Einstein Probe catches ten minutes of soft X-rays after a sub-second gamma-ray burst

Short gamma-ray bursts are thought to come from two compact objects, such as neutron stars, spiralling together and merging, and those events also produce gravitational waves ^[5]. For decades astronomers have found them mainly through the gamma-ray flash ^[14]. The soft X-rays at the start have been hard to catch, because most narrow-field X-ray telescopes need a gamma-ray alert to locate a burst before they can turn toward it ^[11].

"The event initially appeared to be an ordinary short GRB," said An Li, who began the analysis as soon as Einstein Probe's onboard alerts came in ^[8][9]. "However, instead of fading away, the source continued emitting episodes of soft X-rays for nearly ten minutes," Li said ^[10]. Nearly ten minutes behind a flash shorter than half a second is a tail roughly a thousand times the length of the burst ^[15].

Its spectrum explains why earlier missions would have missed it. "Although this long-lasting emission carried substantial energy, its spectrum was so soft that, for a burst at this typical cosmological distance, it would have remained below the detection threshold of conventional gamma-ray instruments, such as Swift's Burst Alert Telescope," said Bin-Bin Zhang of Nanjing University, a co-corresponding author ^[1]. This is the team's estimate of what another instrument would have recorded, worked from this burst's spectrum and distance ^[1].

Tying the X-rays to a merger took a campaign at X-ray, optical and radio wavelengths ^[3]. Spectroscopy from Eleonora Troja's group at the University of Rome Tor Vergata supplied the redshift ^[4]. "They not only allowed us to identify and study the burst's host galaxy and measure its distance, but also enabled us to rule out an accompanying supernova and provide strong evidence linking this extraordinary X-ray emission to a compact object merger," Troja said ^[4]. With no supernova, the combined observations pointed to a compact object merger and away from another type of stellar explosion ^[6].

Yi-Han Iris Yin of the University of Hong Kong, who led the high-energy analysis, based the central-engine reading on three observations: rapidly changing brightness, an evolving spectrum, and later X-ray and optical afterglows ^[12]. Each points to activity continuing after the short flash had faded ^[12].

The thing this doesn't tell you is how often it happens. The sample is one burst. Zhang said the observations show that "what appears to be a typical short gamma-ray burst can actually conceal a much longer and richer episode of activity at soft X-ray energies" ^[2]. With a single event, "can" is as far as the data reach. The proposed pairing with gravitational-wave signals depends on that rate ^[16]. The release does not report a gravitational-wave detection for this burst.