High-energy radiation from pulsed radio pulsars raises new questions for astrophysicists

An X-ray image of a pulser taken by the Chandra Space Telescope. The neutron star is located in the area of ​​a round bright spot in the brightest area of ​​the image. The blurred streaks running from the bottom left to the top right corner of the image are jets emitted by pulsars. Photo courtesy of www.nasa.gov

Collaboration We announced the measurement results of high-energy radiation from the H.E.S.S. Pulse pulsar. The 78 photons captured have energies ranging from a few TeV to 20 TeV, making them the most energetic radiation ever detected from a pulsar. Although the region where the radiation originates could not be determined, changes in the radiation due to the rotation period of the neutron star indicate that the source of the radiation is close to the star itself. This modulation completely eliminates the possibility of radiation occurring in the nebula surrounding the pulsar. Detection of such high-energy radiation provides a unique opportunity to look “deep” into a pulsar’s magnetosphere (just a few thousand kilometers in diameter) and test theories about the acceleration of particles and the origin of their radiation. Theoretical astrophysicists will now have to explain this new energy limit.

radio pulsar Radio pulsars were discovered in 1967-1968 as sources of radio radiation with stable periodicity, rare in astrophysics (for the history of their discovery, see Radio Physics of the Sixties: Two Great Discoveries The Story of Elements (detailed in the Elements article). ”. October 15, 2018). The peculiarity of these objects is encoded in the name radio pulsar. A radio pulsar is a source that periodically generates a strong signal within the radio range. Characteristics of the signal Their periods range from milliseconds to seconds and are surprisingly stable, varying by only 10−11 to 10−8 seconds per year. These properties allow astrophysicists to I quickly understood the nature of the radio pulsar: it is a neutron star rotating in a super strong magnetic field.The periodicity of the radio signal is determined by the star’s rotation, and the duration of each individual radio pulse also depends on the pulsar’s rotation. It is determined by the “thickness” of a narrow radiation region around the magnetic axis (Figure 1). 2). A pulsar can be said to be like a beacon that receives radio waves for a short time when its magnetic axis is directed toward an observer.

According to theoretical estimates, repeatedly confirmed by observations, the magnetic field at the surface of such stars varies between 108 and 1015 Gauss. The mechanisms behind the formation of radio radiation are still not completely understood. Many theoretical models have been proposed over the past 50 years, most of which have been refuted over time by observations and numerical simulations. Astrophysicists don’t even agree on where radio radiation comes from. Most likely, it occurs near the surface of the neutron star, that is, at a distance of several kilometers (remember that the typical radius of a neutron star is 10-15 kilometers). This widely accepted hypothesis is of great importance for the following discussion.

magnetosphere The magnetosphere of a pulsar, the region surrounding a neutron star, has a very interesting structure. The fact is that an electric potential should be generated by the rotation of a magnetized ideal conductor (and this is precisely the surface of a neutron star) – something like a unipolar inductor assembled from wires, batteries and magnets (Fig. 3). . However, this analogy works in reverse because in unipolar gears the presence of voltage (battery) causes the wire to rotate.

However, since a vacuum cannot carry electrical current, the rotation of the pulsar in the absence of a charged particle source does not screen the electric field at all. In fact, it was clear as early as the early 1970s that the necessary plasma could be “robbed” from the atmospheres of neutron stars. Electrons stripped from the surface by the electric field are accelerated to enormous energies (tens of millions of times the residual energy) and produce photons that interact with the ultra-strong magnetic field to produce secondary electron-positron pairs. absorbed by the same electric field. This self-replicating vapor production process, called a cascade, continues several kilometers above the neutron star’s surface, to a region where the electric field is screened by the charges produced by the neutron star. This fills the magnetosphere with electron-positron plasma.

