arXiv subjects
Publications and source records attributed to Vicky Kalogera.
Future GW detector networks and EM observatories will provide a unique opportunity to observe the most luminous events in the Universe involving matter in extreme environs. They will address some of the key questions in physics and astronomy: formation and evolution of compact binaries, sites of formation of heavy elements and the physics of jets.
The field of time-domain astrophysics has entered the era of Multi-messenger Astronomy (MMA). One key science goal for the next decade (and beyond) will be to characterize gravitational wave (GW) and neutrino sources using the next generation of Extremely Large Telescopes (ELTs). These studies will have a broad impact across astrophysics, informing our knowledge of the production and enrichment history of the heaviest chemical elements, constrain the dense matter equation of state, provide independent constraints on cosmology, increase our understanding of particle acceleration in shocks and jets, and study the lives of black holes in the universe. Future GW detectors will greatly improve their sensitivity during the coming decade, as will near-infrared telescopes capable of independently finding kilonovae from neutron star mergers. However, the electromagnetic counterparts to high-frequency (LIGO/Virgo band) GW sources will be distant and faint and thus demand ELT capabilities for characterization. ELTs will be important and necessary contributors to an advanced and complete multi-messenger network.
Black hole (BH) spins in low-mass X-ray binaries (LMXBs) cover a range of values that can be explained by accretion after BH birth. In contrast, the three BH spin measurements in high-mass X-ray binaries (HMXBs) show only values near the maximum and likely have a different origin connected to the BH stellar progenitor. We explore here two possible scenarios to explain the high spins of BHs in HMXBs: formation in binaries that undergo mass transfer (MT) during the main sequence (MS; Case-A MT), and very close binaries undergoing chemically homogeneous evolution (CHE). We find that both scenarios are able to produce high-spin BHs if internal angular momentum (AM) transport in the progenitor star after its MS evolution is not too strong (i.e., weak coupling between the stellar core and its envelope). If instead efficient AM transport is assumed, we find that the resulting BH spins are always too low with respect to observations. The Case-A MT model provides a good fit for the BH spins, the masses of the two components, and the final orbital periods for two of the three BHs in HMXBs with measured spins. For one of them, the mass predicted for the BH companion is significantly lower than observed, but this depends strongly on the assumed efficiency of mass transfer. The CHE models predict orbital periods that are too large for all three cases considered here. We expect the Case-A MT to be much more frequent at the metallicities involved, so we conclude that the Case-A MT scenario is preferred. Finally, we predict that the stellar companions of HMXBs formed through the Case-A MT have enhanced nitrogen surface abundances, which can be tested by future observations.
The origin of the X-ray emission from neutron star coalescence GW170817/GRB170817A is a key diagnostic of the unsettled post-merger narrative, and different scenarios predict distinct evolution in its X-ray light curve. Due to its sky proximity to the Sun, sensitive X-ray monitoring of GW170817/GRB170817A has not been possible since a previous detection at 16 days post-burst. We present new, deep Chandra observations of GW170817/GRB170817A at 109 days post-burst, immediately after Sun constraints were lifted. The X-ray emission has brightened from a 0.3-8.0 keV flux of 3.6 x 10^-15 erg/s/cm^2 at 16 days to 15.8 x 10^-15 erg/s/cm^2 at 109 days, at a rate similar to the radio observations. This confirms that the X-ray and radio emission have a common origin. We show that the X-ray light curve is consistent with models of outflow afterglows, in which the outflow can be a cocoon shocked by the jet, dynamical ejecta from the merger, or an off-axis structured jet. Further deep X-ray monitoring can place powerful constraints on the physical parameters of these models, by both timing the passing of a synchrotron cooling break through the X-ray band, and detecting the associated steepening of the X-ray photon index. Finally, the X-ray brightening strengthens the argument that simple off-axis top-hat jet models are not consistent with the latest observations of GW170817/GRB170817A.
