Black hole electromagnetic spectrum

In summary, the electromagnetic spectrum associated with black holes encompasses a range of wavelengths emitted or influenced by the extreme gravitational and energetic environments surrounding them. This includes X-rays from accretion disks as matter spirals into the black hole, radio waves from jets produced by high-energy processes, and gamma rays from particle interactions. Observations across this spectrum provide insights into black hole properties, their formation, and the dynamics of matter in their vicinity, fostering a deeper understanding of fundamental astrophysical phenomena.
  • #1
snorkack
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Of course neither a single Schwarzschild nor a single Kerr black hole, nor a pair of these has an emission spectrum, other than the Hawking one. (Nordström and Newman holes must have it while in binaries, but they are not common).
But not having emission spectrum does not rule out having absorption and deflection spectra. Nor of shift spectra.

For light of short wavelength compared to Schwarzschild radius, a single Schwarzschild black hole has simple, black spectrum dictated by geometry. GR can treat light as rays and geometrically derive the amount of light intercepted by event horizon, and deflected by any angle.

But what happens when the wavelength is appreciable compared to the Schwarzschild radius? A black ball would deflect such waves by diffraction. A black hole should have interplay of gravity and diffraction. So what does Schwarzschild black hole cross-section for absorption and deflection do for the parts of spectrum where the wavelength is comparable or bigger than Schwartzschild radius?

Now, I mentioned shift spectra.
A lone Schwarzschild black hole would not shift frequency in a reference frame where it is stationary. Waves would be absorbed or deflected but not change frequency. It would Doppler shift frequency of deflected waves in frames where it moves.
Binary black holes are moving in reference frame where their common centre of mass is stationary, so of course they Doppler shift deflected waves.
But how about Kerr black holes? Is there any frequency shift for electromagnetic waves that pass through ergosphere and get out again?
 
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  • #2
snorkack said:
But not having emission spectrum does not rule out having absorption and deflection spectra. Nor of shift spectra.
I'm not sure what you mean. An "absorption spectrum" would mean absorbing some wavelengths but not others. A black hole absorbs all wavelengths.

snorkack said:
For light of short wavelength compared to Schwarzschild radius, a single Schwarzschild black hole has simple, black spectrum dictated by geometry.
Where are you getting this from? Do you have a reference?

snorkack said:
Now, I mentioned shift spectra.
A lone Schwarzschild black hole would not shift frequency in a reference frame where it is stationary. Waves would be absorbed or deflected but not change frequency. It would Doppler shift frequency of deflected waves in frames where it moves.
Again, where are you getting this from? Do you have a reference?
 
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  • #3
PeterDonis said:
I'm not sure what you mean. An "absorption spectrum" would mean absorbing some wavelengths but not others. A black hole absorbs all wavelengths.
Yes, but does the black hole absorb all wavelengths exactly equally?
Even wavelengths that are in the order of magnitude of black hole size?
PeterDonis said:
Where are you getting this from? Do you have a reference?
Reference for what?
For the black hole deflection or absorption of light, a simple example is this:
https://www.spacetimetravel.org/expeditionsl/1
Many references duplicate this simple picture - pure ray geometry.
Which I reworded as "simple black spectrum" - absorption cross-section independent on wavelength.
For the wavelengths where this simple model holds.
What I do not find references for is what happens for wavelengths where this ray optics models necessarily breaks down. A black absorbing obstacle will create a diffraction and interference pattern. Therefore, for a black hole interrogated with a plane wave whose wavelength is appreciable compared to the black hole size, I should expect interplay of gravitational bending with diffraction and interference. Who discuss that?
PeterDonis said:
Again, where are you getting this from? Do you have a reference?
Uh, this?
Basic relativity of light frequency?
If a mirror, in a reference frame where it is stationary leaves the light frequency unaltered then in a reference frame where the mirror moves towards or away from light it Doppler shifts the light - respectively blueshift and redshift.
Therefore, if a Schwartzschild black hole in a reference frame where it is stationary leaves the frequency of deflected light unaltered (it blueshifts when approaching the hole, but redshifts back to exactly the initial frequency when escaping to infinity), in a reference frame where it moves relative to the direction of light direction change it must Doppler shift the light frequency.
 
  • #4
snorkack said:
does the black hole absorb all wavelengths exactly equally?
Any light that goes into the hole is absorbed.

snorkack said:
Even wavelengths that are in the order of magnitude of black hole size?
That would depend on which theoretical model you use.

Observationally we can't detect such long wavelengths, so we have no way of checking our theoretical models in this regime.

snorkack said:
Reference for what?
For your claims about the spectrum of a black hole.

snorkack said:
For the black hole deflection or absorption of light, a simple example is this:
https://www.spacetimetravel.org/expeditionsl/1
That's not a valid reference.

snorkack said:
Many references duplicate this simple picture - pure ray geometry.
Then you should have no trouble finding a textbook or peer-reviewed paper.

snorkack said:
What I do not find references for is what happens for wavelengths where this ray optics models necessarily breaks down.
Then you should not be making claims about what happens in that case.

snorkack said:
Uh, this?
Basic relativity of light frequency?
Sorry, no. Either you have a valid reference to back up your claims, or you don't. Since you have said you don't, your claims are personal speculation, which is off limits here.

Thread closed.
 
  • #5
snorkack said:
if a Schwartzschild black hole in a reference frame where it is stationary leaves the frequency of deflected light unaltered (it blueshifts when approaching the hole, but redshifts back to exactly the initial frequency when escaping to infinity), in a reference frame where it moves relative to the direction of light direction change it must Doppler shift the light frequency.
As a side note, this has nothing whatever to do with the presence of a black hole; it would be the same in flat spacetime. So it has nothing to do with any claims you are making about black holes.
 

FAQ: Black hole electromagnetic spectrum

What is the electromagnetic spectrum of a black hole?

Black holes themselves do not emit electromagnetic radiation directly. However, the material surrounding black holes, such as the accretion disk, can emit radiation across the electromagnetic spectrum, including X-rays, gamma rays, and sometimes visible light, due to the intense gravitational forces and high temperatures involved.

How do black holes affect the electromagnetic spectrum of nearby objects?

Black holes can significantly influence the electromagnetic spectrum of nearby objects through gravitational lensing, where the black hole's gravity bends light from objects behind it. Additionally, as matter accretes onto a black hole, it heats up and emits radiation, primarily in the X-ray and gamma-ray regions of the spectrum.

Can we detect black holes using the electromagnetic spectrum?

Yes, we can detect black holes indirectly by observing the electromagnetic radiation from the accretion disks and jets that form around them. The high-energy X-rays and gamma rays emitted by these regions are often telltale signs of a black hole's presence.

What role does Hawking radiation play in the electromagnetic spectrum of black holes?

Hawking radiation is a theoretical prediction that black holes can emit radiation due to quantum effects near the event horizon. This radiation would span the entire electromagnetic spectrum but is incredibly weak and has not yet been observed directly. It suggests that black holes can slowly lose mass and energy over time.

Why do black holes emit X-rays and gamma rays?

Black holes emit X-rays and gamma rays primarily because of the intense gravitational forces acting on the material in the accretion disk. As this material spirals inward, it reaches extremely high temperatures, causing it to emit high-energy radiation in the form of X-rays and gamma rays.

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