Hybrid GRB 060614: A Long Gamma-Ray Burst Without a Supernova





An HST image of the location of GRB 060614

Credits: image designed by K. Sharon and A. Gal-Yam and produced by K. Sharon using HST images obtained as part of programs 10551 (PI: Kulkarni) and 10624, 10917 (PI: Fox).
These data were published in Nature by Gal-Yam et al. on Dec. 21, 2006.
Embargoed for Release to the popular press at 10 a.m. PST, Wednesday, December 20, 2006



Gamma-Ray Bursts (GRBs) are bright flashes of gamma-ray radiation which are detected, on average, once every day by spacecraft equipped with detectors designed to search for such events. Scientists became curious about the nature of GRBs as these were the brightest sources of Gamma-ray radiation in the Universe. This curiosity intensified when it became clear that GRBs were also very distant, and that the amounts of energy involved in these explosions are huge - apparently similar to the energy one would obtain by converting the entire mass of a star like our sun into pure energy (as implied by Einstein's famous equation E=mc2) and radiating all this energy as gamma-rays. What could be the source of such violent events?

To further complicate the mystery, it turns out that there are two kinds of GRBs: the long ones, which are usually longer than 2 seconds or so, and the short ones, which are, as you'd guess, typically shorter than 2 seconds. During the last 8 years, accumulated evidence indicated that the class of long GRBs (which are the more numerous) come from very massive stars (each one many times more massive than our sun) which, at the end of their short lifetime of but a few million years, explode in a process called a supernova. A small fraction of these supernova explosions of massive stars also make long GRBs.
The nature of the short class of GRBs remained a mystery for many years, until discoveries last year showed that these explosions are not related to massive stars exploding as supernovae, like their longer relatives. Instead, so it appears, short GRBs may happen when two objects called neutron stars (each one a bit more massive than the sun, but only 10 kilometers (6 miles) or so in size) merge together, to form an even more bizarre object, a black hole. Or perhaps, some thought, a previously-formed black hole swallowed a neutron-star companion.
Whatever the case, for a few happy months in 2006 it appeared like the mystery of GRBs was finally solved - long GRBs came from SNe, and short GRBs happened when two compact objects (neutron stars or a neutron star and a black hole) merged. And then ...

GRB 060614 was a long GRB discovered by the Swift space mission on June 14, 2006. This event occurred in a relatively nearby galaxy, slightly less than 2 billion light years away (which is a small distance for events of this type). Over 100 seconds in duration, this burst was surely among the members of the long class of GRBs. And so, everyone thought, it must come from a supernova, which at such nearby galaxy was expected to be bright and easily detectable. However, as days went by, observations from around the world all pointed toward a surprising fact - the expected supernova was just not there. What's going on? Could it be that some massive stars that make long GRBs also undergo a supernova explosion (as seen so far) but some others make a long GRB but not the supernova? Or maybe merging neutron stars, which are expected to make short GRBs lasting for a second or so, without a supernova, can also make much longer GRBs like GRB 060614? Whatever the answer, it was clear that GRB 060614 is telling us that the GRB puzzle is not solved yet.

Among our group of GRB astrophysicists a debate broke out. It looked like the Hubble Space Telescope (HST) would be the most useful tool to study this puzzling burst. But how should we do it? Caltech's Shri Kulkarni has an approved program to study long GRBs with Hubble - and with a duration of 102 seconds, GRB 060614 was definitely long. But long GRBs have supernovae, and this burst did not ... so maybe it is more related to the short GRBs? Our colleague Derek Fox at Penn State University has an approved program to study short GRBs - maybe this program should be activated? At the end, it was my suggestion that prevailed - we should combine the resources of both HST programs to study this hybrid long/short GRB. Having gained the approval of the managers of the Hubble Space Telescope at the Space Telescope Science Institute (STScI), located in the Johns Hopkins University campus in Baltimore, we set about to design our observing campaign of GRB 060614.


We wanted to observe GRB 060614 with Hubble for two main reasons. First, Hubble's sensitivity should allow us to search for a supernova that accompanied this long GRB even if it emitted much less light than the supernova events we have seen before. Maybe this long GRB has a supernova after all, we thought, it was just so faint that everyone were missing it when they were using telescopes on the ground. Our second target was to study the galaxy in which GRB 060614 occurred. This was a midget galaxy, perhaps 100 times smaller than our own majestic Milky-Way Galaxy. From the ground, this small galaxy looked like a featureless little blob. But with Hubble we would be observing from space, above the atmosphere's blurring effects, and Hubble's superb resolution, we hoped, would allow us to resolve this tiny galaxy and look for clues about the puzzling GRB 060614 from its environment. This seemed to us like a promising plan.

