PHYSICS ANALYSIS

In parallel with the detector characterization studies mentioned above, we plan to begin building some algorithms useful for physics analysis. One area of analysis which our group will pursue is the search for gravitational waves associated with gamma ray bursts.

Gamma ray bursts (GRBs), first serendipitously discovered in the 1960s [11] remain largely unexplained. Observations by the BATSE detector, aboard the Compton Gamma Ray Observatory, established that the distribution of GRBs was isotropic, implying that progenitors of GRBs occur at cosmological distances [12]. GRBs detected by BATSE occur at a rate of approximately one day.

Recent observation of optical transients associated with some GRBs have given redshifts to the GRB of between z = 0.8 and z = 4.0 [13]. It should be noted that not all bursts can be associated with optical transients. This opens the possibility that there is more than one class of GRB. The optical transients which have been observed are consistent with the "fire ball" model of gamma ray production by an electron photon plasma, but the observations are not yet able to constrain the progenitors. Possibilities include the death of massive stars and the merger of two massive objects such as neutron stars [14]. The observation of the optical transients also show that the progenators of the GRB, while associated with galaxies, do not necessarily occur at the galactic center and therefore have a distribution characteristic of objects of stellar masses. If GRBs are due to the mergers of neutron stars (NS/NS) or black holes and neutron stars (BH/NS), gravitational radiation in the frequency band of LIGO should be produced. The challenge for the experimentalist is that the distance to the known GRB progenitors is typically a factor of 100 to 500 more than the 15 Mpc initial sensitivity of LIGO to NS/NS binaries without additional coincidence information.

Fortunately the added constraints given by the time and angular position of the GRB can considerably improve the sensitivity of LIGO to gravitation radiation. For a BATSE detection, the direction of the GRB is determined with an error of better than 2 degrees [15]. For those GRBs originating in directions expected to give the largest LIGO signals, the difference in arrival times of the gravity waves at the two LIGO sites can be determined with an accuracy of better than 0.5 ms. The use of this information to help identify gravitational radiation coincident with GRBs has already been discussed in Ref. [16]. For these optimally positioned sources the gravitational signal will be larger, corresponding roughly to the difference between hSB and hrms lines shown in Figure 1. The gravitation strain will also be larger by a factor of 2 to 5 if the GRBs are due to (BH/NS) mergers, rather than for (NS/NS) mergers [17].

Another constraint comes from the observed time of the GRB. The precise delay between the arrival of gravitational radiation at one of the LIGO sites and the arrival of gamma rays at BATSE is not known and will depend on the details of the development of the electron/photon plasma which produces the gamma rays. The search for gravitational wave signals in LIGO can be concentrated in a time window a few minutes before the detection of each burst compared with a search window of one or two years for a search without an external trigger. One also needs to take into account the fact that there are approximately 300 detected GRBs each year, but also be mindful of the possibility that there may be several different types of GRB, with different (or no) gravitation signal. For example, some GRBs have been associated with Soft Gamma Ray Repeaters (SGR) of galatic origin. It has been proposed that gravitational radiation may also be associated with these bursts [18].

Our work on GRBs will initially concentrate on understanding systematic effects which might give correlations between the two sites. These correlations will weaken the sensitivity of the tests proposed in [16]. For example, there may be correlations in the phase of the AC power between the two sites which could give rise to false signals. Other correlations may be introduced through seismic noise or magnetic fields.

We will also concentrate on methods which can efficiently use all the constraints to either discover gravitational wave signal or to put limits on the gravitational energy emitted from gamma ray bursts in the bandwidth of the LIGO detector. These methods should include completely general analyses of the GRB which make no assumptions about the sources and as well as more targeted searches which use all of the available constraints.

As an intial investigation into this topic, M. Ito has scanned the Caltech 40 meter data for effects associated with observed GRBs. Ito and Rahkola will continue such studies with LIGO simulations to prepare for analysis of 4 km data.

11. Klebesadel, R. W. et al 1973, ApJ, 182, L85.

12. Biggs, M et al 1996, ApJ 459 40.

13. Gehrels, N., "Recent Discoveries in Gamma Ray Burst Astronomy, After the dark ages: when glaxies were young (The universe at 2 < z < 5)," eds. Stephen S. Holt, Eric P. Smith, 371.

14. Chris Fryer, S. E. Woosley, Dieter Hartmann, "Formation Rates of Black Hole Accretion Disk Gamma-Ray Bursts," astro-ph/9904122.

15. Briggs, M. S. et al., BATSE GRB Location Errors in Gamma-Ray Bursts: 4th Huntsville Symposium, 1997.

16. L. S. Finn, S. D. Mohanty, and Joseph D Romano, "Detecting an association between Gamma Ray and Gravitational Wave Bursts", gr-qc/9903101.

17. H-Thomas Janka, Thomas Eberl, Maximilian Ruffert and Chris L. Fryer, astro-ph/9908290.

18. H. J. Mosquera Cuesta, J. C. N. de Araujo, O. D. Aguiar, and J. E. Horvath, "Gravitational-wave bursts from soft gamma-ray repeaters: Can they be detected?", Phys. Rev. Lett. 80, 2988 (1998).