arXiv subjects

Hsin-Yu Chen

Publications and source records attributed to Hsin-Yu Chen.

51 records · Page 3Linked to original sources

arXiv2020

Systematic uncertainty of standard sirens from the viewing angle of binary neutron star inspirals

The independent measurement of Hubble constant with gravitational-wave standard sirens will potentially shed light on the tension between the local distance ladders and Planck experiments. Therefore, thorough understanding of the sources of systematic uncertainty for the standard siren method is crucial. In this paper, we focus on two scenarios that will potentially dominate the systematic uncertainty of standard sirens. First, simulations of electromagnetic counterparts of binary neutron star mergers suggest aspherical emissions, so the binaries available for the standard siren method can be selected by their viewing angles. This selection effect can lead to $\gtrsim 2\%$ bias in Hubble constant measurement even with mild selection. Second, if the binary viewing angles are constrained by the electromagnetic counterpart observations but the bias of the constraints is not controlled under $\sim 10^{\circ}$, the resulting systematic uncertainty in Hubble constant will be $>3\%$. In addition, we find that both of the systematics cannot be properly removed by the viewing angle measurement from gravitational-wave observations. Comparing to the known dominant systematic uncertainty for standard sirens, the $\leq 2\%$ gravitational-wave calibration uncertainty, the effects from viewing angle appear to be more significant. Therefore, the systematic uncertainty from viewing angle might be a major challenge before the standard sirens can resolve the tension in Hubble constant, which is currently $\sim$9%.

astro-ph.HE↗

arXiv2020

Cosmological Inference using Gravitational Wave Standard Sirens: A Mock Data Challenge

The observation of binary neutron star merger GW170817, along with its optical counterpart, provided the first constraint on the Hubble constant $H_0$ using gravitational wave standard sirens. When no counterpart is identified, a galaxy catalog can be used to provide the necessary redshift information. However, the true host might not be contained in a catalog which is not complete out to the limit of gravitational-wave detectability. These electromagnetic and gravitational-wave selection effects must be accounted for. We describe and implement a method to estimate $H_0$ using both the counterpart and the galaxy catalog standard siren methods. We perform a series of mock data analyses using binary neutron star mergers to confirm our ability to recover an unbiased estimate of $H_0$. Our simulations used a simplified universe with no redshift uncertainties or galaxy clustering, but with different magnitude-limited catalogs and assumed host galaxy properties, to test our treatment of both selection effects. We explore how the incompleteness of catalogs affects the final measurement of $H_0$, as well as the effect of weighting each galaxy's likelihood of being a host by its luminosity. In our most realistic simulation, where the simulated catalog is about three times denser than the density of galaxies in the local universe, we find that a 4.4\% measurement precision can be reached using galaxy catalogs with 50\% completeness and $\sim 250$ binary neutron star detections with sensitivity similar to that of Advanced LIGO's second observing run.

gr-qc↗

arXiv2020

Distinguishing binary neutron star from neutron star-black hole mergers with gravitational waves

The gravitational-wave signal from the merger of two neutron stars cannot be easily differentiated from the signal produced by a comparable-mass mixed binary of a neutron star and a black hole. Indeed, both binary types can account for the gravitational-wave signal GW170817 even if its electromagnetic counterpart emission is taken into account. We propose a method that {requires neither information from the post-inspiral phase of the binary nor an electromagnetic counterpart} to identify mixed binaries of neutron stars merging with low-mass black holes using gravitational-waves alone. This method is based on the fact that certain neutron star properties that can be measured with gravitational-waves are common or similar for all neutron stars. For example all neutron stars share the same equation of state and if the latter is hadronic, neutron stars have similar radii. If a mixed binary is misidentified as a neutron star binary, the inferred neutron star properties will be misestimated and appear as outliers in a population of low-mass binaries. We show that as few as $\sim 5$ low-mass events will allow for the identification of the type of one event at the $80\%$ confidence level. We model the population of low-mass binaries with a hierarchical mixture model and show that we can constrain the existence of mixed binaries or measure their abundance relative to neutron star binaries to $\sim 0.1$ at the $68\%$ credible level with 100 events.

