I was just watching a TV show that dealt with M1 - The Crab Nebula. They were saying that there is an immensely powerful pulsar in the middle of it that we “see” because as it rotates the “magnetic poles” sweep in our direction.
So, if the poles did not “sweep over” us, would we know a pulsar was there? Lets forget for a moment that the nebula is there at all. If there was just a pulsar like it say, 5 light years away, would we even know it since they are reportedly so small (only a few KM across)?
Neutron stars (basically what a pulsar is except they don’t ’ pulse’) are pretty dim objects. They are very small and tend to be brightest in the x-ray spectrum. I would think one a mere 5 light years away would have been spotted by now but that’s just a guess. Here’s a link to a ‘lone’ neutron star recently found by the Hubble telescope.
They do not know the distance to this star but it can be no more than 400 light years away as it is in front of another object that is that far away (as described in the link).
A pulsar which was not directing an energy beam towards our radio astronomy equipment, but was as close as the OP enquired about, might be detected by other means. Extreme brightness, surrounding debris, other bodies such as planets or part of a binary system, etc.
Actually, we’d definitely know about it. The thing is that it’s far easier to find supernova remnants in general than the neutron stars/pulsars inside them. Clark and Stephenson, The Historical Supernovae (Pergamon, 1977), is now a very dated reference, but still full of good stuff. At the time they were writing, there were about a 100 remnants known in our Galaxy. These had been identified on the basis of four characteristics: a pulsar, an expanding optical nebula, radio emission and X-ray emission (p57). But only the Crab Nebula and the Vela supernova had all four and it was only the radio emission that was typically found in all cases. However, for the hypothetical of looking for stuff nearby, close remnents also tend to be the ones visible in the optical and X-rays.
They don’t really discuss the likelihood of a particular remnant being visible as a pulsar. But there were a half dozen or so cases where the original explosions could be dated from historical records and Clark and Stephenson suggested remnants that were the results. Only in the case of the Crab Nebula had a pulsar been seen in this special - and especially young and close - subset of remnants (p154). To my knowledge, this hasn’t changed in the intervening quarter century.
Actually, there could be a neutron star relatively close without us seeing it at all. If it were old, like billions of years old, it would basically be non-rotating and cold.
The spin slows as the magnetic field of a young neutron star sweeps up the nearby stuff; this acts like a parachute, slowing the pulsar. Eventually the rotation slows to something very slow. I don’t think anyone knows how slow, because old, slow pulsars are virtually impossible to detect! But there are some NS with rotation periods of minutes.
As to the OP, I believe that the beams of a pulsar are wide enough that, in general, one will always sweep over the Earth. However, I’m scratching my head and thinking I am not so sure about that! I’ll have to ask my boss; she knows quite a bit on this subject.
Well, I was wrong! I talked with my boss, and she said the radio beams from neutron stars are fairly narrowly focused. It is not only possible but (as far as I understand it) probable that most pulsars have beams that do not sweep over us; they miss us. Pretty cool.