In astronomy news this week, scientists found a pulsar racing out of the galaxy and mapped a star's magnetic field from 130 light-years away.

Cannonball Pulsar Is Escaping the Galaxy

When a star collapsed 10,000 years ago, the asymmetric explosion gave the stellar core — a spinning neutron star dubbed PSR J0002+6216 — a powerful kick. Now, observations from the Karl G. Jansky Very Large Array and NASA's Fermi Gamma-Ray Space Telescope show that the pulsar is racing away from its one-time home at 1,100 km/s (2.5 million mph), trailing a 13-light-year-long tail of energetic particles and magnetic energy.

Cannonball pulsar
Radio observations (orange) reveal the needle-like trail of pulsar J0002+6216 as it flies away from the shell of its supernova remnant; the bubble-like remnant is shown in an image from the Canadian Galactic Plane Survey. The pulsar escaped the bubble some 5,000 years ago and is currently 6,500 light-years from Earth in the constellation Cassiopeia.
Composite by Jayanne English (Univ. of Manitoba) / F. Schinzel et al. / NRAO / AUI / NSF / DRAO / Canadian Galactic Plane Survey / NASA / IRAS

The citizen-science project Einstein@Home, which uses computer downtime to analyze Fermi data, discovered the pulsar in 2017 by its gamma-ray pulses. As the neutron star spins around roughly every tenth of a second, its jet of energetic particles sweeps over Earth like an extremely rapid lighthouse beam. By exactly timing these pulses, astronomers detected tiny variations that helped them determine how quickly and in what direction the object is moving.

Radio observations show that the pulsar's tail points straight back to its point of origin: the center of a gas bubble known as CTB 1. This supernova remnant emits radio waves as the blown-off outer layers of the progenitor star crash into the interstellar medium. Even though the remnant is still expanding, the kicked-out pulsar overtook the bubble's edge about 5,000 years ago. Now, the pulsar is well outside the bubble, 53 light-years from its center.

Somehow, the supernova blast evicted the core of the star responsible for the explosion. Although other "kicked" pulsars have been found before, this one is the fastest known. Eventually, it'll escape the Milky Way. In the meantime, these observations will help astronomers determine how such kicks occur.

Frank Schinzel (NRAO) and colleagues presented these results at the High Energy Astrophysics Division meeting of the American Astronomical Society, and they have submitted a paper to the Astrophysical Journal Letters. Read more about the pulsar in the NRAO and NASA press releases.

Mapping a Far-Away Star

Astronomers have used a neat new technique to map out the surface of a star 130 light-years away — and its magnetic field too.

Star-spotted star
Doppler imaging of II Pegasi A (HD 224085), an orange subgiant star about three times the Sun's girth.

The technique relies on two effects. The first of these is the Doppler effect, which causes an object to appear redder when it's moving away from us and bluer when it's moving toward us. When astronomers measure a spectral line emitted from a star that's rotating, half of the light will be emitted from the part of the star moving toward us while the other half will come from the part of the star that's moving away. The collective effect is that the line will appear broader than it would otherwise, as it combines the redshifted and blueshifted parts of the star.

Starspots, though, will deform the line. With some rather involved analysis of the line profile, astronomers can back out what the surface of a star looks like. This technique is known as Doppler imaging.

So that's how astronomers map stellar surfaces — mapping surface magnetic fields is another matter. Using the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the 11.8-meter Large Binocular Telescope in Arizona, Klaus Strassmeier (AIP) and colleagues stepped Doppler imaging up a notch by including the polarization of light. In the presence of a magnetic field, a spectral line will split into multiple separate lines, and each of these lines will be polarized differently. This is known as the Zeeman effect. So, by feeding polarized light into the spectrograph, Strassmeier and colleagues were able to transform Doppler imaging into Zeeman Doppler imaging, mapping out the orientation of the magnetic field across the star's surface.

Magnetic map
Zeeman Doppler imaging of II Pegasi A

As expected, what appear as dark spots on the star's surface are indeed magnetically active starspots. It's important to note, though, that these starspots aren't quite the same as what we see on the Sun — they're a thousand times bigger than sunspots and may represent a completely different type of magnetic activity than the Sun's.

Read more about these results in the Large Binocular Telescope Observatory's press release.


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Mark James

March 23, 2019 at 9:30 pm

I wonder whether Doppler imaging could be used to determine the orientation of a star's axis of rotation with respect to the Earth. If applied to a star with known exoplanets (and assuming that the planets orbit in a plane roughly aligned with the star's equator), this would constrain estimates of the planets' masses.

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Monica Young

March 27, 2019 at 8:34 am

Neat idea! I don't see why this wouldn't work — you can definitely see the axis of rotation as the starspots go around. The only drawback is that I believe Doppler imaging has only been applied to giant, nearby stars, since their larger surfaces make them easier to "see," so that limits the playing field (at least for now).

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March 30, 2019 at 7:24 am

Just a note of appreciation for the consistent high quality of Ms. Young's articles/notes. Always clear and accurate. - regards, L. Vertal

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April 2, 2019 at 3:54 pm

I heartily second levertalșm'a comment! I always love reading Dr. Young's pieces!
Thanks so much!

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