@article{NedoraRadiceBernuzzietal.2021, author = {Nedora, Vsevolod and Radice, David and Bernuzzi, Sebastiano and Perego, Albino and Daszuta, Boris and Endrizzi, Andrea and Prakash, Aviral and Schianchi, Federico}, title = {Dynamical ejecta synchrotron emission as a possible contributor to the changing behaviour of GRB170817A afterglow}, series = {Monthly notices of the Royal Astronomical Society}, volume = {506}, journal = {Monthly notices of the Royal Astronomical Society}, number = {4}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stab2004}, pages = {5908 -- 5915}, year = {2021}, abstract = {Over the past 3 yr, the fading non-thermal emission from the GW170817 remained generally consistent with the afterglow powered by synchrotron radiation produced by the interaction of the structured jet with the ambient medium. Recent observations by Hajela et al. indicate the change in temporal and spectral behaviour in the X-ray band. We show that the new observations are compatible with the emergence of a new component due to non-thermal emission from the fast tail of the dynamical ejecta of ab-initio binary neutron star merger simulations. This provides a new avenue to constrain binary parameters. Specifically, we find that equal mass models with soft equations of state (EOSs) and high-mass ratio models with stiff EOSs are disfavoured as they typically predict afterglows that peak too early to explain the recent observations. Moderate stiffness and mass ratio models, instead, tend to be in good overall agreement with the data.}, language = {en} } @phdthesis{Antonelli2021, author = {Antonelli, Andrea}, title = {Accurate waveform models for gravitational-wave astrophysics: synergetic approaches from analytical relativity}, doi = {10.25932/publishup-57667}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-576671}, school = {Universit{\"a}t Potsdam}, pages = {XII, 259, LXXV}, year = {2021}, abstract = {Gravitational-wave (GW) astrophysics is a field in full blossom. Since the landmark detection of GWs from a binary black hole on September 14th 2015, fifty-two compact-object binaries have been reported by the LIGO-Virgo collaboration. Such events carry astrophysical and cosmological information ranging from an understanding of how black holes and neutron stars are formed, what neutron stars are composed of, how the Universe expands, and allow testing general relativity in the highly-dynamical strong-field regime. It is the goal of GW astrophysics to extract such information as accurately as possible. Yet, this is only possible if the tools and technology used to detect and analyze GWs are advanced enough. A key aspect of GW searches are waveform models, which encapsulate our best predictions for the gravitational radiation under a certain set of parameters, and that need to be cross-correlated with data to extract GW signals. Waveforms must be very accurate to avoid missing important physics in the data, which might be the key to answer the fundamental questions of GW astrophysics. The continuous improvements of the current LIGO-Virgo detectors, the development of next-generation ground-based detectors such as the Einstein Telescope or the Cosmic Explorer, as well as the development of the Laser Interferometer Space Antenna (LISA), demand accurate waveform models. While available models are enough to capture the low spins, comparable-mass binaries routinely detected in LIGO-Virgo searches, those for sources from both current and next-generation ground-based and spaceborne detectors must be accurate enough to detect binaries with large spins and asymmetry in the masses. Moreover, the thousands of sources that we expect to detect with future detectors demand accurate waveforms to mitigate biases in the estimation of signals' parameters due to the presence of a foreground of many sources that overlap in the frequency band. This is recognized as one of the biggest challenges for the analysis of future-detectors' data, since biases might hinder the extraction of important astrophysical and cosmological information from future detectors' data. In the first part of this thesis, we discuss how to improve waveform models for binaries with high spins and asymmetry in the masses. In the second, we present the first generic metrics that have been proposed to predict biases in the presence of a foreground of many overlapping signals in GW data. For the first task, we will focus on several classes of analytical techniques. Current models for LIGO and Virgo studies are based on the post-Newtonian (PN, weak-field, small velocities) approximation that is most natural for the bound orbits that are routinely detected in GW searches. However, two other approximations have risen in prominence, the post-Minkowskian (PM, weak- field only) approximation natural