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Highlights · 02 Mar 2021

When Neighbors Talk with Each Other

-- Insight-HXMT and Its Identification of the Sender of FRB 200428, a Cosmic “Telegram”

By SONG Jianlan

 

Insight-HXMT, an astronomical satellite of China working in a very broad X-ray waveband, has helped identify a magnetar as the long-sought-after sender of a fast radio burst. Shown is an artistic illustration for this scenario. (Image credit: IHEP)

Fast radio bursts (FRBs), pulses of brief but extremely bright radio waves blasting from the depth of the cosmos, particularly those repeatedly erupting, have been fascinating astronomers and, astronomical amateurs as well, since their first discovery in 2007. What are they? What, or WHO, are behind these mysterious signals? Could it be a desperate telegram sent by an alien civilization seeking cosmic counterparts smart enough to decipher their message? Unfortunately, such bursts endure only milliseconds and hence are far too brief to allow closer astronomical observation and examination. About their origins, there exist even more theories than observed FRBs, just as a joke goes among astronomers to describe the smorgasbord of models attempting to explain their birth and propagation.

A majority of these theories suggest magnetars – highly magnetized neutron stars newly born from some catastrophic events like supernovae – as promising producers of such radio pulses. They are known to be sources of high-energy radiations like gamma rays and X-rays, and some models predict that young magnetars can give off radio bursts aside from gamma- and X-rays, however, no such radio burst had ever been directly detected from them. On the other hand, nor had any burst been detected from other wavebands in coincidence with any FRB, leaving their origin an enigma.

Long-awaited Evidence

The situation did not change until very lately when the results from a wave of coordinated observations occurring in late April of 2020 eventually got published in succession. These observations, focused on the same soft gamma ray repeater named SGR J1935+2154, known as a magnetar from our Milky Way, altogether helped associate FRBs with magnetars for the first time. Observatories across the world, ground- or space-based, working from different locations and wavebands – gamma rays, X-rays and radios, eventually close the long-existing gap, and partially solve the enigma of FRB origin.

Among them, the observation conducted by a team with the Hard X-ray Modulation Telescope (dubbed “Insight” and here later referred to as Insight-HXMT), an astronomical satellite of China, has offered data of the best quality from an unprecedented broad energy range for wavelengths of X-rays, as well as very accurate localization of the source. For the very first time, it unambiguously identified an X-ray burst from SGR J1935+2154 as the counterpart of FRB 200428, demonstrating that at least some FRBs are produced by magnetars.

Two space observatories, the Neil Gehrels Swift Observatory and the Fermi Gamma-ray Space Telescope first detected some bursts – in X- and gamma-ray bands – from SGR J1935+2154 on April 27, 2020. The source fell into the view of ground-based telescopes located in the Western Hemisphere the following day, and two radio telescopes, namely the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Survey for Transient Astronomical Radio Emission 2 (STARE2) in the United States, detected a radio burst from the same direction as the magnetar SGR J1935+2154. The burst was later named as FRB 200428 after the date it was detected.

During the outburst of FRB 200428, several space observatories reported the reception of an X-ray burst from SGR J1935+2154, including the European Space Agency’s International Gamma-Ray Astrophysics Laboratory (INTEGRAL) space telescope, the Italian Astro-rivelatore Gamma a Immagini LEggero (AGILE) satellite, Russia’s Konus detector aboard NASA’s Wind spacecraft and, Insight-HXMT.

Targeting on the same object SGR J1935+2154, part of the results got published in Nature last November, immediately triggering a new wave of hot debate on physical mechanisms of FRBs. The Insight-HXMT team, led by Prof. ZHANG Shuangnan from the Institute of High Energy Physics (IHEP), Chinese Academy of Science (CAS), finally published their results in Nature Astronomy at sharp 12 am (GMT+8) on February 19, 2021. With the long-awaited direct observational evidence, the team identified the magnetar SGR J1935+2154 as the counterpart of FRB200428, finally unveiling the sender of this cosmic “telegram”.

Highlighted in the lightcurve recorded by the Insight-HXMT team are two steep peaks separated by an interval of arrival time measuring about 34 milliseconds; this twin sub-burst is broadly consistent with the 29 milliseconds interval between the two narrow peaks in FRB200428 as recorded by the teams working with CHIME and STARE 2. Moreover, the apparent time delay between the X-ray signals recorded by the Insight-HXMT and the radio peaks captured by CHIME and STARE2 is also in excellent agreement with the dispersion delay predicted by theoretical calculation.

