Observations at Atacama Large Millimeter/sub-millimeter Array (ALMA), involving study of a GRB at millimeter wavelengths, enabled its first-ever time-lapse movie of a cosmic explosion, which revealed a long-lasting reverse shock-wave from the explosion echoing back through the jets
A reverse shock ensues when material blasted away from a GRB by its jets, runs into the neighboring gas. This meeting slackens down the escaping material, sending a shock-wave back down the jet. Since jets are estimated to last no more than a few seconds, a reverse shock should be an equally short-lived event. But that now seems not to be the case.
Instead, the light from the reverse shock gleams most brightly at the millimeter wavelengths on periods of about a day, which is most likely why it has been so hard to detect hitherto. While the early millimeter light was produced by the reverse shock, the X-ray and visible light came from the blast-wave shock riding ahead of the jet. Understanding the shape and duration of the expenditure from the star is crucial for determining the correct energy of the burst.