Trying to clarify the question
magnetar said:
If two black holes combind together,the main form energy emited by this event is "gravitational waves"?
Unfortunately, I don't think this question has been very well stated. This shouldn't be read as indictment--- it can often be hard to state a question well when you are struggling with something new.
At any event, I guess that the question concerns energy radiated away in some form, not something like (in str terms) kinetic energy wrt a frame comoving with one of two colliding objects, or ejected matter (e.g. from an accretion disk or torus of dust around one of the holes, etc.). Let me try to restate it:
No-one said:
When two astrophysical objects collide, in addition to ejected matter we can expect the emission of various kinds of radiation which carries away "news" of the collision to distant observers. For ordinary stars we would expect most of this radiation to be nongravitational, but when a highly compact object such as a neutron star is involved, we might guess that the fraction of energy radiated as gravitational radiation might be substantial. If both the objects are black holes, the result of the merger is a single black hole and neglecting possible accretion disk we should expect only gravitational radiation in this case.
So, question: in realistic models of the merger of an ordinary star or neutron star with a black hole, what fraction of the combined rest mass of the original objects might be radiated away in the form of gravitational, electromagnetic, or neutrino radiation, etc.?"
A variant of this question: in dense stellar clusters, collisions of stars might sometimes eventually result in the formation of a supermassive black hole. What fraction of the combined rest mass of the "ingredients" can be radiated away as gravitational radiation in such cases?
There are many arXiv eprints which touch on these questions, including:
http://arxiv.org/abs/gr-qc/0507014
http://aps.arxiv.org/abs/astro-ph/9908290
http://www.arxiv.org/abs/gr-qc/9905058
http://de.arxiv.org/abs/astro-ph/9801105
http://arxiv.org/abs/astro-ph/9606181
pervect said:
Offhand, I can think of any other way for them to emit energy, so I would say yes. But I don't have a reference for this statement, unfortunately.
Praetorius 2005 says that in simulations of equal mass nonspinning BH mergers, "a rough calculation suggests on the order of 5% of the initial rest mass of the system is radiated as gravitational waves during the final orbit and ringdown."
Janka et. all 1999 say, concerning NS/BH mergers, "A gas mass between about 0.3 and about 0.7 solar masses is left in an accretion torus around the black hole and radiates neutrinos at a luminosity of several 10^{53} erg/s during an estimated accretion time scale of about 0.1 s."
Totani 1998 points out that due to the strong gravitational fields in NS/NS mergers, time delay plays a significant role: "A unique feature of the binary neutron-star merger scenario, in contrast to other scenarios associated to single stellar collapses, is that a time delay during binary spiral-in phase emitting gravitational waves is not negligible and makes the rate evolution flatter than that of star formation rate."
Ruffert et al. 1996 say "About 10^{-4}-10^{-3}~M_\odot of material lost during the neutron star merging and swept out from the system in a neutrino-driven wind might be a site for nucleosythesis".
Shibata 1997 says that in stellar cluster collision simulations, up to "0.5% of the rest mass energy may be dissipated by the gravitational wave emission in the final phase of the merger."
In one of his papers, Thorne does a back of the envelope computation suggesting that in certain merger events, the gravitational radiation luminosity can indeed exceed the EM luminosity.
Those of the above remarks which don't quite address the question as I stated it, nonetheless suggest how the restated question might be further refined.
FrigginGenius said:
It would seem - to me - that since we are dealing with BHs, that nothing would be emitted no matter what happens to it. Though I think its mass would increase and consequently its gravity. So if you call it gravity waves then I suppose I would agree with the OP that the main if not only (indirectly) observable emission would be gravity waves.
I've never heard anything about the destruction of a BH or its affects on matter and its surrounding space. So I would volunteer that nothing would be emitted in that event. I would guess that the more massive BH would devour the lesser BH and we may see the wobble of a nearby star intensify or some other effect of the gravity increase but nothing from the BH(s) itself.
Please note:
1. The study of binary mergers as treated using a rather subtle and sophisticated theory (gtr) currently involves state of the art numerical and theoretical methods, which smart people spend years mastering in graduate school.
2. Standards here at PF are probably higher than you may have encountered at other web forums (although I think standards can be greatly improved even at PF!).
Given these facts, I'd suggest you try to refrain from stating wild guesses based on (apparently) little or no knowledge of the subject at hand. The OP's question was poorly stated, but I imagine he nonetheless wanted an informed response.
Standards here at PF
https://www.physicsforums.com/showthread.php?t=5374 are probably higher than you may have encountered at other web forums (although I think standards can be greatly improved even at PF!).
