Thursday, August 13, 2015

Gamma Ray Bursts: one of the most disruptive events in the universe!!!

Once a day a remarkable event is observed in the universe: a gamma ray burst (GRB)!
Gamma Ray Bursts  are collimated energy sources which last from seconds to several hours and having the same energy content our Sun has released in 10 Billions of years!!! Just think about!
Are you not impressed? What's wrong with you?

(This artist’s impression of a gamma-ray burst shows the two intense beams of relativistic matter emitted by the black hole. To be visible from Earth, the beams must be pointing directly towards us. (Image: NASA/Swift/Mary Pat Hrybyk-Keith and John Jones) source: universetoday.com)


What does originate a GRB?
Even if these events were discovered in the last '60 no-one knows for sure how they originate.
The best guess is that they come from the collapse of very massive stars. This is also supported by the fact that the GRBs observed until now have been originated in distant galaxies rich in highly massive stars.

All GRBs observed to date have occurred well outside the Milky Way galaxy and have been harmless to Earth. However, if a GRB were to occur within the Milky Way, and its emission were beamed straight towards Earth, the effects could be devastating for the planet. And it is thought this already happened in the past! 



From wikipedia:
The greatest danger is believed to come from Wolf–Rayet stars, regarded by astronomers as likely GRB candidates. When such stars transition to supernovae, they may emit intense beams of gamma rays, and if Earth were to lie in the beam zone, devastating effects may occur. Gamma rays would not penetrate Earth's atmosphere to impact the surface directly, but they would chemically damage the stratosphere.
For example, if WR 104, at a distance of 8,000 light-years, were to hit Earth with a burst of 10 seconds duration, its gamma rays could deplete about 25 percent of the world's ozone layer. This would result in mass extinction, food chain depletion, and starvation. The side of Earth facing the GRB would receive potentially lethal radiation exposure, which can cause radiation sickness in the short term, and, in the long term, results in serious impacts to life due to ozone layer depletion.

Longer-term, gamma ray energy may cause chemical reactions involving oxygen and nitrogen molecules which may create nitrogen oxide then nitrogen dioxide gas, causing photochemical smog. The GRB may produce enough of the gas to cover the sky and darken it. Gas would prevent sunlight from reaching Earth's surface, producing a "cosmic winter" effect – a similar situation to an impact winter, but not caused by an impact. GRB-produced gas could also even further deplete the ozone layer.  This means mass-extinction!


In a paper published on the prestigious Physical Review Letters, the authors claim
 
As a copious source of gamma rays, a nearby galactic gamma ray burst (GRB) can be a threat to life. [..] There is a 50% chance that  a lethal GRB took place during the last 500×106years, causing one of the major mass extinction events. Assuming that a similar level of radiation would be lethal to life on other exoplanets hosting life, we explore the potential effects of GRBs to life elsewhere in the Galaxy and the Universe. We find that the probability of a lethal GRB is much larger in the inner Milky Way [...], making it inhospitable to life. [..] Early life forms must have been much more resilient to radiation.

So finding life on other planets is a difficult tasks, and GRBs make the universe more inhospitable to it. At least they make "hinder" the life as we know it.





This post is dedicated to memory of my friend Alessio, who first told me about gamma ray bursts some time ago. Thanks for all our nice conversations! 
I will miss you. Really. Ciao Alessio!





Sources
http://w.astro.berkeley.edu
https://en.wikipedia.org/wiki/Gamma-ray_burst

http://cerncourier.com
http://www.universetoday.com
http://journals.aps.org

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