APOD: Fermi Epicyles: The Vela Pulsar's Path (2012 May 04)

[APOD Robot]

Fermi Epicyles: The Vela Pulsar's Path

Explanation: Exploring the cosmos at extreme energies, the Fermi Gamma-ray Space Telescope orbits planet Earth every 95 minutes. By design, it rocks to the north and then to the south on alternate orbits in order to survey the sky with its Large Area Telescope (LAT). The spacecraft also rolls so that solar panels are kept pointed at the Sun for power, and the axis of its orbit precesses like a top, making a complete rotation once every 54 days. As a result of these multiple cycles the paths of gamma-ray sources trace out complex patterns from the spacecraft's perspective, like this mesmerising plot of the path of the Vela Pulsar. Centered on the LAT instrument's field of view, the plot spans 180 degrees and follows Vela's position from August 2008 through August 2010. The concentration near the center shows that Vela was in the sensitive region of the LAT field during much of that period. Born in the death explosion of a massive star within our Milky Way galaxy, the Vela Pulsar is a neutron star spinning 11 times a second, seen as the brightest persistent source in the gamma-ray sky.

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[geckzilla]

This is such a strange APOD. Does this have any use other than to explain that Fermi doesn't always look in the same direction and looking cool? Not that it needs a use, I just want to know if it does have one.

[Beyond]

The pattern looks kinda neat, but it deals with gamma rays, so i'm not going to get too close to it. We all know what happened to Bill Bixby when he got too close to gamma rays. He turned into Lou Ferrigno and became green

[Chris Peterson]

This is such a strange APOD. Does this have any use other than to explain that Fermi doesn't always look in the same direction and looking cool? Not that it needs a use, I just want to know if it does have one.

I think the purpose isn't simply to show that it doesn't always look in the same direction, but that its multiple modes of motion result in a very interesting coverage pattern of the sky.

[neufer]

This is such a strange APOD. Does this have any use other than to explain that Fermi doesn't always look in the same direction and looking cool? Not that it needs a use, I just want to know if it does have one.

I think the purpose isn't simply to show that it doesn't always look in the same direction, but that its multiple modes of motion result in a very interesting coverage pattern of the sky.

I think the purpose is that Fermi is NEVER looking in the same direction. Rather, it is performing an cool exotic dance that scans the whole sky about once every 3 hours. We are much more accustomed to space craft that contort themselves in order to track the single object they are focused upon.

_________ The Gymnast

KRAMER: Yeah, it's 3-D art. Computers generate 'em. BIG computers.

MR. PITT: Yes, I've heard about these. How do they work?

KRAMER: Well, you blur your eyes like you're starin' straight through the picture. And you keep your eyes unfocused. And then... (Kramer and Pitt stare at picture) Oh, oh, oh, YEAH!

MR. PITT: I don't see it.

KRAMER: Yeah, it's a spaceship, surrounded by planets, asteroids...

MR. PITT: I still don't see it.

ELAINE: Okay, Kramer, that's enough. Mr. Pitt has got work to do.

KRAMER: Ya' ever dream in 3-D? It's like the boogeyman is comin' right at you.

MR. PITT: A spaceship, where?

KRAMER: (pointing) Right in here. Just keep your eyes unfocused.

[Boomer12k]

Pretty SPIROGRAPH...

:---[===] *

[tnzkka]

Beautifully asymmetric. No spirograph-drawing seen.

[orin stepanek]

This is such a strange APOD. Does this have any use other than to explain that Fermi doesn't always look in the same direction and looking cool? Not that it needs a use, I just want to know if it does have one.

I didn't get much out of it either; but than again much of it was beyond my understanding.

[geckzilla]

Ok, so "never" is a better way of saying "does not always", but I'm not sure it changes the meaning of my post, Neufer. So Fermi has to be paranoid and look everywhere. The gamma rays are out to get it.

[copper5817]

Ok, I have a question for you guys. With the pulsar spinning 11 times a second is the density or the gravitational field of the pulsar what keeps the centrifugal forces from flinging material away? Pulsars are know for having extremely dense structures. With this type of density would the structure of the star be hard like a non-porous surface might be? I guess what I'm really wondering is, how much faster can these things spin, and ultimately does the use of the fuel contained within the pulsar eventually contribute to a density loss that would lead to a catastrophic failure of the structure of the pulsar due to centrifugal forces? I imagine that would be spectacular if it did. Kind of like an accretion disk in reverse!

