Einstein@Home reveals true identity of mysterious gamma-ray source

Gamma-ray – Artist’s impression of PSR J2039−5617 and its companion. The binary system consists of a rapidly rotating neutron.

An international research team including members from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI) in Hannover has shown that a rapidly rotating neutron star is at the core of a celestial object now known as PSR J2039−5617. They used novel data analysis methods and the enormous computing power of the citizen science project Einstein@Home to track down the neutron star’s faint gamma-ray pulsations in data from NASA’s Fermi Space Telescope.

Their results show that the pulsar is in orbit with a stellar companion about a sixth of the mass of our Sun. The pulsar is slowly but surely evaporating this star. The team also found that the companion’s orbit varies slightly and unpredictably over time. Using their search method, they expect to find more such systems with Einstein@Home in the future.

The celestial object has been known since 2014 as a source of X-rays, gamma rays, and light. All evidence obtained so far pointed at a rapidly rotating neutron star in orbit with a light-weight star being at the heart of the source. But clear proof was missing.

Precision observations with optical telescopes

The first step to solving this riddle were new observations of the stellar companion with optical telescopes. They provided precise knowledge about the binary system without which a gamma-ray pulsar search (even with Einstein@Home’s huge computing power) would be unfeasible.

The system’s brightness varies during an orbital period depending on which side of the neutron star’s companion is facing the Earth. “For J2039-5617, there are two main processes at work,” explains Dr. Colin Clark from Jodrell Bank Centre for Astrophysics, lead author of the study and former Ph.D. student at AEI Hannover. “The pulsar heats up one side of the light-weight companion, which appears brighter and more bluish. Additionally, the companion is distorted by the pulsar’s gravitational pull causing the apparent size of the star to vary over the orbit.” These observations allowed the team to get the most precise measurement possible of the binary star’s 5.5-hour orbital period, as well as other properties of the system.

Searching with the help of tens of thousands of volunteers

With this information and the precise sky position from Gaia data, the team used the aggregated computing power of the distributed volunteer computing project Einstein@Home for a new search of about 11 years of archival observations of NASA’s Fermi Gamma-ray Space Telescope. Improving on earlier methods they had developed for this purpose, they enlisted the help of tens of thousands of volunteers to search Fermi data for periodic pulsations in the gamma-ray photons registered by the Large Area Telescope onboard the space telescope. The volunteers donated idle compute cycles on their computers’ CPUs and GPUs to Einstein@Home.

This search required combing very finely through the data in order not to miss any possible signals. The computing power required is enormous. The search would have taken 500 years to complete on a single computer core. By using a part of the Einstein@Home resources it was done in 2 months.

With the computing power donated by the Einstein@Home volunteers, the team discovered gamma-ray pulsations from the rapidly rotating neutron star. This gamma-ray pulsar, now known as J2039−5617, rotates about 377 times each second.

Surprising changes of the orbit

“We found that the companion’s orbital period varies slightly and unpredictably over the 11 years. It only changes by up to about ten milliseconds, but since we know the arrival time of every single gamma photon from the pulsar to microsecond precision, even this little is a lot!” says Nieder. These variations of the orbital period could be linked to tiny changes in the shape of the companion caused by its magnetic activity. Similar to our Sun the companion might be going through activity cycles. The changing magnetic field interacts with the plasma inside the star and deforms it. As the shape of the star varies its gravitational field also changes, which in turn affects the pulsar orbit. This could explain the observed orbital period variations.

“Spidery” pulsars consume their mates

While the light-weight stellar companion is orbiting the pulsar, the strong radiation and particle wind from the pulsar evaporate the companion. “This is the reason that astronomers call systems like this one ‘redbacks’ in reference to the Australian redback spiders whose females consume the males after mating,” explains Nieder. In the case of J2039−5617 the matter ablated from the star forms clouds of charged particles in the binary system that absorb radio waves. This is one of the reasons that previous searches for pulsating radio emission from the neutron star failed. With the precise determination of the orbit from the gamma-ray data, it was also possible to detect radio pulsations and this will be published in a separate paper.

