The Shadow Of The First Black Hole We’ve Ever Imaged Has A Wobble

The first-ever image of a black. M87* or Pōwehi was observed in this detail in 2017. Event Horizon Telescope Collaboration.

The extraordinary image taken last year of Pōwehi, or M87*, the supermassive black hole at the center of galaxy M87, opened a new chapter in astronomy and researchers are now sharing new insights gleaned on how the shadow of the black hole changes. Using the lessons learned last year, they analyzed data collected before the historic photo that suggests the shadow has been in its crescent shape for a while – and it wobbles. The findings are reported in The Astrophysical Journal.

"Last year we saw an image of the shadow of a black hole, consisting of a bright crescent formed by hot plasma swirling around M87*, and a dark central part, where we expect the event horizon of the black hole to be," lead author Maciek Wielgus from the Center for Astrophysics | Harvard & Smithsonian, said in a statement.

"But those results were based only on observations performed throughout a one-week window in April 2017, which is far too short to see a lot of changes. Based on last year's results we asked the following questions: is this crescent-like morphology consistent with the archival data? Would the archival data indicate a similar size and orientation of the crescent?"

Snapshots of the M 87* black hole obtained through imaging / geometric modeling, and the EHT array of telescopes in 2009-2017. M. Wielgus, D. Pesce & the EHT Collaboration

To be able to image the event horizon of the black hole, which is located 53.5 million light-years away, researchers had to create the Event Horizon Telescope, a collaboration of radio observatories from across the world that worked together as a single Earth-sized instrument. But researchers have attempted these observations for a long time.

Data from observations between 2009 to 2013 was not good enough to create an image of the black hole but there was enough data to create models that could be compared to what was seen in 2017 (when the observations for that first image were taken). The models show that the bright crescent was there already and is a persistent feature surrounding the black hole. However, they also discovered that the crescent moved, or wobbled, between observations, meaning it changes with time, even across a matter of days.  

This means researchers can get a glimpse of the structure of the accretion flow so close to the event horizon, despite the extreme gravity conditions.

Cartoon showing the consistency of the measured ring diameter and the uncertainties of the orientation measurement. Only the 2017 data have enough quality for imaging, while for the earlier observations a ring model was fitted. M. Wielgus & the EHT Collaboration

"These early EHT experiments provide us with a treasure trove of long-term observations that the current EHT, even with its remarkable imaging capability, cannot match," said Shep Doeleman, Founding Director of the EHT. "When we first measured the size of M87* in 2009, we couldn't have foreseen that it would give us the first glimpse of black hole dynamics. If you want to see a black hole evolve over a decade, there is no substitute for having a decade of data."

The team conducted observations in 2018 too, which are currently being analyzed. New observations are also planned for next year, with the researchers hoping to get more data, and even potentially create not just new images of M87 but videos of how it changes.

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