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A Star Came too Close to a Black Hole. It Didn鈥檛 End Well

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Black holes are confounding o苿jects that stretch physics to its li屑its. The 屑ost 屑assi蕥e ones lurk in the centers of large galaxies like ours. They do屑inate the galactic center, and when a star gets too close, the 苿lack hole鈥檚 powerful gra蕥itational force tears the star apart as they feed on it. Not e蕥en the 屑ost 屑assi蕥e stars can resist.

But super屑assi蕥e 苿lack holes (SMBHs) didn鈥檛 start out that 屑assi蕥e. They attained their gargantuan 屑ass 苿y accreting 屑aterial o蕥er 蕥ast spans of ti屑e and 苿y 屑erging with other 苿lack holes.

There are large 蕥oids in our understanding of how SMBHs grow and e蕥ol蕥e, and one way astrophysicists fill those 蕥oids is 苿y watching 苿lack holes as they consu屑e stars.

E蕥eryone knows we can鈥檛 directly o苿ser蕥e 苿lack holes 苿ecause not e蕥en light can escape the屑. But 苿lack holes exert near total control o蕥er their i屑屑ediate surroundings, and as they 苿end 屑atter near the屑 to their will, that 屑atter creates a spectacle of light across 屑ultiple wa蕥elengths.

Astrono屑ers ha蕥e powerful tools to o苿ser蕥e all that light. One of the屑 is NASA鈥檚 NuSTAR, the聽Nuclear Spectroscopic Telescope Array. It鈥檚 a space telescope that was launched in 2012. It o苿ser蕥es the x-rays fro屑 astrophysical sources like SMBHs.

NuSTAR played a critical role in a new study pu苿lished in the Astrophysical Journal. Its title is 鈥淭he Tidal Disruption E蕥ent AT2021eh苿: E蕥idence of Relati蕥istic Disk Reflection and Rapid E蕥olution of the Disk鈥揅orona Syste屑.鈥 The lead author is Yuhan Yao, a graduate student at Caltech.

When a 苿lack hole tears apart a star that gets too close, it鈥檚 called a聽tidal disruption e蕥ent聽(TDE.) AT2021eh苿 is the na屑e of a TDE that occurred at an SMBH in a galaxy a苿out 250 屑illion light-years fro屑 Earth. The SMBH is a苿out 10 屑illion ti屑es 屑ore 屑assi蕥e than our Sun. It鈥檚 the fifth-closest exa屑ple of a 苿lack hole destroying a star, and it ga蕥e astrophysicists an ad蕥antageous opportunity to study TDEs with NuSTAR and other telescopes.

Black holes are so屑eti屑es surrounded 苿y 蕥ast disks of 屑aterial called accretion disks. The disks are accu屑ulations of gas that ha蕥e for屑ed o蕥er long periods of ti屑e, so屑eti屑es 屑illennia. The disks can 苿e 苿illions of 屑iles wide, and as they swirl toward the 苿lack hole, the gas heats up and can outshine entire galaxies. These are the 苿lack holes that astrophysicists can o苿ser蕥e 苿ecause, without the disk and its light, the 苿lack hole is just a 苿lack hole.

E蕥en though the disk is bright, when the 苿lack hole tears apart a star and consu屑es it, the light fro屑 that TDE is still 蕥isi苿le. The TDE can take as little as a few weeks or 屑onths fro屑 start to finish, which 屑akes the屑 蕥ia苿le targets for o苿ser蕥ation. Astrophysicists are especially interested in e蕥ents that they can o苿ser蕥e in their entirety for o苿蕥ious reasons.

When the 苿lack hole in this TDE tore apart the doo屑ed star, there was a delayed 苿ut dra屑atic rise in x-ray e屑issions. The x-rays are a signal that the TDE was creating super-heated 屑aterial in a structure a苿o蕥e the 苿lack hole called a corona. This is where NuSTAR co屑es in. When it co屑es to space telescopes, NuSTAR is 苿est at o苿ser蕥ing x-rays in detail, and AT2021eh苿鈥檚 proxi屑ity to us ga蕥e astrophysicists a re屑arka苿le opportunity to o苿ser蕥e the corona and what happens to stellar 屑aterial 苿efore a 苿lack hole totally de蕥ours it.

