Black holes and x-rays

It is my understanding that while obviously not visible in a normal sense, black holes can be detected by the x-rays they give off. Physics and space aren’t really my strong point, but does this mean x-rays are able to escape a situation in which light cannot? Are x-rays subject to gravity? Am I missing something here, like maybe the x-rays are emitted somewhere beyond the event horizon?

thanks in advance.

Yes, this is it. Stuff getting sucked in to the BH gets really dense and hot as it approaches the event horizon, so hot that it gives off huge amounts of x-rays. It’s this superheated glowing matter we are detecting, not something emanating from within the BH itself.

ah, that makes sense. well, that wasn’t much of a thread. Feel free to talk about Hawking radiation and whether or not black holes to give off something to stretch it to a more appropriate length.

X-rays, like any other form of electromagnetic radiation, cannot emerge from the event horizon of a black hole. However, the accretion disk surrounding a black hole where matter is spiraling down into it will produce lots of radiation, from infrared and visible light on up through X- and gamma rays. X-rays are easily detected at long distances, as Q.E.D. describes.

The radiation includes pretty much everything: For instance, the brightest radio source in the sky isn’t the Sun, but rather the black hole at the center of our Galaxy (called Sag A[sup]*[/sup]). There’s visible light, too, but nobody pays that as much attention, since there are plenty of visible light sources in the Universe (like, say, stars), so they don’t stand out as much in those wavelengths.

And Hawking radiation does exist (we’re about as certain of that as we are of anything about black holes), but it’s very, very faint. Black holes, unsurprisingly, emit as perfect blackbodies, and a typical black hole (i.e., one of stellar mass or larger) has a temperature of about a millionth of a Kelvin or less, far colder (and hence darker) than just about anything else in the Universe. We might conceivably detect the Hawking radiation from a primordial black hole (which can presumably be much smaller and therefore much hotter than the stellar ones), but from a normal-sized one, not a chance.

Yeah, a black hole that’s in the process of actively accreting matter will be detectable from the emission from its accretion disc (or just from the stuff flowing onto the black hole if there’s no real accretion disc). We can see this radiation in most wavebands, even the radio, and the spectral features can be used as a diagnostic to determine what’s going on close to the black hole (and indeed, according to some of the modellers I know, stuff about the state of the black hole, such as its spin etc). Additionally, if conditions are right, you can get jets produced by the accretion disc/black hole, which can be detected from the radio through to gamma rays and provide additional information as to what the black hole’s doing (provided suitable assumptions are made and models applied :wink: ).

There isn’t very much “etc” for a black hole. The only properties a black hole can have are mass, angular momentum, electric charge, and magnetic charge. Magnetic charge is believed to be zero for everything in the observable Universe, and electric charge is very seldom large enough to be relevant for anything the size of a black hole, which means that in practical terms, all there is to say about an astronomical black hole is its mass and spin (both of which can be gotten from the accretion behavior).

Why “Sag A”? Why not “Tartarus” or “Sheol” or “Ginnungagap”? Don’t astronomers have any imagination anymore?

Well, Sag is short for Sagittarius, that’s not too bad.

Of course, you could always have names like Shurnarkabtishashutu (Zeta Tauri) or Zubeneschamali (Beta Librae)…

Well, it’s not called Tartarus or the like because it was discovered (and hence named) a good while before it was determined to be a black hole. The name basically just means that it was the first or most prominent radio source discovered in the constellation of Sagitarius. By the same token, Cyg X1 (also a black hole) was the first X-ray source discovered in the constellation of Cygnus.

OK, ‘state of the black hole’ was the wrong phrasing, and indeed as you say, it is just mass and spin that are important for a black hole. I was referring more to the spectral features being used to figure out what’s going on close to the black hole and why some accreting black holes produce large scale jets, others don’t, why some emit more in one waveband than another and things like that, and whilst we’re making some fairly important steps forward in understanding that, we don’t fully get it, and we are definitely trying to do a jigsaw puzzle whilst missing quite a few of the pieces.