Can cosmic collisions be predicted before they happen?

On August 17, 2017, some 70 telescopes combined turned their gazes to a fiery collision between two dead stars millions of light-years away. Telescopes watched the event unfold in a rainbow of wavelengths, from radio waves to visible light to high-energy gamma rays. When a pair of super-dense neutron stars collided with each other, they threw outward debris that glowed for days, weeks, and months. Some telescopes looking at gold, platinum, and uranium were spotted in the scorching blast, confirming that most of the heavy elements in our universe are formed in this type of cosmic collision.

If that were the end of the story, this cosmic event would have been pretty cool in itself, but three other detectors were there for the astronomical gathering that day — two belonged to LIGO (Laser Interferometer Gravitational-Wave Observatory) and one belonged to the European Virgo. LIGO and Virgo are not noticed light waves But gravitational waves, or shivers in space and time produced by massive, accelerating objects. When neutron stars spiral together, they generate gravitational waves before merging and being blasted with light. It was the LIGO–Virgo gravitational wave network that alerted dozens of telescopes around the world that something amazing was happening in the skies above. Without LIGO and Virgo, August 17, 2017 would have been a typical astronomy day.

Since that time, the LIGO–Virgo network has detected only one neutron star merger; In this case, which occurred in 2019, optical telescopes were not able to observe the event. (LIGO-Virgo has also detected dozens of binary black hole mergers, but they are not expected to produce light in most cases.) With LIGO-Virgo scheduled to return again in May, astronomers are excitedly preparing for more explosive neutron star mergers. One burning question on the minds of some members of the LIGO team is: Can they detect these events sooner — perhaps even before dead stars Collide?

To this end, researchers are developing early warning software to alert astronomers of neutron star mergers up to a full seconds or even a minute before collision.

“It’s a race against time,” says Ryan Magee, a postdoctoral scientist at Caltech who co-leads early warning software development with Surabi Sachdev, a professor at the Georgia Institute of Technology. “We’re missing valuable time to understand what happens just before and after these mergers,” he says.

After 11 hours, the source was found

Once LIGO detects a potential neutron star collision, the race is on for telescopes on Earth and in space to follow up and locate it. The LIGO–Virgo network, consisting of three gravitational wave detectors, helps narrow down the approximate location of where the fireworks occur while light-based telescopes are needed to pinpoint the exact galaxy where the neutron stars reside.

For the Aug. 17 event, known as GW170817, most light-based telescopes weren’t able to start looking for the source of the gravitational-wave event until nine hours later. The LIGO-Virgo team sent its first alert to the astronomical community 40 minutes after the neutron star impact and the first sky maps, determining the approximate location of the event, 4.5 hours after the event occurred.

But by then, the region of interest in the southern sky had lurked below the horizon and out of view for southern telescopes capable of seeing it. Astronomers will have to wait anxiously until nine hours after the event to start combing the sky. About 11 hours after the neutron star collision, several ground-based optical telescopes finally located the source of the waves: a galaxy called NGC 4993, located about 130 million light-years away.

Preparing for the next round

With 11 hours missing from the story of how neutron stars collide with each other and seed the universe with them heavy itemsAstronomers are eagerly awaiting more neutron star crashes. For the upcoming LIGO–Virgo run, which will also include observations provided by Japan’s KAGRA, the detectors have undergone a series of upgrades to make them better at picking up gravitational wave events and thus neutron star mergers. The team expects to detect four to 10 neutron star mergers in the next round and up to 100 in the fifth observing round of the current advanced detector network, which is planned to start in 2027. Future runs with more advanced detectors are scheduled for 2030.

One of the new features that will be used in the next run is the early warning system. The specialized software will complement the main one that has been routinely used to detect all gravitational wave events to date.

The main program, also called the search pipeline, looks for weak gravitational-wave signals buried in the noisy LIGO data by matching the data to a library of known signals, or waveforms, that represent different types of events, such as black hole and neutron star mergers. If a match is found and confirmed, an alert will be sent to the astronomical community. Early Warning works the same way but it only uses shortened versions of waveforms so it can work faster.

“The detectors are constantly taking in new data in the process of monitoring, and we compare our waveforms to the data as they come in. If we use truncated waveforms, we don’t have to wait until a lot of data has been collected to do the comparison,” Magee says. “The trade-off is that the signal has to be high enough to be detected using clipped waveforms. It is important to keep running the main pipelines in conjunction with the early warning pipeline to catch the weaker signals and get the best final localization.” Magee, Sachdev, and their colleagues are working on an early warning pipeline called GSTLAL; Additional early warning pipelines for LIGO – Virgo are also in the works.

before the fireworks

As neutron stars circle each other like a pair of ice dancers, they spin faster and faster and go off gravitational waves of increasingly high frequencies. The final dance between neutron stars lasts longer than that between black holes, up to several minutes in the frequency bands LIGO is most sensitive to, and this gives LIGO and Virgo more time to catch the stars’ dramatic end. In the case of GW170817, the pair of mixed neutron stars spent six minutes in the frequency bands detectable by LIGO-Virgo before the two objects finally combined.

The truncated waveforms of the LIGO Early Warning Program are designed to capture snippets of this last dance; In fact, the researchers believe the program will eventually detect a neutron star merger up to a minute before the collision. If so, that would give telescopes around the world more time to find and study the explosions.

“In the next round, we might catch one of the neutron star mergers 10 seconds ahead of time,” says Sachdev. “In round five, we think we can catch one of them with a full minute of caution.”

For astronomers, one minute is a lot of time. Caltech astronomy professor Greg Hallinan and director of Caltech’s Owens Valley Radio Observatory says early warnings of impending neutron star mergers will be especially important for gamma-ray, X-ray and radio telescopes because collisions can explode at these correct wavelengths. . in the beginning.

“Radio telescope arrays such as the Owens Valley Radio Observatory’s Long Wavelength Array (OVRO-LWA) and Caltech’s Future Deep Synoptic Array (DSA-2000) containing 2,000 antennas may be able to detect a radio flash presumably occurring at that time. Neutron stars Some forms merge during the final muse before merging,” Hallinan says. This will teach us about the immediate environments of these devastating large-scale events. Furthermore, seeing radio flashes can help us quickly locate mergers.”

Caltech graduate student Shriya Anand says early optical and ultraviolet observations of mergers can reveal new information about their evolution, such as how elements form in the fast-moving matter emitted by the collisions.

Anand, who works in the group of Caltech astronomy professor Mansi Kasliwal (MS ’07, Ph.D. ’11), is busy developing software herself, not for early warning systems but to search the sky for neutron star mergers and more. cosmic events as soon as you receive an alert from LIGO. Kasliwal’s group is currently developing software for the Zwicky Transient Facility (ZTF) and Wide-field Infrared Transient ExploreR (WINTER), survey instruments located at Caltech’s Palomar Observatory. ZTF and WINTER can follow LIGO alert to find a file neutron star merger. Anand is developing software that will accelerate this research.

“Our algorithms figure out the best way to cover different areas of the sky and how long to ensure the maximum chance of finding the target,” she adds. “We’re missing interesting physics in the early stages of mergers. The LIGO team’s early warning program and our telescope research program will speed up our chances of finding an event early. This will ultimately give us a more complete picture of what’s going on.”

early warning Study led by Maggie appeared in Astrophysical Journal Letters in 2021 A study led by Sachdev Also appeared in Astrophysical Journal Letters in 2020.