astro-ph.HE↗

arXiv2020

Searching for exotic cores with binary neutron star inspirals

We study the feasibility of detecting exotic cores in merging neutron stars with ground-based gravitational-wave detectors. We focus on models with a sharp nuclear/exotic matter interface, and assume a uniform distribution of neutron stars in the mass range $[1,2] M_\odot$. We find that the existence of exotic cores can be confirmed at the 70% confidence level with as few as several tens of detections. Likewise, with such a sample, we find that some models of exotic cores can be excluded {with high confidence}.

astro-ph.HE↗

arXiv2019

Viewing angle of binary neutron star mergers

The joint detection of the gravitational wave (GW) GW170817 and its electromagnetic (EM) counterparts GRB170817A and kilonova AT 2017gfo has triggered extensive study of the EM emission of binary neutron star mergers. A parameter which is common to and plays a key role in both the GW and the EM analyses is the viewing angle of the binary's orbit. If a binary is viewed from different angles, the amount of GW energy changes (implying that orientation and distance are correlated) and the EM signatures can vary, depending on the structure of the emission. Information about the viewing angle of the binary orbital plane is therefore crucial to the interpretation of both the GW and the EM data, and can potentially be extracted from either side. In the first part of this study, we present a systematic analysis of how well the viewing angle of binary neutron stars can be measured from the GW data. We show that if the sky position and the redshift of the binary can be identified via the EM counterpart and an associated host galaxy, then for 50$\%$ of the systems the viewing angle can be constrained to $\leq 7^{\circ}$ uncertainty from the GW data, independent of electromagnetic emission models. On the other hand, if no redshift measurement is available, the measurement of the viewing angle with GW alone is not informative, unless the true viewing angle is close to $90^{\circ}$. This holds true even if the sky position is measured independently. Then, we consider the case where some constraints on the viewing angle can be placed from the EM data itself. We show that the EM measurements can then be used in the analysis of GW data to improve the precision of the luminosity distance, and hence of the Hubble constant, by a factor of 2 to 3.

astro-ph.HE↗

arXiv2019

Measuring the delay time distribution of binary neutron stars. II. Using the redshift distribution from third-generation gravitational wave detectors network

We investigate the ability of current and third-generation gravitational wave (GW) detectors to determine the delay time distribution (DTD) of binary neutron stars (BNS) through a direct measurement of the BNS merger rate as a function of redshift. We assume that the DTD follows a power law distribution with a slope $Γ$ and a minimum merger time $t_{\rm min}$, and also allow the overall BNS formation efficiency per unit stellar mass to vary. By convolving the DTD and mass efficiency with the cosmic star formation history, and then with the GW detector capabilities, we explore two relevant regimes. First, for the current generation of GW detectors, which are only sensitive to the local universe, but can lead to precise redshift determinations via the identification of electromagnetic counterparts and host galaxies, we show that the DTD parameters are strongly degenerate with the unknown mass efficiency and therefore cannot be determined uniquely. Second, for third-generation detectors such as Einstein Telescope (ET) and Cosmic Explorer (CE), which will detect BNS mergers at cosmological distances, but with a redshift uncertainty inherent to GW-only detections ($δ(z)/z\approx 0.1z$), we show that the DTD and mass efficiency can be well-constrained to better than 10\% with a year of observations. This long-term approach to determining the DTD through a direct mapping of the BNS merger redshift distribution will be supplemented by more near term studies of the DTD through the properties of BNS merger host galaxies at $z\approx 0$ (Safarzadeh & Berger 2019).

astro-ph.HE↗

arXiv2019

Astro 2020 Science White Paper: Joint Gravitational Wave and Electromagnetic Astronomy with LIGO and LSST in the 2020's