However, when we were finally about to begin our observing sequence, we ran into an unexpected obstacle. We planned to observe GRB 060614 with Hubble's newest, most sensitive instrument: the Advanced Camera for Surveys (ACS). However, just a few days after GRB 060614 was detected, and before we managed to get our first observations, the ACS malfunctioned, and all science observations with the new camera were postponed. What shall we do now? It may take weeks to fix the camera remotely, as the Hubble Space Telescope floats high up in space, but supernovae quickly decay away, and it may be too late to search for the one that may be responsible for GRB 060614, by the time ACS is back on line. In the GRB game faster is better, we well knew, and so, with dedicated help from STScI's science team, we called upon another of HST's suite of instruments - the old, almost retired Wide Field and Planetary Camera 2 (WFPC2). This old, venerable instrument has served as Hubble's main work horse for years, but has been used only rarely since the more sensitive ACS has been installed. Well, we needed its services again - and it performed just great. We observed GRB 060614 with WFPC2 late in June, and got excellent data. By the time we needed to observe it again ACS was back on line, having been remotely fixed by NASA engineers, and we got some more observations of the GRB location with this sensitive camera as well. Comparing our observations obtained shortly after the GRB and data we continued to accumulate during the coming months (with the fixed ACS performing perfectly) we could undertake a sensitive search for a supernova associated with the long GRB 060614. If it was there at all, even if very faint - we were sure we'll be able to see it.


HST images of the location of GRB 060614. We use a negative representation here, so black objects are actually sources of light. The first images (on the left), obtained with the WFPC2 camera, show the afterglow, a short-lived burst of visible light which is emitted following a Gamma-Ray Burst. The location of the afterglow tells us where exactly the GRB happened, and where we should look for the signs of a supernova. The middle panels show images obtained about two weeks later with ACS, which should have shown a bright supernova inside the empty circle (but don't). These images and additional we obatined later on show that there was no supernova associated with this GRB. On the right you see the result of a digital subtraction: what you get when you subtract the light in the middle panels from the light in the left panels. Constant sources of light (like the host galaxy of the GRB) are removed by this process, but the afterglow (which is only visible early on) remains in the difference image.

Well - we did not find any evidence for a supernova explosion or even to an event that is a hundred times fainter than supernova explosions previously associated with long GRBs. In fact, any supernova ever observed by anyone would have been seen - so we could definitely say that GRB 060614 had no supernova, at least, nothing like any we have ever seen before.

How about the galaxy in which GRB 060614 happened? You can see its image at the top of this page, and the red crosshairs tell you where the GRB happened, and where no supernova is seen. Well, our Hubble images and additional data we got from the Gemini observatory tell us that it is quite different from many other galaxies in which long GRBs happened, which were also studied using Hubble by Andy Fruchter at STScI and his collaborators. Usually, long GRBs happen in galaxies that have lots of young, massive stars. GRB 060614 occurred in a galaxy with few massive stars, and appears, as you can see in the image above, to have happened quite far away from where most of those few massive stars that do exist in that small galaxy live - at its center. It's just the wrong neighborhood for a long GRB.

So what have we learned from it all? We found a long GRB with no supernova. If it comes from a massive star like other long GRBs (that do have supernovae), it must be a different kind of massive star, one that lives, so to speak, in a different neighborhood than the stars that make the "usual" long GRBs, and that explodes quite differently, without a supernova. Or maybe this GRB also comes from two neutron stars or a neutron star and a black hole like the members of the short GRB class? These have no supernova, and do live in galaxies with few massive stars ... but theoretical models tell us that merging neutrons stars will only radiate for a second or so, and GRB 060614 lasted for over 100 seconds - so we would need to change our theories ... Or perhaps this is the first example of a new kind of GRBs, coming neither from massive stars that explode as supernovae, or from merging binary compact objects, but from something totally new. Anyway you look at the data we collected, we concluded, there must be a new process at work behind GRB 060614. So the GRB puzzle not quite solved ... not yet, at least.


Links:

  • Final versions of the paper and supplementary information.
  • Caltech press release .
  • NASA press release .
  • NASA/Goaddrad feature story and the Swift version.
  • Nature News and Views .
  • ANU press release .
  • Additional papers about this burst published in the same volume by Gehrels et al., Fynbo et al. and Della Valle et al. .
  • Color images of the location of GRB 060614 can be found here and here.


    Team:

    Avishay Gal-Yam (Caltech)
    D. B. Fox (Penn State)
    P. A. Price (IfA)
    E. O. Ofek (Caltech)
    M. R. Davis (SDSU)
    D. C. Leonard (SDSU)
    A. M. Soderberg (Caltech)
    E. Nakar (Caltech)
    B. P. Schmidt (ANU)
    K. M. Lewis (ANU)
    B. A. Peterson (ANU)
    S. R. Kulkarni (Caltech)
    E. Berger (OCIW/Princeton)
    S. B. Cenko (Caltech)
    R. Sari (Caltech)
    K. Sharon (TAU)
    D. Frail (NRAO)
    D.-S. Moon (U. Toronto)
    P. J. Brown (Penn State)
    A. Cucchiara (Penn State)
    F. Harrison (Caltech)
    T. Piran (HUJI)
    S. E. Persson (OCIW)
    P. J. McCarthy (OCIW)
    B. E. Penprase (Pomona)
    R. A. Chevalier (U. Virginia)
    A. I. MacFadyen (IAS and NYU)



    Constructed: December 2006, by: Avishay Gal-Yam , E-Mail: avishay@astro.caltech.edu and Keren Sharon , E-Mail: kerens@wise.tau.ac.il