The Puzzling Long GRB 191019A: Evidence for Kilonova Light

Abstract GRB 191019A was a long gamma-ray burst (GRB) lasting ~65 s and, as such, originally thought to be linked to a core-collapse supernova. However, even though follow-up observations identified the optical counterpart close to the bright nucleus of a nearby ancient galaxy ( z = 0.248), no associated supernova was found. This led to the suggestion that the burst was caused by the merger of two compact stellar objects, likely in a dense circumnuclear environment. By using a recently developed diagnostic tool based on prompt emission temporal properties, we noticed that GRB 191019A falls among those long GRBs which are associated with compact mergers and with evidence of kilonova light. We thus reanalyzed unpublished GROND multicolor ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>g</mml:mi> <mml:mi>′</mml:mi> <mml:mi>r</mml:mi> <mml:mi>′</mml:mi> <mml:mi>i</mml:mi> <mml:mi>′</mml:mi> <mml:mi>z</mml:mi> <mml:mi>′</mml:mi> <mml:mi>J</mml:mi> <mml:mi>H</mml:mi> <mml:msub> <mml:mrow> <mml:mi>K</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>s</mml:mi> </mml:mrow> </mml:msub> </mml:math> ) data obtained between 0.4 and 15 days posttrigger. Image subtraction confirmed the optical counterpart in all four optical bands, with GROND tracking its fading until 1.5 days postburst. Incorporating publicly available Swift-XRT data, a joint fit of an afterglow plus a kilonova model revealed a better match than an afterglow-only scenario. The resulting kilonova properties resemble those of AT2017gfo associated with the binary neutron star merger GW170817, with a total ejected mass of ~0.06 M ⊙ . Contrary to previous findings inferring a high-density circumburst environment ( n 0 ~ 10 7−8 cm −3 ), our analysis finds standard conditions ( n 0 ~ 1 cm −3 ), suggesting the long duration of GRB 191019A was intrinsic rather than due to jet interaction with a dense external medium.