BEGIN:VCALENDAR VERSION:2.0 PRODID:-//linuxsoftware.nz//NONSGML Joyous v1.4//EN BEGIN:VEVENT SUMMARY:Backreaction of perturbations around a stable Light Ring DTSTART:20250410T143000Z DTEND:20250410T160000Z DTSTAMP:20260802T202405Z UID:682214d0-9977-4b24-87f8-a98e4c26078d SEQUENCE:2 CREATED:20250408T110737Z DESCRIPTION:Light rings (LRs) - closed circular orbits of null geodesics - are key features of both black holes and horizonless ultracompact objects . While unstable LRs are relevant for the observation of black hole images \, stable LRs have been suspected to trigger instabilities\, namely in exo tic compact objects that could mimic black holes. The underlying mechanism behind this instability remains poorly understood\, but a key missing pie ce is how the backreaction of a perturbation around the stable LR modifies the surrounding spacetime. In this talk\, some progress in this direction is discussed by examining a conceptually simple\, yet instructive\, toy m odel: continuum-shell stars\, supported solely by tangential pressures. Us ing both analytical and numerical methods\, we show how perturbations arou nd the stable LR deepen the geodesic potential and shifts its location inw ard\, potentially amplifying any instability associated with the LR. By th en extending the analysis to more general stars with nonzero radial pressu re\, we find that the same phenomenon can be expected to persist under r easonable assumptions. LAST-MODIFIED:20250408T110749Z LOCATION:DF Seminar Room (2-8.3)\, 2nd floor of Physics Building URL:http://df.vps.tecnico.ulisboa.pt/pt/eventos/backreaction-of-perturbati ons-around-a-stable-light-ring/ X-ALT-DESC;FMTTYPE=text/html:

Light rings (LRs) - closed circular orbits of null geodesics - are key features of both black holes and horizonless ultracompact objects. While unstable LRs are releva nt for the observation of black hole images\, stable LRs have been suspect ed to trigger instabilities\, namely in exotic compact objects that could mimic black holes.

The underlying mechanism behind this instabil ity remains poorly understood\, but a key missing piece is how the backrea ction of a perturbation around the stable LR modifies the surrounding spac etime. In this talk\, some progress in this direction is discussed by exam ining a conceptually simple\, yet instructive\, toy model: continuum-shell stars\, supported solely by tangential pressures.

Using both an alytical and numerical methods\, we show how perturbations around the stab le LR deepen the geodesic potential and shifts its location inward\, poten tially amplifying any instability associated with the LR. By then extendin g the analysis to more general stars with nonzero radial pressure\, we fin d that the same phenomenon can be expected to persist under reasonable  assumptions.

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