Mysterious Galactic Signals: Magnetar Observations Shed New Light on Fast Radio Bursts

Mysterious Galactic Signals: Insights from Magnetar Observations Illuminate the Nature of Fast Radio Bursts

A report by an international team of researchers delves into the radio pulsar phase exhibited by a Galactic magnetar, which emitted a fast radio burst (FRB) in 2020. These observations have brought forth distinctive origins for both “bursts” and “pulses,” contributing to the ongoing theoretical framework for the formation of FRBs.

More than a decade and a half following the initial detection of fast radio bursts (FRBs)—brief, millisecond-long cosmic eruptions of electromagnetic radiation from the depths of space—astronomers worldwide have been diligently surveying the cosmos in search of clues that could elucidate the mechanisms and reasons behind their emergence.

Predominantly, FRBs that have been identified originate in the distant realms beyond our own Milky Way galaxy. This status quo shifted in April 2020, when the first-ever Galactic FRB, designated as FRB 20200428, was pinpointed. Intriguingly, this particular FRB was traced back to a magnetar known as SGR J1935+2154, a neutron star of remarkable density, comparable in size to a city, and distinguished by its extraordinarily potent magnetic field.

This groundbreaking revelation instigated speculation that FRBs discerned at cosmic distances outside our galaxy might also be generated by magnetars. Nevertheless, the pivotal evidence to substantiate this hypothesis, namely the detection of a rotation period stemming from the magnetar’s spin, has thus far eluded capture. Recent investigations centered around SGR J1935+2154 have shed light on this intriguing incongruity.

A recent installment of the journal Science Advances houses a collaborative effort by scientists of international repute, including astrophysicist Bing Zhang from UNLV. The report details the sustained monitoring of SGR J1935+2154 subsequent to the April 2020 FRB event. Significantly, these observations led to the identification of an additional cosmic occurrence—a radio pulsar phase—five months later.

Untangling a Cosmic Enigma

In their quest for answers, astronomers draw upon the capabilities of potent radio telescopes such as the expansive Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China. By utilizing FAST, researchers ascertained that FRB 20200428 and the subsequent pulsar phase originated from discrete zones within the ambit of the magnetar. This differentiation alludes to distinct points of origin.

Weiwei Zhu, the primary author of the paper hailing from the National Astronomical Observatory of China (NAOC), elucidates, “FAST detected 795 pulses in 16.5 hours over 13 days from the source. These pulses show different observational properties from the bursts observed from the source.”

This divergence in emission characteristics from the magnetosphere region contributes to astronomers’ comprehension of where and how FRBs and associated phenomena manifest within our galaxy and potentially beyond, in the cosmic expanse.

Decoding Radio Pulses and Magnetars

Analogous to FRBs, radio pulses denote cosmic electromagnetic eruptions, albeit emitting radiance approximately ten orders of magnitude less potent than an FRB. While these pulses are customarily identified in entities other than magnetars, such as rotating neutron stars named pulsars, Zhang explains that most magnetars do not exhibit radio pulses as a consistent trait, likely due to their incredibly robust magnetic fields. Notably, magnetars occasionally transiently adopt the guise of radio pulsars after undergoing bursts of activity, a phenomenon exemplified by SGR J1935+2154.

An additional disparity lies in the emission “phases” of bursts and pulses—i.e., the temporal windows during which radio emissions occur within each emission cycle.

Zhang elaborates, “Analogous to pulses in radio pulsars, the magnetar pulses are confined within a narrow phase interval during each cycle. This is recognized as the ‘lighthouse’ effect, signifying that the emission beam sweeps the observer’s line of sight only once per cycle, within a brief temporal span. This is when the pulsed radio emission can be observed.”

In contrast, the April 2020 FRB and subsequent, less energetic bursts exhibited random phases that did not coincide with the pulse window noted during the pulsar phase.

Zhang concludes, “This substantial incongruence suggests that pulses and bursts emanate from diverse locales within the magnetar’s magnetosphere, thereby potentially implying distinct emission mechanisms for these two phenomena.”

Implications for Cosmological FRBs

The meticulous scrutiny of a Galactic FRB source, as demonstrated in this research, bestows fresh insights into the enigmatic nature of FRBs dispersed across cosmological distances.

Numerous instances of cosmological FRBs—those transpiring beyond the bounds of our galaxy—have demonstrated a propensity to repeat. In certain cases, FAST has discerned thousands of recurrent bursts from a handful of sources. Extensive searches for periodicity on the order of seconds have been conducted using these bursts, yielding no definitive periods thus far.

According to Zhang, this casts uncertainty upon the prevailing conjecture that repeating FRBs are fueled by magnetars.

“Our revelation that bursts tend to emerge in random phases presents a natural explanation for the absence of periodicity in repeating FRBs,” Zhang asserts. “For reasons not yet fully comprehended, bursts exhibit a propensity to radiate in all directions from a magnetar, rendering the identification of periods from FRB sources an unattainable endeavor.”

Reference: “A radio pulsar phase from SGR J1935+2154 provides clues to the magnetar FRB mechanism” by Weiwei Zhu, Heng Xu, Dejiang Zhou, Lin Lin, Bojun Wang, Pei Wang, Chunfeng Zhang, Jiarui Niu, Yutong Chen, Chengkui Li, Lingqi Meng, Kejia Lee, Bing Zhang, Yi Feng, Mingyu Ge, Ersin Göğüş, Xing Guan, Jinlin Han, Jinchen Jiang, Peng Jiang, Chryssa Kouveliotou, Di Li, Chenchen Miao, Xueli Miao, Yunpeng Men, Chenghui Niu, Weiyang Wang, Zhengli Wang, Jiangwei Xu, Renxin Xu, Mengyao Xue, Yuanpei Yang, Wenfei Yu, Mao Yuan, Youling Yue, Shuangnan Zhang and Yongkun Zhang, 28 July 2023, Science Advances.
DOI: 10.1126/sciadv.adf6198

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More about Cosmic Phenomena

• Understanding Fast Radio Bursts (FRBs)
• Magnetars: The Most Magnetic Stars in the Universe
• Five-hundred-meter Aperture Spherical radio Telescope (FAST)
• UNLV Astrophysics Research
• Science Advances Journal