Astron. Astrophys. 364, 655-659 (2000)

4. Discussion

In this paper, we have shown that GRBs may be accompanied by very powerful short pulses of low-frequency radio emission. In our consideration we based on recent simulations of interaction between a relativistic, strongly magnetized wind and an ambient gas (Smolsky & Usov 1996, 2000; Usov & Smolsky 1998). In these simulations it was shown that particles of the ambient gas are reflected from the wind front, and outflowing electrons may be accelerated up to the energy of protons. These electrons may be responsible for the non-thermal emission of GRBs. The current that separates the strongly magnetized matter of the outflowing wind and the ambient gas where the magnetic field strength is negligible varies in time because of nonstationarity of the wind-ambient gas interaction and generates low-frequency waves. The high-frequency tail of these waves may reach a few tens of MHz and be detected.

High-energy particles reflected from the wind front interact with the ambient gas. This interaction is a very poor studied process. There are two possibilities. The first possibility assumed in the GRB model of Smolsky & Usov (1996, 2000) is that a collisionless shock does not form ahead of the wind front, and the wind front interacts with the ambient gas during all time of GRB generation. In this case, the undispersed duration of a low-frequency radio pulse is of the order of the GRB duration (see above). The second possibility is that a collisionless shock forms ahead of the wind front as it is assumed in the standard external shock model of GRBs (for a review, see Piran 1999). If the last is true, simulations of Smolsky & Usov (1996, 1998, 2000) relate only to the first stage of the wind-ambient gas interaction when the shock is only forming. In this case, the undispersed duration of a low-frequency radio pulse is about the characteristic time of the shock formation and may be much shorter than the GRB duration. Therefore, observations of such radio pulses may be used for diagnostics of interaction between a relativistic strongly magnetized wind and an ambient gas.

It is noted above that our mechanism for production of short pulses of low-frequency radio emission applies very generally. All GRBs generated by relativistic, strongly magnetized winds may be accompanied by short pulses of low-frequency radio emission irrespective of the mechanism of GRB generation. In the case, for example, if the GRB radiation (or its main part) is produced by internal shocks (e.g., Piran 1999and references therein), not all energy of the outflowing wind is converted to -rays. At the distance the process of the wind deceleration becomes essential, and a short low-frequency radio pulse may be generated as we discuss.

For detection of short low-frequency radio pulses it may be necessary to perform observations at lower frequencies than are generally used in radio astronomy, which are limited by the problem of transmission through and refraction by the ionosphere. In particular, observations from space are free of ionospheric refraction and are shielded by the ionosphere from terrestrial interference. Although even harder to predict, detection from the ground at higher frequencies may also be possible.

The possibility of detecting of a coherent radio pulse generated in the initial explosion of a supernova has been discussed in (Colgate 1975; Meikle & Colgate 1978 and references therein). However, the mechanism for the production of such a pulse is entirely different from that of our paper.

Space observations are possible at frequencies down to that at which the interstellar medium becomes optically thick to free-free absorption. This frequency is

where cm^-3) and K (Spitzer 1962), and T are the interstellar electron density and temperature, respectively, and is the Galactic latitude. The expression may be , but could be substantially larger if the electrons are strongly clumped or cold. In fact, determination of by space measurements of interstellar absorption of bright extragalactic radio sources is a useful probe of the spatial and thermal structure of the interstellar electron gas, giving information which cannot be obtained from dispersion measure and scintillation studies alone. The frequency dependences of interstellar inverse bremsstrahlung absorption and self-absorption at the source are quite different, so these two processes should be distinguishable in accurate data obtained at several well-chosen frequencies.

Much of the interstellar volume is filled with very hot (K) and radio-transparent gas. Pressure balance arguments suggest that this has a very low density ( cm^-3) and probably contains very little of the electron column density. Most of the interstellar dispersion must be attributed to denser and more radio-opaque regions. Intergalactic absorption poses a somewhat less restrictive condition than interstellar absorption because of the likely high temperature (K), and low density of intergalactic gas.

At the low frequencies we suggest for observations of coherent radio emission from GRBs, and even at tens of MHz, interstellar scintillation (Goodman 1997) will be very large. Successful observations of this emission would not only illuminate the physical conditions in the radiating regions, but would determine (through the dispersion measure) the mean intergalactic plasma density and (through the scintillation) its spatial structure.

The flux density implied by Eq. (14) appears impressively large, but it applies only to the brief period when the dispersed signal is sweeping through the bandwidth of observation, so that it is unclear if it is, in fact, excluded by the very limited data (Cortiglioni 1981) available. Further, the extrapolation of the radiated spectrum to is very uncertain. Finally, we have also not considered the (difficult to estimate) temporal broadening of this brief transient signal by intergalactic scintillation, which will both reduce its amplitude and broaden its time-dependence.

Searches for radio pulses started about 50 years ago, prior to the discovery of GRBs. During wide beam studies of ionospheric scintillations, simultaneous bursts at 45 MHz of 10-20 s duration were reported by Smith (1950) at sites 160 km apart. These events were detected at night, approximately once a week. The origin of these bursts was never determined. The results of Smith (1950) were not confirmed by subsequent observations at frequencies of 150 MHz or higher.

Modern observations (Dessenne et al. 1996 at 151 MHz; Balsano et al. 1998 at 74 MHz; Benz & Paesold 1998 broadband; see Frail 1998 for a review) set upper bounds to the brightnesses of some GRB at comparatively high frequencies. These bounds are not stringent, and do not exclude extrapolations of the lower frequency fluxes suggested here. There are few data at lower frequencies.

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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