Neutron Stars - re 'Dragon's Egg'

[GeeKay]

I am currently writing an SF novel whose primary setting is a neutron star (this is essentially a projected sequel of the two novels 'Dragon's Egg' and 'Starquake' by Robert L Forward). The question that's bothering me at present concerns the likely effects a serious of asteroids would have were they to be directed down on to the neutron star itself. I am aware that the effects any such impacts might have could be extremely minimal, or else restricted to the star's magnetic poles. Nevertheless, I would welcome any views or opinions this question might raise.

[mathematic]

Neutron stars have more mass than the sun. Asteroids are pretty small. I doubt if there would be any noticable effect.

[GeeKay]

Mathematic - many thanks for the comment. Yes, I'm inclined to agree with you there. However, as so often seems to be the case whenever I post a question in forums such as these, it didn't take me all that long afterwards to find myself stumbling upon answers of sorts while hunting around elsewhere on the net. Here, I refer to the so-called 'Christmas Burst' of 2010. Back then two explanations were offered up for the cause of this extraordinarily lengthy gamma ray outburst. For me, the most telling of these two explanations pointed to a cometary impact upon an otherwise quiescent neutron star located within our own galaxy. Since then this notion has given way to the other explanation, then doing the rounds: namely that the cause for this GRB was due to a considerably more distant supernova explosion. Even so, the fact that the astrophysicists were willing to entertain a (large) cometary impact as a possible cause for this GRB does leave me wondering whether such strikes are as benign as is so often thought. Unfortunately, I simply don't have the necessary number-crunching abilities to sweat the answer out for myself - at least not yet! In the meantime, I offer the link below as supporting evidence for my query.

 

http://www.universetoday.com/91406/did-a-neutron-star-create-the-christmas-burst/

[Delta1212]

Comparing asteroid impacts with cometary impacts is like comparing fireworks to nuclear missiles. Yes, they're both basically exploding rockets, and yes, you don't want to be next to either when they go off, but the scale is just a tad different.

[Moontanman]

Comparing asteroid impacts with cometary impacts is like comparing fireworks to nuclear missiles. Yes, they're both basically exploding rockets, and yes, you don't want to be next to either when they go off, but the scale is just a tad different.

 

 

Any chance you could clarify the difference a little more precisely?

[Royston]

Mathematic - many thanks for the comment. Yes, I'm inclined to agree with you there. However, as so often seems to be the case whenever I post a question in forums such as these, it didn't take me all that long afterwards to find myself stumbling upon answers of sorts while hunting around elsewhere on the net. Here, I refer to the so-called 'Christmas Burst' of 2010. Back then two explanations were offered up for the cause of this extraordinarily lengthy gamma ray outburst. For me, the most telling of these two explanations pointed to a cometary impact upon an otherwise quiescent neutron star located within our own galaxy. Since then this notion has given way to the other explanation, then doing the rounds: namely that the cause for this GRB was due to a considerably more distant supernova explosion. Even so, the fact that the astrophysicists were willing to entertain a (large) cometary impact as a possible cause for this GRB does leave me wondering whether such strikes are as benign as is so often thought. Unfortunately, I simply don't have the necessary number-crunching abilities to sweat the answer out for myself - at least not yet! In the meantime, I offer the link below as supporting evidence for my query.

 

http://www.universetoday.com/91406/did-a-neutron-star-create-the-christmas-burst/

 

The energy in a comet impact is bugger all compared to a GRB, the last 2/3's of your paragraph is nonsense. GRB's are not really comparable, the energies and the subsequent processes are boring.

Edited August 18, 2013 by Royston

[BearOfNH]

 

The energy in a comet impact is bugger all compared to a GRB

 

Pardon my asking, but English slang is foreign to me.

 

Does the above mean comet >> GRB, or comet << GRB?

 

I'd guess a GRB starts out with tremendous energy which then rapidly dissipates over time and space. Comets, different.

[imatfaal]

x is bugger all to y means x<<y.

[GeeKay]

All other things being equal, I guess any differences in explosive impact between an asteroid and a comet must come down to mass and velocity.

Finding yourself stopping a high-velocity snowball might be no less painful than being struck by a slow-moving pebble. Yes? No?