for unbound (scattering) orbits and the small-mass-ratio (SMR) approximation typical of binaries in which the mass of one body is much bigger than the other. These are most appropriate to binaries with high asymmetry in the masses that challenge current waveform models. Moreover, they allow one to "cover" regions of the parameter space of coalescing binaries, thereby improving the interpolation (and faithfulness) of waveform models. The analytical approximations to the relativistic two-body problem can synergically be included within the effective-one-body (EOB) formalism, in which the two-body information from each approximation can be recast into an effective problem of a mass orbiting a deformed Schwarzschild (or Kerr) black hole. The hope is that the resultant models can cover both the low-spin comparable-mass binaries that are routinely detected, and the ones that challenge current models. The first part of this thesis is dedicated to a study about how to best incorporate information from the PN, PM, SMR and EOB approaches in a synergistic way. We also discuss how accurate the resulting waveforms are, as compared against numerical-relativity (NR) simulations. We begin by comparing PM models, whether alone or recast in the EOB framework, against PN models and NR simulations. We will show that PM information has the potential to improve currently-employed models for LIGO and Virgo, especially if recast within the EOB formalism. This is very important, as the PM approximation comes with a host of new computational techniques from particle physics to exploit. Then, we show how a combination of PM and SMR approximations can be employed to access previously-unknown PN orders, deriving the third subleading PN dynamics for spin-orbit and (aligned) spin1-spin2 couplings. Such new results can then be included in the EOB models currently used in GW searches and parameter estimation studies, thereby improving them when the binaries have high spins. Finally, we build an EOB model for quasi-circular nonspinning binaries based on the SMR approximation (rather than the PN one as usually done). We show how this is done in detail without incurring in the divergences that had affected previous attempts, and compare the resultant model against NR simulations. We find that the SMR approximation is an excellent approximation for all (quasi-circular nonspinning) binaries, including both the equal-mass binaries that are routinely detected in GW searches and the ones with highly asymmetric masses. In particular, the SMR-based models compare much better than the PN models, suggesting that SMR-informed EOB models might be the key to model binaries in the future. In the second task of this thesis, we work within the linear-signal ap- proximation and describe generic metrics to predict inference biases on the parameters of a GW source of interest in the presence of confusion noise from unfitted foregrounds and from residuals of other signals that have been incorrectly fitted out. We illustrate the formalism with simple (yet realistic) LISA sources, and demonstrate its validity against Monte-Carlo simulations. The metrics we describe pave the way for more realistic studies to quantify the biases with future ground-based and spaceborne detectors.}, language = {en} } @phdthesis{Vu2022, author = {Vu, Nils Leif}, title = {A task-based parallel elliptic solver for numerical relativity with discontinuous Galerkin methods}, doi = {10.25932/publishup-56226}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-562265}, school = {Universit{\"a}t Potsdam}, pages = {172}, year = {2022}, abstract = {Elliptic partial differential equations are ubiquitous in physics. In numerical relativity---the study of computational solutions to the Einstein field equations of general relativity---elliptic equations govern the initial data that seed every simulation of merging black holes and neutron stars. In the quest to produce detailed numerical simulations of these most cataclysmic astrophysical events in our Universe, numerical relativists resort to the vast computing power offered by current and future supercomputers. To leverage these computational resources, numerical codes for the time evolution of general-relativistic initial value problems are being developed with a renewed focus on parallelization and computational efficiency. Their capability to solve elliptic problems for accurate initial data must keep pace with the increasing detail of the simulations, but elliptic problems are traditionally hard to parallelize effectively. In this thesis, I develop new numerical methods to solve elliptic partial differential equations on computing clusters, with a focus on initial data for orbiting black holes and neutron stars. I develop a discontinuous Galerkin scheme for a wide range