GW230529: unveiling the hidden realm of the anisotropic neutron stars in the lower mass gap with gravitational wave echoes

Abstract Recent discovery of a compact binary coalescence through GW 230529 by LIGO has indicated the merger event of a compact object of mass between $$2.5{-}4.5~M_{\odot }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>2.5</mml:mn> <mml:mo>-</mml:mo> <mml:mn>4.5</mml:mn> <mml:mspace/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:mrow> </mml:math> with a neutron star of mass between $$1.2{-}2.0~M_{\odot }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>1.2</mml:mn> <mml:mo>-</mml:mo> <mml:mn>2.0</mml:mn> <mml:mspace/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:mrow> </mml:math> . The mass of the unknown compact object makes it within the heaviest neutron star never tracked out or the lightest black hole ever detected. Here, we have shown that such a mass gap neutron star with this observed mass ( $$2.5{-}4.5~M_{\odot }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>2.5</mml:mn> <mml:mo>-</mml:mo> <mml:mn>4.5</mml:mn> <mml:mspace/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:mrow> </mml:math> ) can be explained consistently by f ( R ) gravity. We have also adopted the presence of pressure anisotropy inside the neutron star which supports a massive neutron star with mass more than $$2.6~M_{\odot }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>2.6</mml:mn> <mml:mspace/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:mrow> </mml:math> , compatible with LIGO data ( GW 190814). Further, the modified Tollman–Oppenheimer–Volkoff equations have acknowledged such a compact stellar structure that can produce gravitational wave echoes (frequencies remain in the range of $$3{-}6$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> </mml:math> kHz).