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

## Echoes of quantum black holes from Kerr/CFT

## Next Journal Club

## Discussion of the paper "Gravitational Radiation from Colliding Black Holes" by S. W. Hawking

## Black Hole Workshops

Next workshop:

XIII Black Holes Workshop, Lisbon,

21-22 December 2020

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

## Numerical data

## FunFiCO Network

Our group coordinates the Marie Curie RISE FunFiCO network (Dec 2017- Dec 2021)

More info here

## GWVerse COST

## StronGrHEP Network

Our group is 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.