of elliptic equations, and a stack of task-based parallel algorithms for their iterative solution. The resulting multigrid-Schwarz preconditioned Newton-Krylov elliptic solver proves capable of parallelizing over 200 million degrees of freedom to at least a few thousand cores, and already solves initial data for a black hole binary about ten times faster than the numerical relativity code SpEC. I also demonstrate the applicability of the new elliptic solver across physical disciplines, simulating the thermal noise in thin mirror coatings of interferometric gravitational-wave detectors to unprecedented accuracy. The elliptic solver is implemented in the new open-source SpECTRE numerical relativity code, and set up to support simulations of astrophysical scenarios for the emerging era of gravitational-wave and multimessenger astronomy.}, language = {en} } @article{HainichOskinovaShenaretal.2018, author = {Hainich, Rainer and Oskinova, Lida and Shenar, Tomer and Marchant Campos, Pablo and Eldridge, J. J. and Sander, Andreas Alexander Christoph and Hamann, Wolf-Rainer and Langer, Norbert and Todt, Helge Tobias}, title = {Observational properties of massive black hole binary progenitors}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {609}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201731449}, pages = {62}, year = {2018}, abstract = {Context: The first directly detected gravitational waves (GW 150914) were emitted by two coalescing black holes (BHs) with masses of ≈ 36 M⊙ and ≈ 29 M⊙. Several scenarios have been proposed to put this detection into an astrophysical context. The evolution of an isolated massive binary system is among commonly considered models. Aims: Various groups have performed detailed binary-evolution calculations that lead to BH merger events. However, the question remains open as to whether binary systems with the predicted properties really exist. The aim of this paper is to help observers to close this gap by providing spectral characteristics of massive binary BH progenitors during a phase where at least one of the companions is still non-degenerate. Methods: Stellar evolution models predict fundamental stellar parameters. Using these as input for our stellar atmosphere code (Potsdam Wolf-Rayet), we compute a set of models for selected evolutionary stages of massive merging BH progenitors at different metallicities. Results: The synthetic spectra obtained from our atmosphere calculations reveal that progenitors of massive BH merger events start their lives as O2-3V stars that evolve to early-type blue supergiants before they undergo core-collapse during the Wolf-Rayet phase. When the primary has collapsed, the remaining system will appear as a wind-fed high-mass X-ray binary. Based on our atmosphere models, we provide feedback parameters, broad band magnitudes, and spectral templates that should help to identify such binaries in the future. Conclusions: While the predicted parameter space for massive BH binary progenitors is partly realized in nature, none of the known massive binaries match our synthetic spectra of massive BH binary progenitors exactly. Comparisons of empirically determined mass-loss rates with those assumed by evolution calculations reveal significant differences. The consideration of the empirical mass-loss rates in evolution calculations will possibly entail a shift of the maximum in the predicted binary-BH merger rate to higher metallicities, that is, more candidates should be expected in our cosmic neighborhood than previously assumed.}, language = {en} } @phdthesis{Cotesta2021, author = {Cotesta, Roberto}, title = {Multipolar gravitational waveforms for spinning binary black holes and their impact on source characterization}, doi = {10.25932/publishup-50823}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-508236}, school = {Universit{\"a}t Potsdam}, pages = {XII, 274, LVI}, year = {2021}, abstract = {In the last five years, gravitational-wave astronomy has gone from a purerly theoretical field into a thriving experimental science. Several gravitational- wave signals, emitted by stellar-mass binary black holes and binary neutron stars, have been detected, and many more are expected in the future as consequence of the planned upgrades in the gravitational-wave detectors. The observation of the gravitational-wave signals from these systems, and the characterization of their sources, heavily relies on the precise models for the emitted gravitational waveforms. To take full advantage of the increased detector sensitivity, it is then necessary to also improve the accuracy of the gravitational-waveform models. In this work, I present an updated version of the waveform models for spinning binary black holes within the effective-one-body formalism. This formalism is based on the