I Why are continuously emitting radio pulsars not detected?

[Line_112]

TL;DR Summary

As far as I've read, all discovered pulsars have pulsating radiation, although according to probability theory, some of them should have continuous radiation. It looks more like the pulsations are not related to the rotation of the neutron star, but to true pulsations of radiation.

As far as I have read, no continuously emitting radio pulsar has been discovered, although the probability of detecting such a pulsar with an arbitrary arrangement of magnetic poles is approximately 1 in 100. If this probability is realized, the Earth should be constantly in the radiation cone when the pulsar is directed at it with its magnetic pole, and the magnetic pole coincides with the geographic pole. Logically, the magnetic poles should generally coincide with the geographic poles, so the actual probability of detecting a continuously emitting pulsar will be even higher. But if there are none, then it is logical to assume that the pulse radiation of pulsars is not related to their rotation, but to the periodicity of the radiation itself. Plus, each pulsar has its own individual pulse profile, but with continuous radiation this should not have happened.
The presence of an intermediate low-amplitude pulse between the main ones is also easier to explain by a true oscillation in the radiation of pulsars than by the effect of rotation of the emitting magnetic poles.
I also read about the coherence of radio pulsar radiation, but I still haven't figured out how it manifests itself in pulsars. If someone understands this, I'd be glad to get a hint. As far as I know, coherent radiation is characteristic of artificial objects, not natural ones.

 

[Baluncore]

If it does not pulse, then it is a steady radiation source, not a pulsar.

 

[martynk]

A pulsar pulses because it is a rotating object transmitting radiation in a similar fashion to the way a traditional lighthouse flashes as the rotating mirror concentrates the light in a specific direction.

We only detect the radiation when the light beam is pointing towards us or in the case of a pulsar when the radio beam is pointing towards us.

 

[Line_112]

If it does not pulse, then it is a steady radiation source, not a pulsar.

That's the point, there are no such steadily emitting radio neutron stars. At least, I haven't read about any. But if they do exist, then the question is exhausted and the topic is closed.

 

[Baluncore]

That's the point, there are no such steadily emitting radio neutron stars. At least, I haven't read about any.

There are many steady, non-pulsating, astronomical RF energy sources.
How can you possibly identify, what proportion of that energy, comes from non-rotating neutron stars.

 

[davenn]

That's the point, there are no such steadily emitting radio neutron stars. At least, I haven't read about any. But if they do exist, then the question is exhausted and the topic is closed.

It appears that you dont understand/know what a pulsar is/works

 

[sophiecentaur]

There are many steady, non-pulsating, astronomical RF energy sources.
How can you possibly identify, what proportion of that energy, comes from non-rotating neutron stars.

Perfect logic.
If it doesn't quack then how do you know it's not a duck?

 

[Line_112]

There are many steady, non-pulsating, astronomical RF energy sources.
How can you possibly identify, what proportion of that energy, comes from non-rotating neutron stars.

I can't find anything about such non-pulsating sources, even the Google search engine recognizes the query as incorrect, redirecting me straight to the query: "stable pulsating sources of radio emission". It is possible to determine that this is the same star as a pulsar by the spectrum of the emission and its intensity.

 

[phinds]

I can't find anything about such non-pulsating sources,

Seriously??? How hard did you look?

 

[sophiecentaur]

Seriously??? How hard did you look?

Astronomers don't have to look too hard because they can spot regular changes in signal levels much easier than constant level signals. A constant level signal; doesn't draw attention to itself.

It's the same with comets and asteroids. Sequences of images show tracks of pretty faint objects moving day by day.

 

[Line_112]

Seriously??? How hard did you look?

But what's interesting is that none of these sources are non-pulsating neutron stars.

 

[Line_112]

Astronomers don't have to look too hard because they can spot regular changes in signal levels much easier than constant level signals. A constant level signal; doesn't draw attention to itself.

It's the same with comets and asteroids. Sequences of images show tracks of pretty faint objects moving day by day.

But in this case we are talking about sources of radio emission that a person cannot see, and a radio telescope does not care whether it pulsates or not. It seems to me that in this case a stable source would be even easier to detect than a variable one, which can be missed when the telescope moves across the sky if it passes a star in the interval between pulses.

 

[phinds]

But what's interesting is that none of these sources are non-pulsating neutron stars.

Right. Also, none of them are pink unicorns either, but so what? Are you suggesting that there ARE no such things as non-pulsar neutron stars, or non-pulsars that do not emit RF?

