Discussion on the paper "Stationary black holes and light rings", ArXiv:2003.06445 to appear in Physical Review Letters, as an Editor's Suggestion. The ringdown and shadow of the astrophysically significant Kerr Black Hole (BH) are both intimately connected to a special set of bound null orbits known as Light Rings (LRs). Does it hold that a generic equilibrium BH must possess such orbits? In this letter we prove the following theorem. A stationary, axi-symmetric, asymptotically flat black hole spacetime in 1+3 dimensions, with a non-extremal, topologically spherical, Killing horizon admits, at least, one standard LR outside the horizon for each rotation sense. The proof relies on a topological argument and assumes C2-smoothness and circularity, but makes no use of the field equations. The argument is also adapted to recover a previous theorem establishing that a horizonless ultra-compact object must admit an even number of non-degenerate LRs, one of which is stable.

# Stationary black holes and light rings

## Next Seminar

## The polihomogeneity of the gravitational field and conserved charges at null infinity

## Next Journal Club

Session to be resumed.

## Black Hole Workshops

Next workshop:

XVII Black Holes Workshop, Aveiro,

19-20 December 2024

Previous workshops in the series:

I Black Holes Workshop, Porto, 2008

II Black Holes Workshop, Lisbon, 2009

III Black Holes Workshop, Minho, 2010

IV Black Holes Workshop, Aveiro, 2011

V Black Holes Workshop, Lisbon, 2012

VI Black Holes Workshop, Minho 2013

VII Black Holes Workshop, Aveiro 2014

VIII Black Holes Workshop, Lisbon 2015

IX Black Holes Workshop, Minho 2016

X Black Holes Workshop, Aveiro 2017

XI Black Holes Workshop, Lisbon, 2018

XII Black Holes Workshop, Minho, 2019

XIII Black Holes Workshop, Lisbon, 2020

XIV Black Holes Workshop, Aveiro, 2021

XV Black Holes Workshop, Lisbon, 2022

XVI Black Holes Workshop, Porto, 2023

## Numerical data

## NewFunFICO network

Our group coordinates the Marie Sklodowska Curie Staff Exchange NewFunFiCO network (Jan 2023- Dec 2026)

More info here

## FunFiCO Network

Our group coordinates the Marie Curie RISE FunFiCO network (Dec 2017- Dec 2023)

More info here

## EuCAPT Consortium

## GWVerse COST

## StronGrHEP Network

Our group was part of the RISE StronGrHEP network (2016-2019)

Meetings:

Paris, 12-13 May 2016

Azores, 3-8 July 2017

Osaka, 4-8 September 2017

## NRHEP Network Meetings

Our group coordinated the "Numerical Relativity and High Energy Physics" IRSES network (2012-2015). Here is a list of the global network meetings organized:

First Meeting:

9-13 July 2012, Aveiro, Portugal

Second Meeting:

11-14 March 2013, Lisbon, Portugal

Third Meeting:

6-10 January 2014, Mississippi, USA

Fourth Meeting:

7-10 July 2015, Rome, Italy

Fifth Meeting:

28 Sep-2 Oct 2015, Belém, Brazil

## Ph.D. and Post-doctoral opportunities

Contact us If you are interested in pursuing graduate studies/research in our group.

Information for prospective Ph.D. students can be found here.

Past Ph.D. theses from our group can be found here.

Working as a researcher in Portugal - a quick guide.