Bence Kocsis

Finding the electromagnetic counterparts of cosmological standard sirens

The gravitational waves (GWs) emitted during the coalescence of supermassive black holes (SMBHs) in the mass range ~104-107 M?/(1 + z) will be detectable out to high redshifts with the future Laser Interferometer Space Antenna (LISA). The distance and direction to these standard sirens can be inferred directly from the GW signal, with a precision that depends on the masses, spins, and geometry of the merging system. In a given cosmology, the LISA-measured luminosity distance translates into a redshift shell. We calculate the size and shape of the corresponding three-dimensional error volume in which an electromagnetic counterpart to a LISA event could be found, taking into account errors in the background cosmology (as expected by the time LISA flies), weak gravitational lensing (de)magnification due to inhomogeneities along the line of sight, and potential source-peculiar velocities. Weak-lensing errors largely exceed other sources of uncertainties (by a factor of ~7 for typical sources at z = 1). Under the plausible assumption that SMBH-SMBH mergers are accompanied by gas accretion leading to Eddington-limited quasar activity, we then compute the number of quasars that would be found in a typical three-dimensional LISA error volume, as a function of BH mass and event redshift. Low redshifts offer the best opportunities to identify quasar counterparts to cosmological standard sirens. For mergers of ~4 ? (105-107) M? SMBHs, the LISA error volume will typically contain a single near-Eddington quasar at z ~ 1. If SMBHs are spinning rapidly, the error volume is smaller and may contain a unique quasar out to redshift z ~ 3. This will allow a straightforward test of the hypothesis that GW events are accompanied by bright quasar activity and, if the hypothesis proves correct, will guarantee the identification of a unique quasar counterpart to a LISA event, with a B-band luminosity of LB ~ (1010-1011) L?. Robust counterpart identifications would allow unprecedented tests of the physics of SMBH accretion, such as precision measurements of the Eddington ratio. They would clarify the role of gas as a catalyst in SMBH coalescences and would also offer an alternative method to constrain cosmological parameters.

DISRUPTED GLOBULAR CLUSTERS CAN EXPLAIN the GALACTIC CENTER GAMMA-RAY EXCESS

The Fermi satellite has recently detected gamma ray emission from the central regions of our Galaxy. This may be evidence for dark matter particles, a major component of the standard cosmological model, annihilating to produce high-energy photons. We show that the observed signal may instead be generated by millisecond pulsars that formed in dense star clusters in the Galactic halo. Most of these clusters were ultimately disrupted by evaporation and gravitational tides, contributing to a spherical bulge of stars and stellar remnants. The gamma ray amplitude, angular distribution, and spectral signatures of this source may be predicted without free parameters, and are in remarkable agreement with the observations. These gamma rays are from fossil remains of dispersed clusters, telling the history of the Galactic bulge.

Rapid and Bright Stellar-mass Binary Black Hole Mergers in Active Galactic Nuclei

The Laser Interferometer Gravitational-Wave Observatory, LIGO, found direct evidence for double black hole binaries emitting gravitational waves. Galactic nuclei are expected to harbor the densest population of stellar-mass black holes. A significant fraction (

DETECTION RATE ESTIMATES OF GRAVITY WAVES EMITTED DURING PARABOLIC ENCOUNTERS OF STELLAR BLACK HOLES IN GLOBULAR CLUSTERS

\sim30\%

Observable signatures of extreme mass-ratio inspiral black hole binaries embedded in thin accretion disks

) of these black holes can reside in binaries. We examine the fate of the black hole binaries in active galactic nuclei, which get trapped in the inner region of the accretion disk around the central supermassive black hole. We show that binary black holes can migrate into and then rapidly merge within the disk well within a Salpeter time. The binaries may also accrete a significant amount of gas from the disk, well above the Eddington rate. This could lead to detectable X-ray or gamma-ray emission, but would require hyper-Eddington accretion with a few percent radiative efficiency, comparable to thin disks. We discuss implications for gravitational wave observations and black hole population studies. We estimate that Advanced LIGO may detect

DYNAMICAL FORMATION SIGNATURES OF BLACK HOLE BINARIES IN THE FIRST DETECTED MERGERS BY LIGO

\sim20

Black Hole Mergers in Galactic Nuclei Induced by the Eccentric Kozai–Lidov Effect

such, gas-induced binary mergers per year.