 

[Drakkith]

If this probability is realized, the Earth should be constantly in the radiation cone when the pulsar is directed at it with its magnetic pole, and the magnetic pole coincides with the geographic pole.

I wonder if a rotating neutron star would emit much RF radiation if its magnetic poles coincide with its physical poles. @Ken G any idea?

 

[Hornbein]

I wonder if a rotating neutron star would emit much RF radiation if its magnetic poles coincide with its physical poles. @Ken G any idea?

I think it wouldn't make any difference.

A non-rotating neutron star is extremely unlikely. Every heavenly body is rotating. A coincidence of poles is very unlikely as well.

 

[Line_112]

Right. Also, none of them are pink unicorns either, but so what? Are you suggesting that there ARE no such things as non-pulsar neutron stars, or non-pulsars that do not emit RF?

Let's just say I suspect it, since there are other arguments in favor of hypotheses true radio pulses (that is, not related to the rotation of the star).

 

[sophiecentaur]

and a radio telescope does not care whether it pulsates or not

Hmmm. Are you sure about that? A very low level narrow band amplitude modulated signal is far easier to detect than a CW signal. Such a signal spectrum stands out amidst a host of other continuous signals.

The signal processing in a radio telescope can be very discriminatory- much more than the output of an optical photodetector.

 

[Line_112]

A coincidence of poles is very unlikely as well.

Wikipedia: "according to theoretical estimates, the number of observable radio pulsars in the Galaxy is estimated at (24±3)⋅103, and their total number is (240±30)⋅103." That is, every tenth of the pulsars is observable, which, without additional conditions, means that the width of the radiation cone is 1/10 of the length of the meridian between the pole and the equator. Since the magnetic poles gravitate toward the geographic poles, then for many neutron stars the cone of radiation from the magnetic pole should overlap with the geographic pole.

 

[sophiecentaur]

I think it wouldn't make any difference.

'Thinking' is risky here, without some detailed knowledge of the model used. I guess you could look at the various frequencies of pulsars and look for any correlation between frequency and received signal level. If the average level goes down with frequency then you might conclude it being zero level for zero frequency.

But the point has already been made that 'everything' out there has angular momentum. I must say, the frequencies of some pulsars really freaks me out when you realise how massive they are.

 

[Hornbein]

'Thinking' is risky here, without some detailed knowledge of the model used. I guess you could look at the various frequencies of pulsars and look for any correlation between frequency and received signal level. If the average level goes down with frequency then you might conclude it being zero level for zero frequency.

But frequency of rotation wasn't the issue. It was whether emissions would be reduced if "its magnetic poles coincide with its physical poles."

But the point has already been made that 'everything' out there has angular momentum. I must say, the frequencies of some pulsars really freaks me out when you realise how massive they are.

The period is limited to a millisecond because with period less than that the star ovalizes and dissipates energy via gravity waves.

 

[Hornbein]

Since the magnetic poles gravitate toward the geographic poles

I don't believe that they do. The star is created in a cataclysm. The magnetic field is "frozen" into the newly rigid outer layer of the star. The magnetic pole is wherever it happens to be at this time. The surface can move due to a starquake or whatever but there is no reason such should be in the direction of the rotational poles.

Our Sun has that same freezing of magnetic lines into the highly conductive material of the Sun but it's a plasma so the material can flow slowly. This movement causes the magnetic poles of the Sun to flip ever eleven years.

 

[Line_112]

I don't believe that they do. The star is created in a cataclysm -- the magnetic pole is wherever it happens to be after this. The magnetic field is "frozen" into the extremely rigid outer layer of the star. The surface can move due to a starquake or whatever but there is no reason such should be in the direction of the rotational poles.

Answer from AI (may contain errors):
The Earth's magnetic pole is not attracted to the geographic pole, but rather is located near it, but does not coincide. The reason for this is that the Earth's magnetic field is created by the movement of liquid iron-nickel alloy in the Earth's outer core, and not by the planet's electric charge. This "dynamo effect" leads to the fact that the magnetic poles can be located in different points, but, as a rule, they are located near the geographic poles.

I will add on my own that, the magnetic field of a neutron star is obviously also related to its rotation, so rapidly rotating magnetars have the strongest magnetic field.

 

[Line_112]

Right. Also, none of them are pink unicorns either, but so what? Are you suggesting that there ARE no such things as non-pulsar neutron stars, or non-pulsars that do not emit RF?

