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Multi-messenger astronomy has been all the rage lately. It involves capturing data on the gravitational and electromagnetic signals from catastrophic cosmic events. However, with that newfound interest comes required updates to infrastructure. Gravitational wave detectors have been upgraded and will be even more sensitive soon. But to realize the promise of multi-messenger astronomy, scientists must have a fleet of spacecraft watching the entire sky for high-energy signals indicative of the events that cause gravitational waves. At least, that is the team's long-term plan behind the High Energy Rapid Modular Ensemble of Satellites Pathfinder (HERMES-PF) mission, which successfully launched in March and is currently undergoing commissioning.
The motivation behind HERMES-PF is simple: collecting electromagnetic and gravitational wave data on the same event opens up a whole new world of understanding of the forces underlying the event that has never been possible to explore. With that data, we could dramatically increase our knowledge of events like black-hole mergers, neutron star and black hole mergers, or other cataclysmic events that have never been observed in such detail.
However, we need an accurate idea of where the signal is coming from to do so. Gravitational wave detectors are relatively direction agnostic, however electromagnetic detectors are very dependent on which angle they are pointed towards. If they happen to be pointed away from a signal, they might miss it altogether. Even simply pointing in the general direction rather than spot on at the source of the signal could result in less-than-ideal data.
Fraser and Pamela discuss Multi Messenger Astronomy and what it means for the future of the discipline.
But limiting the area of a high-energy source is difficult. Efforts to do so have run into problems with relative timing between detectors and a lack of sensitive instruments. HERMES-PF hopes to fix that by setting up a system that can accurately detect where a source is coming from to within one degree of the sky and by having precise time monitoring to ensure the data is linked up with other detectors correctly.
To do this, HERMES-PF will utilize six different 3U CubeSats, mainly using Commercial-Off-The-Shelf (COTS) parts that don't have to be radiation hardened (read: expensive). Each 3U CubeSat will include 60 GAGG:Ce scintillator crystals and 12 silicon drift detectors that will allow the system to capture a wide range of possible energy spectra and provide high-resolution temporal data.
HERMES-PF's sensor system has been operating on another CubeSat mission since 2023. That mission, known as the Space Industry Responsive Intelligent Thermal nanosatellite (SpIRIT), has already begun collecting data. However, some issues with its cooling system and S-band antenna downlink have limited the time it has spent observing.
Integrated HERMES-PF spacecraft.
Credit - F. Fiore et al.
The six CubeSats in the HERMES-PF system proper hope to avoid that fate. Unlike SpIRIT's single detector setup, the HERMES-PF system proper uses a triangulation technique to determine where a signal source is coming from. Ideally, with enough of the constellation in space, these types of detectors should provide "all-sky" monitoring, ensuring that no matter where an event happens, its electromagnetic data is captured.
This will be critically important as new gravitational wave detectors like the Einstein Telescope come online. They are expected to catch as many as 100 new gravitational wave (GW) events yearly - an order of magnitude greater than today's detectors can. With proper support from a constellation like HERMES-PF watching the sky for the GW's electromagnetic equivalent, multi-messenger astronomy could take off to a new level of sophistication.
Learn More:
F. Fiore et al. - HERMES Pathfinder & SpIRIT: a progress report
HERMES-PF Team - HERMES Pathfinder on its way to orbit: successful launch!
UT - First Cosmic Event Observed in Both Gravitational Waves and Light
UT - There's a Chorus of Gravitational Waves Coming from the Core of the Milky Way. Will We Hear Them?