notion that the solution to the relativistic two- body problem varies smoothly with the mass ratio of the binary system, from the equal-mass regime to the test-particle limit. For this reason, it provides an elegant method to combine, under a unique framework, the solution to the relativistic two-body problem in different regimes. The main two regimes that are combined under the effective-one-body formalism are the slow-motion, weak field limit (accessible through the post-Newtonian theory), and the extreme mass-ratio regime (described using the black-hole- perturbation theory). This formalism is nevertheless flexible enough to integrate information about the solution to the relativistic two-body problem obtained using other techniques, such as numerical relativity. The novelty of the waveform models presented in this work is the inclusion of beyond-quadupolar terms in the waveforms emitted by spinning binary black holes. In fact, while the time variation of the source quadupole moment is the leading contribution to the waveforms emitted by binary black holes observable by LIGO and Virgo detectors, beyond-quadupolar terms can be important for binary systems with asymmetric masses, large total mass, or observed with large inclination angle with respect to the orbital angular momentum of the binary. For this purpose, I combine the approximate analytic expressions of these beyond-quadupolar terms, with their calculations from numerical relativity, to develop an accurate waveform model including inspiral, merger and ringdown for spinning binary black holes. I first construct this model in the simplified case of black holes with spins aligned with the orbital angular momentum of the binary, then I extend it to the case of generic spin orientations. Finally, I test the accuracy of both these models against a large number of waveforms obtained from numerical relativity. The waveform models I present in this work are the state of the art for spinning binary black holes, without restrictions in the allowed values for the masses and the spins of the system. The measurement of the source properties of a binary system emitting gravitational waves requires to compute O(107 - 109) different waveforms. Since the waveform models mentioned before can require O(1 - 10)s to generate a single waveform, they can be difficult to use in data-analysis studies given the increasing number of sources observed by the LIGO and Virgo detectors. To overcome this obstacle, I use the reduced-order-modeling technique to develop a faster version of the waveform model for black holes with spins aligned to the orbital angular momentum of the binary. This version of the model is as accurate as the original and reduces the time for evaluating a waveform by two orders of magnitude. The waveform models developed in this thesis have been used by the LIGO and Virgo collaborations in the inference of the source parameters of the gravitational-wave signals detected during the second observing run (O2), and first half of the third observing run (O3a) of LIGO and Virgo detectors. Here, I present a study on the source properties of the signals GW170729 and GW190412, for which I have been directly involved in the analysis. In addition, these models have been used by the LIGO and Virgo collaborations to perform tests on General Relativity employing the gravitational-wave signals detected during O3a, and to analyze the population of the observed binary black holes.}, language = {en} } @article{HosseinzadehCowperthwaiteGomezetal.2019, author = {Hosseinzadeh, Griffin and Cowperthwaite, Philip S. and Gomez, Sebastian and Villar, Victoria Ashley and Nicholl, Matt and Margutti, Raffaella and Berger, Edo and Chornock, Ryan and Paterson, Kerry and Fong, Wen-fai and Savchenko, Volodymyr and Short, Phil and Alexander, Kate D. and Blanchard, Peter K. and Braga, Joao and Calkins, Michael L. and Cartier, Regis and Coppejans, Deanne L. and Eftekhari, Tarraneh and Laskar, Tanmoy and Ly, Chun and Patton, Locke and Pelisoli, Ingrid Domingos and Reichart, Daniel E. and Terreran, Giacomo and Williams, Peter K. G.