Black holes and neutron star collisions

The recent observations of gravitational waves from cosmic events opens a dramatic new window on the universe (first in 2015).  The first four events observed by gravitational waves were colliding black holes with masses between 7.5 and 35 solar masses. As black holes do not emit any radiation, there is no electromagnet radiation from the resultant black holes although the actual collision produces a gamma ray pulse which has been detected for one of the events. The amount of energy radiated away as gravitational energy from these black hole collisions is immense: between 1 and 3 solar masses of energy emitted in a short time of order or less than 100 seconds.

Even more interesting in terms of observations was the GW170817 event (published on 16 Oct 2017) which was caused by the collision of two neutron stars approximately 130 million years ago. 

GW170817 in NGC4993 (NASA APOD)

For the GW170817 event, a gamma ray burst was detected 1.7 seconds after the gravitational wave ended.  Using the information from gravitational and gamma ray observations, several observatories were able to identify a optical object  in the galaxy NGC4993 as the source of the gravitational waves.  Radio waves from the event have also been observed.

Analysis of the ultraviolet, visible and infrared data are consistent with a large amount of fast-moving neutron-rich debris ejected from the collision.  The cooling of this neutron-rich debris creates wide range of elements including elements heavier than iron.  It is estimated that 1600 times the mass of the earth of elements heavier than iron were formed.

The detection of both gravitational waves and gamma rays from the GW170817 event showed that gravitational waves do travel at the speed of light to an accuracy of 4.5 x 10-16.  This puts limits on theories of dark energy [] [ New Scientist 28 Oct 2017]




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