[cxd207]

Just a beautiful picture. A spirograph in space.

[DCStone]

Just a beautiful picture. A spirograph in space.

An awfully expensive spirograph, but still fun!

[emc]

I see a dancing Owl

[Chris Peterson]

I think the purpose is that Fermi is NEVER looking in the same direction.

Every place on the image where there is an intersection represents a point in the sky where Fermi is looking twice. Given the field-of-view of the camera, what this image demonstrates is that, despite the exotic dance, Fermi spends a lot of time looking at the same places repeatedly.

[neufer]

Ok, so "never" is a better way of saying "does not always", but I'm not sure it changes the meaning of my post, Neufer.

'Does not always' is pretty vague.

Billions and Billions: <<I never said it. Honest. Oh, I said there are maybe 100 billion galaxies and 10 billion trillion stars. It's hard to talk about the Cosmos without using big numbers. I said 'billion' many times on the Cosmos television series, which was seen by a great many people. But I never said 'billions and billions.' For one thing, it's imprecise. How many billions are 'billions and billions'? A few billion? Twenty billion? A hundred billion? 'Billions and billions' is pretty vague... For a while, out of childish pique, I wouldn't utter the phrase, even when asked to. But I've gotten over that. So, for the record, here it goes: 'Billions and billions.'>>

So Fermi has to be paranoid and look everywhere. The gamma rays are out to get it.

Just because Fermi is paranoid doesn't mean that gamma rays are not out to get it.

[geckzilla]

Given Chris's contradiction to your statement, perhaps it is necessary for it to be vague.

[neufer]

I think the purpose is that Fermi is NEVER looking in the same direction.

Every place on the image where there is an intersection represents a point in the sky where Fermi is looking twice. Given the field-of-view of the camera, what this image demonstrates is that, despite the exotic dance, Fermi spends a lot of time looking at the same places repeatedly.

I think the purpose is that Fermi is NEVER looking in precisely the same direction with the same orientation.

[Chris Peterson]

Ok, I have a question for you guys. With the pulsar spinning 11 times a second is the density or the gravitational field of the pulsar what keeps the centrifugal forces from flinging material away? Pulsars are know for having extremely dense structures. With this type of density would the structure of the star be hard like a non-porous surface might be? I guess what I'm really wondering is, how much faster can these things spin, and ultimately does the use of the fuel contained within the pulsar eventually contribute to a density loss that would lead to a catastrophic failure of the structure of the pulsar due to centrifugal forces? I imagine that would be spectacular if it did. Kind of like an accretion disk in reverse!

A pulsar is a rotating neutron star. Neutron stars are inert; they have no fuel left to burn and are, therefore, stable. The stars are incredibly dense (although they may have a fluid surface), and have very strong gravitational fields, which hold them together against centripetal breakup. The theoretical maximum speed of rotation for a neutron star (pulsar) is around 1500 rotations per second, although they probably can't get much above 1000 rps in practice because of energy loss by gravitational radiation. Many pulsars are known that rotate hundreds of times per second.

[Chris Peterson]

I think the purpose is that Fermi is NEVER looking in precisely the same direction with the same orientation.

I don't know what that means. The diagram shows where the camera is aimed; the rotation angle of the camera about the optical axis isn't indicated. But certainly, every intersection on the diagram is a point where the camera is pointing in precisely the same direction twice. The very high density of intersections in certain areas reveals that the camera field (if not the exact center of the field) has high repeat coverage in those areas- a point made specifically in the image caption.

[neufer]

I think the purpose is that Fermi is NEVER looking in precisely the same direction with the same orientation.

I don't know what that means. The diagram shows where the camera is aimed; the rotation angle of the camera about the optical axis isn't indicated.

The diagram shows where the Vela Pulsar is located vis-a-vis as to where the LAT camera is aimed and how the camera is oriented (at least, when the Vela Pulsar is NOT located in the center of the field of view). Given the complexity of the pattern it is unlikely that LAT is EVER looking in precisely looking the same direction with the same orientation.

[Spin Dizzy]

Ok, I have a question for you guys. With the pulsar spinning 11 times a second is the density or the gravitational field of the pulsar what keeps the centrifugal forces from flinging material away? Pulsars are know for having extremely dense structures. With this type of density would the structure of the star be hard like a non-porous surface might be? I guess what I'm really wondering is, how much faster can these things spin, and ultimately does the use of the fuel contained within the pulsar eventually contribute to a density loss that would lead to a catastrophic failure of the structure of the pulsar due to centrifugal forces? I imagine that would be spectacular if it did. Kind of like an accretion disk in reverse!

