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.