Is the image at left an accurate depiction of what triggers at least some of the gamma-ray bursts we’re now detecting? Or is it a model now in need of serious revision? We’re looking at an artist’s conception of the merger of two neutron stars, an event that produces gamma rays (note the jets emanating from the center). Such a scenario may be the cause of short gamma ray bursts (GRBs). But NASA’s Swift satellite and the Gemini Observatory (Hawaii) have detected one such burst that takes us farther back in time than ever before, some 7.4 billion years. And therein lies a tale.
GRB 070714B was detected last July 14, the second burst of the day (note the terminal B in the designation). Short bursts are those lasting less than three seconds, the most popular theory for their formation being neutron star merger and collapse into a black hole, with consequent ejection of energy. Such bursts are obviously tricky to study because their short duration calls for immediate follow-up with optical telescopes. In this case, the 4-meter William Herschel Telescope was able to locate the optical afterglow at the burst location.
Image: One possibility for the GRB 070714B event, the merger of two neutron stars. Credit: NASA/Dana Berry.
With the host galaxy now identified, researchers could use the Gemini North telescope to reveal the spectral line of ionized hydrogen in that galaxy, tagging its redshift at 0.92. That makes GRB 070714B half the age of the universe. Thus we can move the era during which we know short GRBs were a factor significantly back in time. In fact, this burst is almost twice as distant as the short GRB previously considered the record holder.
Nor is this burst, which occurred in Taurus, average in any other way. Its energy is about one hundred times the norm for short bursts, a level more typical of a long GRB (duration greater than three seconds). Does it make sense for there to be such variation in the neutron-star merger model? Says Swift lead scientist Neil Gehrels (NASA GSFC):
“It is unclear whether another mechanism is needed to explain this explosion, such as a neutron star-black hole merger. Or it could be that there are a wide range of energies for neutron star-neutron star mergers, but that seems unlikely.”
The take on long GRBs is increasingly that they are the result of the collapse and subsequent explosion of massive stars. But the neutron star smash-up is only one of the possible models designed to explain the short bursts. Did one of the beams generated by the event happen to be aimed directly at our planet? It’s a possibility, and would account for the detection of more powerful energies than would otherwise have been the case. But we know so little about the mechanism at play here that short GRBs should provide fertile ground for research for some time to come.