Gamma-ray bursts (GRBs) are the most powerful explosions the Universe has seen since the Big Bang. They occur approximately once per day and are brief, but intense, flashes of gamma radiation. They come from all different directions of the sky and last from a few milliseconds to a few hundred seconds. So far scientists do not know what causes them. Do they signal the birth of a black hole in a massive stellar explosion? Are they the product of the collision of two neutron stars? Or is it some other exotic phenomenon that causes these bursts?
With Swift, a NASA mission with international participation, scientists have a tool dedicated to answering these questions and solving the gamma-ray burst mystery. Its three instruments give scientists the ability to scrutinize gamma-ray bursts like never before. Within seconds of detecting a burst, Swift relays its location to ground stations, allowing both ground-based and space-based telescopes around the world the opportunity to observe the burst's afterglow. Swift is part of NASA's medium explorer (MIDEX) program and was launched into a low-Earth orbit on a Delta 7320 rocket on November 20, 2004. The Principal Investigator is Dr. Brad Cenko (NASA-GSFC).
Swift resumed observing pre-planned science targets at about 19:57 UT on September 11, but the BAT instrument is not triggering on new transients such as GRBs. The Swift team is working to return the BAT to normal operations.
On August 21, 2016 NASA's Neil Gehrels Swift Observatory discovered a new gamma-ray burst, named GRB160821B, and began tracking its light minutes after it was detected. The burst, which lasted less than two seconds, was localized in the outskirts of a distant spiral galaxy, 2.5 billion years away from Earth. By using a wide array of ground-based and space-based telescopes, astronomers followed the evolution of its light and found the best evidence for a kilonova - a turbocharged explosion that instantly forged several hundred planets' worth of gold and platinum. Astronomers suspect that all of the gold and platinum on Earth formed as a result of ancient kilonovae created during the collisions of neutron stars.
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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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