arXiv subjects

Johan Samsing

Publications and source records attributed to Johan Samsing.

At least 73 records · Page 4Linked to original sources

arXiv2018

Black Hole Mergers From Globular Clusters Observable by LISA I: Eccentric Sources Originating From Relativistic $N$-body Dynamics

We show that nearly half of all binary black hole (BBH) mergers dynamically assembled in globular clusters have measurable eccentricities ($e>0.01$) in the LISA band ($10^{-2}$ Hz), when General Relativistic corrections are properly included in the $N$-body evolution. If only Newtonian gravity is included, the derived fraction of eccentric LISA sources is significantly lower, which explains why recent studies all have greatly underestimated this fraction. Our findings have major implications for how to observationally distinguish between BBH formation channels using eccentricity with LISA, which is one of the key science goals of the mission. We illustrate that the relatively large population of eccentric LISA sources reported here originates from BBHs that merge between hardening binary-single interactions inside their globular cluster. These results indicate a bright future for using LISA to probe the origin of BBH mergers.

astro-ph.HE↗

arXiv2018

Eccentric Black Hole Mergers Forming in Globular Clusters

We derive the probability for a newly formed binary black hole (BBH) to undergo an eccentric gravitational wave (GW) merger during binary-single interactions inside a stellar cluster. By integrating over the hardening interactions such a BBH must undergo before ejection, we find that the observable rate of BBH mergers with eccentricity $>0.1$ at $10\ \rm{Hz}$ relative to the rate of circular mergers can be as high as $\sim 5\%$ for a typical globular cluster (GC). This further suggests that BBH mergers forming through GW captures in binary-single interactions, eccentric or not, are likely to constitute $\sim 10\%$ of the total BBH merger rate from GCs. Such GW capture mergers can only be probed with an $N$-body code that includes General Relativistic corrections, which explains why recent Newtonian cluster studies not have been able to resolve this population. Finally, we show that the relative rate of eccentric BBH mergers depends on the compactness of their host cluster, suggesting that an observed eccentricity distribution can be used to probe the origin of BBH mergers.

astro-ph.HE↗

arXiv2018

Post-Newtonian Dynamics in Dense Star Clusters: Highly-Eccentric, Highly-Spinning, and Repeated Binary Black Hole Mergers

We present models of realistic globular clusters with post-Newtonian dynamics for black holes. By modeling the relativistic accelerations and gravitational-wave emission in isolated binaries and during three- and four-body encounters, we find that nearly half of all binary black hole mergers occur inside the cluster, with about 10% of those mergers entering the LIGO/Virgo band with eccentricities greater than 0.1. In-cluster mergers lead to the birth of a second generation of black holes with larger masses and high spins, which, depending on the black hole natal spins, can sometimes be retained in the cluster and merge again. As a result, globular clusters can produce merging binaries with detectable spins regardless of the birth spins of black holes formed from massive stars. These second-generation black holes would also populate any upper mass gap created by pair-instability supernovae.

astro-ph.HE↗

arXiv2018

Black Hole Mergers from Globular Clusters Observable by LISA and LIGO: Results from post-Newtonian Binary-Single Scatterings

We study the gravitational wave (GW) frequency and chirp mass distribution of binary black hole (BBH) mergers assembled through three-body interactions in globular clusters (GCs), when GW emission at the 2.5 post-Newtonian (PN) level is included in the $N$-body equation-of-motion (EOM). From performing $\sim 2.5\times10^{6}$ PN binary-single interactions based on GC data from the `MOCCA-Survey Database I' project, and by the use of analytical methods, we find that $5-10\%$ of all the three-body assembled GC BBH mergers have a GW frequency at formation that is $\gtrsim 10^{-1}$ Hz, implying they enter the LIGO band without having drifted through the LISA band first. If PN terms are not included in the EOM, one finds instead that all BBH mergers drifts through both LISA and LIGO. As the fraction of BBH mergers that only show up in LIGO is expected to be $\sim 0\%$ for standard field binary BBH mergers, future joint measurements with LISA and LIGO can be used to gain insight into the formation of BBH mergers.

