The Question

It is my understanding that observation of x-rays have been used in the past in the search for black holes. I have been trying to find out whether there has been any reasonable evidence found for their existence. Having glanced through numerous out of date texts on the subject, it seems people are unwilling to say one way or another. If there is any generally accepted evidence I would be most grateful if you could let me know.

The Answer

http://lheawww.gsfc.nasa.gov/users/nandra/pubs/paper2/figure.html
http://lheawww.gsfc.nasa.gov/users/turner/agn_reproc.html
http://lheawww.gsfc.nasa.gov/users/ptak/agn/liner_llagn.html

You may also want to revisit Imagine the Universe! at:

http://imagine.gsfc.nasa.gov

In addition to the discussions of black holes and x-ray astronomy you will find there, check out the archive of past 'Ask a High-Energy Astronomer' questions and answers. Many of those under 'black holes' are relevant to your question.

There is now a strong consensus that black holes are present in the Universe, both small ones associated with stars in binary systems (Cygnus X-1 is a convincing candidate, for example) and much larger ones in the centers of many galaxies (including our own). The best lines of evidence are currently coming from determinations of the masses of non-stellar perturbing objects from their effects on the motions of stars (measured from Doppler shiftss in their spectra). In many cases the unseen perturbing object is so massive that it is hard not to conclude that it is a black hole.

As an indication of how rapidly the evidence is accumulating, only a couple of hours ago NASA issued a Press Release on recent results from the New Hubble Instruments. Here's an extract:

The imaging spectrograph, which just last month demonstrated its efficiency as a black hole hunter, now shows the results when the power of a black hole is unleashed into its surrounding environment. In a single observation, the spectrograph measured the velocities of hundreds of gas blobs caught up in a twin-cone beam of radiation emanating from a supermassive black hole at the core of galaxy NGC 4151. Follow-up observations reveal hot gas emanating from deep within the throat of the beam, near the vicinity of the black hole. These observations also allow scientists to map the mass outflows near the black hole. The surprisingly complex motion may offer clues to the galaxy's stellar population, the orientation of the beam in the past, or evidence of some kind of backflow of gas into the central cone regions.

For pictures see:

http://oposite.stsci.edu/pubinfo/Pictures.html

Best wishes,

Paul Butterworth and Andy Ptak
for the 'Ask a High-Energy Astronomer' team

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