The blossoming field of joint gravitational wave and electromagnetic (GW-EM) astronomy is one of the most promising in astronomy. The first, and only, joint GW-EM event GW170817 provided remarkable science returns that still continue to this day. Continued growth in this field requires increasing the sample size of joint GW-EM detections. In this white paper, we outline the case for using some percentage of LSST survey time for dedicated target-of-opportunity follow up of GW triggers in order to efficiently and rapidly identify optical counterparts. We show that the timeline for the LSST science survey is well matched to the planned improvements to ground based GW detectors in the next decade. LSST will become particularly crucial in the later half of the 2020s as more and more distant GW sources are detected. Lastly, we highlight some of the key science goals that can be addressed by a large sample of joint GW-EM detections.

astro-ph.HE↗

arXiv2018

A 2 per cent Hubble constant measurement from standard sirens within 5 years

Gravitational wave coalescence events provide an entirely new way to determine the Hubble constant, with the absolute distance calibration provided by the theory of general relativity. This standard siren method was utilized to measure the Hubble constant using LIGO-Virgo's detection of the binary neutron-star merger GW170817, as well as optical identifications of the host galaxy, NGC 4993. The novel and independent measurement is of particular interest given the existing tension between the value of the Hubble constant determined using Type Ia supernovae via the local distance ladder ($73.24 \pm 1.74$) and that from Cosmic Microwave Background observations ($66.93 \pm 0.62$) by $\sim 3$ sigma. Local distance ladder observations may achieve a precision of $1\%$ within 5 years, but at present there are no indications that further observations will substantially reduce the existing discrepancies. In addition to clarifying the discrepancy between existing low and high-redshift measurements, a precision measurement of the Hubble constant is of crucial value in elucidating the nature of the dark energy. Here we show that LIGO and Virgo can be expected to constrain the Hubble constant to a precision of $\sim2\%$ within 5 years and $\sim1\%$ within a decade.

astro-ph.CO↗

arXiv2018

Measuring the Hubble constant with neutron star black hole mergers

The detection of GW170817 and the identification of its host galaxy have allowed for the first standard-siren measurement of the Hubble constant, with an uncertainty of $\sim 14\%$. As more detections of binary neutron stars with redshift measurement are made, the uncertainty will shrink. The dominating factors will be the number of joint detections and the uncertainty on the luminosity distance of each event. Neutron star black hole mergers are also promising sources for advanced LIGO and Virgo. If the black hole spin induces precession of the orbital plane, the degeneracy between luminosity distance and the orbital inclination is broken, leading to a much better distance measurement. In addition neutron star black hole sources are observable to larger distances, owing to their higher mass. Neutron star black holes could also emit electromagnetic radiation: depending on the black hole spin and on the mass ratio, the neutron star can be tidally disrupted resulting in electromagnetic emission. We quantify the distance uncertainty for a wide range of black hole mass, spin and orientations and find that the 1-$σ$ statistical uncertainty can be up to a factor of $\sim 10$ better than for a non-spinning binary neutron star merger with the same signal-to-noise ratio. The better distance measurement, the larger gravitational-wave detectable volume, and the potentially bright electromagnetic emission, imply that spinning black hole neutron star binaries can be the optimal standard siren sources as long as their astrophysical rate is larger than $O(10)$ Gpc$^{-3}$yr$^{-1}$, a value allowed by current astrophysical constraints.