astro-ph.HE↗

arXiv2018

MOCCA-SURVEY Database I: Eccentric Black Hole Mergers During Binary-Single Interactions In Globular Clusters

We estimate the population of eccentric gravitational wave (GW) binary black hole (BBH) mergers forming during binary-single interactions in globular clusters (GCs), using ~ 800 GC models that were evolved using the MOCCA code for star cluster simulations as part of the MOCCA-Survey Database I project. By re-simulating binary-single interactions (only involving 3 BHs) extracted from this set of GC models using an N-body code that includes GW emission at the 2.5 post-Newtonian level, we find that ~ 10% of all the BBHs assembled in our GC models that merge at present time form during chaotic binary-single interactions, and that about half of this sample have an eccentricity > 0.1 at 10 Hz. We explicitly show that this derived rate of eccentric mergers is ~ 100 times higher than one would find with a purely Newtonian N-body code. Furthermore, we demonstrate that the eccentric fraction can be accurately estimated using a simple analytical formalism when the interacting BHs are of similar mass; a result that serves as the first successful analytical description of eccentric GW mergers forming during three-body interactions in realistic GCs.

astro-ph.HE↗

arXiv2017

Topology of Black Hole Binary-Single Interactions

We present a study on how the outcomes of co-planar binary-single black hole (BH) interactions distribute as a function of the orbital initial conditions. We refer to this distribution as the topology. Using an $N$-body code that includes BH finite sizes and gravitational wave (GW) emission in the equation-of-motion (EOM), we perform more than a million three-body scatterings to explore the topology of both the classical and the relativistic limit. We further describe how the inclusion of GW emission in the EOM naturally leads to scenarios where the binary-single system undergoes two successive GW mergers. The time span from the first to the second GW merger is shortest in co-planar interactions, and are for some configurations even short enough for LIGO to observe both events. We note that co-planar interactions could be frequent in environments such as active galactic nuclei discs.

astro-ph.HE↗

arXiv2017

Dissipative Evolution of Unequal Mass Binary-Single Interactions and its Relevance to Gravitational Wave Detections

We present a study on binary-single interactions with energy loss terms such as tidal dissipation and gravitational wave emission added to the equation-of-motion. The inclusion of such terms leads to the formation of compact binaries that form during the three-body interaction through two-body captures. These binaries predominantly merge relative promptly at high eccentricity, with several observable and dynamical consequences to follow. Despite their possibility for being observed in both present and upcoming transient surveys, their outcomes are not firmly constrained. In this paper we present an analytical framework that allows to estimate the cross section of such two-body captures, which permits us to study how the corresponding rates depends on the initial orbital parameters, the mass hierarchy, the type of interacting objects, and the energy dissipation mechanism. This formalism is applied here to study the formation of two-body gravitational wave captures, for which we estimate absolute and relative rates relevant to Advanced LIGO detections. It is shown that two-body gravitational wave captures should have compelling observational implications if a sizable fraction of detected compact binaries are formed via dynamical interactions.

astro-ph.HE↗

arXiv2017

On the Assembly Rate of Highly Eccentric Binary Black Hole Mergers

In this {\it Letter} we calculate the fraction of highly eccentric binary black hole (BBH) mergers resulting from binary-single interactions. Using an $N$-body code that includes post-Newtonian correction terms, we show that $\gtrsim 1\%$ of all BBH mergers resulting from this channel will have an eccentricity $e>0.1$ when coming into the LIGO frequency band. As the majority of BBH mergers forming in globular clusters are assembled through three-body encounters, we suggest that such interactions are likely to dominate the population of high eccentricity BBH mergers detectable by LIGO. The relative frequency of highly eccentric events could eventually help to constrain the astrophysical origin of BBH mergers.

astro-ph.HE↗

arXiv2016

Formation of Tidal Captures and Gravitational Wave Inspirals in Binary-Single Interactions