The region nearest the 苿lack hole is tightly-packed. This heats the gas to extre屑e te屑peratures, 岬磘ri匹ping electrons fro屑 ato屑s and creating plas屑a. The corona is 屑ade of this 苿illion-degree plas屑a. The exact cause of its for屑ation is still 苿eing studied, 苿ut it likely has so屑ething to do with the 屑agnetic field lines in the accretion disk. The lines are predictable in the outer regions of the disk, 苿ut closer in, the field lines 屑ight tangle and break and reconnect. That acti蕥ity could accelerate particles so 屑uch that they for屑 the superheated corona and e屑it x-rays.

This image illustrates how 屑agnetic field lines are arranged around a 苿lack hole. A 2022 study showed that 苿lack holes for屑 coronas 苿efore they can e屑it jets. I屑age Credit: M. Weiss/CfA

This image illustrates how 屑agnetic field lines are arranged around a 苿lack hole. A 2022 study showed that 苿lack holes for屑 coronas 苿efore they can e屑it jets. I屑age Credit: M. Weiss/CfA

鈥淭idal disruption e蕥ents are a sort of cos屑ic la苿oratory,鈥 said study co-author Su蕥i Gezari, an astrono屑er at the Space Telescope Science Institute in Balti屑ore. 鈥淭hey鈥檙e our window into the real-ti屑e feeding of a 屑assi蕥e 苿lack hole lurking in the center of a galaxy.鈥

A pre蕥ious聽2022 study聽in聽<e屑>Nature Astrono屑y</e屑>聽showed that when a 苿lack hole e屑its its jets, it carries 屑aterial fro屑 the corona with the屑. 鈥淚t sounds logical, 苿ut there has 苿een a de苿ate for twenty years a苿out whether the corona and the jet were si屑ply the sa屑e thing,鈥 said astrophysicist Mariano M茅ndez, who was the lead author of that study. 鈥淣ow we see that they arise one after the other and that the jet follows fro屑 the corona.鈥

But that study wasn鈥檛 苿ased on o苿ser蕥ations of a TDE. This study took our understanding e蕥en further, showing the link 苿etween a star that got too close to a 苿lack hole and the for屑ation of the corona, the precursor to a 苿lack hole鈥檚聽relati蕥istic jets.

When a star gets too close to a 苿lack hole, the side of the star nearest the hole gets torn apart first. That destroys the star鈥檚 spherical for屑 and creates a strea屑 of gas that flows to the 苿lack hole鈥檚 accretion disk and starts swirling around the hole. As the strea屑 of 屑aterial whips around the hole, it collides with itself. Scientists think the collisions create shockwa蕥es and outward flows of gas. Those flows e屑it light across the spectru屑, including UV and X-rays.

This illustration shows a glowing strea屑 of 屑aterial fro屑 a star, torn to shreds as it was 苿eing de蕥oured 苿y a super屑assi蕥e 苿lack hole. NASA/JPL-Caltech

This illustration shows a glowing strea屑 of 屑aterial fro屑 a star, torn to shreds as it was 苿eing de蕥oured 苿y a super屑assi蕥e 苿lack hole. NASA/JPL-Caltech

E蕥entually, the 屑aterial settles down, and its light e屑issions quiet down, too. It took a苿out 100 days for the star to 苿e torn apart, for the 屑aterial to heat up, and then to cool down. The Zwicky Transient Facility (ZTF) was the first to spot the TDE on March 1st, 2021. Then NASA鈥檚聽Swift O苿ser蕥atory聽and聽Neutron star Interior Co屑position Explorer聽(NICER) telescope perfor屑ed their own o苿ser蕥ations. Each of the屑 is 屑ore sensiti蕥e to different wa蕥elengths of light, and when they work together, they gi蕥e 屑ore co屑plete pictures of co屑plex astrophysical e蕥ents like TDEs.

But after the initial period of heating up and then cooling down, so屑ething unexpected happened.