}, title = {Follow-up of the Neutron Star Bearing Gravitational-wave Candidate Events S190425z and S190426c with MMT and SOAR}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters}, volume = {880}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {2041-8205}, doi = {10.3847/2041-8213/ab271c}, pages = {14}, year = {2019}, abstract = {On 2019 April 25.346 and 26.640 UT the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo gravitational-wave (GW) observatory announced the detection of the first candidate events in Observing Run 3 that contained at least one neutron star (NS). S190425z is a likely binary neutron star (BNS) merger at d(L) = 156 +/- 41 Mpc, while S190426c is possibly the first NS-black hole (BH) merger ever detected, at d(L) = 377 +/- 100 Mpc, although with marginal statistical significance. Here we report our optical follow-up observations for both events using the MMT 6.5 m telescope, as well as our spectroscopic follow-up of candidate counterparts (which turned out to be unrelated) with the 4.1 m SOAR telescope. We compare to publicly reported searches, explore the overall areal coverage and depth, and evaluate those in relation to the optical/near-infrared (NIR) kilonova emission from the BNS merger GW170817, to theoretical kilonova models, and to short gamma-ray burst (SGRB) afterglows. We find that for a GW170817-like kilonova, the partial volume covered spans up to about 40\% for S190425z and 60\% for S190426c. For an on-axis jet typical of SGRBs, the search effective volume is larger, but such a configuration is expected in at most a few percent of mergers. We further find that wide-field gamma-ray and X-ray limits rule out luminous on-axis SGRBs, for a large fraction of the localization regions, although these searches are not sufficiently deep in the context of the gamma-ray emission from GW170817 or off-axis SGRB afterglows. The results indicate that some optical follow-up searches are sufficiently deep for counterpart identification to about 300 Mpc, but that localizations better than 1000 deg(2) are likely essential.}, language = {en} } @article{AbdallaAbramowskiAharonianetal.2017, author = {Abdalla, Hassan E. and Abramowski, Attila and Aharonian, Felix A. and Benkhali, Faical Ait and Anguner, Ekrem Oǧuzhan and Arakawa, M. and Arrieta, M. and Aubert, Pierre and Backes, Michael and Balzer, Arnim and Barnard, Michelle and Becherini, Yvonne and Tjus, J. Becker and Berge, David and Bernhard, Sabrina and Bernl{\"o}hr, K. and Blackwell, R. and B{\"o}ttcher, Markus and Boisson, Catherine and Bolmont, J. and Bonnefoy, S. and Bordas, Pol and Bregeon, Johan and Brun, Francois and Brun, Pierre and Bryan, Mark and Buechele, M. and Bulik, Tomasz and Capasso, M. and Caroff, S. and Carosi, A. and Casanova, Sabrina and Cerruti, M. and Chakraborty, N. and Chaves, Ryan C. G. and Chen, Andrew and Chevalier, J. and Colafrancesco, Sergio and Condon, B. and Conrad, Jan and Davids, I. D. and Decock, J. and Deil, C. and Devin, J. and deWilt, P. and Dirson, L. and Djannati-Atai, A. and Donath, A. and Dutson, K. and Dyks, J. and Edwards, T. and Egberts, Kathrin and Emery, G. and Ernenwein, J. -P. and Eschbach, S. and Farnier, C. and Fegan, S. and Fernandes, M. V. and Fiasson, A. and Fontaine, G. and Funk, S. and Fuessling, M. and Gabici, S. and Gallant, Y. A. and Garrigoux, T. and Gate, F. and Giavitto, G. and Giebels, B. and Glawion, D. and Glicenstein, J. F. and Gottschall, D. and Grondin, M. -H. and Hahn, J. and Haupt, M. and Hawkes, J. and Heinzelmann, G. and Henri, G. and Hermann, G. and Hinton, James Anthony and Hofmann, W. and Hoischen, Clemens and Holch, Tim Lukas and Holler, M. and Horns, D. and Ivascenko, A. and Iwasaki, H. and Jacholkowska, A. and Jamrozy, M. and Jankowsky, D. and Jankowsky, F. and Jingo, M. and Jouvin, L. and Jung-Richardt, I. and Kastendieck, M. A. and Katarzynski, K. and Katsuragawa, M. and Katz, U. and Kerszberg, D. and Khangulyan, D. and Khelifi, B. and King, J. and Klepser, S. and Klochkov, D. and Kluzniak, W. and Komin, Nu. and Kosack, K. and Krakau, S. and Kraus, M. and Kruer, P. P. and Laffon, H. and Lamanna, G. and Lau, J. and Lees, J. -P. and Lefaucheur, J. and Lemiere, A. and Lemoine-Goumard, M. and Lenain, J. -P. and Leser, Eva and Lohse, T. and Lorentz, M. and Liu, R. and Lopez-Coto, R. and Lypova, I. and Malyshev, D. and Marandon, V. and Marcowith, A. and Mariaud, C. and Marx, R. and Maurin, G. and Maxted, N. and Mayer, M. and Meintjes, P. J. and Meyer, M. and Mitchell, A. M. W. and Moderski, R. and Mohamed, M. and Mohrmann, L. and Mora, K. and Moulin, E. and Murach, T. and Nakashima, S. and de Naurois, M. and Ndiyavala, H. and Niederwanger, F. and Niemiec, J. and Oakes, L. and Odaka, H. and Ohm, S. and Ostrowski, M. and Oya, I. and Padovani, M. and Panter, M. and Parsons, R. D. and Pekeur, N. W. and Pelletier, G. and Perennes, C. and Petrucci, P. -O. and Peyaud, B. and Piel, Q. and Pita, S. and Poireau, V. and Poon, H. and Prokhorov, D. and Prokoph, H. and Puehlhofer, G. and Punch, M. and Quirrenbach, A. and Raab, S. and Rauth, R. and Reimer, A. and Reimer, O. and Renaud, M. and de los Reyes, R. and Rieger, F. and Rinchiuso, L. and Romoli, C. and Rowell, G. and Rudak, B. and Rulten, C. B. and Sahakian, V. and Saito, S. and Sanchez, David M. and Santangelo, A. and Sasaki, M. and Schlickeiser, R. and Schussler, F. and Schulz, A. and Schwanke, U. and Schwemmer, S. and Seglar-Arroyo, M. and Settimo, M. and Seyffert, A. S. and Shafi, N. and Shilon, I. and Shiningayamwe, K. and