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Is the amount of energy a black hole emit through polar jets comparable to the mass it accretes or is it negligible? Here I am mostly concerned about the observational findings as opposed to the theoretical ones.

In other words, any mass in the accretion disk has two possible fate: ending up inside the event horizon or escaping through jets. What is the possible probability range of the second happening to each unit of mass?

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Real-world black holes accrete matter and this matter gets heated before it is devoured by the black hole so it radiates. We may observe this accretion. The radiation coming from this heated matter is qualitatively similar to the radiation in any accretion disk, even in the absence of a black hole.

But the very reason why the observations lead us to be near-certain that there are black holes is that we observe less radiation than what any star-like object would emit. That's because the accreted matter is hiding behind the event horizon. See e.g. this paper from 1997-1998

http://arxiv.org/abs/astro-ph/9706112

about the evidence supporting the existence of the black hole in our galactic center. The models discussed over there are observationally driven.

Concerning your "percentage" question, it is estimated that only 1% of the accreting matter ever crosses the event horizon, see e.g.

http://arstechnica.com/science/2013/08/the-milky-ways-black-hole-like-cookie-monster-loses-more-than-it-eats/

But this number is so tiny only because of an inclusive enough definition of the accretion disk. Within the matter that is really close to the event horizon, a much larger percentage gets devoured.

There is no experimental observation of the Hawking radiation as of today; that is unrealistic for astronomical sizes of black holes and for smaller black holes where it is imaginable, such a discovery would be a huge breakthrough.

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So, do these observations leave open the possibility that relativistic jets instead of event horizons are responsible for that 1% disappearing mass that would allow the black hole to cool down? I would tend to think the answer is yes, as our knowledge of polar jets has uncertainities both from theoretical and observational sides. –  Jani Kovacs Feb 1 at 17:53
    
I. e. as far as I know in 1998 there was no observational evidence that sgr a* has jets and so the paper you link to does not account for that. "[...]this region of the spectrum may possibly arise from a separate component such as a jet which is outside the scope of our model" –  Jani Kovacs Feb 1 at 17:55
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