Stange X-ray Surges Detected in Crab Pulsar’s Radio Bursts by NASA’s NICER

An International science association using data from NASA’s Neutron star Interior Composition Explorer (NICER) telescope on the International Space Station has found X-ray waves accompanying radio bursts from the pulsar in the Crab Nebula. The discovery reveals that these bursts, called giant radio pulses, release far more energy than previously assumed.

A pulsar is a kind of quick rotating neutron star, the crushed, city-sized core of a star that exploded as a supernova. An isolated, young neutron star can rotate dozens of times each second, and its rotating magnetic field powers beams of X-rays, visible light, and gamma rays, and radio waves. The astronomers classify the object as a pulsar if these beams sweep past Earth and emit clock-like pulses.

“Out of more than 2,800 pulsars recorded, the Crab pulsar is one of only a few that emit giant radio pulses, which occur sporadically and can be thousand times more luminous than the usual pulses,” said lead astrologist Teruaki Enoto at the RIKEN Cluster for Pioneering Research in Wako, Saitama prefecture, Japan. “After years of observations, only the Crab has been shown to intensify its enormous radio pulses with emission from other portions of the spectrum.”

Experts examining data from NASA’s Neutron star Interior Composition Explorer (NICER) telescope on the International Space Station has detected X-ray surges accompanying radio bursts from the pulsar in the Crab Nebula. The discovery reveals that these bursts, called giant radio pulses, release far more energy than previously assumed. Source: NASA’s Goddard Space Flight Center

The new research, issued on April 9, 2021, in the publication of the journal Science, examined the largest amount of synchronous X-ray and radio data ever collected from a pulsar. It prolongs the detected energy range correlated with this enhancement phenomenon by thousands of times.

Located approximately 6,500 light-years away in the constellation Taurus, the Crab Nebula and its pulsar appeared in a supernova whose light touched Earth in July 1054. The neutron star rotates 30 times each second, and at radio and X-ray wavelengths it is among the brightest pulsars in the sky.

Between August 2017 and August 2019, Enoto and his associates used NICER to frequently examine the Crab pulsar in X-rays with energies up to thousands of times that of visible light, or 10,000 electron volts. While NICER was observing, the group also analyzed the object using at least one of two ground-based radio telescopes in Japan – the 64-meter dish at the Japan Aerospace Exploration Agency’s Usuda Deep Space Center and the 34-meter dish at the Kashima Space Technology Center, both working at a frequency of 2 gigahertz.

The combined dataset adequately provided the scientists almost a day and a half of synchronous X-ray and radio coverage. All told, they netted some 26,000 giant radio pulses and captured activity across 3.7 million pulsar rotations. Enormous pulses erupt suddenly, spiking in millionths of a second, and happen unpredictably. Yet, when they occur, they accord with the usual clockwork pulsations.

Between 2017 and 2019, radio telescopes in Japan and NASA’s Neutron star Interior Composition Explorer (NICER) analyzed the Crab pulsar concurrently. In this visualization, which has just 13 minutes of NICER observations, hundreds of thousands of X-rays are plotted corresponding to the pulsar’s rotational phase, which is focused on the strongest radio emission. For simplicity, two full rotations are shown. As the pulsar beams sweep across our line of sight, they produce two peaks for each rotation, with the brighter one correlated with higher numbers of large radio pulses. For the first time, NICER data show a slight improvement in X-ray emission connected with these events. Source: NASA’s Goddard Space Flight Center/Enoto et al. 2021

NICER records the appearance time of each X-ray it identifies to within 100 nanoseconds, but the telescope’s timing accuracy isn’t its only benefit for this research.

“NICER’s potential for recognizing bright X-ray sources is almost four times higher than the combined brightness of both the pulsar and its nebula,” said Zaven Arzoumanian, the project’s science lead at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “So these observations were considerably unaffected by pileup – where a detector counts two or more X-rays as a single event – and other problems that have hampered earlier studies.”

Enoto‘s team combined all of the X-ray data that matched with large radio pulses, showing an X-ray boost of about 4% that occurred in synch with them. It’s exceptionally similar to the 3% rise in visible light also connected with the phenomenon, discovered in 2003. Compared to the brightness difference between the large pulses and Crab’s regular, these variations are exceptionally small and provide a challenge for theoretical models to explain.

The improvements hint that large pulses are a sign of underlying processes that produce emission spanning the electromagnetic spectrum, from radio to X-rays. And because X-rays pack millions of times the punch of radio waves, even a modest increase represents a large energy contribution. The scientist concludes that the total emitted energy associated with a large pulse is dozens to hundreds of times higher than previously expected from the optical and radio data alone.

“We still don’t know how or where pulsars produce their wide-ranging and complicated emission, and it’s satisfying to have contributed another piece to the multiwavelength mystery of these interesting objects,” Enoto said.

Journal Reference:

“Enhanced x-ray emission coinciding with giant radio pulses from the Crab Pulsar” by Teruaki Enoto, Toshio Terasawa, Shota Kisaka, Christian Malacaria, Paul S. Ray, Wynn C.G. Ho, Alice K. Harding, Takashi Okajima, Zaven Arzoumanian, Keith C. Gendreau, Yasuhiro Murata, Hiroshi Takeuchi, Kazuhiro Takefuji, Zorawar Wadiasingh, Craig B. Markwardt, Yang Soong, Steve Kenyon, Slavko Bogdanov, Chin-Ping Hu, Sebastien Guillot, Natalia Lewandowska, Walid A. Majid, Tolga Güver, Gaurav K. Jaiswal, Rick Foster, Mamoru Sekido, Yoshinori Yonekura, Hiroaki Misawa, Fuminori Tsuchiya, Takahiko Aoki, Munetoshi Tokumaru, Mareki Honma, Osamu Kameya, Tomoaki Oyama, Katsuaki Asano, Shinpei Shibata, and Shuta J. Tanaka, 9 April 2021, Science. DOI: 10.1126/science.abd4659

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