Scientists around Felix Aharonian from the MPI for Nuclear Physics and the Dublin Institute for Advanced Studies now present an explanation for this radiation. It is based on the abrupt acceleration of an ultrafast wind of “cold” electrons and positrons, taking place at a distance of some Earth’s diameters from the pulsar. (Nature, 15.02.2012 online)
Fig. 1: The Crab nebula (M1) in the Taurus constellation, taken by the Hubble space telescope (lower left). The details to the right are showing a composite of visible light (red) and X-rays (blue) with the pulsar as central star. At the shock front in 0.3 light-years distance from the pulsar, the ultrarelativistic wind of electrons and positrons collides with the surrounding nebula.
Graphics: MPIK, source: NASA
Fig. 2: Schematic evolution of the pulsar wind (electrons and positrons: e–, e+). High-energy gamma quanta (ã) are created in the acceleration zone by inverse Compton scattering of the pulsar wind with X-ray quanta (X) from the magnetosphere as well as at large distance at the shock front to the interstellar medium.
The Crab pulsar, a fast-rotating, highly magnetized neutron star, is a product of the historical supernova observed in the constellation Taurus in 1054 AD. Its mass amounts to 1.4 to 2 times that of the Sun and its diameter is only 28 to 30 km. Together with its surrounding nebula, it is one of the best investigated astronomical objects (Fig. 1).
The generally accepted paradigm postulates the existence of a relativistic wind of electrons and their antiparticles, positrons, which originates in the pulsar’s magnetosphere and terminates in the interstellar medium. The evolution of the wind is characterized by three consecutive processes (Fig. 2): At a distance of about 1000 km from the pulsar, the pulsar’s rotational energy is transformed into electromagnetic energy, which in turn is converted to kinetic energy of bulk motion, i.e. acceleration of the wind. Finally, the wind terminates by collision with matter in a standing reverse shock about 0.3 light years away. Thereby, the electrons and positrons are accelerated up to extremely high energies, resulting in an extended non-thermal source: the Crab nebula. All three processes need to proceed with incredibly high (close to 100 %) efficiency in order to explain the observational data.
Both the Crab pulsar and the Crab nebula are bright gamma-ray sources. While the pulsar emits in the high energies, the radiation of the nebula is released predominantly at the very-high-energy band. Meanwhile, the third key component, the wind, via which the transfer of energy from the pulsar to the nebula is realized, at first glance seems to be an ‘invisible substance’. Indeed, despite the relativistic speed of the wind, in the frame of the outflow the electrons are ‘cold’, meaning they move together with the wind’s magnetic field and therefore do not emit radiation. The wind, however, can radiate high-energy gamma-rays through the mechanism of inverse Compton scattering in which ultrafast electrons and positrons of the wind are illuminated by X-ray photons originating in the pulsar’s magnetosphere and/or the surface of the neutron star. In a paper published in Nature, Felix Aharonian, Sergey Bogovalov and Dmitry Khangulyan argue that recent reports of the surprise detection of pulsed, very-high-energy gamma radiation from Crab by the VERITAS and MAGIC atmospheric Cherenkov telescopes are best explained by inverse Compton scattering. Pulsed X-ray photons of the pulsar interact with ultrafast electrons of the wind predominantly in their acceleration zone. The wind, therefore, is the source of the pulsed gamma radiation and explains the observations with only three parameters: site of the acceleration of the wind, its final velocity, and the level of anisotropy.
If this interpretation is correct, then detection of the pulsed very-high-energy gamma-ray emission implies the first observational evidence of the formation of a cold ultrafast electron-positron wind from the Crab pulsar. The reported gamma-ray data allow us to localize, with a good precision, the site and estimate the speed with which the electromagnetic energy is transformed into the kinetic energy of the wind’s bulk motion. The results show that the acceleration of the wind to ultrarelativistic velocities should take place abruptly in a narrow cylindrical zone of radius between 20 and 50 thousand kilometers centered on the rotation axis of the pulsar. Although the ultrafast nature of the wind does support the general paradigm of pulsar winds, the requirement of the very fast acceleration of the wind in a narrow zone not very far from the pulsar challenges current models.Weitere Informationen:
Dr. Bernold Feuerstein | Max-Planck-Institut
Study offers new theoretical approach to describing non-equilibrium phase transitions
27.04.2017 | DOE/Argonne National Laboratory
SwRI-led team discovers lull in Mars' giant impact history
26.04.2017 | Southwest Research Institute
More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.
Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...
Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.
"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...
The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.
Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...
The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...
Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.
Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...
20.04.2017 | Event News
18.04.2017 | Event News
03.04.2017 | Event News
27.04.2017 | Life Sciences
27.04.2017 | Physics and Astronomy
27.04.2017 | Earth Sciences