 

As for any associations between GRBs and cometary material impacting upon neutron stars, well, like the White Queen in 'Alice Through the Looking Glass', I too can believe in six impossible things before breakfast. Seriously, though, if professional astrophysicists across the world can raise such a possibility about GRBs - as they did about the 2010 Christmas Burst - who am I (with my humble GCSE in Maths) to disparage their line of enquiry?

[Royston]

 

The energy in a comet impact is bugger all compared to a GRB, the last 2/3's of your paragraph is nonsense. GRB's are not really comparable, the energies and the subsequent processes are boring.

 

Oh dear, I don't even recall writing this, it doesn't make sense. Going by the date I was celebrating a friends birthday that day...(IOW I was mentally incapacitated). Sorry about that.

 

By way of redemption, neutron stars have exceptionally strong gravitational fields, so anything travelling towards one (be it a comet or asteroid) has the potential to accelerate to a very high velocity (up to a large percent of the speed of light). Energy is conserved, so the kinetic energy of the body impacting the neutron star (which will be huge at those speeds) is balanced by thermal energy et.c on impact, and hence a very high energy explosion.

 

A body in orbit around a star that tips over it's Roche limit will be broken up by tidal forces, and will (for the most part) join the accretion disk. Comets have a low tensile strength, so for the bulk of the debris to crash into the neutron star and not join the accretion disk, would probably require some unique initial conditions.

 

Finding yourself stopping a high-velocity snowball might be no less painful than being struck by a slow-moving pebble. Yes? No?

 

 

This seems to be along the right lines.

 

EDIT: I don't find GRB's boring, quite the opposite...I have no idea where that came from .

Edited August 24, 2013 by Royston

[Moontanman]

All other things being equal, I guess any differences in explosive impact between an asteroid and a comet must come down to mass and velocity.

Finding yourself stopping a high-velocity snowball might be no less painful than being struck by a slow-moving pebble. Yes? No?

 

As for any associations between GRBs and cometary material impacting upon neutron stars, well, like the White Queen in 'Alice Through the Looking Glass', I too can believe in six impossible things before breakfast. Seriously, though, if professional astrophysicists across the world can raise such a possibility about GRBs - as they did about the 2010 Christmas Burst - who am I (with my humble GCSE in Maths) to disparage their line of enquiry?

 

 

At low speeds like throwing snow balls or pebbles on the earth there wouldn't be much difference but at the speeds of orbiting materials what the object made of is not a factor, pebble or snow ball, the same mass equals the same impact energy at the same speeds.

[GeeKay]

Yes, in the case of neutron stars, I can well understand why any infalling material - whether cometary or asteroidal - would be accelerated to a substantial fraction of the speed of light, this regardless of initial velocities. I guess too in view of Roche limits having been exceeded, that the kinetic energies resulting from such an impact would be much the same, even if (as seems highly likely) an infalling comet or asteroid had by this stage been reduced to plasma. Another thought: would an accretion disc orbiting a neutron star likewise be reduced to plasma? If so, would the resulting radiation output cause the star to 'behave' like a pulsar, albeit temporarily - hence the GRB effect? Or is this taking things too far?

 

Moontanman: yes, I fully agree with your comment. It's just that I was trying to distinguish between mass and velocity, that's all. In doing so, I was basing this notion on the observable fact that the orbital velocities of comets tend to be be higher than those of asteroids. Thus there may be no difference in terms of explosive impact between (say) a high-velocity low-mass comet and a comparatively slow-moving high-mass asteroid. Sorry about labouring the point.

[Airbrush]

Think about an asteroid impacting a neutron star. Yes it will be accelerated to great speed and will hit the neutron star's surface with great energy, but the energy will get sucked up like a vacuum cleaner sucking up dust (very much like a black hole). The effects will be hardly noticeable, even if you somehow could install a camera on the surface of the neutron star. The explosion would be so brief that it would look like the asteroid just vanished into thin air.

[GeeKay]

Ah, yes, I guess you're right, there. Shame really. Still, it probably helps to make the universe a safer environment from the point of view of life on Earth (and elsewhere?) - a lot safer than having asteroid-primed GRBs popping off every other minute. Oh, well, back to the drawing board.

[imatfaal]

Think about an asteroid impacting a neutron star. Yes it will be accelerated to great speed and will hit the neutron star's surface with great energy, but the energy will get sucked up like a vacuum cleaner sucking up dust (very much like a black hole). The effects will be hardly noticeable, even if you somehow could install a camera on the surface of the neutron star. The explosion would be so brief that it would look like the asteroid just vanished into thin air.

