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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Scientists map gusty winds in a far-off neutron star system    
    The 2D map of this 'disk wind' may reveal clues to galaxy formation.   
      
      
     Date:   
         April 10, 2023   
     Source:   
         Massachusetts Institute of Technology   
     Summary:   
         Astronomers have mapped the 'disk winds' associated with the   
         accretion disk around Hercules X-1, a system in which a neutron   
         star is drawing material away from a sun-like star. The findings may   
         offer clues to how supermassive black holes shape entire galaxies.   
      
      
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   FULL STORY   
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   An accretion disk is a colossal whirlpool of gas and dust that gathers   
   around a black hole or a neutron star like cotton candy as it pulls in   
   material from a nearby star. As the disk spins, it whips up powerful   
   winds that push and pull on the sprawling, rotating plasma. These   
   massive outflows can affect the surroundings of black holes by heating   
   and blowing away the gas and dust around them.   
      
      
   ==========================================================================   
   At immense scales, "disk winds" can offer clues to how supermassive black   
   holes shape entire galaxies. Astronomers have observed signs of disk winds   
   in many systems, including accreting black holes and neutron stars. But   
   to date, they've only ever glimpsed a very narrow view of this phenomenon.   
      
   Now, MIT astronomers have observed a wider swath of winds, in Hercules   
   X-1, a system in which a neutron star is drawing material away from a   
   sun-like star.   
      
   This neutron star's accretion disk is unique in that it wobbles, or   
   "precesses," as it rotates. By taking advantage of this wobble, the   
   astronomers have captured varying perspectives of the rotating disk and   
   created a two- dimensional map of its winds, for the first time.   
      
   The new map reveals the wind's vertical shape and structure, as well   
   as its velocity -- around hundreds of kilometers per second, or about   
   a million miles per hour, which is on the milder end of what accretion   
   disks can spin up.   
      
   If astronomers can spot more wobbling systems in the future, the team's   
   mapping technique could help determine how disk winds influence the   
   formation and evolution of stellar systems, and even entire galaxies.   
      
   "In the future, we could map disk winds in a range of objects and   
   determine how wind properties change, for instance, with the mass of   
   a black hole, or with how much material it is accreting," says Peter   
   Kosec, a postdoc in MIT's Kavli Institute for Astrophysics and Space   
   Research. "That will help determine how black holes and neutron stars   
   influence our universe."  Kosec is the lead author of a study appearing   
   in Nature Astronomy. His MIT co- authors include Erin Kara, Daniele   
   Rogantini, and Claude Canizares, along with collaborators from multiple   
   institutions, including the Institute of Astronomy in Cambridge, U.K.   
      
   Fixed sight Disk winds have most often been observed in X-ray binaries --   
   systems in which a black hole or a neutron star is pulling material from   
   a less dense object and generating a white-hot disk of inspiraling matter,   
   along with outflowing wind.   
      
   Exactly how winds are launched from these systems is unclear. Some   
   theories propose that magnetic fields could shred the disk and expel some   
   of the material outward as wind. Others posit that the neutron star's   
   radiation could heat and evaporate the disk's surface in white-hot gusts.   
      
   Clues to a wind's origins may be deduced from its structure, but the   
   shape and extent of disk winds has been difficult to resolve. Most   
   binaries produce accretion disks that are relatively even in shape, like   
   thin donuts of gas that spins in a single plane. Astronomers who study   
   these disks from far-off satellites or telescopes can only observe the   
   effects of disk winds within a fixed and narrow range, relative to their   
   rotating disk. Any wind that astronomers manage to detect is therefore   
   a small sliver of its larger structure.   
      
   "We can only probe the wind properties at a single point, and we're   
   completely blind to everything around that point," Kosec notes.   
      
   In 2020, he and his colleagues realized that one binary system could   
   offer a wider view of disk winds. Hercules X-1 has stood out from most   
   known X-ray binaries for its warped accretion disk, which wobbles as it   
   rotates around the system's central neutron star.   
      
   "The disk is really wobbling over time every 35 days, and the winds   
   are originating somewhere in the disk and crossing our line of sight at   
   different heights above the disk with time," Kosec explains. "That's a   
   very unique property of this system which allows us to better understand   
   its vertical wind properties."  A warped wobble In the new study,   
   the researchers observed Hercules X-1 using two X-ray telescopes --   
   the European Space Agency's XMM Newton and NASA's Chandra Observatory.   
      
   "What we measure is an X-ray spectrum, which means the amount of X-ray   
   photons that arrive at our detectors, versus their energy. We measure the   
   absorption lines, or the lack of X-ray light at very specific energies,"   
   Kosec says. "From the ratio of how strong the different lines are, we can   
   determine the temperature, velocity, and the amount of plasma within the   
   disk wind."  With Hercules X-1's warped disk, astronomers were able to   
   see the line of the disk moving up and down as it wobbled and rotated,   
   similar to the way a warped record appears to oscillate when seen from   
   edge-on. The effect was such that the researchers could observe signs   
   of disk winds at changing heights with respect to the disk, rather than   
   at a single, fixed height above a uniformly rotating disk.   
      
   By measuring X-ray emissions and the absorption lines as the disk wobbled   
   and rotated over time, the researchers could scan properties such as the   
   temperature and density of winds at various heights with respect to its   
   disk and construct a two-dimensional map of the wind's vertical structure.   
      
   "What we see is that the wind rises from the disk, at an angle of about   
   12 degrees with respect to the disk as it expands in space," Kosec   
   says. "It's also getting colder and more clumpy, and weaker at greater   
   heights above the disk."  The team plans to compare their observations   
   with theoretical simulations of various wind-launching mechanisms, to   
   see which could best explain the wind's origins. Further out, they hope   
   to discover more warped and wobbling systems, and map their disk wind   
   structures. Then, scientists could have a broader view of disk winds,   
   and how such outflows influence their surroundings - - particularly at   
   much larger scales.   
      
   "How do supermassive black holes affect the shape and structure   
   of galaxies?"  poses Erin Kara, the Class of 1958 Career Development   
   Assistant Professor of Physics at MIT. "One of the leading hypotheses   
   is that disk winds, launched from a black hole, can affect how galaxies   
   look. Now we can get a more detailed picture of how these winds are   
   launched, and what they look like."  This research was supported in part   
   by NASA.   
      
       * RELATED_TOPICS   
             o Space_&_Time   
                   # Galaxies # Black_Holes # Stars # Astrophysics   
                   # Astronomy # Space_Telescopes # Nebulae #   
                   Extrasolar_Planets   
       * RELATED_TERMS   
             o Galaxy o Supernova o Quasar o Alpha_Centauri o   
             Gravitational_wave o Globular_cluster o Astronomy o   
             Blue_supergiant_star   
      
   ==========================================================================   
   Story Source: Materials provided by   
   Massachusetts_Institute_of_Technology. Note: Content may be edited for   
   style and length.   
      
      
   ==========================================================================   
   Journal Reference:   
      1. Kosec, P., Kara, E., Fabian, A.C. et al. Vertical wind structure   
      in an X-   
         ray binary revealed by a precessing accretion disk. Nat Astron,   
         2023 DOI: 10.1038/s41550-023-01929-7   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2023/04/230410111703.htm   
      
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