Gravitational-to-electromagnetic wave conversion and gamma-ray bursts calorimetry: The GRB980425/SN 1998bw∼1049ergradio emission

Abstract

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 ${E}_{\mathrm{radio}}g~{10}^{49}\mathrm{erg}$ [S. Kulkarni et al., Nature (London) 395, 663 (1998)], while the apparent expansion velocity of its ejecta $(\ensuremath{\sim}{10}^{\ensuremath{-}5}{M}_{\ensuremath{\bigodot}})$ suggests a shock wave moving relativistically ${(V}_{\mathrm{exp}}\ensuremath{\sim}2c).$ 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 (\ensuremath{\sim}2 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 \ensuremath{\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 $\ensuremath{\Delta}{E}_{\mathrm{GW}}\ensuremath{\sim}{10}^{53}\mathrm{erg}\ifmmode\times\else\texttimes\fi{}{(M/10M}_{\ensuremath{\bigodot}}),$ 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. Bj\ornsson 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.