Quote:
Originally Posted by Quarkus
Cheers Steve,
(that would be one smooshed shuttle! Maybe the radiation would cripple it before it had a chance to smoosh on landing...?)
I understand that some neutron stars emit pulsing magnetic radiation (as you describe) and we call these pulsars. I (sort of) get why and how this is so. But, Pamela says: not all neutron stars are pulsars. I'm asking, if we can only detect these pulses if our planet lies (to use their charming analogy) in the sweeping beam of the lighthouse, how can we say that every neutron star isn't a pulsar? For her to have said that, there must be other ways of detecting the difference, but what are they?
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The short answer is x-rays and gamma-rays.
I think the thing is that
all neutron stars emit heaps of radiation - because they all spin rapidly, and they all have gigantic magnetic fields, so they are all like powerful electric generators. So in some sense
all neutron stars
are pulsars... just in subtly different ways.
Most, but not all, neutron stars emit this radiation from their magnetic poles in powerful radio beams, that wobble because the magentic poles are not aligned with the axis of rotation. Because of the wobble, the radio beam appears to pulse to observers.
Objects that are known to be neutron stars, but for which no pulsing radio beam has been observed, are known as 'radio-quiet neutron stars' or RQNS.
Then it gets harder to follow, at least with my limited resources. I have only enough knowledge to be a danger to myself and others.
It could be the case with some RQNS, that if Earth happens to be aligned with the axis of rotation of the star, the beam simply misses us, because it wobbles around the axis of rotation. It seems others just don't have
typical magnetic fields and/or radiation and particle emissions.
This is what they say here:
http://einstein.phys.uwm.edu/Partial...lts/node4.html
"It is believed that our Galaxy contains a hundred thousand or more rapidly-spinning neutron stars, most of which are (as yet) undetected. This is because either the pulsar's beam misses the Earth, the radio pulses are smeared out by cosmic clouds or dust, or the spinning neutron star does not have an intense enough magnetic field to produce a beam."
Here's the story of Geminga, the first RQNS to be observed:
A couple of sattelites spotted unidentified gamma-rays in the seventies. Some years later the Einstein x-ray satellite detected 'soft x-rays' from the same area of space. Then, in the nineties, the ROSAT x-ray satellite observed faint x-ray pulsations, and the gamma-ray observations were refined, such that astronomers resolved that the source of the emissions was a RQNS. Geminga has even been observed at optical wave-lengths (it's only 500 lightyears away)... but no radio emissions have been detected.
XMM-Newton is observing Geminga in even greater detail, for example the so-called 'hot-spots', which are described as an 'own-goal' where Geminga re-absorbs some of its own particle/radiation emissions, probably due to it's powerful magnetic fields.
Even though Geminga is radio quiet, it seems many astronomers still refer to it as a 'pulsar'.
These guys say this:
http://arxiv.org/PS_cache/physics/pd.../0503245v2.pdf
"Neutron stars were first proposed by Baade and Zwicky
in 1934 in their pioneering paper on supernovae, and con-
siderable theoretical work on their properties, beginning
with calculations by Oppenheimer and Volkoff in 1939,
was carried out prior to their actual observation. It was
not until the discovery in 1967 by Bell and Hewish of ra-
dio pulsars – stars whose radio emission appears to blink
on and off – and their identification by Gold as rotat-
ing neutron stars, that the existence of neutron stars was
established. Since that time neutron stars have become
cosmic laboratories for testing fundamental physics, in-
cluding relativistic theories of gravity and the properties
of matter at extreme densities. Neutron stars also play
a crucial astrophysical role as the objects underlying a
wide variety of highly energetic compact radio, x-ray, and
gamma-ray sources. Radio pulsars are rotation-powered,
are found both in isolation and in binary star systems,
and are observed to emit radiation at all frequencies from
radio to optical to gamma rays. Neutron stars are also
found in luminous compact x-ray binaries in which they
accrete matter from a companion star. About 1500 neu-
tron stars have so far been detected in the galaxy as radio
pulsars, including about 125 such pulsars with millisec-
ond periods. More than 200 accretion powered neutron
stars have been detected in x-ray binary systems; about
50 are x-ray pulsars and a similar number produce intense
x-ray bursts powered by thermonuclear flashes."
X-ray pulsars (or accretion-powered pulsars) pulse x-rays (they would wouldn't they!) and are neutron stars in orbit around a normal star. They have different spin, and generate their pulses in a different way to radio pulsars.
Hope that helps?