The Shape of the Brightest Burst
On October 9, 2022, a cosmic explosion, now known as GRB 221009a, blasted earth with dangerous high-energy radiation. This blast came from a gamma-ray burst, the most powerful class of cosmic explosions. Astronomers believe that this particular gamma-ray burst was produced by the death of a star a few dozen times more massive than the Sun. The star evidently ran out of fuel and collapsed into a black hole some two billion years ago. The collapse of this star produced a tremendous outburst and powerful beam of high energy particles and radiation pointed, as it happened, right at us. As the X-rays from the burst made their two-billion year journey to earth, they were scattered by dust sheets within the Milky Way, producing weird concentric rings of X-rays centered on the burst. GRB 221009a got the attention of the entire astronomical community, and it was observed by nearly every operating high-energy telescope (and many ground based facilities as well). For the first time, the fleet of high energy space observatories included IXPE, the Imaging X-ray Polarimeter Explorer. IXPE is able to determine the polarization, or plane of vibration, of the electromagnetic waves of X-rays produced by extreme objects like exploding stars, black holes and other powerful cosmic phenomena. Polarization therefore provides important information of the geometry of these sources, information that's impossible to obtain in any other way. IXPE began to observe GRB 221009a only two days after the outburst. The image above shows the concentric rings of scattered X-rays from GRB 221009a as seen by IXPE, on three days from October 11 to October 14 (note that the bright emission from GRB 221009a at the center has been removed). IXPE resolved 2 distinct rings at a distance from earth of about 47,000 lightyears for the inner ring and about 12,000 lightyears for the outer ring. The IXPE images show the fading of the ring. Because the rings are produced by the time-delayed scattered radiation from the burst, they help reveal how the intrinsic emission of the burst changes with time. Analysis of the IXPE polarization data from the burst constrains the width of the jet to be greater than 1.5 degrees, and helps determine the orientation of the magnetic field which constrains the jet as well.
The IXPE View of GRB 221009A ~ Michela Negro et al
- Astrophysical Journal Letters 946(1):L21 (2023 Mar 20) DOI: 10.3847/2041-8213/acba17
- arXiv > astro-ph > arXiv:2301.01798 > 04 Jan 2023 (v1), 09 Feb 2023 (v2)
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