I don't know English very well, so I may not understand the text on the screenshot quite correctly. But as I understood from this piece (which is an answer from AI, often containing gross errors), some neutron stars do emit continuous radio radiation, but it is much weaker in strength, and therefore may not be related to the radiation of pulsars. Magnetars are given as an example, but Wikipedia says that they do not emit in the radio range:
"They are observed in gamma radiation close to X-rays, and they do not emit radio radiation [4]. The life cycle of a magnetar is quite short."
No matter how much I search on the Internet, I can't find anything about non-pulsating neutron stars. It seems that one article mentions accreting X-ray sources, but there they associate it with the features of accretion on them, and this is a completely different topic and a different type of stars (X-ray accreting pulsars). The AI could have come up with something, as it often does.

 

[Hornbein]

Answer from AI (may contain errors):
The Earth's magnetic pole is not attracted to the geographic pole, but rather is located near it, but does not coincide. The reason for this is that the Earth's magnetic field is created by the movement of liquid iron-nickel alloy in the Earth's outer core, and not by the planet's electric charge. This "dynamo effect" leads to the fact that the magnetic poles can be located in different points, but, as a rule, they are located near the geographic poles.

The magnetic field of the Earth is generated by a geodynamo. Most neutron stars never have a geodynamo. The exception is magnetars. They have a short-lived geodynamo that is so powerful it generates a superpowerful magnetic field that persists for millions of years.

I will add on my own that, the magnetic field of a neutron star is obviously also related to its rotation, so rapidly rotating magnetars have the strongest magnetic field.

Magnetars are usually the slowest rotating neutron stars, as much of the rotational energy went into the magnetic field via the geodynamo.

Wikipedia says " Magnetars are differentiated from other neutron stars by having even stronger magnetic fields, and by rotating more slowly in comparison. Most observed magnetars rotate once every two to ten seconds, whereas typical neutron stars, observed as radio pulsars, rotate one to ten times per second."

I don't find Wikipedia to be all that reliable but it's better than AI.

I learned about neutron stars by reading real scientific articles in arXiv and elsewhere. I can't understand the math at all, I just read the intro and conclusion. Here's an example. https://academic.oup.com/mnras/article/378/1/159/1154016. They say that the former idea that the magnetic field is inherited from the original pre-supernova star is incorrect. Though this does occur the field would be too weak. Instead for non-magnetars it might be due to the strong inherited magnetic field inducing ninety times stronger Pauli paramagnetic magnetization of the electrons in the star.

I haven't looked at this stuff for a long time. This is inspiring me to catch up. The physics involved is truly extreme so no one is certain what really happens.

 

[weirdoguy]

Answer from AI

Is forbidden per PF rules.

 

[sophiecentaur]

The period is limited to a millisecond because

I would say that was scarily quick! It shows just how strong gravity can be when 'up close'. (Yes, I know the inverse square law (plus some GR, no doubt) would predict it - but still. . . . .

And that limit - and the reason - is also pretty scary.

 

[Line_112]

Most observed magnetars rotate once every two to ten seconds, whereas typical neutron stars, observed as radio pulsars, rotate one to ten times per second."

The Russian Wikipedia says otherwise:
"Most of the known magnetars rotate very quickly, at least several revolutions around the axis per second[3]."
This link leads to the English source: Mark A. Garlick. Magnetar (1999) (англ.). www.space-art.co.uk.
Link to the Wikipedia article on magnetars: https://ru.wikipedia.org/wiki/Магнитар

 

[Ken G]

It does seem odd that no neutron stars have been detected with continuous radio emission that looks like a normal pulsar beam except that it is aligned with the rotation axis, given that it seems the probability the beam crosses Earth is about 1/10. It is important to realize we should not infer from this that 1/10 of pulsars should have their rotation axis within their radio beam, because the radio beam is much smaller than 1/10 of the sky. However, the beam does seem to be maybe 0.5-1% of the sky, so if the rotation axis was random with respect to the beam axis, perhaps 1/200 or even 1/100 of radiating neutron stars should be seen as non-pulsing sources. That means we should have detected a few dozen of them by now. That's a small enough number that maybe we've just missed them because we were not looking for them (since they don't pulse), but a big enough number that maybe we have data that could be mined to find them.

My conclusion is not that we should be looking for a different explanation of pulsars, one that does not connect the pulsing to the rotation period (after all there has been a Nobel prize awarded for connecting the changes in time of the pulses with the changes in rotation rate), but rather, we should be looking for continuous radio sources that could be "pulsars" whose beam aligns with the rotation axis. If we don't find them, it would mean that something about the beam mechanism doesn't work unless it is misaligned, and I'm not sure if we know enough about the beam mechanism to answer that without an observational inquiry.

