Gamma-ray Outbursts Shed New Light on Pulsars

by Ray Sanders on June 15, 2012

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Researchers using the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope have developed a new method to detect a special class of stellar remnant, known as pulsars. A pulsar is a special type of neutron star, which spin hundreds of times per second. When the intense spin is combined with beams of energy caused by intense magnetic fields, a “lighthouse” pulse is generated. When the “lighthouse” beam sweeps across Earth’s field of view, the object is referred to as a pulsar.

Led by Matthew Kerr (Kavli Institute for Particle Astrophysics and Cosmology), and Fernando Camilo (Columbia University), a research team recently announced a new method for detecting pulsars. How will Kerr’s research help astronomers better understand (and locate) these small, elusive stellar remnants?

Every three hours, the LAT surveys the entire sky, searching for the high energy signatures associated with gamma-ray outbursts. In general the energy levels of the photos detected by the LAT are 20 million to over 300 billion times as energetic as the photons associated with visible light.

By combining observations from the LAT and data obtained from the Parkes radio telescope in Australia, the team is able to detect pulsar candidates. The team’s approach combines a “wide area” approach of an all-sky telescope like the LAT with the sensitivity of a radio telescope. So far, the team’s discovery of five “millisecond” class pulsars, including one unusual pulsar has proven their technique to be successful.

The unusual pulsar, officially named PSR J0101–6422, had an additional 35 days of study devoted to better understanding its properties. Once the radio pulsation period and phase were determined, an incredible amount of data, including data on its gamma-ray pulsations was obtained. Using the data, the team was able to determine PSR J0101–6422 is roughly 1750 light-years away from Earth, and has an unusual light curve which features a “sandwich” of two gamma-ray peaks with an intense radio peak in the center, like a cosmic ham sandwich.

The team was unable to explain the phenomenon with standard pulsar emission models.which the team could not explain with standard geometric pulsar emission models, and have proposed that J0101–6422 is a new hybrid class of pulsar that features radio emissions that originate from low and high altitudes above the neutron star.

If you’d like to learn more about the Fermi Gamma-ray space telescope, visit:

The results of Kerr’s research have been published in the Astrophysical Journal.

Image #1 Caption:Clouds of charged particles move along the pulsar’s magnetic field lines (blue) and create a lighthouse-like beam of gamma rays (purple) in this illustration. Image Credit: NASA


In addition to being a published astronomer specializing in variable stars, Ray Sanders has blogged for Universe Today, and The Planetary Society blog, among others. He runs his own blog, Dear Astronomer, teaches classes for CosmoQuest, and is a NASA/JPL Solar System Ambassador.

Jamie Disqus June 16, 2012 at 7:12 AM

Can anyone tell me if there are neutron stars that do not have the ‘lighthouse beam’ mentioned in the first paragraph of this article?

zetetic elench June 16, 2012 at 1:32 PM

quiescent vs. accreting pulsar? wouldn’t it have more pronounced radio lobes if it was spinning up? talk about complex objects! wow.

Rick Holcomb June 16, 2012 at 1:59 PM

I’ve heard that pulsars could be used as a solar system, or even galactic, GPS type location system. Is this possible?

wjwbudro June 16, 2012 at 2:08 PM

Anyone using FF or Chrome and having a problem viewing the new Comments layout in particular the “invisible” log in icons? You can find them if you run your mouse over that area. I have allowed all scripts and turned off add blockers and still have the viewing issue. Not so with IE.

lcrowell June 16, 2012 at 4:11 PM
DrFlimmer June 16, 2012 at 9:13 PM

Yes, there are. ;-)

Colin Burnett June 17, 2012 at 1:50 AM

DrFilmmer, Do you mean those instances where the pulsar pulses are not aligned for our observation, or if there are ones that have significantly slowed over the age of the universe so that the neuron stars longer emit a pulse, or is their some other mechanism at play?

DrFlimmer June 17, 2012 at 7:52 AM

In fact, both versions are possible.
Neutron stars come in several flavors, so to speak. They can be ordinary neutron stars, with a rather normal magnetic field strength, and no lighthouse beam. Then we have pulsars, which do emit the lighthouse beam. And finally, we have magnetars: Neutron stars with a magnetic field of 10^12 Tesla or so. This is an unbelievably strong magnetic field.
We also observed “star quakes” on neutron stars. Imagine: A shift of the crust of only a few cm or so, but releasing an energy that is on the order of a supernova explosion (not exactly, but maybe only an order or two of magnitude below).

lcrowell June 17, 2012 at 1:13 PM

I have found myself working with two other gentlemen on a problem of why neutron star rotation or pulsar frequencies tend to not exceed the millisecond range. This is a form of the Birkhoff theorem for extending a vacuum solution of the Einstein field equation, the Popapetrou metric with Ernst potentials, into a medium. The medium is the neutron star itself. This is a hard and complicated problem as it turns out. The magnetic fields of 10^{15} Gauss and the frame dragging of a fast rotating neutron star begins to cause shearing of the material at fast rotations.


lcrowell June 17, 2012 at 1:06 PM

A neutron star produces the beam of electromagnetic energy from the interaction with gas around it. The magnetic field of the neutron star causes charged particles (protons and electrons) to enter into a spiraling trajectory. A charge particle that is accelerated emits electromagnetic radiation, called Bremsstrahlung radiation, where a charged particle on a circular orbit is under a centripetal acceleration. The sorts of kHz whistles and noise one hears on a radio is due to analogous processes with charged particles in the Earth’s magnetic field.

