When two neutron stars collide and merge, what do you get? A beefier neutron star or a tiny black hole? A May 2018 paper looking into last year's historic neutron star collision suggests the latter.
On Aug. 17, 2017, the U.S.-based Laser Interferometer Gravitational-wave Observatory (LIGO) and Italian Virgo gravitational wave observatory detected what is arguably the most significant astronomical event in modern times: a neutron star smashup. That smashup created a gravitational wave signal named GW170817. Unlike the handful of gravitational wave signals that came before it, GW170817 wasn't generated by a merging black hole.
Three gravitational wave observatories (the two LIGO stations in Washington and Louisiana, plus the single Virgo detector) detected the signal in concert, so scientists were able to triangulate the approximate location in the sky where the gravitational wave signal came from. Then, at roughly the same time, NASA's Fermi space telescope detected a short gamma-ray burst (GRB) in that patch of sky. Scientists had theorized that such bursts were triggered by two neutron stars colliding, and through the analysis of GW170817, they confirmed the neutron star merger scenario.
Astronomers made many scientific discoveries in the wake of this astronomical event, but GW170817 just keeps on giving. With the help of NASA's Chandra space telescope, which continued to study the site of the neutron star merger in the days, weeks and months afterward, astronomers now think that the neutron star merger birthed a baby black hole. And we've never seen that before.
From the LIGO studies, astronomers already had a pretty good idea as to the mass of the colliding neutron stars and the mass of the object that they ought to produce post-collision. By their estimates, the merged object would have a mass of around 2.7 times that of our sun. This is an interesting mass as it's right on the edge of either being the most massive neutron star or the lowest mass black hole ever discovered. To work out whether the event created a monster neutron star or a tiny black hole, astronomers needed to study the X-rays being generated, and that's where Chandra helped out.
"While neutron stars and black holes are mysterious, we have studied many of them throughout the Universe using telescopes like Chandra," said Dave Pooley of Trinity University in San Antonio, Texas, who led the study. "That means we have both data and theories on how we expect such objects to behave in X-rays."