We report Chandra observations of GW170817, the first neutron star-neutron star merger discovered by the joint LIGO-Virgo Collaboration, and the first direct detection of gravitational radiation associated with an electromagnetic counterpart, Fermi short gamma-ray burst GRB 170817A. The event occurred on 2017 August 17 and subsequent observations identified an optical counterpart, SSS17a, coincident with NGC 4993 (~10 arcsec separation). Early Chandra (Δt ~ 2 days) and Swift (Δt ~ 1-3 days) observations yielded non-detections at the optical position, but ~9 days post-trigger Chandra monitoring revealed an X-ray point source coincident with SSS17a. We present two deep Chandra observations totaling ~95 ks, collected on 2017 September 01-02 (Δt ~ 15-16 days). We detect X-ray emission from SSS17a with L_{0.3-10 keV} = 2.6^{+0.5}_{-0.4} x 10^38 ergs, and a power law spectrum of Gamma = 2.4 +/- 0.8. We find that the X-ray light curve from a binary NS coalescence associated with this source is consistent with the afterglow from an off-axis short gamma-ray burst, with a jet angled >~23 deg from the line of sight. This event marks both the first electromagnetic counterpart to a LIGO-Virgo gravitational-wave source and the first identification of an off-axis short GRB. We also confirm extended X-ray emission from NGC 4993 (L_{0.3-10 keV} ~ 9 x 10^38 ergs) consistent with its E/S0 galaxy classification, and report two new Chandra point sources in this field, CXOU J130948 and CXOU J130946.
Non-cosmic, non-Gaussian disturbances known as "glitches", show up in gravitational-wave data of the Advanced Laser Interferometer Gravitational-wave Observatory, or aLIGO. In this paper, we propose a deep multi-view convolutional neural network to classify glitches automatically. The primary purpose of classifying glitches is to understand their characteristics and origin, which facilitates their removal from the data or from the detector entirely. We visualize glitches as spectrograms and leverage the state-of-the-art image classification techniques in our model. The suggested classifier is a multi-view deep neural network that exploits four different views for classification. The experimental results demonstrate that the proposed model improves the overall accuracy of the classification compared to traditional single view algorithms.
(abridged for arXiv) With the first direct detection of gravitational waves, the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) has initiated a new field of astronomy by providing an alternate means of sensing the universe. The extreme sensitivity required to make such detections is achieved through exquisite isolation of all sensitive components of LIGO from non-gravitational-wave disturbances. Nonetheless, LIGO is still susceptible to a variety of instrumental and environmental sources of noise that contaminate the data. Of particular concern are noise features known as glitches, which are transient and non-Gaussian in their nature, and occur at a high enough rate so that accidental coincidence between the two LIGO detectors is non-negligible. In this paper we describe an innovative project that combines crowdsourcing with machine learning to aid in the challenging task of categorizing all of the glitches recorded by the LIGO detectors. Through the Zooniverse platform, we engage and recruit volunteers from the public to categorize images of glitches into pre-identified morphological classes and to discover new classes that appear as the detectors evolve. In addition, machine learning algorithms are used to categorize images after being trained on human-classified examples of the morphological classes. Leveraging the strengths of both classification methods, we create a combined method with the aim of improving the efficiency and accuracy of each individual classifier. The resulting classification and characterization should help LIGO scientists to identify causes of glitches and subsequently eliminate them from the data or the detector entirely, thereby improving the rate and accuracy of gravitational-wave observations. We demonstrate these methods using a small subset of data from LIGO's first observing run.
Using numerical models for star clusters spanning a wide range in ages and metallicities (Z) we study the masses of binary black holes (BBHs) produced dynamically and merging in the local universe ($z\lesssim0.2$). After taking into account cosmological constraints on star-formation rate and metallicity evolution, which realistically relate merger delay times obtained from models with merger redshifts, we show here for the first time that while old, metal-poor globular clusters can naturally produce merging BBHs with heavier components, as observed in GW150914, lower-mass BBHs like GW151226 are easily formed dynamically in younger, higher-metallicity clusters.More specifically, we show that the mass of GW151226 is well within $1σ$ of the mass distribution obtained from our models for clusters with Z/Zsun$\gtrsim0.5$. Indeed dynamical formation of a system like GW151226 likely requires a cluster that is younger and has a higher metallicity than typical Galactic globular clusters. The LVT151012 system, if real, could have been created in any cluster with Z/Zsun$\lesssim0.25$. On the other hand, GW150914 is more massive (beyond $1σ$) than typical BBHs from even the lowest-metallicity (Z/Zsun=0.005) clusters we consider, but is within $2σ$ of the intrinsic mass distribution from our cluster models with Z/Zsun$\lesssim0.05$; of course detection biases also push the observed distributions towards higher masses.