Gravitational Waves and Intermediate Massive Black Hole Retention in Globular Clusters

The rapid advance of gravitational wave (GW) detector facilities makes it very important to estimate the event rates of possible detection candidates. We consider an additional possibility of GW bursts produced during parabolic encounters (PEs) of stellar-mass compact objects in globular clusters (GCs). We estimate the rate of successful detections for specific detectors: the initial Laser Interferometric Gravitational-Wave Observatory (InLIGO), the French-Italian gravitational wave antenna VIRGO, the near-future Advanced-LIGO (AdLIGO), the space-based Laser Interferometric Space Antenna (LISA), and the Next Generation LISA (NGLISA). Simple GC models are constructed to account for the compact object mass function, mass segregation, number density distribution, and velocity distribution. We both calculate encounters classically and account for general relativistic corrections by extrapolating the results for infinite mass ratios. We also include the cosmological redshift of waveforms and event rates. We find that typical PEs with masses m1 = m2 = 40 M? are detectable with matched filtering over a signal-to-noise ratio S/N = 5 within a distance dL ~ 200 Mpc for InLIGO and VIRGO, z = 1 for AdLIGO, 0.4 Mpc for LISA, and 1 Gpc for NGLISA. We estimate single data stream detection rates of 5.5 ? 10-5 yr-1 for InLIGO, 7.2 ? 10-5 yr-1 for VIRGO, 0.063 yr-1 for AdLIGO, 2.9 ? 10-6 yr-1 for LISA, and 1.0 yr-1 for NGLISA, for reasonably conservative assumptions. These estimates are subject to uncertainties in the GC parameters, most importantly the total number and mass distribution of BHs in the cluster core. In reasonably optimistic cases, we get 1 detection for AdLIGO per year. We expect that a coincident analysis using multiple detectors and accounting for GW recoil capture significantly increases the detection rates. The regular detection of GWs during PEs would provide a unique observational probe for constraining the stellar BH mass function of dense clusters.

Gamma-ray and X-ray emission from the Galactic centre: hints on the nuclear star cluster formation history

United States. National Aeronautics and Space Administration (Einstein Postdoctoral Fellowship Award No. PF9-00063)

Testing the Binary Hypothesis: Pulsar Timing Constraints on Supermassive Black Hole Binary Candidates

The dynamical formation of stellar-mass black hole-black hole binaries has long been a promising source of gravitational waves for the Laser Interferometer Gravitational-Wave Observatory (LIGO). Mass segregation, gravitational focusing, and multibody dynamical interactions naturally increase the interaction rate between the most massive black holes in dense stellar systems, eventually leading them to merge. We find that dynamical interactions, particularly three-body binary formation, enhance the merger rate of black hole binaries with total mass M_tot roughly as ~M_tot^beta, with beta >~ 4. We find that this relation holds mostly independently of the initial mass function, but the exact value depends on the degree of mass segregation. The detection rate of such massive black hole binaries is only further enhanced by LIGOs greater sensitivity to massive black hole binaries with M_tot <~ 80 solar masses. We find that for power-law BH mass functions dN/dM ~ M^-alpha with alpha <~ 2, LIGO is most likely to detect black hole binaries with a mass twice that of the maximum initial black hole mass and a mass ratio near one. Repeated mergers of black holes inside the cluster result in about ~5% of mergers being observed between two and three times the maximum initial black hole mass. Using these relations, one may be able to invert the observed distribution to the initial mass function with multiple detections of merging black hole binaries.