A recent discovery has revealed a supermassive black hole tearing apart a large star, three times the mass of the Sun, in a death scene known as a tidal disruption event (TDE). But this particular case, called ASASSN–14li, is unique for a multitude of reasons.
Two crucial factors: This star, undergoing a TDE, holds the record as one of the largest ever observed and stands out as one of the closest discoveries in the past decade, positioned just 290 million light-years away. Due to the relatively close proximity and unusual size of the star, astronomers were able to obtain key details to some unanswered mysteries behind the process of this event. The team published their results Aug. 20 in The Astrophysical Journal Letters.
Detecting TDEs
Although it’s a common belief that nothing can escape a black hole’s grasp, a black hole can interact with a luminous object without causing it to disappear into the black hole.
As an object nears a black hole, it experiences a tidal force that can stretch it apart, causing it to shed material in a TDE. But astronomers are not certain how this process begins. Hypothetically, once a star enters the tidal disruption radius of a black hole, the black hole turns the star inside-out, and its material starts moving at incredibly high speeds, forming a bright stream toward the black hole.
Alternatively, the material can also become unbound. Even as this happens, the star continues its orbit and undergoes a TDE each trip around the black hole, which eventually forms a small accretion-like disk.
Forensic telescopes
Now clues — in the form of X-rays from ASASSN–14li — have confirmed this idea. X-ray data captured using NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton space telescopes have allowed scientists to retrace the scene of the crime for this massive star.
“These X-ray telescopes can be used as forensic tools in space,” said co-author Brenna Mockler of Carnegie Observatories and the University of California, Los Angeles, in a press release.
Analyzing hot spots in X-ray observations reveals particles reaching high temperatures due to either massive explosions or intense gravitational fields. Additionally, using these telescopes to observe at different wavelengths produces an X-ray spectrum for the scene, showing the chemical composition of the material that’s glowing.
The Chandra spectrum reveals that “the relative amount of nitrogen to carbon that we found points to material from the interior of a doomed star,” said Mockler. Fortunately for us, th material is being pushed toward Earth and away from the supermassive black hole, as confirmed by a blueshift in the color of the light. (A more familiar redshift occurs when material is moving away from Earth.) The spectrum accentuates an abundance of nitrogen and the lack of carbon, which validates the star’s mass of three times the Sun’s mass. Now, astronomers can fine-tune any models they use to more accurately narrate the ongoing story of the stellar victim’s demise.
“ASASSN–14li is exciting because one of the hardest things with tidal disruptions is being able to measure the mass of the unlucky star,” said co-author Enrico Ramirez-Ruiz of the University of California, Santa Cruz.
Reconstructing the crime scene
Analyzing data from ASASSN–14li and other Tidal Disruption Events (TDEs) can yield improved models, estimating the presence of nitrogen and carbon around the black hole. Particularly, using NASA’s Neil Gehrels Swift Observatory, scientists have already been able to create an automated search of X-ray emitting TDEs to keep a vigilant watch on these phenomena.
And with a combination of findings, astronomers will have the chance to identify the possible presence of star clusters surviving in the harsh environment around supermassive black holes in distant galaxies.
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