In short, I think the OP has identified an interesting hole in our current observations, but not a reason to doubt the basic pulsar model.

 

[Baluncore]

That's a small enough number that maybe we've just missed them because we were not looking for them (since they don't pulse), but a big enough number that maybe we have data that could be mined to find them.

Many astronomical radio sources have a wideband thermal noise energy flux. Pulsars have a very wide spectrum from about 3MHz to 3GHz, much wider than most RA receivers in that range.

If a sufficiently wideband receiver was made, it would be so inherently noisy due to V_RMS α √BW_Hz, that it would be blind. It would also be de-sensed by radio interference from sources outside the preserved quiet RA bands. Broadband observations of pulsar spectra would need to simultaneously use several different band receivers, then interpolate across the gaps in the spectrum.

It would be impossible to identify the constant flux, of a non-rotating pulsar, against the many other white noise sources in the RA sky.

There are many signals with a spectrum that could be identified as not pulsar, such as H-line, and molecular MASER sources, but there is still a background of noise sources behind those, that may or may not originate from hypothetical non-rotating pulsars.

There are a few pulsars that generate a double pulse, or a pulse with a dip in the centre. That pulse looks like a section through a hollow conical beam, similar to that generated by a travelling wave antenna. The central dip, in the conical beam from a TWA, is due to the dipole pattern of radiation being aligned with the axial conductor. To observe the central dip, those pulsars would need to sweep almost directly across the observer.

 

[Ken G]

Many astronomical radio sources have a wideband thermal noise energy flux. Pulsars have a very wide spectrum from about 3MHz to 3GHz, much wider than most RA receivers in that range.

If a sufficiently wideband receiver was made, it would be so inherently noisy due to V_RMS α √BW_Hz, that it would be blind. It would also be de-sensed by radio interference from sources outside the preserved quiet RA bands. Broadband observations of pulsar spectra would need to simultaneously use several different band receivers, then interpolate across the gaps in the spectrum.

It would be impossible to identify the constant flux, of a non-rotating pulsar, against the many other white noise sources in the RA sky.

There are many signals with a spectrum that could be identified as not pulsar, such as H-line, and molecular MASER sources, but there is still a background of noise sources behind those, that may or may not originate from hypothetical non-rotating pulsars.

There are a few pulsars that generate a double pulse, or a pulse with a dip in the centre. That pulse looks like a section through a hollow conical beam, similar to that generated by a travelling wave antenna. The central dip, in the conical beam from a TWA, is due to the dipole pattern of radiation being aligned with the axial conductor. To observe the central dip, those pulsars would need to sweep almost directly across the observer.

Interesting point about the difficulty in identifying a neutron star from its radio spectrum alone. It might still be possible in the time domain, however. I think what you'd look for is a nearly continuous but still periodically varying source, as if the beam was nearly, but not exactly, aligned with the rotation axis. If there is nothing physical that prevents that, there should be examples that might escape a typical pulsar search algorithm but still be quite easy to see if you are looking for it, though I don't know what algorithms are currently used to find pulsars in existing archives.

 

[Baluncore]

It might still be possible in the time domain, however. I think what you'd look for is a nearly continuous but still periodically varying source, as if the beam was nearly, but not exactly, aligned with the rotation axis.

A pulsar is a noise source that pulses in a noisier background. The detected signal from the brightest pulsar, fed in real time to an amplifier and speaker, can just be heard in the background noise.

Given sufficient time, you can gather data with about a 2 kHz BW, then FFT, and Power Spectrum Accumulate, deep into the noise, to see what is hidden in there. If you find a blip in the PS, you can run it again on the same patch of sky, to see if it is still there, and repeatable. Once you have an idea of the pulse rate, you can take longer time samples, to get better estimates of the rate. You can then accumulate power in a circular time buffer, to see the shape of the pulse. But what if the pulse is an almost flat sinusoid?

Either you have reliably detected a pulsar, and measured its period, or you have not. I see no way to tell the difference between a dim pulsar in noise, a pulsar being seen side-on in noise, or one almost end-on in noise.

The search for the pulsar with the flattest sinusoidal pulse, will take forever, and cannot resolve any question.

Observatory time is expensive. There is little point in investing the huge amount of observation time, needed to dig the smallest signals out of noise, if those signals are so small, that they cannot be categorised meaningfully.