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:20260813T193230Z UID:682214d0-9977-4b24-87f8-a98e4c26078d SEQUENCE:1 CREATED:20250408T110727Z DESCRIPTION: Light rings (LRs) - closed circular orbits of null geodesics - are key features of both black holes and horizonless ultracompact object s. While unstable LRs are relevant for the observation of black hole image s\, stable LRs have been suspected to trigger instabilities\, namely in ex otic compact objects that could mimic black holes. The underlying mechanis m behind this instability remains poorly understood\, but a key missing pi ece is how the backreaction of a perturbation around the stable LR modifie s the surrounding spacetime. In this talk\, some progress in this directio n is discussed by examining a conceptually simple\, yet instructive\, toy model: continuum-shell stars\, supported solely by tangential pressures. U sing both analytical and numerical methods\, we show how perturbations aro und the stable LR deepen the geodesic potential and shifts its location in ward\, potentially amplifying any instability associated with the LR. By t hen extending the analysis to more general stars with nonzero radial press ure\, we find that the same phenomenon can be expected to persist under  reasonable assumptions. LAST-MODIFIED:20250408T110727Z LOCATION:DF Seminar Room (2-8.3)\, 2nd floor of Physics Building URL:http://df.vps.tecnico.ulisboa.pt/en/events/backreaction-of-perturbatio ns-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 blac k holes and horizonless ultracompact objects. While unstable LRs are relev ant for the observation of black hole images\, stable LRs have been suspec ted to trigger instabilities\, namely in exotic compact objects that could mimic black holes.

The underlying mechanism behind this instabi lity remains poorly understood\, but a key missing piece is how the backre action of a perturbation around the stable LR modifies the surrounding spa cetime. In this talk\, some progress in this direction is discussed by exa mining a conceptually simple\, yet instructive\, toy model: continuum-shel l stars\, supported solely by tangential pressures.

Using both a nalytical and numerical methods\, we show how perturbations around the sta ble LR deepen the geodesic potential and shifts its location inward\, pote ntially amplifying any instability associated with the LR. By then extendi ng the analysis to more general stars with nonzero radial pressure\, we fi nd that the same phenomenon can be expected to persist under reasonable  assumptions.

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