A pulsar is a rotating neutron star. Neutron stars are inert; they have no fuel left to burn and are, therefore, stable. The stars are incredibly dense (although they may have a fluid surface), and have very strong gravitational fields, which hold them together against centripetal breakup. The theoretical maximum speed of rotation for a neutron star (pulsar) is around 1500 rotations per second, although they probably can't get much above 1000 rps in practice because of energy loss by gravitational radiation. Many pulsars are known that rotate hundreds of times per second.

How is the gamma ray energy generated if the neutron star is no longer burning fuel? Is it like a black hole's Hawking radiation where matter/energy is being sucked in, mangled and blasted out as gamma energy? Or perhaps stored energy from before its collapse that's now bleeding out?

[neufer]

A pulsar is a rotating neutron star. Neutron stars are inert; they have no fuel left to burn and are, therefore, stable. The stars are incredibly dense (although they may have a fluid surface), and have very strong gravitational fields, which hold them together against centripetal breakup. The theoretical maximum speed of rotation for a neutron star (pulsar) is around 1500 rotations per second, although they probably can't get much above 1000 rps in practice because of energy loss by gravitational radiation. Many pulsars are known that rotate hundreds of times per second.

It should be noted that rapidly rotating neutron stars are also constantly gaining rotational momentum/energy by being fed from the outside; and that they start to get into trouble not so much by overwhelming centrifugal accelerations as by relativistic equatorial velocities.

<<PSR J1748-2446ad is the fastest-spinning pulsar known, at 716 Hz. This pulsar was discovered by Jason W. T. Hessels of McGill University on November 10, 2004 and confirmed on January 8, 2005. It has been calculated that the neutron star contains slightly less than two times the mass of the Sun, which is approximately the same for all neutron stars. Its radius is constrained to be less than 16 km. At its equator it is spinning at approximately 24% of the speed of light, or over 70,000 km per second.

The pulsar is located in a globular cluster of stars called Terzan 5, located approximately 18,000 light-years from Earth in the constellation Sagittarius. It is part of a binary system and undergoes regular eclipses with an eclipse fraction of about 40%. Its orbit is highly circular with a 26 hour period. The other object is about 0.14 solar masses, with a radius of 5–6 solar radii. Hessels states that the companion may be a "bloated main-sequence star, possibly still filling its Roche Lobe". Hessels goes on to speculate that gravitational radiation from the pulsar might be detectable by LIGO.>>

[neufer]

How is the gamma ray energy generated if the neutron star is no longer burning fuel? Is it like a black hole's Hawking radiation where matter/energy is being sucked in, mangled and blasted out as gamma energy? Or perhaps stored energy from before its collapse that's now bleeding out?

It is like a black hole's gamma radiation where matter is being sucked in, mangled and blasted out as gamma energy.

However, this has nothing to do a typical black hole's negligible black body Hawking radiation.

[Chris Peterson]

The diagram shows where the Vela Pulsar is located vis-a-vis as to where the LAT camera is aimed and how the camera is oriented (at least, when the Vela Pulsar is NOT located in the center of the field of view). Given the complexity of the pattern it is unlikely that LAT is EVER looking in precisely looking the same direction with the same orientation.

Ah... I misread things and took this to be where the camera points in the sky. So I'll agree that any particular source (in this case the Vela pulsar) is unlikely to ever be in exactly the same place on the focal plane. I don't think that's remotely the point of the image, however. What it illustrates, if anything, is how much time this object spends on the camera's detector despite the complex path it follows over the sky.

[Chris Peterson]

How is the gamma ray energy generated if the neutron star is no longer burning fuel? Is it like a black hole's Hawking radiation where matter/energy is being sucked in, mangled and blasted out as gamma energy? Or perhaps stored energy from before its collapse that's now bleeding out?

High energy radiation is produced from the kinetic energy released as external material falls onto the surface of the neutron star (the same thing that allows us to detect black holes; Hawking radiation has never been observed). The neutron star is also hot because of the energy it contained at the time it's progenitor ceased fusing. It is cooling, but only very slowly. We see that thermal energy as light, but it's not hot enough to produce gamma rays.