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The Wake of a Pulsar

When a neutron star gives off pulses of radiation every time it rotates, it’s called a pulsar. The radiation, which moves along the star’s magnetic field lines, is often compared to a lighthouse beam sweeping across an ocean. Now a pulsar called Geminga has been found to leave a comet-like trail of high-energy electrons as it muscles its way through the nearby interstellar medium at about 120 kilometers per second. Geminga is close in interstellar terms, a mere 500 light years from Earth, and because it is moving across our line of sight, it offers unprecedented material for observation.

The ‘cometary’ tail shows up on data gathered by the Chandra X-ray Observatory; the same team found twin x-ray tails stretching billions of kilometers behind the object in 2003, using data from ESA’s XMM-Newton. As for Geminga itself, this incredibly dense core of an exploded star is about 20 kilometers across but contains the mass of our Sun. Although most pulsars emit radio waves, Geminga is silent at those wavelengths and shows up as a gamma-ray source that was only later identified in the x-ray and optical wavelengths. The gamma rays are evidently produced from the acceleration of electrons and positrons as Geminga spins at a rate of four times per second.

Besides being fascinating in its own right, this fast-moving object gives insights into the interstellar medium through which it passes. Its bowshock compresses the gas and magnetic fields it encounters by a factor of four; its tails are the bright edges of the shockwave it carves out as it moves. The current work suggests that high-energy electrons escaping the pulsar’s magnetosphere are creating the complex cometary and x-ray tails now being observed.

“Astronomers have known that only a fraction of these accelerated particles produce gamma rays, and they have wondered what happens to the remaining ones,” said Dr. Patrizia Caraveo of the Italian National Institute for Astrophysics (INAF), a co-author on the Astronomy & Astrophysics article that reports this work. “Thanks to the combined capabilities of Chandra and XMM-Newton, we now know that such particles can escape. Once they reach the shock front, created by the supersonic motion of the star, the particles lose their energy, radiating X-rays.”

The paper is De Luca, A., Caraveo, P.A., et al., “On the complex X-ray structure tracing the motion of Geminga,” Astronomy & Astrophysics 445 (2006), L9-L13. From the abstract: “Geminga is thus the first neutron star to show a clear X-ray evidence of a large-scale, outer bow-shock as well as a short, inner cometary trail.” An INAF press release (Italian only) is here. ESA reports on the discovery of ‘hot spots’ on Geminga and two other neutron stars in this April news release.

Centauri Dreams note: I love the name Geminga. INAF reports that the discoverer of this pulsar, Giovanni Bignami (Centre d’Etude Spatiale des Rayonnements, Toulouse) named the object for ‘Gemini gamma-ray source’ in 1973, but in his Milanese dialect, ‘ghè minga’ means ‘it is not there.’ The reference is to the fact that Geminga showed up as a gamma-ray only source until 1993, when x-ray and optical measurements were finally made.

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  • ljk January 8, 2009, 17:58

    January 6, 2009

    “Lighthouse” Analogy No Longer Works for Pulsars

    Written by Nancy Atkinson

    NASA’s Fermi Gamma-ray Space Telescope has found 12 previously unknown gamma-ray only pulsars, as well as identifying gamma-ray emissions from 18 known or suspected radio pulsars. And what the telescope is finding is changing the way we think of these stellar cinders.

    The old analogy for pulsars was a lighthouse: gamma-rays were thought to pulse out in a narrow beam from the neutron star’s magnetic poles. But this new research shows that cannot be the case.

    A new class of gamma-ray-only pulsars shows that the gamma rays must form in a broader region than the lighthouse-like radio beam.

    “We used to think the gamma rays emerged near the neutron star’s surface from the polar cap, where the radio beams form,” says Alice Harding of NASA’s Goddard Space Flight Center. “The new gamma-ray-only pulsars put that idea to rest.” She and Roger Romani from Stanford University in California spoke today at the American Astronomical Society meeting.

    Full article here:


  • ljk January 3, 2010, 0:42

    High-energy gamma-ray observations of Geminga with the Fermi Large Area Telescope

    Authors: M. Razzano, D. Dumora, F. Gargano (on behalf of the Fermi LAT Collaboration)

    (Submitted on 30 Dec 2009)

    Abstract: Geminga is the second brightest persistent source in the GeV gamma-ray sky. Discovered in 1975 by SAS-2 mission, it was identified as a pulsar only in the 90s, when ROSAT detected the 237 ms X-ray periodicity, that was later also found by EGRET in gamma rays.

    Even though Geminga has been one of the most intensively studied isolated neutron star during the last 30 years, its interest remains intact especially at gamma-ray energies, where instruments like the Large Area Telescope (LAT) aboard the Fermi mission will provide an unprecedented view of this pulsars.

    We will report on the preliminary results obtained on the analysis of the first year of observations. We have been able to do precise timing of Geminga using solely gamma rays, producing a timing solution and allowing a deep study of the evolution of the light curve with energy.

    We have also measured and studied the high-energy cutoff in the phase-averaged spectrum and produced a detailed study of the spectral evolution with phase.

    Comments: 6 pages, 4 figures, 2009 Fermi Symposium, eConf Proceedings C091122

    Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

    Cite as: arXiv:0912.5442v1 [astro-ph.HE]

    Submission history

    From: Massimiliano Razzano [view email]

    [v1] Wed, 30 Dec 2009 10:31:31 GMT (103kb)


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