[Paper Review] An X-Ray Burst from a Magnetar Enlightening the Mechanism of Fast Radio Bursts

M. Tavani, C. Casentini+61 more|arXiv (Cornell University)|May 25, 2020

Pulsars and Gravitational Waves ResearchPhysics and Astronomy8 references38 citations

TL;DR

The paper reports an X-ray burst detected by AGILE coincident with a bright radio burst from the Galactic magnetar SGR 1935+2154, constraining FRB–magnetar connections and energetics.

ABSTRACT

Fast radio bursts (FRBs) are short (millisecond) radio pulses originating from enigmatic sources at extragalactic distances so far lacking a detection in other energy bands. Magnetized neutron stars (magnetars) have been considered as the sources powering the FRBs, but the connection is controversial because of differing energetics and the lack of radio and X-ray detections with similar characteristics in the two classes. We report here the detection by the AGILE satellite on April 28, 2020 of an X-ray burst in coincidence with the very bright radio burst from the Galactic magnetar SGR 1935+2154. The burst detected by AGILE in the hard X-ray band (18-60 keV) lasts about 0.5 seconds, it is spectrally cutoff above 80 keV, and implies an isotropically emitted energy ~ $10^{40}$ erg. This event is remarkable in many ways: it shows for the first time that a magnetar can produce X-ray bursts in coincidence with FRB-like radio bursts; it also suggests that FRBs associated with magnetars may emit X-ray bursts of both magnetospheric and radio-pulse types that may be discovered in nearby sources. Guided by this detection, we discuss SGR 1935+2154 in the context of FRBs, and especially focus on the class of repeating-FRBs. Based on energetics, magnetars with fields B ~ $10^{15}$ G may power the majority of repeating-FRBs. Nearby repeating-FRBs offer a unique occasion to consolidate the FRB-magnetar connection, and we present new data on the X-ray monitoring of nearby FRBs. Our detection enlightens and constrains the physical process leading to FRBs: contrary to previous expectations, high-brightness temperature radio emission coexists with spectrally-cutoff X-ray radiation.

Motivation & Objective

• Motivate the magnetar–FRB connection by presenting a coordinated X-ray and radio burst event.
• Quantify the X-ray burst properties and energetics to assess compatibility with FRB models.
• Discuss implications for repeating FRBs and the role of magnetars with ~10^15 G fields.

Proposed method

• Report and analysis of the AGILE satellite detection of an X-ray burst in the 18–60 keV band.
• Characterization of burst duration (~0.5 s) and spectral cutoff above ~80 keV.
• Estimate of isotropic emitted energy (~10^40 erg) for the X-ray event.
• Contextual discussion linking the X-ray burst to FRB-like radio emission from the magnetar SGR 1935+2154.

Experimental results

Research questions

• RQ1 Can a magnetar produce X-ray bursts coincident with FRB-like radio bursts and what are the energetics involved?
• RQ2 What do such multi-wavelength detections imply about the emission mechanisms of FRBs, especially for nearby repeating FRBs?
• RQ3 Are magnetars with ~10^15 G fields capable of powering the majority of repeating FRBs based on energetics?

Key findings

• An X-ray burst in the 18–60 keV band lasted ~0.5 s and was spectrally cutoff above ~80 keV.
• Isotropic emitted energy for the X-ray burst is ~10^40 erg.
• The event demonstrates, for the first time, that a magnetar can produce X-ray bursts in coincidence with FRB-like radio bursts.
• The detection supports a magnetar–FRB connection and suggests FRBs associated with magnetars may emit X-ray bursts of both magnetospheric and radio-pulse types.
• Nearby repeating-FRBs offer a testbed to consolidate the FRB–magnetar link and motivate further X-ray monitoring.