accretion/event horizon

just reading about the chandra observations of the event horizons of
xray novae. whilst reading it a question came to mind; have all the
previous 'observations' of black holes been based only upon the
observing of relative motions created by the enormous densities that
define black holes? have we not directly seen structures of the black
holes themselves until this point? what about jets like m87 (?) that
come to mind; wouldnt we be able to discern where the event horizon
would be there, or are we not certain that is a black hole?
excuse the ignorance and thanks in advance.

-e.

wrote:
just reading about the chandra observations of the event horizons of
xray novae. whilst reading it a question came to mind; have all the
previous 'observations' of black holes been based only upon the
observing of relative motions created by the enormous densities that
define black holes? have we not directly seen structures of the black
holes themselves until this point? what about jets like m87 (?) that
come to mind; wouldnt we be able to discern where the event horizon
would be there, or are we not certain that is a black hole?
excuse the ignorance and thanks in advance.

-e.


What has been observed are phenomena that are modeled (some say best modeled) by
a black hole. In the case of x-rays coming from what appear to be normal stars,
the model in some cases is a black hole orbiting that star and the x-rays coming
from an accretion disk around it. In the case of the active cores of galaxies,
the best model consistent with the observations (and our current understanding
of physics) is a black hole, supermassive in this case (millions to 100's of
millions of solar masses or more), again with an accretion disk orbiting around
it. There have even been observations hinting at detecting the central
accretion disk.

But, in all of those cases reported so far, all that is said is that the best
model for explaining the observed phenomenon, whatever it is, is a black hole
interacting in some way with its environment. We have not observed directly a
black hole or its event horizon (go back and check the wording on the Chandra
announcement).

So, the correct assessment is that we have plenty of candidates that might best
be explained as the result of a black hole.

"s" == spiderrescue writes:

s just reading about the chandra observations of the event horizons
s of xray novae. whilst reading it a question came to mind; have all
s the previous 'observations' of black holes been based only upon the
s observing of relative motions created by the enormous densities
s that define black holes?

I'd phrase it as something like the following. Assuming that general
relativity is the correct description of gravity (at least on
macroscopic scales), the motions of gas or stars in the neighborhood
of certain dark objects is only consistent with those objects being
black holes.

In other words, their presence has been inferred not observed directly.

s have we not directly seen structures of the black holes themselves
s until this point? what about jets like m87 (?) that come to mind;
s wouldnt we be able to discern where the event horizon would be
s there, or are we not certain that is a black hole?

The future may be different. There are plans to use the technique of
very long baseline interferometry (VLBI) at sub-millimeter wavelengths
to image the neighborhood of the object Sgr A* at the center of the
Milky Way Galaxy. If everything works, the observations should be
able to detect the "shadow" of the black hole against its accretion
disk. See Volume 18 of GCNEWS,
URL:http://www.aoc.nrao.edu/~gcnews/gcnews/Vol.18/gcnews.shtml.

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