Monday, June 26, 2023

4067 - GAMMA RAY BURSTS - how to explain them?

 

-    4067  -  GAMMA  RAY  BURSTS  -   how to explain them?    Fifty years ago, on June 1, 1973, astronomers around the world were introduced to a powerful and perplexing new phenomenon called GRBs (gamma-ray bursts). Today sensors on orbiting satellites like NASA's Swift and Fermi missions detect a GRB somewhere in the sky about once a day on average. Astronomers think the bursts arise from catastrophic occurrences involving stars in distant galaxies, events thought to produce new black holes.


------------   4067   -  GAMMA  RAY  BURSTS  -   how to explain them?  

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-    GRBs occur so far beyond our galaxy that even the closest-known burst exploded more than 100 million light-years away. Each burst produces an initial pulse of gamma rays, the highest-energy form of light, that typically lasts from milliseconds to minutes. This emission comes from a jet of particles moving close to the speed of light launched in our direction, and the closer we are to looking straight down the barrel, the brighter it appears.

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-     Following this prompt emission is a fading afterglow of gamma rays, X-rays, ultraviolet, visible, infrared, and radio light that astronomers may be able to track for hours to months.

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-   Even half a century on, GRBs offer up surprises. One recent burst was so bright it temporarily blinded most of the gamma-ray detectors in space. Nicknamed the BOAT (for brightest of all time), the 7-minute blast may have been the brightest GRB in the past 10,000 years. It also showed that scientists' most promising models of these events are nowhere near complete.

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-    While theorists proposed 100 models in an effort to explain GRBs, most involving neutron stars in our own galaxy, observational progress was slow despite the growing number of detections by different spacecraft. Gamma rays can't be focused like visible light or X-rays, making precise localizations quite difficult. Without them, it was impossible to search for GRB counterparts in other wavelengths using larger telescopes in space or on the ground.

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-  In 1991, NASA launched the Compton Gamma Ray Observatory, which included an instrument named BATSE (Burst and Transient Science Experiment) dedicated to exploring GRBs.   BATSE was about 10 times more sensitive than previous GRB detectors. Over Compton's nine-year mission, BATSE detected 2,704 bursts, which gave astronomers a rich set of observations made with the same instrument.

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-    In its first year, BATSE data showed that bursts were distributed all over the sky instead of in a pattern that reflected the structure of our Milky Way galaxy.  This suggested that they were coming from distant galaxies, and that meant they were more energetic than most scientists thought possible.

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-   Burst durations clustered into two broad groups,one lasting less than two seconds, the other lasting longer than two seconds, and that short bursts produced higher-energy gamma rays than long ones.

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-    So both temporal and spectral properties agreed in identifying two separate groups of GRBs: short and long.  Theorists associated long GRBs with the collapse of massive stars and short ones with binary neutron star mergers.

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-    When a burst occurred in the field of view X-ray cameras, the spacecraft could locate it well enough over a couple of hours that additional instruments could be brought to bear. Whenever BeppoSAX turned to a GRB's position, its instruments found a rapidly fading and previously unknown high-energy source,the X-ray afterglow theorists had predicted.

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-    These positions enabled large ground-based observatories to discover long GRB afterglows in visible light and radio waves, and also permitted the first distance measurements, confirming that GRBs were truly far-away events.

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-    Although most GRBs originate from exploding massive stars or neutron-star mergers, the researchers concluded that “GRB 191019A” instead came from the collision of stars or stellar remnants in the jam-packed environment surrounding a supermassive black hole at the core of an ancient galaxy.

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-   This study will be published June 22, 2023, in the journal Nature Astronomy. This remarkable discovery grants astronomers a tantalizing glimpse into the intricate dynamics at work within these cosmic environments, establishing them as factories of events that would otherwise be deemed impossible.

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-   Most stars die, according to their mass, in one of three predictable ways. When relatively low-mass stars like our sun reach old age, they shed their outer layers, eventually fading to become white dwarf stars. More massive stars, on the other hand, burn brighter and explode faster in cataclysmic supernovae explosions, creating ultra-dense objects like neutron stars and black holes. The third scenario occurs when two such stellar remnants form a binary system and eventually collide.

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-    But the new study finds there might be a fourth option.  Stars can meet their demise in some of the densest regions of the universe, where they can be driven to collide. Long past their star-forming prime, ancient galaxies have few, if any, remaining massive stars. Their cores, however, teem with stars and a menagerie of ultra-dense stellar remnants, such as white dwarfs, neutron stars and black holes.

