Is one of the galaxy’s mightiest particle engines secretly hiding in plain sight? Composite new observations of the pulsar wind nebula MSH 15-52, aka the “cosmic hand,” indicate that even after decades of research, astronomers are only just starting to unravel the physics governing its ethereal shape.
At the center of this system is pulsar B1509-58, a neutron star only 12 miles wide but rotating nearly seven times per second. Formed when a massive parent star collapsed in a supernova approximately 1,700 years ago, it has a magnetic field tens of millions stronger than any continuous magnet ever constructed on Earth. Such powerful fields, along with the high-speed rotation, render it one of the Milky Way’s strongest natural dynamos, propelling a ceaseless flow of electrons, positrons, and potentially heavier particles into the surrounding space.
The new image of MSH 15-52 is made by merging high-resolution X-ray observations from NASA’s Chandra X-ray Observatory with radio maps from the Australia Telescope Compact Array. In the composite, hydrogen gas is a gold glow, radio emission is red, and X-rays are a blue, orange, and yellow brilliance. Where radio and X-ray features overlap with each other, the nebula has a purple color. But the information also show a mysterious gap: the bright X-ray features like the pulsar’s jet and the inner “fingers” do not produce a corresponding radio emission, suggesting they are energized by higher-energy particles that emit only in X-rays before they lose energy.
The “fingers” themselves follow lines of the magnetic field, likely shaped by a shock wave similar to a sonic boom. When the pulsar wind collides with the ambient medium, particles sneak out along these fields, emitting X-rays but very little in radio. The supernova remnant RCW 89, a clumpy shell of debris associated with the explosion, demonstrates tight agreement between radio, X-ray, and optical knots. But the radio glow spreads far beyond the X-ray limits, a characteristic that scientists struggle to account for. Lack of radio emission along a sharp X-ray edge, believed to be the blast wave of the supernova, is especially puzzling for such a young remnant.
These discrepancies are more than an imaging enigma. They address the little-understood interaction between supernova ejecta and pulsar winds a dynamic system that, in accordance with newly developed models, can be capable of generating the most powerful cosmic rays within the galaxy. Within this scheme, the pulsar wind nebula first expands into the freely moving stellar ejecta. It eventually catches up to the reverse supernova remnant shock, where particles are accelerated by multiple crossings between the two. When the nebula is later squeezed by the shocked shell of the supernova, particles contained within are accelerated from approximately 0.1 petaelectronvolts (PeV) to almost 1 PeV, energies near the so-called “knee” of the cosmic ray spectrum.
Simulations indicate that the magnetic fields in both the shocked supernova and the young nebula hundreds of microgauss in strength are capable of scattering and holding back these particles while they’re accelerated. Turbulence produced by the explosive interaction can carry a large portion of particles from the forward shock to the reverse shock, where they are injected into the pulsar wind nebula to be further energized. This process may also accelerate heavy nuclei originating in the metal-rich supernova ejecta, potentially explaining the composition of the highest-energy cosmic rays reaching Earth.
The morphology of MSH 15-52 offers a rare laboratory for testing these ideas. The nebula spans about 150 light-years far larger than the famous Crab Nebula yet is powered by a compact engine whose rotational energy output rivals the luminosity of entire star clusters. The filamentary radio filaments unveiled in the new observations are parallel to the nebula’s magnetic field and suggest a role for field geometry in directing particle streams. Their genesis could be connected with the impact of pulsar wind on the dense knots in RCW 89, where shock waves and instabilities would be able to amplify magnetic fields and initiate new acceleration sites.
Knowledge of such systems also involves studying their birthplaces. Other extremely magnetized neutron stars, or magnetars, offer evidence that the most massive of these progenitors, stars with 30 to 40 solar masses, will lose sufficient mass prior to explosion to be left with neutron stars rather than black holes. Birth rotation rates of hundreds of revolutions per second could fuel a dynamo that can generate magnetic fields a quadrillion times more powerful than the Earth’s. Although B1509-58 is not a magnetar, its high field strength and spin rate make it in the vicinity of the typical upper limits for normal pulsars, making the distinction between these stellar remnants less clear.
Until now, the discordant maps of MSH 15-52 are both a technical achievement of multiwavelength astronomy and a test for theory. Only by combining more penetrating observations with sophisticated magnetohydrodynamic simulations will scientists come to understand why some structures glow in X-rays alone, why radio radiation extends far beyond boundaries predicted by theory, and how such a small creature can shape a nebula that appears to reach out toward the fabric of space itself.
