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    Behold: 'The Screams of a Star Being Ripped Apart'

    Written by

    Michael Byrne


    There is more to the violence of a black hole than the cataclysmic inward suck of infinite gravity. There is a yang to the yin of a black hole's inward attraction (accretion), which takes the form of relativistic jets that blast outward from the black hole stretching for sometimes millions of light-years.

    A recent discovery, described in a paper published Friday in Science, offers a new perspective on these jets thanks to a passing star unfortunate enough to wander into the outflowing blast from a quasar (either a supermassive black hole or a binary black hole system found at the center of a galaxy) spotted by the All-Sky Automated Survey for Supernovae (ASAS-SN) project. The event is known as ASASSN-14li.

    The star in question was promptly ripped to shreds, resulting in a tidal disruption flare (TDF) as its remains fell inward and were then blasted back out into space by the energy of the black hole's jet. A TDF is a very rare event—occurring about once every 100,000 years or so per galaxy—and one highly sought after by astronomers trying to generally explain the accretion/outflow balance of black holes. This is an ongoing challenge for the simple reason that the processes involved typically evolve at timescales much longer than a human lifespan.

    TDFs, however, offer a much shorter timeframe, an accelerated perspective on the otherwise masked accretion/outflow processes of a supermassive black hole. "TDFs are events associated with the gravitational disruption of a star making a close passage to a super massive black hole," explains astrophysicist Geoffrey C Bower in the awesomely titled Science perspective, "The Screams of a star Being Ripped Apart." "If a star passes within the tidal radius of a black hole then the differential gravity on the star will rip the star apart."

    "As the stellar remnant approaches the black hole," Bower writes, "its gravitational potential energy is converted into heat through viscous effects. The accretion flow will reach a temperature of 105 K and glow brightly at optical, ultraviolet, and x-ray wavelengths for about 100 days."

    This is a funny thing about black holes—they have so much gravitational pull that all of the infalling material winds up smashing up against other infalling material, with the result being tremendous amounts of energy released via friction. Not everything just falls into the void. These are the jets we're talking about, and they're what ripped apart a star to result in the TDF event dubbed ASASSN-14li.

    This isn't the first TDF identified, but, as the researchers explain, ASASSN-14li is the first observed within 30 days of its peak. Most have been bagged only years later. The ASAS-SN group was able to rule out jets fueled by more typical everyday accretion because this one erupted and then "switched off" so abruptly. There is still a lot to learn about TDFs and black hole jets generally.

    The current ASAS-SN results, Bower concludes, "suggest that new discoveries may also come from large-area surveys at radio wavelengths. Powerful telescopes such as the Very Large Array, as well as new telescopes optimized for surveys such as the Australian Square Kilometer Array Pathfinder and MeerKAT in South Africa, will have a rich future in searching for TDFs."