NICER / ISS Science Nugget
for May 29, 2025

Ashes in the wind

Where a black hole has an event horizon, a neutron star has a surface. This distinction becomes most important in binary systems where the object's strong gravity draws matter from a companion star. Instead of plunging into the unfathomable spacetime beyond the event horizon, matter accreting onto a neutron star accumulates, sometimes after being channeled onto a small area by the star's strong magnetic field. The result, in a crushing gravitational environment, is often a thermonuclear runaway, a fusion-driven explosion that combines light elements (hydrogen, helium, sometimes carbon) into heavier ones. This is the origin of so-called Type I X-ray bursts, and probing the fusion process, the elemental makeup of the accreted fuel, and other aspects of nuclear burning in extreme physical environments has been a long-sought goal. A key approach involves measuring the distribution of X-ray photon energies during the rise, peak, and decay of Type I bursts to search for signs of spectral features - emission and/or absorption lines - that can be ascribed to the presence of specific atomic elements. A few tantalizing detections of spectral lines have been reported, with NICER and with earlier telescopes, but many questions remain.

In a peer-reviewed paper recently accepted for publication in The Astrophysical Journal, G. Jaisawal (Technical Univ. of Denmark) and collaborators analyze 15 Type I bursts captured by NICER, between 2017 and 2021, from the binary system 4U 1820-30, which has an orbital period of just 11.4 minutes. These bursts typically last just a few seconds (a decaying glow can last up to a minute), but the team also examined NICER data from the aftermath of an especially energetic "superburst," lasting a couple of hours, detected by JAXA's MAXI payload in August 2021, which NICER began observing two ISS orbits after the burst peak. The team reports finding spectral lines in 12 of the short-duration bursts: emission near 1 keV, and absorption at three energies that can be identified as arising from silicon, argon, and calcium atoms. Small shifts in the absorption line energies are also evident as the bursts progress, a combination of "blue shifts" to higher energies because the explosion debris is moving toward us at relativistic velocities, and red-shifts because the absorbing material is still deep in the neutron star's gravitational well. Similar absorption features are seen in the long-burst aftermath, where the accretion disk is quickly reconstituted after being disrupted by the explosion. In both scenarios - short and long bursts - the evidence points to nuclear-burning ashes that have been transported from the neutron star's surface to its near environment by a "mushroom cloud." The atomic identifications and relative abundances of the ashes appear to be consistent with predictions from nuclear-burning theory.