Listening to multiple cosmic messengers to get the full story
Cosmic messengers?
Astronomers tend to think of four cosmic messengers: light, gravitational waves, neutrinos, and cosmic rays. Each of these, when detected with some instrument, tells us something about the Universe. In multi-messenger astronomy, we try to detect multiple messengers from the same astrophysical source.
With a multi-messenger approach, the insights gained are more than the sum of their parts. For example, a merger of two neutron stars emits both gravitational waves and light. If we detect both gravitational waves and light from the same merger, and we figure out the redshift of the object from its light, that gives us independent measurements of distance and redshift which can be used to measure the Hubble constant. We've only been able to do this with one object so far—the event GW170817—but even a few dozen such measurements could resolve the longstanding tension in the Hubble constant!
Gravitational wave events come with a localization region: the detectors' best estimate for where in the sky the gravitational waves came from. In the case of gravitational waves, these detectors are currently the LIGO and Virgo detectors. Detectors like IceCube are used to observe neutrinos, and they provide an estimate of where the neutrino came from in the sky, too.
I first searched for an electromagnetic counterpart to a gravitational wave event in 2019, when LIGO and Virgo saw the event GW190814: a potential merger between a neutron star and black hole. I coordinated the search for a counterpart in optical and infrared light with MegaCam on the Canada-France-Hawaii Telescope. We didn't find a counterpart (nobody did!), but our observations provided important constraints on the properties of the system. Nonetheless, we and others in the community have continuously raised the question:
How can we do better?
My current focus is on how to optimize our strategies for searching for electomagnetic counterparts to gravitational wave sources. Remember GW190814—that event was exceptionally well-localized by gravitational wave standards, and yet, the community found more than a hundred candidate electromagnetic counterparts in the tiny localization region of that event! Astronomers were able to discard many candidates by establishing that they were supernovae, or lived in galaxies too close or far away compared to the gravitational waves, or through other disqualifying traits, but many candidates were never classified. We cannot observe and classify all candidate counterparts to a gravitational wave event for the simple reason that our observing resouces are finite.
So, how do we decide which candidates to follow up on?
I'm currently working primarily on the Multi-messenger Treasure TROVE: a Tool for Rapid Object Vetting and Examination. We're building TROVE to look at the tens to hundreds of candidate counterparts to gravitational wave (or neutrino, or gamma ray burst!) events and assign them each a score which quantifies how likely they are to be the true counterpart. TROVE will be available to the community for optimizing and coordinating follow up efforts so that we can gain those insights available only through a multi-messenger approach! Beyond developing a tool, we will also produce lots of documentation and tutorials on how to use the TROVE. We'll lead workshops on how to use the TROVE. Finally, we'll have a Slack space for TROVE users. Our goal is not just to build a tool, but to make the barrier to entry to multi-messenger astronomy lower so that everyone can contribute to this exciting new field.
I've listed papers I'm on below which tackle follow-up coordination and begin to introduce the TROVE. You can also visit the TROVE website yourself! Keep an eye on it—we'll need beta testers!
Image Credit: Alex Andrix
Relevant publications
Optimizing Kilonova Searches: A Case Study of the Type IIb SN 2025ulz in the Localization Volume of the Low-significance Gravitational Wave Event S250818k
Franz et al. 2025. ApJL, 994, 2, L45
Work led by Noah Franz, a graduate student at University of Arizona. We introduce the Tool for Rapid Object Vetting and Examination (TROVE) and apply it to the gravitational wave event S250818k, a potential neutron star merger. We score all of the candidate counterparts to this event based on their position in the sky, distance, absence of nearby point sources or minor planets, and resemblence to a kilonova. We thoroughly investigate the likely SN IIb AT2025ulz, which received special attention from the community during follow-up efforts.
A Deep CFHT Optical Search for a Counterpart to the Possible Neutron Star - Black Hole Merger GW190814
N. Vieira, J. J. Ruan, D. Haggard, M. R. Drout et al. 2020. ApJ, 895, 96
Our CFHT MegaCam optical/near-infrared imaging campaign in search of the first high-significance candidate neutron star - black hole merger, GW190814. This work was summarized in a CFHT news release.
The LIGO/Virgo collaboration announced in June of 2020 that this event was a merger between a black hole 23 times as massive as the Sun and a compact object 2.6 times as massive as the Sun. This makes the lighter object either the heaviest neutron star or the lightest black hole ever! See this excellent summary, and check out this beautiful animation of one of the many possible scenarios for this event.