The recent detections of the binary black hole mergers GW150914 and GW151226 have inaugurated the field of gravitational-wave astronomy. For the two main formation channels that have been proposed for these sources, isolated binary evolution in galactic fields and dynamical formation in dense star clusters, the predicted masses and merger rates overlap significantly, complicating any astrophysical claims that rely on measured masses alone. Here, we examine the distribution of spin- orbit misalignments expected for binaries from the field and from dense star clusters. Under standard assumptions for black-hole natal kicks, we find that black-hole binaries similar to GW150914 could be formed with significant spin-orbit misalignment only through dynamical processes. In particular, these heavy-black-hole binaries can only form with a significant spin-orbit anti-alignment in the dynamical channel. Our results suggest that future detections of merging black hole binaries with measurable spins will allow us to identify the main formation channel for these systems.
The intermediate mass-ratio inspiral of a stellar compact remnant into an intermediate mass black hole (IMBH) can produce a gravitational wave (GW) signal that is potentially detectable by current ground-based GW detectors (e.g., Advanced LIGO) as well as by planned space-based interferometers (e.g., eLISA). Here, we present results from a direct integration of the post-Newtonian $N$-body equations of motion describing stellar clusters containing an IMBH and a population of stellar-mass black holes (BHs) and solar mass stars. We take particular care to simulate the dynamics closest to the IMBH, including post-Newtonian effects up to order $2.5$. Our simulations show that the IMBH readily forms a binary with a BH companion. This binary is gradually hardened by transient 3-body or 4-body encounters, leading to frequent substitutions of the BH companion, while the binary's eccentricity experiences large amplitude oscillations due to the Lidov-Kozai resonance. We also demonstrate suppression of these resonances by the relativistic precession of the binary orbit. We find an intermediate mass-ratio inspiral in one of the 12 cluster models we evolved for $\sim 100$ Myr. This cluster hosts a $100 M_\odot$ IMBH embedded in a population of 32 $10M_\odot$ BH and 32,000 $1M_\odot$ stars. At the end of the simulation, after $\sim 100$ Myr of evolution, the IMBH merges with a BH companion. The IMBH--BH binary inspiral starts in the eLISA frequency window ($\gtrsim 1\rm mHz$) when the binary reaches an eccentricity $1-e\simeq 10^{-3}$. After $\simeq 10^5$ years the binary moves into the LIGO frequency band with a negligible eccentricity. We comment on the implications for GW searches, with a possible detection within the next decade.
We explore the possibility that GW150914, the binary black hole merger recently detected by Advanced LIGO, was formed by gravitational interactions in the core of a dense star cluster. Using models of globular clusters with detailed $N$-body dynamics and stellar evolution, we show that a typical cluster with a mass of $3\times10^5M_{\odot}$ to $6\times10^5M_{\odot}$ is optimal for forming GW150914-like binary black holes that will merge in the local universe. We identify the most likely dynamical processes for forming GW150914 in such a cluster, and we show that the detection of GW150914 is consistent with the masses and merger rates expected for binary black hole mergers from globular clusters. Our results show that dynamical processes provide a significant and well-understood pathway for forming binary black hole mergers in the local universe. Understanding the contribution of dynamics to the binary black hole merger problem is a critical step in unlocking the full potential of gravitational-wave astronomy.
