Superradiant scattering of electromagnetic fields from ringing black holes

Abstract Detection of gravitational waves (GWs) paves the beginning of a new era of gravitational wave astronomy. Black holes (BHs) in their ringdown phase (ringing BHs) provide the cleanest signal of emitted GWs that imprint the fundamental nature of BHs under low energy perturbation. Apart from GWs, any complementary signature of ringing BHs can be of paramount importance. Motivated by this, we analyzed the scattering of electromagnetic waves in such a background and demonstrated that the absorption cross section of a ringing Schwarzschild BH can be superradiant. It appears that superradiance in the ringdown phase results from the stimulated growth of the matter field, driven by the oscillating gravitational background. Therefore, the amplification is inherently transient, with a characteristic time scale equal to the GW oscillation time scale. We have further analysed the frequency ranges of such amplified transient signals for a wide range of BH masses. In this context, we propose the premise that the primordial black hole (PBH) may undergo a merging phase and exhibit superradiance. We point out that the existing ground-based Low Frequency Array (LOFAR), radio telescopes could be able to detect such transient signals from PBHs, with mass range $$M\sim 10^{-1} - 10^{-2} M_{\odot }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>M</mml:mi> <mml:mo>∼</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mo>-</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:mrow> </mml:math> , going through the ringdown phase. Our present result, therefore, opens up an intriguing possibility of observing the PBH-PBH merging phenomena through electromagnetic waves.