We perform the first systematic study on how dynamical stellar tides and general relativistic (GR) effects affect the dynamics and outcomes of binary-single interactions. For this, we have constructed an N-body code that includes tides in the affine approximation, where stars are modeled as self-similar ellipsoidal polytropes, and GR corrections using the commonly-used post-Newtonian formalism. Using this numerical formalism, we are able resolve the leading effect from tides and GR across several orders of magnitude in both stellar radius and initial target binary separation. We find that the main effect from tides is the formation of two-body tidal captures that form during the chaotic and resonant evolution of the triple system. The two stars undergoing the capture spiral in and merge. The inclusion of tides can thus lead to an increase on the stellar coalescence rate. We also develop an analytical framework for calculating the cross section of tidal inspirals between any pair of objects with similar mass. From our analytical and numerical estimates we find that the rate of tidal inspirals relative to collisions increases as the initial semi-major axis of the target binary increases and the radius of the interacting tidal objects decreases. The largest effect is therefore found for triple systems hosting white dwarfs and neutron stars. In this case, we find the rate of highly eccentric white dwarf - neutron star mergers to likely be dominated by tidal inspirals. While tidal inspirals occur rarely, we note that they can give rise to a plethora of thermonuclear transients such as Ca-rich transients.

astro-ph.HE↗

arXiv2015

Extracting Periodic Transit Signals from Noisy Light Curves using Fourier Series

We present a simple and powerful method for extracting transit signals associated with a known transiting planet from noisy light curves. Assuming the orbital period of the planet is known and the signal is periodic, we illustrate that systematic noise can be removed in Fourier space at all frequencies, by only using data within a fixed time frame with a width equal to an integer number of orbital periods. This results in a reconstruction of the full transit signal which on average is unbiased, despite that no prior knowledge of either the noise or the transit signal itself is used in the analysis. The method has therefore clear advantages over standard phase folding, which normally requires external input such as nearby stars or noise models for removing systematic components. In addition, we can extract the full orbital transit signal ($360$ degrees) simultaneously, and \emph{Kepler} like data can be analyzed in just a few seconds. We illustrate the performance of our method by applying it to a dataset composed of light curves from \emph{Kepler} with a fake injected signal emulating a planet with rings. For extracting periodic transit signals, our presented method is in general the optimal and least biased estimator and could therefore lead the way toward the first detections of, e.g., planet rings and exo-trojan asteroids.

astro-ph.EP↗

arXiv2014

A Gravitational Double Scattering Mechanism for Generating High Velocity Objects

We present a dynamical model describing how halo particles can receive a significant energy kick from the merger between their own host halo and a target halo. This is highly relevant for understanding the growth of cosmological halos, and could especially provide an explanation for some high velocity objects. The model we present includes a \emph{double scattering mechanism}, where a halo particle is given a significant energy kick by undergoing two subsequent gravitational deflections during the merger. The first deflection is by the potential of the target halo, whereas the second is by the potential of the particle's original host halo. The resultant energy kick arises because the two halos move relative to each other during the two deflections. To our knowledge, this mechanism has never been characterized in this context before. We derive analytically a halo particle's total kick energy, which is composed of energy from the double scattering mechanism and energy release from tidal fields, as a function of its position in its original host halo. In the case of a $1:10$ merger between two Hernquist halos, we estimate that the presented mechanisms can generate particles with a velocity $\sim 2$ times the virial velocity of the target halo measured at its virial sphere. This motivates us to suggest that the high velocity of the recently discovered globular cluster HVCG-1 \citep{2014ApJ...787L..11C} can be explained by a head-on halo merger. Finally, we illustrate the orbital evolution of particles outside the virial sphere of the target halo, by solving the equation of motion in an expanding universe. We find a 'sweet spot' around a scale factor of $0.3-0.5$ for ejecting particles into large orbits, which easily can reach beyond $\sim 5$ virial radii.

astro-ph.GA↗

arXiv2014

Illuminating Massive Black Holes With White Dwarfs: Orbital Dynamics and High Energy Transients from Tidal Interactions

