by E.H. Shepard from A. A. Milne's Winnie-the-Pooh books
Can we detect gravitational waves and/or neutrinos from gamma-ray bursts?
Barbara Jo Mattson
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by E.H. Shepard from A. A. Milne's Winnie-the-Pooh books |
Can we detect gravitational waves and/or neutrinos from gamma-ray bursts?
Barbara Jo Mattson |
Two types of astrophysics that are just beginning to come into their own are gravitational wave and neutrino astrophysics. The LIGO I gravitational wave (GW) detector is in the commissioning phase and will begin its science run in 2002. A large-volume neutrino detector in the Antarctica ice, AMANDA-B10, began taking data in 1997 and construction is underway for a larger detector (AMANDA II, and eventually a much larger detector, IceCube).
Gamma-ray bursts (GRBs) are one of the hot topics in high energy astronomy right now. These mysterious events occur about once per day in a random part of the sky. The long bursts (those with a duration longer than about 30 seconds) have been seen to have an "afterglow" that starts in the gamma-ray/X-ray region of the EM spectrum and falls down to the radio several weeks later. Redshift measurements have shown that these long bursts are coming from cosmological distances, making the energy output of GRBs enormous (a single GRB is the brightest high energy source we witness, paled only by the Big Bang itself).
Current theories of GRB formation include models which should produce both gravitational waves and neutrinos. The question, then, is whether this current/up-coming generation of gravitational wave and neutrino detectors will be able to detect events from a GRB.
Can we detect the GWs emitted in such a merger with the current first generation GW detectors? Unfortunately, the answer is no -- not directly. Lee Samuel Finn from Penn State recently (20 June 2001) gave a talk at Goddard's Laboratory for High Energy Astrophysics. He demonstrated that for GWs emitted during a merger, the signal would be several orders of magnitude too small to be detected by either LIGO I or LIGO II.
However, this does not mean that this first generation of GW detectors can not be used to study GW from GRBs. In fact, once both LIGO detectors are running (or even using LIGO I and one of the other GW detectors), it should be possible to show statistically that GW are produced in GRBs. The general idea is to perform a cross-correlation of the signals from two different detectors for both on- and off-source detections (the off-source detections would not have to be time-coincident with the burst itself). By building up a number of observations (hundreds of burst observations), it could be shown to a 95% confidence that GW are emitted in the gamma-ray burst formation process and information could be gained about when in relation to the burst these GWs are emitted.
The brochure for LISA (a space-based GW telescope/detector currently scheduled for a 2011 launch) indicates that it will observe GW from Galactic binary system mergers, so it would seem that LISA will not have the sensitivity to detect GWs from similar systems billions of light years away, so direct detection of GW from GRB will be a long time in coming.
Are there enough neutrinos of the right energy to be detected in the current and up-coming generation of neutrino detectors? In a paper published on the web, Gilles Barouch et al. quotes Monte Carlo results for the AMANDA array in the Antarctic. They found that this rate depends strongly on the bulk Lorentz factor (G) of the expanding fireball. If all bursts were detected, AMANDA-B10 array would expect to detect 18 events for G = 10, 0.4 events for G = 300 and 0.003 events for G = 1000. (A note on the AMANDA array: the array consists of strings of detectors deposited deep in the Antarctic ice. Each season more strings are implanted into the ice, and at the end of the season, the current array can be used to detect neutrinos until the next drilling season. AMANDA-B10 refers to the detector which ran during the austral winter 1997.) This does not take into account the zenith angle dependence of detecting an event (so the actual numbers are significantly smaller). Since, to produce the observed gamma-ray burst, G must be larger than 300, the prospects of observing GRB neutrino events with AMANDA do not look promising.
Larger neutrino detectors are currently being planned -- for example, IceCube, a one-kilometer cubed detector in the Antarctic ice. Since the detection of neutrinos depends strongly on the volume of the detector material (neutrinos do not interact with matter very often, so the more stuff you can watch, the higher the probability that you will see an event), these new detectors should have the capability to detect more neutrino events from GRBs. According to the NSF proposal for IceCube , using a standard event calculation, the event rate for a km3 neutrino detector is 50 events per year with "a hard spectrum extending well beyond the atmospheric background and, even more important, some of the high-energy GRB neutrino events should coincide with observed GRB photon events within a narrow time window." In fact, the rate may be as much as 200 events per year, if the non uniformity of GRBs is taken into account.
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Created: 03 July 2001