 

Shown here are the lightcurves of the X-ray burst from SGR J1935+2154 as observed by Insight-HXMT, covering the whole duration of this burst in coincidence with the radio burst of FRB 200428. Highlighted is the twin sub-burst with a 34-millisecond separation in arriving time of signal.  The refined structure, in excellent consistency with the twin peak detected in the radio burst detected by CHIME and STARE2, unambiguously confirms the association between the two. (Credit: IHEP)

The team also gave the localization of the X-ray burst, at an accuracy far better than STARE2 and CHIME, two ground-based radio telescopes. The location is perfectly consistent with the previously known position of the magnetar SGR J1935+2154 in the sky. Given that SGR J1935+2154 was the only active magnetar at the time of observation falling in the small localization region within the field of view of Insight-HXMT, the team unambiguously identified that the short X-ray burst “was emitted by the Galactic magnetar SGR J1935+2154 and produced almost simultaneously with FRB 200428 in a single explosive event.”

 

The accurate localization (white contours at the center of the inset) given by Insight-HXMT of the source SGR J1935+2154 (the red cross on the left) coincident with FRB 200428, in comparison with those given by STARE 2 (the dot-line circle on the left) and CHIME (the red circle on the left, also shown in the inset). (Credit: IHEP)


The team does not exclude the possibility of other origins for the majority of FRBs, however.

“For the burst we caught with Insight-HXMT, we would say it is very unlikely to have been sent by an alien,” concludes ZHANG Shuangnan, principal scientist of the telescope, in a humorous tone when inquired of the possibility. “The gravity and magnetic fields of magnetars are extremely strong,” he explains, “too strong for any life to survive.”

Its gravity is so strong that the gravitational pull between two points in a very tiny distance could be extremely large – any object there would be ripped apart by the intense action introduced by this difference, or tidal dynamics. The magnetic field on the surface of a magnetar can go as strong as about 1.0×108 Tesla or even larger. As a sharp contrast, the magnetic field of our home planet, the Earth, is much milder, roughly ranging from 3.5×10-5 Tesla around the equator to 7.0×10-5 Tesla in the polar regions on the surface.

“Atypical” Telegram Sender

It might not be typical for magnetars to burst out both X-rays and radio pulses, however.

When analyzing the data, the team fitted different models and found out that the spectrum follows a cutoff power-law model the best, with an acceptable statistical reliability. Based on this, they concluded that the integrated spectrum could be dominated by a power law covering at least the 1?100-keV energy range, which means, this burst is primarily non-thermal in nature.

 

Covering an energy band from 1 to 250 keV, the spectrum observed with Insight-HXMT follows a cutoff power law model the best, indicating its unusual nature in terms of physical mechanism. (Credit: IHEP)

“Magnetar outbursts are mostly thermal, therefore this burst is rare for magnetars,” explains Prof. XIONG Shaolin, ZHANG’s IHEP colleague and coauthor of the paper. “Non-thermal spectra are believed to be related to high-energy particles. However, how the particles are accelerated to this high energy is still not clear.” “On the other hand, radio pulses are of non-thermal nature,” adds ZHANG Shuangnan, “and this may explain why the X-ray burst came together with a radio pulse. Concerning the acceleration of particles, either an electric field or a magnetic field can do the job. It is very interesting an issue to explore.”

Models for magnetar origin of FRBs roughly fall into two categories, one attributing the FRB emission to coherent curvature radiation of electron-positron pairs from the magnetosphere of the magnetar, while the other attributing it to the synchrotron maser emission produced by a relativistic, magnetized shock. Given that the double peaks appear in both X-ray and radio parts of the spectrum are structurally consistent, they are very likely produced by the same explosion; if the hard X-ray part of the spectrum is more likely to be produced in the magnetosphere, it lends more support to the magnetospheric category of models, analyzed the Insight-HXMT team in the Nature Astronomy paper.

Generally, normal short bursts from magnetars are of thermal origin, as the Insight-HXMT team reported in their paper. They gave a conservative e