Tech UPTechnologyThe first mergers of black holes and neutron stars

The first mergers of black holes and neutron stars

This week has been a glorious one for gravitational wave astronomy. For the first time, not one but two new collisions of massive astronomical objects have been detected. The novelty is not that there have been two, but that they are of the same type and that this has not been observed so far.

Indeed, in both cases it is a question of the merger (although perhaps the word “heartbreaking encounter” is better here) of a black hole with a neutron star. In the first of the two collisions (codename GW200105), the black hole was about 10 times the mass of the Sun and the neutron star was twice as massive as ours. In the second (GW200115), both objects were slightly smaller. What is supposed to happen in these cases is that the neutron star ends up being “dislodged” and its remains end up scattered in orbit around the black hole. Despite how large they may seem to us, their diameter hardly exceeds the metropolitan area of Madrid.

eye! This is not traditional astronomy. Here we are not working with light, but with the measurement of very subtle disturbances of space-time . When it comes to the first, we have hundreds, perhaps thousands of sensors (telescopes, satellites, etc.) to capture the information that comes to us. In gravitational wave astrophysics, we only have a handful of detection instruments. Perhaps the most famous are Ligo (in the US) and Virgo (in Italy). Well, these three devices (Ligo is composed, in turn, of two) detected, in January 2020 (yes, before the pandemic that we all want to leave behind), two cataclysmic events, which occurred with just a few days of difference. But… wait: weren’t they discovered this week?

Not quite. What has happened in these past days has been the verification of the two events. How? Almost a year and a half later? Why? Very easy. Everything has to do with the word that we have used previously: “very subtle.” So it is: the signal left by the passage of a gravitational wave when it permeates the Earth is so imperceptible that it is necessary to perform extremely complex numerical and computational analyzes to reveal the existence of a real astronomical event. The signal analyzed is none other than the variation in the distance between two mirrors, measured with the precision of a beam of laser light and with an astonishing frequency, which translates into terabytes and terabytes of information. When a cosmic incident such as the one described occurs, it leaves an imprint in the sky that we “suffer” millions of years later (900 in this case) as an accordion effect in space-time. Then we need another year or so of calculations and calculations to ratify it.

Truth be told, this has not been the first time that a detection of this type has occurred, but it is the first time that there is almost total certainty. Two similar events were found in 2019 but were ruled out for prudence. In gravitational wave astrophysics, the signal-to-noise ratio is very important. If this does not exceed a certain threshold (on the other hand, very demanding), an event is discarded without regard. In these two recent mergers, this relationship was exceptionally good (and yet it has taken 18 months to identify!).

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