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).
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Using data from NASA's Neil Gehrels Swift Observatory and ESA's XMM-Newton, researchers found evidence of an unusual long decay of X-ray emission coming from the active galaxy GSN 069. The X-ray signal was first seen in July 2010 by XMM-Newton. These observations showed that the source became at least 240 times brighter in X-rays. Since this initial detection, Swift and XMM-Newton observed GSN 069 multiple times and caught the long decay behavior of this outburst spanning about a decade. The X-ray data also indicate that the X-ray spectra are ultra-soft, i.e. steeply decreasing towards higher energies. All these properties point to a rare long-lived tidal disruption event (TDE). A few dozen of these events are known, but are mostly short-lived. The unusual long decay of the X-ray signal in GSN 069 suggests that either this was a late stage of evolution from a typical TDE, or the rare case where a long sustained TDE was caught in a galaxy with a pre-existing AGN. A paper describing these results was published in the journal Astrophysical Journal Letters.
As a star passes too close to a black hole, the black hole’s gravitational pull generates tidal forces on the star, similar to the way in which the moon stirs up tides on Earth. However, a black hole’s gravitational forces are so immense that they can disrupt the star, stretching and flattening it like a pancake and eventually shredding the star to pieces. In the aftermath, a shower of stellar debris rains down and gets caught up in an accretion disk — a swirl of cosmic material that eventually funnels into and feeds the black hole. This entire tidal disruption process generates colossal bursts of energy across the electromagnetic spectrum.
Scientists from MIT and Johns Hopkins University used Swift's X-ray Telescope (XRT) to monitor the X-ray emission arising from ASASSN-14li, a tidal disruption event 300 million light years away discovered in 2014 by the global telescope network ASASSN (All-sky Automated Survey for Supernovae). They noticed that the same fluctuations observed in the X-ray signal appeared 13 days later in the radio band. These radio “echoes,” which are more than 90 percent similar to the event’s X-ray light, are more than a passing coincidence. Instead, they appear to be evidence of a giant jet of highly energetic particles streaming out from the black hole as stellar material is falling in. The highly similar patterns between the X-ray and radio signals suggest that the power of the jet shooting out from the black hole is somehow controlled by the rate at which the black hole is feeding on the obliterated star.
While waiting for new gamma-ray bursts, Swift's Burst Alert Telescope (BAT) continuously scans the hard X-ray sky and accumulates data in survey mode covering a fraction between 50 and 80% of the sky every day. The data collected during the first 105 months of the Swift mission were published in the new Swift BAT 105-month catalog. This catalog is a uniform hard X-ray all-sky survey with a sensitivity of 8.40 x 10-12 erg s-1 cm-2 over 90% of the sky in the 14–195 keV band, and provides 422 new hard X-ray sources identified as Seyfert active galactic nuclei (AGN) in nearby galaxies (34%), X-ray binaries (7%) and blazars/BL Lac objects (10%). A large fraction of these sources remains unidentified. As part of this new edition of the Swift BAT catalog, the eight-channel spectra and monthly sampled light curves for each object are released on the Swift-BAT 105-month website.
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