Three years back, astronomers spotted the nearest example to date of a star that was shredded or “spaghettified” after approaching too close to a massive black hole. A Sun-like star was tidally disrupted by a black hole which is one million times more massive than the star. Material being blown away from a black hole after it is torn apart is known as tidal disruption, according to NASA.
The “spaghettification” took place 215 million light years from Earth. Before this, the closest tidal disruption to be discovered in the recent past took place in the centre of a galaxy about 290 million light years away from Earth.
Tidal Disruption
According to NASA, early in the evolution of a tidal disruption, material from the shredded star should be pulled towards the black hole at a high rate, generating a huge amount of light. As the disrupted material falls into the black hole, the amount of light should decline.
Astronomers from the University of California, Berkeley, were able to study the optical light from the stellar death because this was the first such event bright enough to make it possible to analyse the light.
The astronomers also studied the light’s polarisation, to understand what happened after the star was torn apart. When light reflects off the surface of particles, it can become polarised, which means that its electric fields line up together in the same direction. Normally, electric fields are oriented in all directions.
The Star Is Surrounded By A Spherical Cloud
The astronomers observed the tidal disruption of the star 215 million light years away on October 8, 2019. These observations suggest that a lot of the star’s material was blown away at high up speed, up to 10,000 kilometres per second. The star’s material formed a spherical cloud of gas that blocked most of the high-energy emissions produced as the black hole grabbed the remainder of the star.
The blast was called AT2019qiz. Observations of optical light from the blast had revealed earlier that much of the star’s matter was launched outward in a powerful wind. The light’s polarisation was essentially zero at visible or optical wavelengths when the event was at its brightest. The new data on the light’s polarisation tells astronomers that the cloud of gas was likely spherically symmetric.
The study describing the results was recently published in the journal Monthly Notices of the Royal Astronomical Society.
What Makes The Study Unique?
In a statement released by University of California, Berkeley, Alex Filippenko, one of the authors on the paper, said this is the first time anyone has deduced the shape of the gas cloud around a tidally spaghettified star.
The new study gives an answer to why astronomers do not see high-energy radiation, such as X-rays, from many of the dozens of tidal disruption events observed to date. According to the study, the X-rays, which are produced by material ripped from the star and dragged into an accretion disk around the black hole before falling inward, are obscured from view by the gas blown outward by powerful winds from the black hole.
Kishore Patra, the lead author on the paper, said the observation rules out a class of solutions that have been proposed theoretically and gives astronomers a stronger constraint on what happens to gas around a black hole.
He further said that people have been seeing other evidence of wind coming out of these events, and that the polarisation study definitely makes that evidence stronger, in the sense that one would not get a spherical geometry without having a sufficient amount of wind. He explained that interestingly, a significant fraction of the material in the star that is spiralling inward does not eventually fall into the black hole. Rather, it is blown away from the black hole.
How Light Polarisation Reveals Symmetry
According to theorists, stellar debris forms an eccentric, asymmetric disk after tidal disruption. However, an eccentric disk is expected to show a relatively high degree of polarisation, which would mean that several per cent of the total light is polarised. But this was not observed for the tidal disruption event studied.
Wenbin Lu, one of the authors on the paper, said one of the craziest things a supermassive black hole can do is to shred a star by its enormous tidal forces. He explained that stellar tidal disruption events are one of very few ways astronomers know the existence of supermassive black holes at centres of galaxies and measure their properties. However, astronomers still do not understand the complicated processes after a tidal disruption because of the extreme cost in numerically simulating such events.
Light Was Found To Be Slightly Polarised In A Different Set Of Observations
On November 26, 2019, astronomers made a second set of observations. These observations, made 29 days after the October observation, revealed that the light was very slightly polarised, about one per cent. This suggests that the cloud had thinned enough to reveal the asymmetric gas structure around the black hole.
Both the October and November observations were made using the three-metre Shane telescope at Lick Observatory near San Jose, California. The telescope is fitted with the Kast spectrograph, an instrument that can determine the polarisation of light over the full optical spectrum. According to the study, the light becomes polarised when it scatters off electrons in the gas cloud.
Patra said the accretion disk is hot enough to emit most of its light in X-rays, but that light has to come through this cloud. Also, there are many scatterings, absorptions and reemissions of light before it can escape out of the cloud.
He further said that with each of these processes, the light loses some of its photon energy, going all the way down to ultraviolet and optical energies. The final scatter then determines the polarisation state of the photon. Therefore, by measuring polarisation, one can deduce the geometry of the surface where the final scatter happens.
According to Patra, this deathbed scenario may apply only to normal tidal disruptions, and not “oddballs”, in which relativistic jets of material are expelled out of the poles of the black hole. The question can be answered only after more measurements of the polarisation of light are made.
Patra said polarisation studies are an “uncharted territory” for tidal disruption events.
Gas Cloud Around The Star Is 100 Times Larger Than Earth’s Orbit
The astronomers calculated that the polarised light was emitted from the surface of a spherical cloud with a radius of about 100 astronomical units. An astronomical unit is a unit of measurement equal to 149.6 million kilometres, the mean distance from the centre of the Earth to the centre of the Sun.
The surface of the spherical cloud was 100 times farther from the star than Earth is from the Sun. According to the study, an optical glow from hot gas emanated from a region at about 30 astronomical units.
The 2019 spectropolarimetric observations were of AT2019qiz, a tidal disruption event located in a special galaxy in the constellation of Eridanus. Spectropolarimetric observation is a technique that measures polarisation across many wavelengths of light.
According to the study, the zero polarisation of the entire spectrum in October indicates a spherically symmetrical cloud gas. This is because all the polarised photons balance one another.
In the November measurements, a slight polarisation was observed. This indicates a small asymmetry.
The tidal disruptions appear as only a point of light because they occur very far away, in the centres of distant galaxies. Therefore polarisation is one of few indications of the shapes of objects, the study said.
Filippenko explained that these disruption events are so far away that one cannot really resolve them. So, one cannot study the geometry of the event or the structure of these explosions.
However, studying polarised light actually helps researchers to deduce some information about the distribution of the matter in that explosion, he said. In this case, studying the polarised light helped astronomers deduce information about how the gas, and possibly the accretion disk, around the black hole is shaped.