On April 1st, 2019, LIGO and its Italian partner, VIRGO began their search for gravitational waves, called O3 for third observing run. During the first month of operations the LIGO-VIRGO network spotted five stellar collisions. The Neil Gehrels Swift Observatory responded to the alerts and observed thousands of galaxies within the LIGO-VIRGO localizations in order to find the luminous electromagnetic counterparts of the gravitational wave signal.
On April 26th, 2019, the LIGO-VIRGO network issued an alert for the possible collision of a neutron star with a black hole. This event was named S190426c. Swift began pointed UV and X-ray tiling of the localization region of S190426c 142 minutes after the GW alert. The campaign continued for 2 days and observed over 32% of the galaxy-convolved localization in >800 fields covering thousands of massive galaxies.
The Burst Alert Telescope aboard the Neil Gehrels Swift Observatory continuously scans the sky producing one of the most sensitive all-sky surveys in the hard X-rays. The survey is especially useful at finding nearby active galaxies, even those obscured at other wavelengths by gas and dust. Using high resolution IR observations of active galaxies detetected with BAT, a team of astronomers found a surprising number of galaxies in the final stages of merging together into single, larger galaxies. Peering through thick walls of gas and dust surrounding the merging galaxies' messy cores, the research team captured pairs of supermassive black holes drawing closer together before they coalescence into one giant black hole. It is not easy to find galactic nuclei so close together. Most prior observations of merging galaxies have caught the coalescing black holes at earlier stages, when they were about 10 times farther away. When the black holes finally do collide, they will unleash enormous energy in the form of gravitational waves.
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About a year ago, astronomers excitedly reported the first detection of electromagnetic waves, or light, from a gravitational wave source. Now, a year later, researchers are announcing the existence of a cosmic relative to that historic event. GRB150101B was a gamma-ray burst that lasted less than a second, localized by the NASA's Neil Gehrels Swift Observatory in an old elliptical galaxy 1.7 billion years away. The apparent match between GRB 150101B and GW170817 is striking: both produced an unusually faint and short-lived gamma ray burst, and both were a source of bright, blue optical light lasting a few days, and X-ray emission lasted much longer. The host galaxies are also remarkably similar. Both are bright elliptical galaxies with a population of stars a few billion years old and displaying no evidence for new stars forming. A paper reporting the results was published today in Nature Communications.
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