The magnetic field is not necessarily oriented parallel to the axis of rotation, which means the lobe of electromagnetic radiation emitted can do this lighthouse sweeping. An observer with the right equipment, such as a radio telescope, at a distant position that is on the line of sight will detect this sweeping, termed a pulsar, which when converted to an audio signal often sounds like a repeating pattern of bongo drumming. If you are not on that line of sight the neutron star is not detected this way.

A neutron star in near pure vacuum will not produce any EM radiation. These rogue neutron stars are fairly invisible. It is not well known what their “population census” is in the galaxy.

Aqua4U June 18, 2012 at 9:21 PM

Non rotating neutron stars anyone? I only got 143,000 hits on Google for that one. Would a non rotating neutron star be impossible to detect?

magnus.nyborg June 19, 2012 at 9:27 AM

In what external frame of reference would a neutron star be able to be non-rotating?
A neutron star that is not accreting will slow down its rotation, but the rate of the slowdown decreases rapidly so coming to a full stop will never be reached. And the only known formation mecanisms for neutron star involves a relatively rapidly rotating start.

Peter Croft June 17, 2012 at 10:26 AM

The kinetic energy involved in rotations of hundreds or thousands per second must be enormous (ha! is that word inadequate!). I’ve never understood where this energy comes from. Is it just the skater analogy? Are these failed stars or dying stars? How did they get to this point?

lcrowell June 17, 2012 at 1:19 PM

Neutron stars are the left over of a star core after it exhausted its nuclear fuel. The end of the star burning phase causes the core to implode and send a huge shock wave outwards with rapid fusion in these outer layers as the shock wave propagates. It is actually rather similar to a chemical explosion, which is a combustion that propagates through a medium of fuel faster than the speed of sound through that medium. This is a supernova that marks the end of a star. If that core has a mass larger than the Chandrasekhar limit of 1.4 times the mass of the sun, which is also necessary for a supernova, it will collapse into a neutron star. A more modest core will just produce a white dwarf. A collapsing core larger than the Oppenheimer-Volkov limit of 3.1 times the mass of the sun forms a black hole.


Peter Croft June 19, 2012 at 1:14 AM

Thanks LC, I get that, but what starts it spinning, and in particular, why do they spin in such a perfect way? Why is such a mighty explosion so symmetrical that it makes the pulsar rotate about a single axis without being chaotic? Or did a little asymmetry in the explosion kick off the rotation? Or am I, er, completely “off beam”?

I studied one year of uni physics in 1965! Our texts were the Feynman Lectures on Physics, just published then. Then I found electronics.

lcrowell June 19, 2012 at 3:46 AM

Most stars rotate or have some angular momentum. When they collapse they speed up, like a skater drawing her arms in to spin faster. As Torbjörn Larsson points out the sun rotates every 25 Earth days and if compacted into a neutron star that would be 2 seconds. Actually it would be faster, for the density profile of a neutron increases with decreasing radius.

Any rotating object in the absence of perturbing influences will have a constant angular momentum. Earth’s angular momentum is fairly constant, with a weak precession of its axis due to the moon and solar gravity. A neutron star in the absence of any perturbing influence, and that has largely stopped its “starquakes” will rotate with perfect precision for a vast period of time.


Torbjörn Larsson June 17, 2012 at 4:50 PM

Just for kicks, I want to run a back-of-the-envelope.

- I don’t want to model a star. Hence I am not going to estimate the real angular momentum from a dense remaining core of a red giant. I will scale the Sun to a 1.4 solar mass main sequence star.

Realistically there is some conversion with gravitational potential energy as well. We will have to keep in mind that this will be not even order of magnitude correct.

- A ball angular momentum is I = 2mr^2/5, its angular velocity w = 2pi/t, its kinetic energy of rotation K = Iw^2/2 = 2mr^2/5*(2pi/t)^2/2 ~ (r/T)^2.

Conservation of kinetic energy makes the rotation time scale as r/t = R/T ; r,t post, R, T pre collapse; or t = R/rxT.

The Sun has a radius ~ 7×10^8 m and a rotation time of ~ 25 days. Hence I take precollapse radius R ~ 10^9 m and rotation time T ~ 25x24x60x60 ~ 2×10^6 s.

The post-collapse radius will be “a few km”. Say 1 km then, giving a postcollapse rotation time of t ~ 10^9/10^3*2×10^6 ~ 2 s.

Pulsars ranges between ms to s, so not entirely out of the target range.

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