The amplitude of the magnetic field is so large that the particles are effectively “frozen” within the field lines. Near the star’s surface, plasma driven by a strong magnetic field rotates around the star at the star’s own angular velocity. However, for the distance cP∗/2π From the axis of rotation, here P∗ – Rotation period of the star. Rotating with the star means it travels faster than the speed of light. Behind this surface, called the cylinder of light, the magnetic field lines “extend” to infinity, while the plasma “slides” along them, moving roughly radially outward at speeds close to the speed of light. For a typical pulsar with a rotation period of 1 second, the column of light is about 50,000 km from the surface (about 5,000 times the radius of the star itself). For fast pulsars like the Crab Nebula pulsar, this value is close to 1500 km, and for pulsed pulsars it is about 2.5 times larger. The cylinders of light traditionally correspond to the inner and outer boundaries of the co-rotating magnetosphere, where the plasma generated forms so-called pulsar winds.

High energy radiation In addition to radio emissions, higher frequency emissions have also been detected in many pulsars and repeat with the same periodicity (in the optical, X-ray, and gamma-ray ranges). Interestingly, high-energy radiation often exhibits a rotational phase lag compared to radio. This directly indicates that the source of high-energy radiation is located far from the surface of the neutron star, and that the radio emission is likely to occur nearby, as explained above.

A second important indicator that radiofrequency radiation is not being generated at the surface is the high intensity of this radiation. Fermi telescope observations in the gamma-ray range show that in pulsars, gamma-rays reach energies from 0.1 to several GeV. The intensity of this radiation is 0.1 to 10% of the pulsar’s stopping power, that is, the rotational energy lost by the neutron star per unit time due to a decrease in its rotational angular velocity (Figure 6). In other words, the source that produces the high-energy radiation somehow consumes a large portion of the total amount of available electromagnetic energy that the pulsar releases by braking.

These two facts (the phase shift relative to the radio emission and the intensity of the gamma-ray emission) indicate that the high-energy radiation in the pulsar’s optical to gamma-ray range must be generated in the outer magnetosphere behind the pulsar’s surface. I am. In a so-called cylinder of light in a current sheet, magnetic lines of opposite polarity from the northern and southern hemispheres converge in a very narrow area in the equatorial plane. Scientists have “discovered” a process that dissipates electromagnetic energy within solar sheets. When magnetic lines of opposite polarity converge in a fairly small area where the magnetic field is effectively zero, the plasma becomes essentially demagnetized and magnetic reconnection occurs, converting the energy of the magnetic field into the energy of the particles, which Accelerated to ultra-relativistic energy. The details of this process are explained in the magnetic reconnection problem.

Magnetic reconnection solves both of the above problems simultaneously. First, the plasma is ultrarelativistic, so the magnetic field lines converge toward the current layer at a speed of about 10% of the speed of light (also known as the reconnection speed). This actually allows reconnection to rapidly dissipate a large portion of the electromagnetic energy available in the magnetosphere, ultimately being observed as high-energy radiation. Second, the energy with which a particle, primarily an electron-positron pair, can be accelerated in such a process is determined by the ratio of the magnetic energy and the rest energy of the particle. For a typical young pulsar that exhibits gamma rays, this number exceeds its rest mass by five to six orders of magnitude. In other words, the energy of an electron-positron pair can reach several TeV. Therefore, the main mechanism of radiation in the energy range up to a few GeV is the synchrotron radiation of such ultrarelativistic pairs, which loses energy by emitting gamma-ray photons approximately as fast as they are accelerated. lose.

Parousi’s pulsar These considerations lead us to new observations of pulsed pulsars. The H.E.S.S. telescope system consists of four 12 meter diameter mirrors (CT1-4) placed in the corners of a 120 meter square and a 28 meter diameter mirror (CT5) placed in the central square. . H.E.S.S. is located in Namibia. All five telescopes are modular and made up of many small mirrors (essentially pixels), each of which redirects light to a central detector that detects photons within its optical range. An attentive reader may notice that the optical range, i.e. photons with energy measured in eV, is quite far from the TeV photons we are interested in. The difference is 12 orders of magnitude. The answer is that these telescopes do not “capture” high-energy photons directly, but through the products of their interaction with the atmosphere.