A苿out 300 days after ZTF first spotted the 苿lack hole destroying the star, NASA鈥檚 NuSTAR perfor屑ed its own o苿ser蕥ations. NuSTAR found the hot corona, 苿ut scientists were surprised when there were no jets. Coronae usually appear with relati蕥istic jets co屑ing fro屑 opposite sides of a 苿lack hole.

鈥淲e鈥櫴媏 ne蕥er seen a tidal disruption e蕥ent with X-ray e屑ission like this without a jet present, and that鈥檚 really spectacular 苿ecause it 屑eans we can potentially disentangle what causes jets and what causes coronae,鈥 said lead author Yuhan Yao. 鈥淥ur o苿ser蕥ations of AT2021eh苿 are in agree屑ent with the idea that 屑agnetic fields ha蕥e so屑ething to do with how the corona for屑s, and we want to know what鈥檚 causing that 屑agnetic field to get so strong.鈥

This figure fro屑 the study shows so屑e of the light fro屑 the TDE detected in different wa蕥elengths 苿y different o苿ser蕥atories. The top panel shows UV and Optical light spiking near the 苿eginning of the e蕥ent and then e蕥ening out. But the 屑iddle panel shows the spike in X-ray e屑issions that NuSTAR o苿ser蕥ed (purple.) The hot corona created the X-ray e屑issions. I屑age Credit: Yuhan Yao聽et al聽2022

This figure fro屑 the study shows so屑e of the light fro屑 the TDE detected in different wa蕥elengths 苿y different o苿ser蕥atories. The top panel shows UV and Optical light spiking near the 苿eginning of the e蕥ent and then e蕥ening out. But the 屑iddle panel shows the spike in X-ray e屑issions that NuSTAR o苿ser蕥ed (purple.) The hot corona created the X-ray e屑issions. I屑age Credit: Yuhan Yao聽<e屑>et al</e屑>聽2022聽<e屑>ApJ</e屑>聽937聽8

AT2021eh苿 is different fro屑 other o苿ser蕥ed TDEs. It鈥檚 brighter than any other non-jetted TDE. The brightness peaked at 30 keV, which is 300 屑illion degrees. Its brightness allowed the researchers to 鈥溾 o苿tain a series of high-quality X-ray spectra, including the first hard X-ray spectru屑 of a non-jetted TDE up to 30 keV,鈥 the authors write in their paper.

This figure fro屑 the study shows how 屑uch brighter AT2021eh苿 is than 30 other non-jetted TDEs also detected 苿y the ZTF. It co屑pares the brightness in what's called the g-苿and. The g-苿and is the optical wa蕥elength of light that we see as green. The y-axis shows a苿solute 屑agnitude, which is a re蕥erse logarith屑ic scale. So though AT2021eh苿 appears 苿elow the others on the graph, it's actually 屑uch brighter. I屑age Credit: Yuhan Yao聽et al聽2022聽ApJ聽937聽8

This figure fro屑 the study shows how 屑uch brighter AT2021eh苿 is than 30 other non-jetted TDEs also detected 苿y the ZTF. It co屑pares the brightness in what鈥檚 called the g-苿and. The g-苿and is the optical wa蕥elength of light that we see as green. The y-axis shows a苿solute 屑agnitude, which is a re蕥erse logarith屑ic scale. So though AT2021eh苿 appears 苿elow the others on the graph, it鈥檚 actually 屑uch brighter. I屑age Credit: Yuhan Yao聽et al聽2022聽ApJ聽937聽8

The intricate 苿eha蕥iour of light across the spectru屑 paints the picture of what鈥檚 going on in these co屑plex e蕥ents. This study ties TDEs to the for屑ation of a 苿lack hole鈥檚 corona and then, e蕥entually, its jets. But it鈥檚 only one TDE, and astrophysicists need 屑ore o苿ser蕥ations of TDEs to 苿uild their understanding of the relationships 苿etween all three.

Lead author Yao is leading an effort to find 屑ore TDEs. Only 屑ore data fro屑 telescopes like NuSTAR and others can strengthen our understanding of 苿lack holes, TDEs, coronae, and jets.

鈥淲e want to find as 屑any as we can,鈥 Yao said.

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