“We know dozens of similar gamma-ray sources found by the Fermi Space Telescope, for which the true identity is still unclear,” says Prof. Dr. Bruce Allen, director at the Max Planck Institute for Gravitational Physics in Hannover and director and founder of Einstein@Home. “Many might be pulsars hidden in binary systems and we will continue to chase after them with Einstein@Home,” he adds.

What are gamma-rays?

Gamma-rays are a form of electromagnetic radiation, as are radio waves, infrared radiation, ultraviolet radiation, X-rays and microwaves. Gamma-rays can be used to treat cancer, and gamma-ray bursts are studied by astronomers.

Electromagnetic (EM) radiation is transmitted in waves or particles at different wavelengths and frequencies. This broad range of wavelengths is known as the electromagnetic spectrum. The spectrum is generally divided into seven regions in order of decreasing wavelength and increasing energy and frequency. The common designations are radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma-rays.

Gamma-rays fall in the range of the EM spectrum above soft X-rays. Gamma-rays have frequencies greater than about 10^19 cycles per second, or hertz (Hz), and wavelengths of less than 100 picometers (pm), or 4 x 10^9 inches. (A picometer is one-trillionth of a meter.)

Gamma-rays and hard X-rays overlap in the EM spectrum, which can make it hard to differentiate them. In some fields, such as astrophysics, an arbitrary line is drawn in the spectrum where rays above a certain wavelength are classified as X-rays and rays with shorter wavelengths are classified as gamma-rays. Both gamma-rays and X-rays have enough energy to cause damage to living tissue, but almost all cosmic gamma-rays are blocked by Earth’s atmosphere.

Discovery of gamma-rays

Gamma-rays were first observed in 1900 by French chemist Paul Villard when he was investigating radiation from radium, according to the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA). A few years later, New Zealand-born chemist and physicist Ernest Rutherford proposed the name “gamma-rays,” following the order of alpha rays and beta rays — names given to other particles that are created during a nuclear reaction — and the name stuck.

Gamma-ray sources and effects

Gamma-ray – Gamma-rays are produced primarily by four different nuclear reactions: fusion, fission, alpha decay and gamma decay.

Gamma-ray
Artist impression of gamma ray burst. 

Nuclear fusion is the reaction that powers the sun and stars. It occurs in a multistep process in which four protons, or hydrogen nuclei, are forced under extreme temperature and pressure to fuse into a helium nucleus, which comprises two protons and two neutrons. The resulting helium nucleus is about 0.7 percent less massive than the four protons that went into the reaction. That mass difference is converted into energy, according to Einstein’s famous equation E=mc^2, with about two-thirds of that energy emitted as gamma-rays. (The rest is in the form of neutrinos, which are extremely weakly interacting particles with nearly zero mass.) In the later stages of a star’s lifetime, when it runs out of hydrogen fuel, it can form increasingly more massive elements through fusion, up to and including iron, but these reactions produce a decreasing amount of energy at each stage.

Gamma-ray – Another familiar source of gamma-rays is nuclear fission. Lawrence Berkeley National Laboratory defines nuclear fission as the splitting of a heavy nucleus into two roughly equal parts, which are then nuclei of lighter elements. In this process, which involves collisions with other particles, heavy nuclei, such as uranium and plutonium, are broken into smaller elements, such as xenon and strontium. The resulting particles from these collisions can then impact other heavy nuclei, setting up a nuclear chain reaction. Energy is released because the combined mass of the resulting particles is less than the mass of the original heavy nucleus. That mass difference is converted to energy, according to E=mc^2, in the form of kinetic energy of the smaller nuclei, neutrinos and gamma-rays.. Read more about gamma-rays > livescience.com

Article source: phys.org

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