astro-ph.CO↗

arXiv2016

Finding the One: Identifying the Host Galaxies of Gravitational-Wave Sources

We explore the localization of compact binary coalescences with ground-based gravitational-wave detector networks. We simulate tens of thousands of binary events, and present the distributions of localization sky areas and localization volumes for a range of sources and network configurations. We show that generically there exists a tail of particularly well-localized events, with 2D and 3D localizations of $<10\,\mbox{deg}^2$ and $<1000\,\mbox{Mpc}^3$ achievable, respectively, starting in LIGO/Virgo's third observing run. Incorporating estimates for the galaxy density and the binary event rates, we argue that future gravitational-wave detector networks will localize a small number of binary systems per year to a sufficiently small volume that the unique host galaxy might be identified. For these golden events, which are generally the closest and loudest ones, the gravitational-wave detector networks will point (in 3D; the length of the finger matters) directly at the source. This will allow for studies of the properties of the host galaxies of compact binary mergers, which may be an important component in exploring the formation channels of these sources. In addition, since the host will provide an independent measurement of the redshift, this will allow the use of the event as a standard siren to measure cosmology. Furthermore, identification of a small number of host galaxies can enable deep follow-up searches for associated electromagnetic transients.

astro-ph.HE↗

arXiv2016

Observational Selection Effects with Ground-based Gravitational Wave Detectors

Ground-based interferometers are not perfectly all-sky instruments, and it is important to account for their behavior when considering the distribution of detected events. In particular, the LIGO detectors are most sensitive to sources above North America and the Indian Ocean and, as the Earth rotates, the sensitive regions are swept across the sky. However, because the detectors do not acquire data uniformly over time, there is a net bias on detectable sources' right ascensions. Both LIGO detectors preferentially collect data during their local night; it is more than twice as likely to be local midnight than noon when both detectors are operating. We discuss these selection effects and how they impact LIGO's observations and electromagnetic follow-up. Beyond galactic foregrounds associated with seasonal variations, we find that equatorial observatories can access over $80\%$ of the localization probability, while mid-latitudes will access closer to $70\%$. Facilities located near the two LIGO sites can observe sources closer to their zenith than their analogs in the South, but the average observation will still be no closer than $44^\circ$ from zenith. We also find that observatories in Africa or the South Atlantic will wait systematically longer before they can begin observing compared to the rest of the world, although there is a preference for longitudes near the LIGOs. These effects, along with knowledge of the LIGO antenna pattern, can inform electromagnetic follow-up activities and optimization, including the possibility of directing observations even before gravitational-wave events occur.

astro-ph.HE↗

arXiv2015

Facilitating follow-up of LIGO-Virgo events using rapid sky localization

Fast and effective localization of gravitational wave (GW) events could play a crucial role in identifying possible electromagnetic counterparts, and thereby help usher in an era of GW multi-messenger astronomy. We discuss an algorithm for accurate and very low latency ($<$ 1 second) localization of GW sources using only the relative times of arrival, relative phases, and relative signal-to-noise ratios for pairs of detectors. The algorithm is independent of distances and masses to leading order, and can be generalized to all discrete sources detected by ground-based detector networks. Our approach, while developed independently, is similar to that of BAYESTAR with a few modifications in the algorithm which result in increased computational efficiency. For the LIGO two detector configuration (Hanford+Livingston) expected in late 2015 we find a median 50\% (90\%) localization of 143 deg$^2$ (558 deg$^2$) for binary neutron stars (for network SNR threshold of 12, corresponding to a horizon distance of $\sim 130$ Mpc), consistent with previous findings. We explore the improvement in localization resulting from high SNR events, finding that the loudest out of the first 4 (or 10) events reduces the median sky localization area by a factor of 1.9 (3.0) for the case of 2 GW detectors, and 2.2 (4.0) for 3 detectors. We consider the case of multi-messenger joint detections in both the GW and the electromagnetic (EM) spectra. We specifically explore the case of independent, and possibly highly uncertain, localizations, showing that the joint localization area is significantly reduced. We also show that a prior on the binary inclination, potentially arising from GRB observations, has a negligible effect on GW localization. Our algorithm is simple, fast, and accurate, and may be of particular utility in the development of multi-messenger astronomy.

