Cosmic Flashes We Don't See in Full express
A gamma-ray burst is a short but incredibly powerful explosion in a distant galaxy. In a few seconds, it releases as much energy as the Sun will emit over billions of years. The Neil Gehrels Swift satellite catches such flashes with a special telescope called BAT, which detects gamma rays.
Astronomers decided to test what would happen if the same burst occurred farther away. They took 26 bright bursts with measured distances and, using a computer, 'pushed' them several times farther. This created artificial distant bursts, like watching the same fireworks from a greater distance.
It turned out that for distant bursts, the measured duration becomes shorter than the true one. The energy is also underestimated — sometimes by a factor of two, and the duration even by tens of times. Moreover, each burst behaves differently: it all depends on the shape of its light curve — how exactly the brightness changed over time.
Then the artificial distant bursts were compared with real distant ones already observed by Swift. They turned out to be similar. This means that distant bursts appearing shorter and fainter is not a property of the explosions themselves, but simply an effect of distance. We see only the tip of the iceberg, while most of the flash remains underwater.
Original abstract
as written by the authors arXiv:2602.03032
We investigated how Gamma-Ray Burst (GRB) prompt emission measurements are affected by increasing distance to the source. We selected a sample of 26 bright GRBs with measured redshifts $$z<1$$ observed by the Burst Alert Telescope (BAT) on board the Neil Gehrels Swift Observatory (Swift) and simulated what BAT would have observed if the GRBs were at larger redshifts. We measured the durations of the simulated gamma-ray signals using a Bayesian block approach and calculated the enclosed fluences and peak fluxes. As expected, we found that almost all durations (fluences) measured for simulated high-$$z$$ GRBs were shorter (less) than their true durations (energies) due to low signal-to-noise ratio emission becoming completely dominated by background, i.e., the ``tip-of-the-iceberg'' effect. This effect strongly depends on the profile and intensity of the source light curve. Due to the uniqueness of GRB light curves, there is no common behavior in the evolution of measured durations with redshift. We compared our synthetic high-$$z$$ (i.e., $$z>3$$) GRBs to a sample of 72 observed high-$$z$$ bursts and found that the two samples were not inconsistent with being drawn from the same underlying population. We conclude that: (i) prompt emission durations (fluences) of high-$$z$$ GRBs observed by Swift/BAT are most likely underestimations, sometimes by factors of $$\sim$$several tens ($$\sim2$$), and (ii) changes in the average GRB prompt emission duration and fluence with increasing redshift are consistent with the tip-of-the-iceberg effect.
The most distant known gamma-ray burst came from a galaxy whose light traveled for more than 13 billion years — almost as long as the Universe has existed.
In Carl Sagan's novel 'Contact,' the protagonist picks up an extraterrestrial signal that at first seems like noise, just as astronomers separate the faint tail of a gamma-ray burst from the background.