they decided to make a video of the GRB and allowed me to narrate the Spanish version of the video. That felt surreal. As surreal as a 0.65 sec duration collapsar.

By Ariel Goobar

We were thrilled to find the first strongly lensed Type Ia supernova with ZTF last year, SN Zwicky. The lens, a galaxy in the line of sight, split the supernova light into four images separated by just a fraction of an arcsecond and were ultimately spatially resolved with laser-guided adaptive optics at Keck in the Near-IR and HST at optical wavelengths. One key measurement of SN Zwicky was carried out by OKC student Ana Sagués Carracedo: the difference in arrival times between the images, known as “time-delays”. Before SN Zwicky, the prevalent impression in the community was that an accurate measurement of the time-delays between the supernova images would not be possible unless we could carry out follow-up observations of the resolved images. In other words, it requires high spatial resolution over multiple epochs from repeated observations with Keck or HST. These are very expensive and hard to get, especially for a large sample. Instead, Ana showed that a single high-resolution image from Keck of SN Zwicky, identifying the four images and their relative fluxes, combined with the unresolved ZTF images was sufficient to measure time-delays in this case. Ana’s results are very encouraging and show the potential to uncover important cosmological information from gravitationally lensed supernova based on ZTF data, or similar.