Observations reveal that mass-transferring binary systems may have non-zero orbital eccentricities. The time-evolution of the orbital semi-major axis and eccentricity of mass-transferring eccentric binary systems is an important part of binary evolution theory and has been widely studied. However, various different approaches and assumptions on the subject have made the literature difficult to comprehend and comparisons between different orbital element time-evolution equations not easy to make. Consequently, no self-consistent treatment of this phase has been ever included in binary population synthesis codes. In this paper, we present a general formalism to derive the time-evolution equations of the binary orbital elements, treating mass-loss and mass-transfer as perturbations to the general two-body problem. We present the self-consistent form of the perturbing acceleration and the phase-dependent time-evolution equations for the orbital elements under different mass-loss/transfer processes. First, we study the cases of isotropic and anisotropic wind mass-loss. Then, we proceed with the non-isotropic ejection and accretion in conservative as well as non-conservative manner for both point masses and extended bodies. Comparison of the derived equations with similar work in the literature is made and explanation of the existing discrepancies is provided.
Finite eccentricities in mass-transferring eccentric binary systems can be explained by taking into account mass-loss and mass-transfer processes that often occur in these systems. These processes can be treated as perturbations to the general two-body problem. The time-evolution equations for the semi-major axis and the eccentricity derived from perturbative methods are in general phase-dependent. The osculating semi-major axis and eccentricity change over the orbital timescale and they are not easy to implement in binary evolution codes like MESA. However, the secular orbital element evolution equations can be simplified averaging over the rapidly varying true anomalies. In this paper, we derive the secular time-evolution equations for the semi-major axis and the eccentricity for various mass-loss/transfer processes using either the adiabatic approximation or the assumption of delta-function mass-loss/transfer at periastron. We begin with the cases of isotropic and anisotropic wind mass-loss. We continue with conservative and non-conservative non-isotropic mass ejection/accretion (including RLOF) for both point-masses and extended bodies. We conclude with the case of phase-dependent mass accretion. Comparison of the derived equations with similar work in the literature is included and explanation of the existing discrepancies is provided.
Among the most eagerly anticipated opportunities made possible by Advanced LIGO/Virgo are multimessenger observations of compact mergers. Optical counterparts may be short-lived so rapid characterization of gravitational wave (GW) events is paramount for discovering electromagnetic signatures. One way to meet the demand for rapid GW parameter estimation is to trade off accuracy for speed, using waveform models with simplified treatment of the compact objects' spin. We report on the systematic errors in GW parameter estimation suffered when using different spin approximations to recover generic signals. Component mass measurements can be biased by $>5σ$ using simple-precession waveforms and in excess of $20σ$ when non-spinning templates are employed. This suggests that electromagnetic observing campaigns should not take a strict approach to selecting which LIGO/Virgo candidates warrant follow-up observations based on low-latency mass estimates. For sky localization, we find searched areas are up to a factor of ${\sim}$2 larger for non-spinning analyses, and are systematically larger for any of the simplified waveforms considered in our analysis. Distance biases for the non-precessing waveforms can be in excess of 100\% and are largest when the spin angular momenta are in the orbital plane of the binary. We confirm that spin-aligned waveforms should be used for low-latency parameter estimation at the minimum. Including simple precession, though more computationally costly, mitigates biases except for signals with extreme precession effects. Our results shine a spotlight on the critical need for development of computationally inexpensive precessing waveforms and/or massively parallel algorithms for parameter estimation.
Hierarchical triple-star systems are expected to form frequently via close binary-binary encounters in the dense cores of globular clusters. In a sufficiently inclined triple, gravitational interactions between the inner and outer binary can cause large-amplitude oscillations in the eccentricity of the inner orbit ("Lidov-Kozai cycles"), which can lead to a collision and merger of the two inner components. In this paper we use Monte Carlo models of dense star clusters to identify all triple systems formed dynamically and we compute their evolution using a highly accurate three-body integrator which incorporates relativistic and tidal effects. We find that a large fraction of these triples evolve through a non-secular dynamical phase which can drive the inner binary to higher eccentricities than predicted by the standard secular perturbation theory (even including octupole-order terms). We place constraints on the importance of Lidov-Kozai-induced mergers for producing: (i) gravitational wave sources detectable by Advanced LIGO (aLIGO), for triples with an inner pair of stellar black holes; and (ii) blue straggler stars, for triples with main-sequence-star components. We find a realistic aLIGO detection rate of black hole mergers due to the Lidov-Kozai mechanism of 1yr^-1, with about 20% of these having a finite eccentricity when they first chirp into the aLIGO frequency band. While rare, these events are likely to dominate among eccentric compact object inspirals that are potentially detectable by aLIGO. For blue stragglers, we find that the Lidov-Kozai mechanism can contribute only up to ~10% of their total numbers in globular clusters.
