Compact objects as astrophysical and gravitational probes

2-­6 February 2015 @ Oort


Gravitational-­wave astronomy and novel techniques to scrutinize compact astrophysical

electromagnetic sources will revolutionize our understanding of the Universe before the end of this decade and will provide us with precious information on the nature of nearby massive black holes like SgrA* and M87. In recent years, significant progress has been made in understanding the formation and cosmological evolution of massive black holes, by measuring their spins and by studying their interplay with the gas present in galactic centers. It is thus an exciting period for strong-­field gravity and the recent flurry of activity calls for a joint effort between previously disjoint communities embracing areas as diverse as high-­energy astrophysics, electromagnetic observations of compact objects and tests of General Relativity with gravitational-­wave detectors.


The main goal of this workshop was to bring together leading experts and young researchers in these areas, with the goal of stimulating new collaborations and discussing the potential of

gravitational-­wave detectors such as Advanced Virgo/LIGO and eLISA, and electromagnetic

ones such as Athena+, GRAVITY and the Event Horizon Telescope, for the following specific

themes: (i) The role of the black hole spin;; (ii) Physics of jets and outflows;; (iii) Properties of

compact objects and tests of gravity.


The workshop brought together around 60 participants from different communities working on gravitational-­wave astronomy, high-­energy astrophysics of compact objects and fundamental physics. The most critical aspect of the organization was to devise specific measures in order to facilitate discussions and foster collaborations between the gravitational-­wave community and the astrophysical compact object community. Indeed, such communities are historically rather disjoint although they have various points of common interest and they could benefit enormously from mutual interaction.


On each day, the workshop’s program was opened by five senior moderators (one per day) whose duties actually started one month before the workshop. Indeed, moderators have been asked to collect questions and open problems from all participants, especially those of common interest between the different communities and those that would particularly require a joint effort and a share of expertise in order to be tackled. Moderators have been chosen carefully based on their proven ability to talk to all the communities involved due to their broad and interdisciplinary work.


Moderator have opened the program by listing the questions collected in the previous weeks

and by providing their personal take on the topic of the day. After the moderator’s introduction, the morning program was organized around introductory talks. The scope of the introductory talks was to explain the technical jargon and to set a common starting point for the interactions of the different groups. Indeed, although the communities involved share the objects of study(i.e. black holes and neutron stars), their motivation to study them, their approaches and their very jargon are extremely different and sometimes almost unintelligible to the other community.


Furthermore, speakers were instructed to provide questions rather than answers, so as to engage the participants in a discussion. With the same goal, all participants were offered lunch vouchers for the whole workshop, so that the group would not disperse and the interactions could effectively continue over lunch time in a very informal and relaxed environment. The proposed structure turned out to be quite successful. Most participants were very eager to interact and learn, and were engaged in very active discussions. Ample time was allocated for discussions, both in the middle of the program and at the end of the day. All discussion sections have been attended by the entire group of participants and systematically lasted longer than expected. Despite the quite intense schedule, discussions essentially went on during the entire day, including coffee breaks, lunches and shifts from the Lorentz Center to the hotel. More specifically, critical issues at the interface between gravitational physics and astrophysics that are relevant for the workshop’s themes were identified, namely


1) the relation between jets and compact-­object spin, which is not yet understood. Accurate measurements of spins by gravitational-­wave detectors will help shed light on this issue


2) The mechanism converting energy from the Poynting fluxes produced by compact objects to observable electromagnetic radiation. This is a critical missing link in our understanding of jets, outflows, and feedback, and one on which possible detections of gravitational waves by advanced detectors in coincidence with gamma-­ray bursts will probably provide breakthrough advancements before the end of the decade


3) VLBI observations of SgrA* and M87 were identified as the most promising way of

testing gravity in strong field regimes with electromagnetic data in the next few years. In

this respect, the critical issue was identified to be the uncertainty in our the knowledge of

the accretion flows in the galactic center, whose impact needs to be carefully assessed.


Discussions on a shared roadmap for the future have resulted in a plan to have a follow-­up

workshop in 2-­3 years from now, when advanced gravitational-­wave detectors will be finally

operational and very-­long baseline interferometers are expected to achieve enough resolution to resolve the region near the black-­hole horizon of SgrA* and M87. We hope that this workshop has paved the way for more fruitful collaborations between the different communities involved in these studies and we expect that several publications will emerge from the collaborations initiated at this workshop.


Enrico Barausse (IAP, France)

Tamara Bogdanovic (Georgia Tech, USA)

Vitor Cardoso (IST, Lisbon)

Paolo Pani (Rome Sapienza, Italy)

Elena Maria Rossi (Leiden U., The Netherlands)