White dwarfs (WDs) can be tidally disrupted only by massive black holes (MBHs) with masses less than $\sim10^5 M_\odot$. These tidal interactions feed material to the MBH well above its Eddington limit, with the potential to launch a relativistic jet. The corresponding beamed emission is a promising signpost to an otherwise quiescent MBH of relatively low mass. We show that the mass transfer history, and thus the lightcurve, are quite different when the disruptive orbit is parabolic, eccentric, or circular. The mass lost each orbit exponentiates in the eccentric-orbit case leading to the destruction of the WD after several tens of orbits. We examine the stellar dynamics of clusters surrounding MBHs to show that single-passage WD disruptions are substantially more common than repeating encounters. The $10^{49}$ erg s$^{-1}$ peak luminosity of these events makes them visible to cosmological distances. They may be detectible at rates of as many as tens per year by instruments like Swift. In fact, WD-disruption transients significantly outshine their main-sequence star counterparts, and are the most likely tidal interaction to be detected arising from MBHs with masses less than $10^5 M_\odot$. The detection or non-detection of such WD-disruption transients by Swift is, therefore, a powerful tool to constrain lower end of the MBH mass function. The emerging class of ultra-long gamma ray bursts all have peak luminosities and durations reminiscent of WD disruptions, offering a hint that WD-disruption transients may already be present in existing datasets.

astro-ph.HE↗

arXiv2013

The Formation of Eccentric Compact Binary Inspirals and the Role of Gravitational Wave Emission in Binary-Single Stellar Encounters

The inspiral and merger of eccentric binaries leads to gravitational waveforms distinct from those generated by circularly merging binaries. Dynamical environments can assemble binaries with high eccentricity and peak frequencies within the {\it LIGO} band. In this paper, we study binary-single stellar scatterings occurring in dense stellar systems as a source of eccentrically-inspiraling binaries. Many interactions between compact binaries and single objects are characterized by chaotic resonances in which the binary-single system undergoes many exchanges before reaching a final state. During these chaotic resonances, a pair of objects has a non-negligible probability of experiencing a very close passage. Significant orbital energy and angular momentum are carried away from the system by gravitational wave (GW) radiation in these close passages and in some cases this implies an inspiral time shorter than the orbital period of the bound third body. We derive the cross section for such dynamical inspiral outcomes through analytical arguments and through numerical scattering experiments including GW losses. We show that the cross section for dynamical inspirals grows with increasing target binary semi-major axis, $a$, and that for equal-mass binaries it scales as $a^{2/7}$. Thus, we expect wide target binaries to predominantly contribute to the production of these relativistic outcomes. We estimate that eccentric inspirals account for approximately one percent of dynamically assembled non-eccentric merging binaries. While these events are rare, we show that binary-single scatterings are a more effective formation channel than single-single captures for the production of eccentrically-inspiraling binaries, even given modest binary fractions.

astro-ph.HE↗

arXiv2013

New Constraints on the Early Expansion History

Cosmic microwave background measurements have pushed to higher resolution, lower noise, and more sky coverage. These data enable a unique test of the early universe's expansion rate and constituents such as effective number of relativistic degrees of freedom and dark energy. Using the most recent data from Planck and WMAP9, we constrain the expansion history in a model independent manner from today back to redshift z=10^5. The Hubble parameter is mapped to a few percent precision, limiting early dark energy and extra relativistic degrees of freedom within a model independent approach to 2-16% and 0.71 equivalent neutrino species respectively (95% CL). Within dark radiation, barotropic aether, and Doran-Robbers models, the early dark energy constraints are 3.3%, 1.9%, 1.2% respectively.