Inspirals and mergers of black hole (BHs) and/or neutron star (NSs) binaries are expected to be abundant sources for ground-based gravitational-wave (GW) detectors. We assess the capabilities of Advanced LIGO and Virgo to measure component masses using inspiral waveform models including spin-precession effects using a large ensemble of GW sources {\bf randomly oriented and distributed uniformly in volume. For 1000 sources this yields signal-to-noise ratios between 7 and 200}. We make quantitative predictions for how well LIGO and Virgo will distinguish between BHs and NSs and appraise the prospect of using LIGO/Virgo observations to definitively confirm, or reject, the existence of a putative "mass gap" between NSs ($m\leq3\ M_\odot$) and BHs ($m\geq 5\ M_\odot$). We find sources with the smaller mass component satisfying $m_2 \lesssim1.5\ M_\odot$ to be unambiguously identified as containing at least one NS, while systems with $m_2\gtrsim6\ M_\odot$ will be confirmed binary BHs. Binary BHs with $m_2<5\ M_\odot$ (i.e., in the gap) cannot generically be distinguished from NSBH binaries. High-mass NSs ($2<m<3$ $M_\odot$) are often consistent with low-mass BH ($m<5\ M_\odot$), posing a challenge for determining the maximum NS mass from LIGO/Virgo observations alone. Individual sources will seldom be measured well enough to confirm objects in the mass gap and statistical inferences drawn from the detected population will be strongly dependent on the underlying distribution. If nature happens to provide a mass distribution with the populations relatively cleanly separated in chirp mass space, as some population synthesis models suggest, then NSs and BHs are more easily distinguishable.
We analyze binary population models of double-neutron stars and compare results to the accurately measured orbital periods and eccentricities of the eight known such systems in our Galaxy. In contrast to past similar studies, we especially focus on the dominant evolutionary channels (we identify three); for the first time, we use a detailed understanding of the evolutionary history of three double neutron stars as actual constraints on the population models. We find that the evolutionary constraints derived from the double pulsar are particularly tight, and less than half of the examined models survive the full set of constraints. The top-likelihood surviving models yield constraints on the key binary evolution parameters, but most interestingly reveal (i) the need for electron-capture supernovae from relatively low-mass degenerate, progenitor cores, and (ii) the most likely evolutionary paths for the rest of the known double neutron stars. In particular, we find that J1913+16 likely went through a phase of Case BB mass transfer, and J1906+0746 and J1756-2251 are consistent with having been formed in electron-capture supernovae.
The recent discovery of a neutron star accretor in the ultra-luminous X-ray source M82 X-2 challenges our understanding of high-mass X-ray binary formation and evolution. By combining binary population synthesis and detailed mass-transfer models, however, we show that the binary parameters of M82 X-2 are not surprising provided non-conservative mass transfer is allowed. Specifically, the donor-mass lower limit and orbital period measured for M82 X-2 lie near the most probable values predicted by population synthesis models, and systems such as M82 X-2 should exist in approximately 13% of the galaxies with a star-formation history similar to M82. We conclude that the binary system that formed M82 X-2 is most likely less than 50 Myr old and contains a donor star which had an initial mass of approximately 8-10 M$_\odot$, while the NS's progenitor star had an initial mass in the $8-25\,\rm M_{\odot}$ range. The donor star still currently resides on the main sequence, and is capable of continued MT on the thermal timescale, while in the ultra-luminous X-ray regime, for as long as 400,000 years.