 

Apart from the gamma rays which trashed everything in a sightline, the ir thats cooks everything, all illuminated by the strongest light you will every record absent getting to super nova. The energy would be given off in the main as EMR and that does not get sucked up by a neutron star - even outside the EH of a BH the collision and heating of the accretion disks make some of the most destructive and energetic events we get; large amounts of mass basically converting to energy in a very small region and being directly radiated. Brutal

 

obligatory xkcd - it's a good one http://what-if.xkcd.com/1/

[Royston]

Geekay, I've dug out the original paper by Campana et al, if it's any use. Note, that the GRB or I guess pseudo-GRB is a combination of debris hitting the surface and emitted radiation due to accretion. From the paper...http://arxiv.org/pdf/1112.0018v1.pdf

 

 

The observed radiation is emitted both by an accretion disk (with an outer radius twice as large as periastron) and during the impact of matter onto the neutron star surface.

 

Reading back over my post I did not include momentum, which is important, kinetic energy would not be conserved for debris hitting the surface. However the paper describes the debris that becomes unbound and strikes the surface in terms of specific energy (see the expression for fall back time).

[GeeKay]

Royston: many thanks for the comments and for the link - which I've now saved in my favorites for further investigation. immatfaal, what is EMR? (A wild guess: 'Energised Magnetic Radiation' - surely not??)

 

About momentum: at one point in Robert L Forward's 'Dragon's Egg' novel, a meteor (or rather its plasma) strikes the eponymous neutron star at one of its magnetic poles. The force of impact is such that it triggers a brief but locally devastasting gamma ray outburst. I can remember reading this and thinking that if an incoming object the size of a grain of sand can produce this effect, what would happen if an asteroid, or a planetoid with (say) the mass of Vesta, were to impact upon the surface of the aforesaid neutron star, and with the same terminal velocity - in this instance, around 0.3 c. The thing is, Forward was an astrophysicist as well as a SF writer. So he knew his stuff. I just wish I knew enough of it myself, but - hey - that's part of the challenge.

[imatfaal]

Sorry was typing too quickly EMR = Electro-magnetic Radiation ie Gamma Rays through to Radio Waves

[Airbrush]

 

Apart from the gamma rays which trashed everything in a sightline, the ir thats cooks everything, all illuminated by the strongest light you will every record absent getting to super nova. The energy would be given off in the main as EMR and that does not get sucked up by a neutron star - even outside the EH of a BH the collision and heating of the accretion disks make some of the most destructive and energetic events we get; large amounts of mass basically converting to energy in a very small region and being directly radiated. Brutal

 

obligatory xkcd - it's a good one http://what-if.xkcd.com/1/

 

 

On second thought, you are correct. The gamma rays and other electromagnetic radiation would escape the neutron star as a blinding flash. How long would the flash last? Maybe very brief?

[imatfaal]

Brief like a nuclear bomb brief. Not sure the length of the burst lasts - anything in line of sight of a large object impacting at highly relativistic speeds is gonna be toast. They would be anyway being on a neutron star - but this would cook them completely.

[Airbrush]

Nuclear bomb brief? You mean like a nuke going off in outer space, which would be VERY brief. No boiling, rising mushroom cloud in an atmosphere. I think an asteroid, like the one 65 million years ago, crashing into a neutron star, would be a very brief gamma ray and EM burst, but NOT a directional one like from an exploding massive star. What do you think it would appear as?

[imatfaal]

Nuclear bomb brief? You mean like a nuke going off in outer space, which would be VERY brief. No boiling, rising mushroom cloud in an atmosphere. I think an asteroid, like the one 65 million years ago, crashing into a neutron star, would be a very brief gamma ray and EM burst, but NOT a directional one like from an exploding massive star. What do you think it would appear as?

 

Exploding massive stars are not directional - they go off in all directions (there maybe weird cases where this does not apply - but in general). the gamma ray bursts from exploding stars are birectional - but this is not a well understood area. The directional bursts can also be from accreted material spiralling into a neutron star or blackhole and create jets at the poles (similar to accretion powered pulsars - but not regular) which is the closest to this. I have no idea what the blast pattern would be.

 

Nothing like this could make the energy of a "proper gamma ray burst" - the energy release in those babies is quite spectacular; maybe the equivalent of 0.1% of our suns solar mass being converted to energy