Gravitational-to-electromagnetic wave conversion and gamma-ray bursts calorimetry:: The GRB980425/SN 1998bw ∼1049 erg radio emission -: art. no. 064009

The unusual features of supernova (SN) 1998bw and its apparent association with the gamma-ray burst (GRB) event GRB980425 were highlighted by Kulkarni et al. At its peak SN 1998bw was anomalously superluminous in radio wavelengths with an inferred fluence E(radio)greater than or equal to10(49) erg [S. Kulkarni et al., Nature (London) 395, 663 (1998)], while the apparent expansion velocity of its ejecta (similar to10(-5) M.) suggests a shock wave moving relativistically (V(exp)similar to2c). The unique properties of SN 1998bw strengthen the case for it being linked with GRB980425. I present a consistent, novel mechanism to explain the peculiar event SN 1998bw and similar phenomena in GRBs: Conversion of powerful, high frequency (similar to2 kHz) gravitational waves (GWs) into electromagnetic waves [M. Johnston, R. Ruffini, and F. Zerilli, Phys. Rev. Lett. 31, 1317 (1973)] might have taken place during SN 1998bw. Yet, conversion of GRB photons into GWs, as advanced by Johnston, Ruffini, and Zerilli [Phys. Lett. 49B, 185 (1974)], may also occur. These processes can produce GRBs depleted in gamma rays but enhanced in x rays, for instance, or even more plausibly induce dark GRBs, those with no optical afterglow. The class of GWs needed to drive the calorimetric changes of these gamma-ray bursts may be generated by (a) the nonaxisymmetric dynamics of a torus surrounding the hypernova (or failed supernova) magnetized stellar-mass black hole (BH) remnant, as in van Putten's mechanism for driving long GRBs powered by the BH spin energy [Phys. Rev. Lett. 87, 091101 (2001)], or in the van Putten and Ostriker mechanism to account for the bimodal distribution in duration in GRBs [Astrophys. J. Lett. 552, L32 (2001)], where the torus magnetohydrodynamics may be dominated by either hyperaccretion onto a slowly spinning BH or suspended accretion onto a fast rotating BH, or (b) the just formed black hole with electromagnetic structure as in the GRB central engine mechanism of Ruffini et al. [Astrophys. J. Lett. 555, L107 (2001); 555, L113 (2001)], provided the issue concerning the origin of the black hole charge can be suitably clarified. In both of these mechanisms the total energy radiated as GWs is about DeltaE(GW)similar to10(53) erg x (M/10M.), which for the conversion efficiency estimated here turns out to be enough to explain the superluminous radio wavelength emission from SN 1998bw. Thus, I argue this process could have induced the enhancement in the radio luminosity of SN 1998bw as evidenced in its light curve [Fig. 2, in S. Kulkarni et al., Nature (London) 395, 663 (1998)] and optical light curves of GRB980326 [J. Bloom et al., Nature (London) 401, 453 (1998)] and GRB990712 [G. Bjornsson et al., Astrophys. J. Lett. 552, L121 (2001)]. Moreover, GW-driven plasma density perturbations moving at the speed of light may up- (or down-) convert fireball photons, which could cause further substantial modifications of the gamma-ray burst or supernova calorimetry.