PrepTest 137, Section 3, Question 10
A ring of gas emitting X-rays flickering 450(62 more words not shown)
The statements above, if true,(20 more words not shown)
- ABlack holes that(14 more words not shown)
- BOnly rings of(12 more words not shown)
- CThe black hole(13 more words not shown)
- DX-rays emitted by(12 more words not shown)
- EA black hole(20 more words not shown)
Reading the argument
Set aside the unfamiliar astronomy terms for a moment. What would have to be true for this close orbit to work?
We observe a stable ring of gas, with X-rays flickering 450 times per second. The physical theories give us a best explanation for that rate: the ring has a radius of 49 kilometers. Then we're told the ring couldn't stay in an orbit that close unless the black hole were spinning.
What we’re looking for
That points us toward a spinning black hole. The question specifically tells us to assume the theories are correct, so objecting that the theories might be wrong doesn't help. Follow the two steps: the flickering supports a close orbit, and maintaining that close orbit requires spinning.
Working the answers
- A
What happens with rings larger than 49 kilometers? The passage doesn't tell us. A black hole with a more distant ring might spin or might not. Evidence about the observed close orbit gives us no basis for saying that black holes with larger rings are usually stationary.
- B
We have an example of flickering X-rays from a ring in a stable orbit around a black hole. That doesn't establish that only such rings emit flickering X-rays. What about a ring around some other object, for example? The passage doesn't rule out other sources, so the exclusivity in B is unsupported.
- CCorrect
This stays with the particular black hole observed. The accepted theories support placing its gas ring 49 kilometers away, and a ring cannot maintain that close orbit unless the black hole spins. The observations therefore support the conclusion that this black hole is spinning.
- D
Needing the black hole to spin is different from explaining what makes it spin. We aren't told that the emitted X-rays produce the spin. The passage gives a condition for maintaining the orbit, not a causal account of how the black hole starts rotating.
- E
This says every stationary black hole must have a gas ring with a radius greater than 49 kilometers. We don't even know that every stationary black hole has an orbiting gas ring. The restriction on this particular close orbit doesn't establish a requirement that all stationary black holes have larger rings.
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