Imprints of the Janis-Newman-Winicour spacetime on observations related to shadow and accretion

Abstract

The final fate of gravitational collapse of massive stars has been a subject of interest for a long time since such a collapse may lead to black holes and naked singularities alike. Since, the formation of naked singularities is forbidden by the cosmic censorship conjecture, exploring their observational differences from black holes may be a possible avenue to search for these exotic objects. The simplest possible naked singularity spacetime emerges from the Einstein massless scalar field theory with the advantage that it smoothly translates to the Schwarzschild solution by the variation of the scalar charge. This background, known as the Janis-Newman-Winicour spacetime is the subject of interest in this work. We explore electromagnetic observations around this metric which involves investigating the characteristics of black hole accretion and shadow. We compute the shadow radius in this spacetime and compare it with the image of M87*, recently released by the Event Horizon Telescope Collaboration. Similarly, we derive the expression for the luminosity from the accretion disk and compare it with the observed optical luminosity of eighty Palomar Green quasars. Our analysis indicates that the shadow of M87* favors the Schwarzschild background while the quasars on the other hand exhibit the existence of a non-trivial scalar charge from the accretion data thereby supporting the Janis-Newman-Winicour spacetime. The implications of this result are discussed.

Echoes from a singularity

“…In the electromagnetic spectrum, there are numerous observations pertaining to gravitational lensing [29][30][31][32][33][34][35][36], accretion disk [29,[37][38][39][40][41][42][43], images and shadows [44][45][46] that highlights the difference in the properties of naked singularities and black hole spacetimes. In the gravitational wave spectrum such observations are also possible, particularly, in the ringdown phase of a compact binary coalescence when the signal is dominated by the quasinormal modes.…”

“…The JNW spacetime has a surface-like naked singularity that reduces to the well known Schwarzschild solution in the absence of the scalar field. Optical properties of the JNW spacetime has been studied at great lengths [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46]67]. The JNW spacetime has been shown to be stable against scalar field perturbations [68].…”