astro-ph.IM↗

arXiv2014

The Loudest Gravitational Wave Events

As first emphasized by Bernard Schutz, there exists a universal distribution of signal-to-noise ratios for gravitational wave detection. Because gravitational waves (GWs) are almost impossible to obscure via dust absorption or other astrophysical processes, the strength of the detected signal is dictated solely by the emission strength and the distance to the source. Assuming that the space density of an arbitrary population of GW sources does not evolve, we show explicitly that the distribution of detected signal-to-noise (SNR) values depends solely on the detection threshold; it is independent of the detector network (interferometer or pulsar timing array), the individual detector noise curves (initial or Advanced LIGO), the nature of the GW sources (compact binary coalescence, supernova, or some other discrete source), and the distributions of source variables (only non-spinning neutron stars of mass exactly $1.4\,M_\odot$ or a complicated distribution of masses and spins). We derive the SNR distribution for each individual detector within a network as a function of the relative detector orientations and sensitivities. While most detections will have SNR near the detection threshold, there will be a tail of events to higher SNR. We derive the SNR distribution of the loudest (highest SNR) events in any given sample of detections. We find that the median SNR of the loudest out of the first four events should have an $\mbox{SNR}=22$ (for a threshold of 12, appropriate for the Advanced LIGO/Virgo network), increasing to a median value for the loudest SNR of 47 for 40 detections. We expect these loudest events to provide particularly powerful constraints on their source parameters, and they will play an important role in extracting astrophysics from gravitational wave sources. These distributions also offer an important internal calibration of the response of the GW detector networks.

gr-qc↗

arXiv2012

GRB beaming and gravitational-wave observations

Using the observed rate of short-duration gamma-ray bursts (GRBs) it is possible to make predictions for the detectable rate of compact binary coalescences in gravitational-wave detectors. These estimates rely crucially on the growing consensus that short gamma-ray bursts are associated with the merger of two neutron stars or a neutron star and a black hole, but otherwise make no assumptions beyond the observed rate of short GRBs. In particular, our results do not assume coincident gravitational wave and electromagnetic observations. We show that the non-detection of mergers in the existing LIGO/Virgo data constrains the progenitor masses and beaming angles of gamma-ray bursts. For future detectors, we find that the first detection of a NS-NS binary coalescence associated with the progenitors of short GRBs is likely to happen within the first 16 months of observation, even in the case of a modest network of observatories (e.g., only LIGO-Hanford and LIGO-Livingston) operating at modest sensitivities (e.g., advanced LIGO design sensitivity, but without signal recycling mirrors), and assuming a conservative distribution of beaming angles (e.g. all GRBs beamed at θ=30 deg). Less conservative assumptions reduce the waiting time until first detection to weeks to months. Alternatively, the compact binary coalescence model of short GRBs can be ruled out if a binary is not seen within the first two years of operation of a LIGO-Hanford, LIGO-Livingston, and Virgo network at advanced design sensitivity. We also demonstrate that the rate of GRB triggered sources is less than the rate of untriggered events if θ<30 deg, independent of the noise curve, network configuration, and observed GRB rate. Thus the first detection in GWs of a binary GRB progenitor is unlikely to be associated with a GRB.

astro-ph.CO↗

arXiv2012

Galactoseismology: Discovery of Vertical Waves in the Galactic Disk

We present evidence for a Galactic North-South asymmetry in the number density and bulk velocity of solar neighborhood stars. The number density profile, which is derived from main-sequence stars in the Sloan Digital Sky Survey, shows a (North - South)/(North + South) deficit at |z| ~ 400 pc and an excess at |z| ~ 800 pc. The bulk velocity profile, which is derived from the Sloan Extension for Galactic Understanding and Exploration, shows a gradual trend across the Galactic midplane as well as smaller-scale features. We speculate that the North-South asymmetry, which has the appearance of a wavelike perturbation, is intrinsic to the disk. We explore the physics of this phenomenon through an analysis of the linearized Boltzmann and Poisson equations and through one-dimensional simulations. The perturbation may be excited by the passage of a satellite galaxy or dark matter subhalo through the Galactic disk, in which case we are witnessing a recent disk-heating event.

astro-ph.GA↗