SpIRIT & HERMES in the age of Multi-Messenger Astrophysics
Gamma Ray Bursts (GRBs) are some of the most distant and powerful explosions in the Universe, and existing telescopes see hundreds of bright flashes of light every year.
In August 2017, a gravitational wave was detected by gravitational wave detectors around the earth. Simultaneously, a short GRB was also detected by orbiting satellites, Fermi and INTEGRAL. This demonstrated, as had long been theorised, that short GRBs are caused by the merging of Neutron Stars; the source of detected gravitational waves.
This discovery was only possible due to the simultaneous observation of multiple types of signals; gravitational waves and gamma rays, ushering in the era of “multi-messenger Astrophysics”. As the capabilities of gravitational wave detectors dramatically increase in the coming decades, the ability to rapidly search the sky for GRBs will be critical to the continual boundary-pushing of multi-messenger astrophysics.
Existing GRB detection satellites, however, are limited in their capabilities due to either the telescopes pointing the wrong way (because they’re already looking at something else), or because the Earth is in the way. A constellation capable of observing the entire sky at once, therefore, will allow for the quick determination of the direction of Gamma Ray Bursts, and a rapid search in that area of the sky for potential X-ray sources.
The HERMES-TP/SP mini-constellation, carrying miniature X-ray detectors, and consisting of six nanosatellites in complementary orbits around the earth will be able to quickly spot newly-born black holes and other electromagnetic counterparts of Gravitational Wave Events by detecting the X-rays they emit.
SpIRIT & HERMES localisation
The HERMES constellation & SpIRIT will localise X-ray sources by comparing the arrival time of X-ray wavefronts.
Each detector records the intensity of the arriving X-rays, and since the source of the X-rays is the same, the signal received will be identical albeit shifted by some unknown time-delay. The time delay can be recovered by conducting a cross-correlation on the two signals.
By calculating the time-delay for each pair of detectors, the slight time-differences can be used to perform triangulation to determine the direction of the source.