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-    Astronomers have long suspected that in the turbulent beehive of activity surrounding a supermassive black hole, it only would be a matter of time before two stellar objects collided to produce a GRB. But evidence for that type of merger has remained elusive.

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-    On Oct. 19, 2019, astronomers glimpsed the first hints of such an event when NASA’s Neil Gehrels Swift Observatory detected a bright flash of gamma rays that lasted a little over one minute. Any GRB lasting longer than two seconds is considered “long.” Such bursts typically come from the collapse of stars at least 10 times the mass of our sun.

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-   Making long-term observations of the GRB’s fading afterglow enabled the astronomers to pinpoint the location of the GRB to a region less than 100 light-years from the nucleus of an ancient galaxy, very near the galaxy’s supermassive black hole.

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-    The lack of a supernova accompanying the long GRB 191019A tells us that this burst is not a typical massive star collapse.  The location of GRB 191019A, embedded in the nucleus of the host galaxy, teases a predicted but not yet evidenced theory for how gravitational-wave emitting sources might form.

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-    In typical galactic environments, the production of long GRBs from colliding stellar remnants, such as neutron stars and black holes, is incredibly rare. The cores of ancient galaxies, however, are anything but typical, and there may be a million or more stars crammed into a region just a few light-years across.

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-    Such extreme population density may be great enough that occasional stellar collisions can occur, especially under the titanic gravitational influence of a supermassive black hole, which would perturb the motions of stars and send them careening in random directions. Eventually, these wayward stars would intersect and merge, triggering a titanic explosion that could be observed from vast cosmic distances.

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-    It is possible that such events occur routinely in similarly crowded regions across the universe but have gone unnoticed until this point. A possible reason for their obscurity is that galactic centers are brimming with dust and gas, which could obscure both the initial flash of the GRB and the resulting afterglow. GRB 191019A may be a rare exception, allowing astronomers to detect the burst and study its aftereffects.

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-   While this event is the first of its kind to be discovered, it’s possible there are more out there that are hidden by the large amounts of dust close to their galaxies.  If this long-duration event came from merging compact objects, it contributes to the growing population of GRBs that defies our traditional classifications.

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-    In 2000, NASA launched HETE 2, a small satellite designed to detect and localize GRBs. It was the first mission to compute accurate positions onboard and quickly—in tens of seconds—communicate them to the ground so other observatories could study early afterglow phases. The burst it discovered on March 29, 2003, also exhibited definitive supernova characteristics, confirming a suspected relationship between the two phenomena.

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-    In May 2005, Swift was able to pinpoint the first afterglow of a short GRB, showing that these blasts occur in regions with little star formation. This bolstered the model of short bursts as mergers of neutron stars, which can travel far from their birth place over the many millions of years it takes for them to crash together.

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-   In 2008, NASA's Fermi Gamma-ray Space Telescope joined Swift in hunting GRBs and has observed about 3,500 to date. Its GBM (Gamma-ray Burst Monitor) and Large Area Telescope allow the detection and follow-up of bursts from X-rays to the highest-energy gamma rays detected in space, an energy span of 100 million times. This has enabled the discovery of afterglow gamma rays with billions of times the energy of visible light.

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-    In 2017, Fermi and the European INTEGRAL satellite linked a short GRB to a source of gravitational waves, ripples in space-time produced as orbiting neutron stars spiraled inward and merged. This was an important first that connected two different cosmic "messengers," gravity and light. While astronomers haven't seen another "gravity and light" burst since, they hope more will turn up in current and future observing runs of gravitational wave observatories.

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-   “StarBurst” is a small satellite designed to explore GRBs from neutron star mergers. Other missions include “Glowbug”.   “BurstCube” slated for launch in early 2024; “MoonBEAM”, which would orbit between Earth and the Moon and “LEAP”, designed to study GRB jets from the space station.

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-    What will completely revolutionize our understanding of GRBs will be the ability to track them back to when the universe was most intensely forming stars, around 10 billion years ago.

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-    This part of the universe will be probed by the next generation of gravitational wave detectors—10 times more sensitive than what we currently have—and by future gamma-ray missions that can ensure continuity with the fantastic science Swift and Fermi have enabled.

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June 25,  2023       GAMMA  RAY  BURSTS  -   how to explain them?             4067

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