GRB 060614

Light curve of GRB 060614.

GRB 060614 was a remarkable gamma-ray burst (GRB) detected by the Swift satellite on June 14, 2006 with puzzling properties, which challenge current progenitor models.

In particular, the lack of any bright supernova (SN) down to very strict limits and the vanishing spectral lags during the whole burst are typical of short GRBs, strikingly at odds with the long (102s) duration of this event and its origin in a galaxy 1.6 billion light years away in the constellation Indus.

As of December 2006, more than a dozen telescopes, including the Hubble Space Telescope and large ground-based observatories, have studied the burst.

Gamma-ray bursts represent the most powerful known explosions in the universe. Yet they are random and fleeting, never appearing twice. Scientists have only recently begun to understand their nature.

Such bursts typically fall into one of two categories, long or short. The long bursts last more than two seconds and appear to be from the core collapse of massive stars forming a black hole. Most of these bursts come from the edge of the visible universe. The short bursts, which are under two seconds and often last just a few milliseconds, appear to be the merger of two neutron stars or a neutron star with a black hole, which subsequently creates a new or bigger black hole.

The hybrid burst, called GRB 060614, after the date it was detected, originated from within a galaxy 1.6 billion light years away in the southern constellation Indus. The burst lasted for 102 seconds, placing it soundly in long-burst territory. But the burst lacked the hallmark of a supernova, or star explosion, commonly seen shortly after long bursts. Also, the burst's host galaxy has a low star-formation rate with few massive stars that could produce supernovae and long gamma-ray bursts. "This was close enough to detect a supernova if it existed," said Avishay Gal-Yam of Caltech, Pasadena, Calif., lead author on another Nature report. "Even Hubble didn't see anything."

Certain properties of the burst concerning its brightness and the arrival time of photons of various energies, called the lag-luminosity relationship, suggest that burst behaved more like a short burst (from a merger) than a long burst. Yet no theoretical model of mergers can support a sustained release of gamma-ray energy for 102 seconds. "This is brand new territory; we have no theories to guide us," said Gehrels. However, long GRBs from rapidly rotating black holes feature a viscous time-scale of tens of seconds of spin-down against high-density turbulent matter.^[1] As a candidate inner engine of long GRBs in mergers and collapsars alike, it can account naturally for long GRBs with and without supernovae and a diversity in X-ray afterglows representing different host environments, whose signatures in gravitational-waves fall within the bandwidth of sensitivity of LIGO-Virgo ^[2]

The burst is perhaps not unprecedented. Archived data from the Compton Gamma Ray Observatory in the 1990s possibly reveal other hybrid "long-short" bursts, but no follow-up observations are available to confirm this. Johan Fynbo of the Niels Bohr Institute in Copenhagen, also lead author on a Nature report, suggests that a burst from May of this year was also long, but had no associated supernova.

Scientists remain divided on whether this was a long-short burst from a merger or a long burst from a star explosion with no supernova. Most conclude, however, that some new process must be at play – either the model of mergers creating second-long bursts needs a major overhaul, or the progenitor star from an explosion is intrinsically different from the kind that make supernovae.

"We siphoned out all the information we could from GRB 060614," said Massimo Della Valle of the Osservatorio Astrofisico di Arcetri in Firenze, Italy, another lead author on a Nature report. "All we can do now is wait for the next nearby hybrid burst."

References

1. ↑ van Putten, M.H.P.M., & Ostriker, C.E., 2001, ApJ, 552, L31
2. ↑ van Putten, M.H.P.M., 2009, MNRAS Lett., doi:10.1111/j.1745.3933.2009.00666.x

External links

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Coordinates: 21^h 23^m 27.0^s, −53° 02′ 02″