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Accretion, not colliding spaghetti, flares up as star is devoured by black hole

An extremely bright flare, originating from a star being devoured by a supermassive black hole, has been observed in a galaxy 215 million light-years away – making this the nearest tidal disruption event (TDE) ever seen.

The event was spotted by an international team of astronomers, headed by Matt Nicholl at the University of Birmingham. They caught the event well before its climax using instruments at the European Southern Observatory (ESO) in Chile. For the first time, the observations allowed astronomers to connect the characteristic brightening of these events with rapid outflows of material from stars.

If a star wanders too close to the supermassive black hole at the centre of its galaxy, it can experience tidal forces that exceed the gravitational forces holding the star together. As a result, the star will be dramatically shredded into thin streams of debris, through the process of spaghettification. During these TDEs, stellar remnants will flare, emitting large amounts of light.

Colliding spaghetti


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Currently, there are two competing theories for these surges in brightness: either they occur as material accretes onto the black hole; or they result from earlier collisions between spaghettified streams. Although astronomers are now discovering several TDEs every year, they have yet to determine which of these theories is best.

The main problem is that as the stars disintegrate, their colliding streams, combined with strong inflows and outflows of gas, make for a messy debris geometry which is incredibly difficult to disentangle.

In September 2019 the ESO’s Very Large Telescope and New Technology Telescope each spotted a new surge in brightness in a spiral galaxy. Through further calculations, Nicholl’s team concluded that the flash originated from a supermassive black hole as large as 1 million solar masses, as it devoured a star with a similar mass to the Sun.

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Over six months, the instruments recorded the event across multiple regions of the electromagnetic spectrum as its brightness grew and faded. Owing to its proximity, Nicholl and colleagues were able to identify the TDE well before its peak brightness. This allowed them to capture the whole process unfolding, well before the geometry of the debris became too convoluted to untangle.

Sudden transition


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By tracking changes in the blueshift of key absorption lines in the stellar debris, Nicholl’s team showed that the origin of the TDE’s early optical emission was dominated by an outflow of bright material, with speeds reaching roughly 10,000 km/s. Then, after around 30 days, the outflow underwent a sudden transition: first cooling, and then contracting.


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Overall, the size and mass of the outflow remained consistent with the high ratio between optical and -ray emissions observed by the team, as well as in many TDEs from previous studies. This suggested that the outflow was more likely to have been powered by black hole accretion, instead of collisions between spaghettified streams of debris.

With the upcoming launch of ESO’s Extremely Large Telescope, now scheduled for first light in 2025, the team’s findings will provide researchers with key guidance as they uncover increasingly fainter and more rapidly evolving TDEs.

Source:

Physics World

Journal Reference:

An outflow powers the optical rise of the nearby, fast-evolving tidal disruption event AT2019qiz

ABSTRACT:

At 66 Mpc, AT2019qiz is the closest optical tidal disruption event (TDE) to date, with a luminosity intermediate between the bulk of the population and the faint-and-fast event iPTF16fnl. Its proximity allowed a very early detection and triggering of multiwavelength and spectroscopic follow-up well before maximum light.


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The velocity dispersion of the host galaxy and fits to the TDE light curve indicate a black hole mass ≈106 M⊙, disrupting a star of ≈1 M⊙. By analysing our comprehensive UV, optical, and X-ray data, we show that the early optical emission is dominated by an outflow, with a luminosity evolution L ∝ t2, consistent with a photosphere expanding at constant velocity (≳2000 km s−1), and a line-forming region producing initially blueshifted H and He II profiles with v = 3000–10 000 km s−1.

The fastest optical ejecta approach the velocity inferred from radio detections (modelled in a forthcoming companion paper from K. D. Alexander et al.), thus the same outflow may be responsible for both the fast optical rise and the radio emission – the first time this connection has been observed in a TDE. The light-curve rise begins 29 ± 2 d before maximum light, peaking when the photosphere reaches the radius where optical photons can escape.

The photosphere then undergoes a sudden transition, first cooling at constant radius then contracting at constant temperature. At the same time, the blueshifts disappear from the spectrum and Bowen fluorescence lines (N III) become prominent, implying a source of far-UV photons, while the X-ray light curve peaks at ≈1041 erg s−1.

Assuming that these X-rays are from prompt accretion, the size and mass of the outflow are consistent with the reprocessing layer needed to explain the large optical to X-ray ratio in this and other optical TDEs, possibly favouring accretion-powered over collision-powered outflow models.

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