Simoni, R. and Sol, H. and Spanier, F. and Spir-Jacob, M. and Stawarz, L. and Steenkamp, R. and Stegmann, Christian and Steppa, Constantin Beverly and Sushch, Iurii and Takahashi, T. and Tavernet, J. -P. and Tavernier, T. and Taylor, A. M. and Terrier, R. and Tibaldo, L. and Tiziani, D. and Tluczykont, M. and Trichard, C. and Tsirou, M. and Tsuji, N. and Tuffs, R. and Uchiyama, Y. and van der Walt, D. J. and van Eldik, C. and van Rensburg, C. and van Soelen, B. and Vasileiadis, G. and Veh, J. and Venter, C. and Viana, A. and Vincent, P. and Vink, J. and Voisin, F. and Voelk, H. J. and Vuillaume, T. and Wadiasingh, Z. and Wagner, S. J. and Wagner, P. and Wagner, R. M. and White, R. and Wierzcholska, A. and Willmann, P. and Woernlein, A. and Wouters, D. and Yang, R. and Zaborov, D. and Zacharias, M. and Zanin, R. and Zdziarski, A. A. and Zech, Alraune and Zefi, F. and Ziegler, A. and Zorn, J. and Zywucka, N.}, title = {TeV Gamma-Ray Observations of the Binary Neutron Star Merger GW170817 with HESS}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters}, volume = {850}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, organization = {HESS Collaboration}, issn = {2041-8205}, doi = {10.3847/2041-8213/aa97d2}, pages = {628 -- 650}, year = {2017}, abstract = {We search for high-energy gamma-ray emission from the binary neutron star merger GW170817 with the H.E.S.S. Imaging Air Cherenkov Telescopes. The observations presented here have been obtained starting only 5.3 hr after GW170817. The H.E.S.S. target selection identified regions of high probability to find a counterpart of the gravitational-wave event. The first of these regions contained the counterpart SSS17a that has been identified in the optical range several hours after our observations. We can therefore present the first data obtained by a ground-based pointing instrument on this object. A subsequent monitoring campaign with the H.E.S.S. telescopes extended over several days, covering timescales from 0.22 to 5.2 days and energy ranges between 270 GeV to 8.55 TeV. No significant gamma-ray emission has been found. The derived upper limits on the very-high-energy gamma-ray flux for the first time constrain non-thermal, high-energy emission following the merger of a confirmed binary neutron star system.}, language = {en} } @phdthesis{AmaroSeoane2016, author = {Amaro-Seoane, Pau}, title = {Dense stellar systems and massive black holes}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-95439}, school = {Universit{\"a}t Potsdam}, pages = {239}, year = {2016}, abstract = {Gravity dictates the structure of the whole Universe and, although it is triumphantly described by the theory of General Relativity, it is the force that we least understand in nature. One of the cardinal predictions of this theory are black holes. Massive, dark objects are found in the majority of galaxies. Our own galactic center very contains such an object with a mass of about four million solar masses. Are these objects supermassive black holes (SMBHs), or do we need alternatives? The answer lies in the event horizon, the characteristic that defines a black hole. The key to probe the horizon is to model the movement of stars around a SMBH, and the interactions between them, and look for deviations from real observations. Nuclear star clusters harboring a massive, dark object with a mass of up to ~ ten million solar masses are good testbeds to probe the event horizon of the potential SMBH with stars. The channel for interactions between stars and the central MBH are the fact that (a) compact stars and stellar-mass black holes can gradually inspiral into the SMBH due to the emission of gravitational radiation, which is known as an "Extreme Mass Ratio Inspiral" (EMRI), and (b) stars can produce gases which will be accreted by the SMBH through normal stellar evolution, or by collisions and disruptions brought about by the strong central tidal field. Such processes can contribute significantly to the mass of the SMBH. These two processes involve different disciplines, which combined will provide us with detailed information about the fabric of space and time. In this habilitation I present nine articles of my recent work directly related with these topics.