SPACE

# Unraveling the Secrets of the Crab Pulsar

The Crab Nebula, a remnant of a supernova that occurred 7,500 years ago, became visible on Earth in 1054. At the heart of this nebula lies a pulsar, functioning as an extraordinary astrophysical engine capable of generating vast amounts of energy. A recent study from July 2019 revealed the detection of photons from the Crab Nebula with energies surpassing 400 TeV, which is 35 times greater than the maximum energy produced from proton collisions at the Large Hadron Collider (LHC) in Geneva, the most powerful particle accelerator to date.

On July 4, 1054, a bright new star appeared in the sky, situated near the left horn of the Taurus constellation. It was as luminous as Venus and remained visible even during daylight for over three weeks before its brightness began to wane. The star continued to be observable at night for nearly two years. Historical records from Chinese and Arab sources document this event, while Western chronicles overlooked it, likely due to the misconception that the sky remained unchanged.

Centuries later, telescopes revealed that in the location of the bright star from 1054, there existed a nebulous formation. French astronomer Charles Messier cataloged this nebulosity as number 1 in his renowned list of celestial objects that resembled comets but were not. Today, we recognize Messier 1 (M1) as a supernova remnant, the light from a catastrophic cosmic event that occurred a millennium ago. A massive star, having depleted its nuclear fuel, exploded with tremendous force, ejecting its outer layers into space.

After the supernova, the expelled materials formed a complex halo of filaments. In 1844, William Parsons, 3rd Earl of Rosse, observed this nebula with a 0.9-meter telescope and noted the filaments' resemblance to a crab, thus naming it the Crab Nebula. This designation has persisted, and M1 is now universally known by that name.

Almost 1,000 years post-explosion, the remnants continue to expand at a velocity of 1,500 km/s, equivalent to 0.5% of light speed. The mass of the filaments, estimated at 4.6 solar masses, occupies a region of the sky measuring approximately 7×5 arc minutes, translating to about 13×9 light-years from Earth, which is situated roughly 6,500 light-years away from the Crab Nebula.

However, not all matter from the progenitor star was expelled. A dense core, formed from the remnants of nuclear fusion, remained. When the supernova occurred, while the outer layers dispersed rapidly, the stellar core collapsed under its own gravity. This core, possibly exceeding 1.9 solar masses (equivalent to over 600,000 Earths), became confined within a sphere just 10–14 km in radius. The central density reached approximately 2.7×10¹? kg/m³, resulting in a hyperdense state known as neutron “pasta.”

A neutron star emerged from this process. The magnetic field and angular momentum of the original star, now concentrated in the neutron star, became extraordinarily large, leading to the formation of a pulsar—a highly magnetized neutron star spinning rapidly and capable of emitting periodic pulses observable across multiple regions of the electromagnetic spectrum.

The pulsar at the center of the Crab Nebula, originating from the gravitational collapse of the supernova's core in 1054, was discovered in 1968. It stands out as one of the few pulsars visible in visible light, rotating every 33.5 milliseconds, roughly 30 times per second. This rapid spinning is gradually slowing down, with recent observations indicating a daily increase in rotation period of 38 nanoseconds.

The pulsar's exceptionally strong magnetic field, with intensities near its poles reaching 7.6×10¹² Gs (7.6 trillion gauss), acts as a braking mechanism, contributing to the gradual lengthening of its rotation period. This field possesses far greater energy than the Earth’s magnetic field, which peaks at about 0.65 Gs. It has been calculated that a single cubic centimeter of the Crab pulsar's magnetic field harbors an energy density exceeding 5×10¹? W (50 Petawatts).

The pulsar, owing to its rapid rotation and formidable magnetic fields, injects enormous quantities of energy into its environment. As it spins, it generates electric fields with significant voltage differences, which accelerate charged particles, primarily electrons and positrons. These particles, propelled to velocities nearing that of light, become trapped within the pulsar’s magnetic field lines and spiral around them, producing what is known as synchrotron radiation.

For nearly two decades, the Chandra X-ray Observatory has been studying the Crab Nebula, capturing remarkable details of the pulsar's surroundings, which are filled with a lethal wind of highly energetic plasma. Chandra’s observations reveal a cloud of charged particles emitting high-energy X-rays, with ring structures formed by shock waves from the pulsar's high-speed plasma colliding with surrounding gas. A plasma jet can also be seen emanating from one of the pulsar's poles.

The persistent energy activity around the pulsar generates radiation across the entire electromagnetic spectrum, ranging from radio waves to gamma rays. These emissions reach Earth, located 6,500 light-years away, and have been recorded over time using various instruments. The most energetic photons, particularly in the gamma-ray spectrum, are of great interest to astrophysicists, serving as indicators of the phenomena occurring near the Crab pulsar.

Recent experiments aimed at detecting the energy and origin of high-energy gamma photons have identified emissions from the Crab Nebula close to the remarkable threshold of 100 TeV (Teraelectronvolts). To put this in perspective, 1 TeV equals 1,000 billion eV (electron volts), whereas visible light has energies between 1.5 and 3 eV. Therefore, a 100 TeV gamma photon possesses an energy 14 orders of magnitude greater than that of visible light photons.

A study published on July 29 in Physical Review Letters by a group of scientists from the Tibet AS ? Collaboration has set a new record for gamma photon energies from the Crab Nebula.

These high-energy photons were detected by an observatory in Yangbajing, Tibet, located at an elevation of 4,300 meters. The facility comprises two overlapping structures designed for precision. At ground level, a grid of 597 plastic scintillators covers 65,700 m². Beneath this grid, at a depth of 2.4 meters, there are 64 Cherenkov light detectors, which are capable of capturing the impact of muons, charged particles. Each detector consists of a watertight concrete tank filled with water, equipped with a photomultiplier tube that identifies photons generated by the arrival of even a single muon.

The study reports on observations conducted over 716 days between February 2014 and May 2017. During this period, the observatory recorded 24 photons from the Crab Nebula with energies exceeding 100 TeV. Notably, two of these photons had energies surpassing 400 TeV, specifically 449 TeV and 458 TeV. Comparatively, the most powerful particle accelerator on Earth, the LHC at CERN, achieves a maximum energy of 13 TeV in proton collisions. This indicates that the natural processes occurring at the Crab pulsar can accelerate charged particles to produce photons with energies more than 35 times that generated at CERN. Moreover, it remains uncertain if there is an upper limit to the energy of photons produced near a pulsar, suggesting that even more energetic photons could be en route to Earth, possibly in the PeV range (10¹? eV), awaiting detection by sufficiently sensitive instruments.

The astrophysical processes that generate such energetic photons are still being investigated. The authors of the study in Physical Review Letters propose inverse Compton scattering as the likely mechanism behind this phenomenon. This occurs when a low-energy photon interacts with a highly energetic relativistic electron, such as those accelerated by the pulsar's rotating magnetic field. The electron transfers energy to the photon, causing its wavelength to shift from that of radio waves or microwaves to the much shorter gamma-ray wavelength. This process resembles a billiard shot, where the photon's speed remains constant at light speed, and the energy increase is reflected in the decrease of its wavelength.

# Notes

[1] To put it in perspective, this energy could power a billion 100 W light bulbs for nearly six days.