Tim Linden

The Galactic Center Gamma-Ray Excess

The Milky Way Galactic Center

High Mass X-Ray Binaries

Pulsars

Pulsars (short for pulsating neutron stars), are one of the most intriguing objects in our universe. To understand them, we first have to learn a little bit about stellar evolution. Pulsars are a sub-class of neutron stars. Neutron stars are ultra-dense remnants of dead stars. In addition to making all the light and heat from a star like our sun, nuclear fusion plays a critical role in providing energy to keep the star from collapsing on itself. When a star runs out of Hydrogen to fuse, it begins to shrink, until it is stoped by the pressure of the remaining electrons. In most cases this is effective (and it will be for a star the mass of our sun), but for very big stars, the gravitational force is so strong that the repulsion of electrons is incapable of keeping the star from collapsing.

In this case, the star continues to collapse by fusing the protons and electrons inside the star into neutrons, this continues until the star is composed entirely of neutrons, and the pressure from the neutrons themselves stops the star from collapsing further (this pressure, which is also critical in the above case where electrons prevent the star from collapsing, is called degeneracy pressure). In this case the star collapses into an extremely dense object, it is more massive than the Sun, but condensed into a ball with a radius of only about 6 miles (10 km). One teaspoon of a neutron star would weight more than 10 trillion pounds!

This image shows the magnetic fields of a rotating pulsar, in blue, along with a burst of energetic particles (in pink) being accelerated from the pulsars surface. The "pulsing" signal is observed every time the pink beam spins and points in the direction of the Earth.

A pulsar is even more peculiar, it is a neutron star that is spinning rapidly. The fastest pulsars can spin as fast as 1000x a second, which means that the surface of the neutron star is traveling at nearly 38000 miles per second! Pulsars also have extremely strong magnetic fields, the pulsars with the strongest magnetic fields, called magentars, produce a magnetic field 1 trillion times as strong as the strongest magnetic produced on earth. This combination of extremely fast rotation, and extremely strong magnetic fields produces a stunningly strong electric field, which rips charged material off of the surface of the pulsar and accelerates it to very high energies. Sort of like the Large Hadron Collider on Earth, but much much bigger.

Pulsars are interesting to astrophysicists for a number of reasons. First, unlike regular neutron stars, they are easy to see. Neither neutron stars or pulsars produce a lot of light (there is no nuclear fusion in these "dead" stars), so they do not produce much light in the visible spectrum. However, the "beating" pulsar signal is very bright in radio --- and the extremely steady beat makes the pulsar easy to pick out from other types of astrophysical sources.

In fact, the pulsar beat is incredibly steady - better than the best atomic clocks on Earth. The beat from a single pulsar can be measured to a precision of about 100 ns over an hour of observation. Given that there are more than 2500 pulsars currently observed (and using more pulsars provides us with more precise measurements), the observation of pulsars gives us a sort-of Galactic GPS -- allowing us to measure time to nanosecond precision over very long intervals.

Here we have a system consisting of two pulsars that are in a tight orbit around each-other. Knowing that the pulsar "beat" is extremely stable, we can precisely measure the gravitational force between the two pulsars.

Pulsar observations become even more interesting when pulsars are observed in pairs. As the pulsars orbit each other (moving closer and farther away from the Earth), we can observe a change in the "beat" of each pulsar based on the changing distance from us. This allows us to measure the orbit of the pulsar extremely accurately -- determining how close the two systems are, and the strength of the gravitational force that pulls each pulsar towards each other. For most of these systems, we can only observe the emission from one of the two pulsars (the other system may have its pulsar beam pointed in the wrong direction, or it may be a silent neutron star). However, for one extremely exciting system, PSR J0737-3039A/B, we can individually see both pulsars!

A surprise awaited the astronomers Russell Hulse and Joseph Taylor when they discovered the first binary pulsar system (PSR B1913+16) in 1974. While only one of the two systems has a beam oriented towards earth -- they quickly noticed that the pulse period changed due to the orbit of the two systems. More importantly, they soon found that the orbital period of the binary was itself changing -- the two pulsars were caught in a slow death spiral, creeping closer and closer and moving faster and faster. This phenomenon is not expected in classical gravity, but it is expected in General Relativity. In fact, General Relativity makes a precise prediction for how fast two binary neutron stars should inspiral -- losing energy in the form of gravitational radiation along the way. When they measured the effect, they (and many independent researches thereafter) found that the binaries were getting closer at exactly the rate predicted by General Relativity. This stands as one of the most convicing tests of Einstein's theory, and was the first (indirect) discovery of gravitational waves.

The change in the orbital period of the binary pulsar PSR B1913+16. The data points were taken over a period of more than 30 years. The line represents the prediction from General Relativity --- a result that fits the data almost exactly.

Latest Results

TeV Halos

TeV electrons accelerated by pulsars may explain the diffuse TeV excess observed by Milagro.

Dark Kinetic Heating

Dark Matter collisions with neutron stars set a minimum neutron star temperature. This may be observable with next-generation instruments.

Star-Forming Galaxies

An analysis of 584 SFGs finds significant dispersion in their far-IR to gamma-ray correlation. SFGs significantly contribute to the IGRB.