NICER / ISS Science Nugget
for April 18, 2024

A flash triggers a multiwavelength hunt

On April 8, a bright, brief (lasting approximately 20 seconds) X-ray flash was detected by Einstein Probe, an astronomy satellite developed by the Chinese Academy of Sciences and its European partners, dedicated to finding transient X-ray sources and still in its commissioning phase. Follow-up observations within 48 hours with NICER and NASA's Swift observatory showed an X-ray source, now named EP240408a, already 500x fainter than the initial flash, with no ultraviolet counterpart. Comparison with archival X-ray images revealed no prior emission from the source direction, with an upper limit another 300x fainter still.

NICER began a monitoring campaign that indicated the source brightness was slowly varying, perhaps in a short-lived plateau phase, followed by a steady decline to an apparent leveling-off, although in the most recent data there are hints of re-brightening. Early speculation was that the initial flash might have been a thermonuclear burst from a newly active, previously unknown accreting neutron-star binary, so searches of the NICER data were carried out for pulsations, but none were found. The X-ray spectrum measured with NICER shows a suggestive break near 4 keV photon energy that is similar to those seen in line-of-sight "jets" emitted by some types of explosive events, such as gamma-ray bursts and stellar disruptions. In the meantime, ground-based observations identified an infrared counterpart, but the presence of a nearby bright star made spectroscopic measurements difficult; the Southern African Large Telescope (SALT) was able to obtain a spectrum that hints at an extragalactic origin.

These results were reported in Astronomer's Telegram #16589 by N. Rea (Inst. de Ciencias del Espacio, Spain) and collaborators. As of this writing, the nature of EP240408a remains mysterious, but the overall energetics and other available evidence suggest that it may be an unusually bright and long-lived afterglow of a long gamma-ray burst (GRB), the implosion of an extremely massive star likely forming a black hole, in a distant galaxy. Occasional NICER monitoring and ground-based optical observations continue, to look for additional spectroscopic clues and in the event the source brightens again.