In this talk I describe a new class of superradiant instabilities in which ultralight scalar fields extract rotational energy from neutron stars. The instability arises from the mixing of scalar and photon modes in the magnetic field of the neutron star which extract energy from the rotating magnetosphere. Unlike for black holes, where the dissipation required for superradiance is provided by an absorbative horizon, the non-hermitian dynamics in this setup come from the resistivity in the stellar magnetosphere arising from a finite bulk conductivity. This leads to a set of complex eigenfrequencies associated to the instability, which we derive using techniques analogous to quantum mechanical perturbation theory by treating scalar field bound states like a perturbed atomic state. We then compare the instability rate to the characteristic astrophysical time scales of neutron stars given by their spin-down times.

# Axion superradiance in rotating neutron stars

## Next Seminar

## UV completions of effective models explaining B-physics anomalies

## Next Journal Club

Session to be resumed.

## Black Hole Workshops

Next workshop:

XII Black Holes Workshop, Minho, 2019

19-20 December 2019

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

## 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.