NASA’s NICER finds X-ray enhancements within the radio bursts of the crab pulsar – what is the level?
From NASA
A worldwide scientific collaboration using data from NASA’s NICER (Neutron Star Interior Composition Explorer) telescope on the International Space Station has discovered X-ray bursts associated with radio bursts from the pulsar in the Crab Nebula. The finding shows that these bursts, so-called giant radio pulses, release far more energy than previously thought.
A pulsar is a type of rapidly spinning neutron star, the crushed, city-sized core of a star that exploded as a supernova. A young, isolated neutron star can spin dozens of times per second, and its swirling magnetic field propels beams of radio waves, visible light, X-rays, and gamma rays. When these rays pass the earth, astronomers observe clock-like emission pulses and classify the object as a pulsar.
“Of more than 2,800 cataloged pulsars, the crab pulsar is one of the few that emit huge radio pulses that occur sporadically and can be hundreds to a thousand times brighter than the regular pulses,” said senior scientist Teruaki Enoto from the RIKEN cluster for pioneering research in Wako, Saitama Prefecture, Japan. “After decades of observation, it has been shown that only the crab amplifies its huge radio pulses by emitting from other parts of the spectrum.”
The new study, which will appear in the April 9th issue of Science and is now available online, analyzed the largest amount of simultaneous X-ray and radio data ever collected from a pulsar. It expands the observed energy range associated with this improvement phenomenon by a thousand fold.
The Crab Nebula and its pulsar are located about 6,500 light years away in the constellation Taurus and form a supernova whose light reached Earth in July 1054. The neutron star rotates 30 times per second and is one of the brightest pulsars in the sky at X-ray and radio wavelengths.
Between August 2017 and August 2019, Enoto and his colleagues used NICER to repeatedly observe the crab pulsar in X-rays with energies up to 10,000 electron volts, or a thousand times that of visible light. While NICER watched, the team also examined the object using at least one of two ground-based radio telescopes in Japan – the 34-meter dish in the Kashima Space Technology Center and the 64-meter dish in the Usuda Deep of the Japan Aerospace Exploration Agency Space Center, both work at a frequency of 2 gigahertz.
Between 2017 and 2019, NASA’s neutron star Interior Composition Explorer (NICER) and radio telescopes in Japan simultaneously examined the crab pulsar. In this visualization, which shows only 13 minutes of NICER observations, millions of X-rays are recorded relative to the phase of rotation of the pulsar, which focuses on the strongest radio emission. Two full rotations are shown for clarity. As the pulsar beams sweep across our line of sight, they create two peaks for each revolution, the brighter one being associated with a greater number of giant radio pulses. For the first time, NICER data shows a slight increase in X-ray emission related to these events. Credits: NASA’s Goddard Space Flight Center / Enoto et al. 2021
The combined dataset effectively gave researchers nearly a day and a half of simultaneous X-ray and radio coverage. In total, they recorded activities over 3.7 million pulsar revolutions and achieved around 26,000 huge radio pulses.
Giant impulses break out quickly, come to a head in millionths of a second and occur unpredictably. However, when they do occur, they coincide with the regular clockwork pulsations.
NICER records the time of arrival of each X-ray it captures to within 100 nanoseconds, but the telescope’s timing precision isn’t the only benefit for this study.
“NICER’s ability to observe bright X-ray sources is nearly four times greater than the combined brightness of both the pulsar and its nebula,” said Zaven Arzoumanian, project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “So these observations were largely unaffected by clustering – where a detector counts two or more X-rays as a single event – and other problems that complicated previous analyzes.”
Enoto’s team combined all of the X-ray data that coincided with giant radio pulses and resulted in an X-ray surge of about 4% occurring in sync with them. It is remarkably similar to the 3% increase in visible light that is also associated with the phenomenon discovered in 2003. Compared to the difference in brightness between the crab’s regular and giant pulses, these changes are remarkably small and pose a challenge to theoretical models.
The improvements suggest that giant pulses are a manifestation of the underlying processes that create an emission that spans the electromagnetic spectrum from radio to X-rays. And because X-rays are millions of times as powerful as radio waves, even a modest increase means a large contribution to energy. The researchers conclude that the total energy emitted associated with a giant pulse is dozens to hundreds of times higher than previously estimated based on the radio and optical data alone.
“We still do not understand how or where pulsars produce their complex and long-range emission, and it is gratifying to have added another piece to the multi-wavelength puzzle of these fascinating objects,” said Enoto.
NICER is an Astrophysics Mission of Opportunity under NASA’s Explorers program, which provides frequent flight opportunities for world-class scientific investigations from space and uses innovative, optimized and efficient management approaches in the fields of heliophysics and astrophysics. NASA’s Space Technology Mission Directorate supports the SEXTANT component of the mission and demonstrates the pulsar-based navigation of spacecraft.
Banner: The Crab Nebula, the six light-years long cloud of debris from a supernova explosion, houses a neutron star that rotates 30 times per second and is one of the brightest pulsars in the sky in terms of X-ray and radio wavelengths. This compilation of Hubble Space Telescope images shows various gases emitted in the explosion: blue indicates neutral oxygen, green indicates singly ionized sulfur, and red indicates doubly ionized oxygen. Recognition: NASA, ESA, J. Hester and A. Loll (Arizona State University)
By Francis Reddy
NASA’s Goddard Space Flight Center, Greenbelt, Md.
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