}, language = {en} } @phdthesis{Moesta2011, author = {M{\"o}sta, Philipp}, title = {Novel aspects of the dynamics of binary black-hole mergers}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-59820}, school = {Universit{\"a}t Potsdam}, year = {2011}, abstract = {The inspiral and merger of two black holes is among the most exciting and extreme events in our universe. Being one of the loudest sources of gravitational waves, they provide a unique dynamical probe of strong-field general relativity and a fertile ground for the observation of fundamental physics. While the detection of gravitational waves alone will allow us to observe our universe through an entirely new window, combining the information obtained from both gravitational wave and electro-magnetic observations will allow us to gain even greater insight in some of the most exciting astrophysical phenomena. In addition, binary black-hole mergers serve as an intriguing tool to study the geometry of space-time itself. In this dissertation we study the merger process of binary black-holes in a variety of conditions. Our results show that asymmetries in the curvature distribution on the common apparent horizon are correlated to the linear momentum acquired by the merger remnant. We propose useful tools for the analysis of black holes in the dynamical and isolated horizon frameworks and shed light on how the final merger of apparent horizons proceeds after a common horizon has already formed. We connect mathematical theorems with data obtained from numerical simulations and provide a first glimpse on the behavior of these surfaces in situations not accessible to analytical tools. We study electro-magnetic counterparts of super-massive binary black-hole mergers with fully 3D general relativistic simulations of binary black-holes immersed both in a uniform magnetic field in vacuum and in a tenuous plasma. We find that while a direct detection of merger signatures with current electro-magnetic telescopes is unlikely, secondary emission, either by altering the accretion rate of the circumbinary disk or by synchrotron radiation from accelerated charges, may be detectable. We propose a novel approach to measure the electro-magnetic radiation in these simulations and find a non-collimated emission that dominates over the collimated one appearing in the form of dual jets associated with each of the black holes. Finally, we provide an optimized gravitational wave detection pipeline using phenomenological waveforms for signals from compact binary coalescence and show that by including spin effects in the waveform templates, the detection efficiency is drastically improved as well as the bias on recovered source parameters reduced. On the whole, this disseration provides evidence that a multi-messenger approach to binary black-hole merger observations provides an exciting prospect to understand these sources and, ultimately, our universe.}, language = {en} } @phdthesis{Ohme2012, author = {Ohme, Frank}, title = {Bridging the gap between post-Newtonian theory and numerical relativity in gravitational-wave data analysis}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-60346}, school = {Universit{\"a}t Potsdam}, year = {2012}, abstract = {One of the most exciting predictions of Einstein's theory of gravitation that have not yet been proven experimentally by a direct detection are gravitational waves. These are tiny distortions of the spacetime itself, and a world-wide effort to directly measure them for the first time with a network of large-scale laser interferometers is currently ongoing and expected to provide positive results within this decade. One potential source of measurable gravitational waves is the inspiral and merger of two compact objects, such as binary black holes. Successfully finding their signature in the noise-dominated data of the detectors crucially relies on accurate predictions of what we are looking for. In this thesis, we present a detailed study of how the most complete waveform templates can be constructed by combining the results from (A) analytical expansions within the post-Newtonian framework and (B) numerical simulations of the full relativistic dynamics. We analyze various strategies to construct complete hybrid waveforms that consist of a post-Newtonian inspiral part matched to numerical-relativity data. We elaborate on exsisting approaches for nonspinning systems by extending the accessible parameter space and introducing an alternative scheme based in the Fourier domain. Our methods can now be readily applied to multiple spherical-harmonic modes and precessing systems. In addition to that, we analyze in detail the accuracy of hybrid waveforms with the goal to quantify how numerous sources of error in the approximation techniques affect the application of such templates in real gravitational-wave searches. This is of major importance for the future construction of improved models, but also for the correct interpretation of gravitational-wave observations that are made utilizing any complete waveform family. In particular, we comprehensively discuss how long the numerical-relativity contribution to the signal has to be in order to make the resulting hybrids accurate enough, and for currently feasible simulation lengths we assess the physics one can potentially do with template-based searches.}, language = {en} }