University of Arizona researchers are using the Catalina Sky Survey's near-Earth object telescopes to locate the optical counterparts to gravitational waves triggered by massive mergers

The race is on. Since the construction of technology able to detect the ripples in space and time triggered by collisions from massive objects in the universe, astronomers around the world have been searching for the bursts of light that could accompany such collisions, which are thought to be the sources of rare heavy elements.

The University of Arizona's Steward Observatory has partnered with the Catalina Sky Survey, which searches for near-Earth asteroids from atop Mount Lemmon, in an effort dubbed Searches after Gravitational Waves Using ARizona Observatories, or SAGUARO, to find optical counterparts to massive mergers. {module In-article}

"Catalina Sky Survey has all of this infrastructure for their asteroid survey. So we have deployed additional software to take gravitational-wave alerts from LIGO (the Laser Interferometer Gravitational-Wave Observatory) and the Virgo interferometer then notify the survey to search an area of sky most likely to contain the optical counterpart," said Michael Lundquist, postdoctoral research associate and lead author on the study published today in the Astrophysical Journal Letters.

"Essentially, instead of searching the next section of sky that we would normally, we go off and observe some other area that has a higher probability of containing an optical counterpart of a gravitational wave event," said Eric Christensen, Catalina Sky Survey director and Lunar and Planetary Laboratory senior staff scientist. "The main idea is we can run this system while still maintaining the asteroid search."

The ongoing campaign began in April, and in that month alone, the team was notified of three massive collisions. Because it is difficult to tell the precise location from which the gravitational wave originated, locating optical counterparts can be difficult.

According to Lundquist, two strategies are being employed. In the first, teams with small telescopes target galaxies that are at the right approximate distance, according to the gravitational wave signal. Catalina Sky Survey, on the other hand, utilizes a 60-inch telescope with a wide field of view to scan large swaths of sky in 30 minutes.

Three alerts, on April 9, 25 and 26, triggered the team's software to search nearly 20,000 objects. Machine learning software then trimmed down the total number of potential optical counterparts to five.

The first gravitational wave event was a merger of two black holes, Lundquist said.

"There are some people who think you can get an optical counterpart to those, but it's definitely inconclusive," he said.

The second event was a merger of two neutron stars, the incredibly dense core of a collapsed giant star. The third is thought to be a merger between a neutron star and a black hole, Lundquist said.

While no teams confirmed optical counterparts, the UA team did find several supernovae. They also used the Large Binocular Telescope Observatory to spectroscopically classify one promising target from another group. It was determined to be a supernova and not associated with the gravitational wave event.

"We also found a near-Earth object in the search field on April 25," Christensen said. "That proves right there we can do both things at the same time."

They were able to do this because the Catalina Sky Survey has observations of the same swaths of the sky going back many years. Many other groups don't have easy access to past photos for comparison, offering the UA team a leg up.

"We have really nice references," Lundquist said. "We subtract the new image from the old image and use that difference to look for anything new in the sky."

"The process Michael described," Christensen said, "starting with a large number of candidate detections and filtering down to whatever the true detections are, is very familiar. We do that with near-Earth objects, as well."

The team is planning on deploying a second telescope in the hunt for optical counterparts: Catalina Sky Survey's 0.7-meter Schmidt telescope. While the telescope is smaller than the 60-inch telescope, it has an even wider field of view, which allows astronomers to quickly search an even larger chunk of sky. They've also improved their machine-learning software to filter out stars that regularly change in brightness.

"Catalina Sky Survey takes hundreds of thousands of images of the sky every year, from multiple telescopes. Our survey telescopes image the entire visible nighttime sky several times per month, then we are looking for one kind of narrow slice of the pie," Christensen said. "So, we've been willing to share the data with whoever wants to use it."

New telescope network to rapidly follow up on the changing night sky

Supernovae, neutron star mergers, black holes at the center of galaxies, erupting young stars -- these are all examples of objects in the night sky that change their brightness over time. In the coming years, astronomers expect to discover millions of these variable astronomical events with new sensitive telescopes like the Large Synoptic Survey Telescope (LSST). But to characterize these objects and to understand them better, they need to be observed with other, different telescopes soon after they are discovered. {module In-article}

To provide astronomers with a network of world-class telescopes that can be accessed with a touch of a button, four ground-based astronomical observatories have joined forces to set up the Astronomical Event Observatory Network (AEON): Las Cumbres Observatory (LCO), the National Science Foundation's National Optical Astronomy Observatory (NOAO), the SOAR Telescope, and Gemini Observatory. With AEON, astronomers will be able to automatically follow up on their astronomical objects of interest, with a range of 0.4-meter to 8-meter telescopes, observing in UV light to infrared.

The 4.1-meter SOAR telescope at the Cerro Tololo Inter-American Observatory in Chile is the pathfinder facility for AEON. On 6 August, it successfully completed its first observing night for the network, looking at 10 different astronomical objects under excellent sky conditions. The results, which demonstrate successful access to a large telescope outside the LCO network, marks the beginning of a unique new partnership between major astronomical observatories.

"This was the first of 20 nights in which we will follow up on variable objects such as young supernovae and near-earth objects," said SOAR project scientist César Briceño. "SOAR is a great testbed for AEON. It is located on the same mountain as LSST and we have years of experience with remote observations. The telescope also hosts multiple instruments and has been designed to produce the sharpest images possible with any ground-based telescope," he added.

AEON is a highly automated observing system that runs unsupervised software and automatically generates a dynamic and flexible observing schedule roughly every 15 minutes. SOAR observation requests for its first night were submitted via the LCO portal, scheduled by LCO software, and transmitted to SOAR, where they were executed with the help of new software that automates many aspects of running the telescope and instrument. The resulting data were returned through both the LCO and the NOAO archives.

The successful observing night on SOAR is an important milestone in the process towards setting up the AEON network and including larger telescopes such as Gemini. Gemini Observatory, a leader in flexible or queue observing, is currently upgrading its observing system and will build on the SOAR work to create interfaces and an automatic scheduler that will work with AEON and enhance Gemini's capabilities.

"The recent demonstration at SOAR shows that the innovative techniques developed at LCO over many years can now be extended to other observatories, including more powerful telescopes, with different heritage, management, and technology," said NOAO astronomer Stephen Ridgway.

"This milestone marks the beginning of new science opportunities with existing facilities that will continue to grow with the AEON partnership," added the director of LCO, Dr. Lisa Storrie-Lombardi.