News

Origins of gold traced to extreme explosions of collapsed stars

May 1, 2025
admin


Gold may not come from where you think. According to new research, some of the gold found in objects like phones and laptops could have been forged in violent magnetar explosions billions of years ago.

In December 2004, the magnetar SGR 1806-20, 28 400 light-years away, emitted a giant flare observed by NASA’s RHESSI and ESA’s INTEGRAL satellites. A delayed gamma-ray signal, peaking 10 minutes later, indicated rapid neutron capture (r-process) producing gold and other heavy elements.

“It’s answering one of the questions of the century,” said Anirudh Patel, lead author and doctoral student at Columbia University.

The study, published in The Astrophysical Journal Letters, used 20-year-old archival data from NASA and ESA telescopes to estimate that the flare ejected one-millionth of a solar mass of r-process material. This included gold, silver, and uranium, contributing 1% to 10% of the galaxy’s heavy element inventory. The flare’s energy reached 1046 erg in less than a second.

Schematic of the three high-energy emission phases from the 2004 SGR 1806–20 flare: a prompt ≲1 s gamma-ray spike, a minutes-long spin-modulated X-ray tail, and a delayed MeV component peaking at ~600–800 s and fading over hours, possibly powered by r-process material from crustal ejection during magnetic reconnection. Image credit: Direct Evidence for r-process Nucleosynthesis in Delayed MeV Emission from the SGR 1806-20 Magnetar Giant Flare – Anirudh Patel et al.

Magnetars, neutron stars with magnetic fields trillions of times stronger than Earth’s, trigger giant flares when their crust fractures. This ejects material at 10% the speed of light, enabling r-process synthesis of nuclei like gold, with atomic masses above 130. The process involves an alpha-rich freeze-out in the ejected material.

The delayed gamma-ray emission, detected from 400 to 12 000 seconds, showed a power-law decay index of 1.2, matching r-process decay predictions. Its fluence, 9 × 10-4 erg cm-2, supports models of gold and heavy element production.

“It was noted at the time, but nobody had any conception of what it could be,” said co-author Eric Burns on the signal’s initial mystery.

Example ∼MeV gamma-ray light curve from r-process ejecta (Patel et al. 2025) compared to late-time emission from the 2004 SGR 1806–20 flare (INTEGRAL/ACS), showing a decay ∝ t−1.2 t^{-1.2}t−1.2 and cumulative fluence consistent with ACS and RHESSI data for a source distance of 8.7 kpc.Image credit: Direct Evidence for r-process Nucleosynthesis in Delayed MeV Emission from the SGR 1806-20 Magnetar Giant Flare – Anirudh Patel et al.

Konus-WIND data measured a fluence of 2 × 10-4 erg cm-2 in the 80–750 keV range, while RHESSI recorded thermal bremsstrahlung at 1.9 MeV. SkyNet simulations confirmed the gamma-ray signature as r-process decay.

Magnetar flares, unlike delayed neutron star mergers, occur soon after star formation, potentially seeding early stars with gold and other elements. This could explain their presence in metal-poor stars. Only three such flares have been observed in our galaxy and the Large Magellanic Cloud.

Top panel: mass fraction of synthesized nuclei as a function of atomic mass A from our fiducial model that reproduces the late-time gamma-ray emission light curve from SGR 1806–20 (Figure 2). Vertical blue bars (right axis) show the individual percentage contributions of radioactive nuclei to the total gamma-ray energy in the time interval t = 103–104 s. Bottom panel: synthetic gamma-ray spectra from the fiducial model at three snapshots, t = 1000, 3000, 12,000 s, accounting for Doppler broadening due to the ejecta expansion but excluding extinction effects. For the spectrum at t = 3000 s, we also show with lighter blue lines the intrinsic (i.e., nonbroadened) decay lines, which contribute to the total spectrum. A thick dark gray line shows the bremsstrahlung spectral fit made by S. E. Boggs et al. (2007) in the energy range ≈ 0.1–2.5 MeV, while a dotted light gray line shows the same fit outside of the measured energy range. The gray shaded area indicates the ±0.7 MeV uncertainty in the fit.Top panel: mass fraction of synthesized nuclei as a function of atomic mass A from our fiducial model that reproduces the late-time gamma-ray emission light curve from SGR 1806–20 (Figure 2). Vertical blue bars (right axis) show the individual percentage contributions of radioactive nuclei to the total gamma-ray energy in the time interval t = 103–104 s. Bottom panel: synthetic gamma-ray spectra from the fiducial model at three snapshots, t = 1000, 3000, 12,000 s, accounting for Doppler broadening due to the ejecta expansion but excluding extinction effects. For the spectrum at t = 3000 s, we also show with lighter blue lines the intrinsic (i.e., nonbroadened) decay lines, which contribute to the total spectrum. A thick dark gray line shows the bremsstrahlung spectral fit made by S. E. Boggs et al. (2007) in the energy range ≈ 0.1–2.5 MeV, while a dotted light gray line shows the same fit outside of the measured energy range. The gray shaded area indicates the ±0.7 MeV uncertainty in the fit.
Top: Mass fractions and radioactive nuclei contributions to gamma-ray energy (10³–10⁴ s) from the fiducial model matching SGR 1806–20; bottom: synthetic spectra at 1000, 3000, and 12,000 s with Doppler broadening, overlaid with the bremsstrahlung fit from Boggs et al. (2007) and its ±0.7 MeV uncertainty. Image credit: Direct Evidence for r-process Nucleosynthesis in Delayed MeV Emission from the SGR 1806-20 Magnetar Giant Flare – Anirudh Patel et al.

The flare’s ejecta may accelerate heavy nuclei, including gold, into cosmic rays, potentially surpassing other sources. Hydrodynamical simulations supported the ejection mechanism.

“It’s very cool to think about how some of the stuff in my phone or my laptop was forged in this extreme explosion,” Patel said.

NASA’s COSI mission, launching in 2027, will aim to detect specific isotopes in future flares, enhancing gold production studies. A UV/optical signal from these events may also be observable.

References:

1 Direct Evidence for r-process Nucleosynthesis in Delayed MeV Emission from the SGR 1806-20 Magnetar Giant Flare – Anirudh Patel et al. – The Astrophysical Journal Letters – April 29, 2025 – – OPEN ACCESS

2 Where Does Gold Come From? NASA Data Has Clues – NASA – April 29, 2025




Source link