astro-ph.CO↗

arXiv2012

Power Spectrum Precision for Redshift Space Distortions

Redshift space distortions in galaxy clustering offer a promising technique for probing the growth rate of structure and testing dark energy properties and gravity. We consider the issue of to what accuracy they need to be modeled in order not to unduly bias cosmological conclusions. Fitting for nonlinear and redshift space corrections to the linear theory real space density power spectrum in bins in wavemode, we analyze both the effect of marginalizing over these corrections and of the bias due to not correcting them fully. While naively subpercent accuracy is required to avoid bias in the fixed case, in the fitting approach the Kwan-Lewis-Linder reconstruction function for redshift space distortions is found to be accurately selfcalibrated with little degradation in dark energy and gravity parameter estimation for a next generation galaxy redshift survey such as BigBOSS.

astro-ph.CO↗

arXiv2012

Model Independent Early Expansion History and Dark Energy

We examine model independent constraints on the high redshift and prerecombination expansion history from cosmic microwave background observations, using a combination of principal component analysis and other techniques. This can be translated to model independent limits on early dark energy and the number of relativistic species $N_{\rm eff}$. Models such as scaling (Doran-Robbers), dark radiation ($ΔN_{\rm eff}$), and barotropic aether fall into distinct regions of eigenspace and can be easily distinguished from each other. Incoming CMB data will map the expansion history from $z=0$--$10^5$, achieving subpercent precision around recombination, and enable determination of the amount of early dark energy and valuable guidance to its nature.

astro-ph.CO↗

arXiv2012

Measuring the 3D shape of X-ray clusters

Observations and numerical simulations of galaxy clusters strongly indicate that the hot intracluster x-ray emitting gas is not spherically symmetric. In many earlier studies spherical symmetry has been assumed partly because of limited data quality, however new deep observations and instrumental designs will make it possible to go beyond that assumption. Measuring the temperature and density profiles are of interest when observing the x-ray gas, however the spatial shape of the gas itself also carries very useful information. For example, it is believed that the x-ray gas shape in the inner parts of galaxy clusters is greatly affected by feedback mechanisms, cooling and rotation, and measuring this shape can therefore indirectly provide information on these mechanisms. In this paper we present a novel method to measure the three-dimensional shape of the intracluster x-ray emitting gas. We can measure the shape from the x-ray observations only, i.e. the method does not require combination with independent measurements of e.g. the cluster mass or density profile. This is possible when one uses the full spectral information contained in the observed spectra. We demonstrate the method by measuring radial dependent shapes along the line of sight for CHANDRA mock data. We find that at least 10^6 photons are required to get a 5-σ detection of shape for an x-ray gas having realistic features such as a cool core and a double powerlaw for the density profile. We illustrate how Bayes' theorem is used to find the best fitting model of the x-ray gas, an analysis that is very important in a real observational scenario where the true spatial shape is unknown. Not including a shape in the fit may propagate to a mass bias if the x-ray is used to estimate the total cluster mass. We discuss this mass bias for a class of spacial shapes.

astro-ph.CO↗

arXiv2012

Dithering Strategies and Point-Source Photometry

The accuracy in the photometry of a point source depends on the point-spread function (PSF), detector pixelization, and observing strategy. The PSF and pixel response describe the spatial blurring of the source, the pixel scale describes the spatial sampling of a single exposure, and the observing strategy determines the set of dithered exposures with pointing offsets from which the source flux is inferred. In a wide-field imaging survey, sources of interest are randomly distributed within the field of view and hence are centered randomly within a pixel. A given hardware configuration and observing strategy therefore have a distribution of photometric uncertainty for sources of fixed flux that fall in the field. In this article we explore the ensemble behavior of photometric and position accuracies for different PSFs, pixel scales, and dithering patterns. We find that the average uncertainty in the flux determination depends slightly on dither strategy, whereas the position determination can be strongly dependent on the dithering. For cases with pixels much larger than the PSF, the uncertainty distributions can be non-Gaussian, with rms values that are particularly sensitive to the dither strategy. We also find that for these configurations with large pixels, pointings dithered by a fractional pixel amount do not always give minimal average uncertainties; this is in contrast to image reconstruction for which fractional dithers are optimal. When fractional pixel dithering is favored, a pointing accuracy of better than $\sim 0.15$ pixel width is required to maintain half the advantage over random dithers.

astro-ph.IM↗