Session Y16: Gravitational Wave Observations

Live

We present results from an all-sky search for continuous gravitational waves in the LIGO O2 data from the Hanford and Livingston detectors. We search for nearly-monochromatic signals with frequency between 20.0 Hz and 585.15 Hz and spin-down between -2.6e-9 Hz/s and 2.6e-10 Hz/s. We deploy the search on the Einstein@Home volunteer-computing project and follow-up the waveforms associated with the most significant results with eight further search-stages, reaching the best sensitivity ever achieved by an all-sky survey up to 500 Hz. Six of the inspected waveforms pass all the stages but they are all associated with fake signals simulated at the LIGO detector for validation purposes. We recover all these fake signals with consistent parameters. No other waveform survives, so we find no evidence of a continuous gravitational wave signal. We constrain the h0 amplitude of continuous gravitational waves at the detector as a function of the signal frequency, in half-Hz bins. The most constraining upper limit at 163.0 Hz is h0 $=$ 1.3e-25, at the 90{\%} confidence level. Our results exclude neutron stars rotating faster than 5 ms with equatorial ellipticities larger than 1e-7 closer than 100 pc.   

Live

The third observing run of Advanced LIGO and Virgo brought a great wealth of new gravitational-wave (GW) detections. One of the most important discoveries was GW190521, the heaviest binary black-hole merger detected to date with a remnant mass of about 140 solar masses. This observation is the first strong evidence for the existence of intermediate-mass black holes, heavier than stellar mass and lighter than supermassive black holes. The significance of GW190521 was determined by the coherent WaveBurst (cWB) pipeline - a search algorithm which operates with minimal assumptions. I will present the capabilities of cWB to detect unexpected GW sources. Since cWB is not dependent on a signal model, it can detect unusual events, such as binaries with highly precessing spins and eccentricities, or when exact signal models are not available. GW190521 is such an exceptional event and its underlaying physics may indicate the merger history or the stellar environment. Based on the cWB reconstruction I will show that GW190521 properties can be explained more accurately by models which incorporate the effects of precession and higher order modes.   

Live

By probing the population of binary black hole (BBH) mergers detected by LIGO-Virgo, we can infer properties of the underlying black hole formation channels. A mechanism known as pair-instability (PI) supernova is expected to prevent the formation of black holes from stellar collapse with mass greater than $\sim 40-65\,M_\odot$ and less than $\sim 120\,M_\odot$. Any BBH merger detected by LIGO-Virgo with a component black hole in this gap, known as the PI mass gap, likely originated from an alternative formation channel. Here, we identify outlier BBH events of the population of binaries which consist of black holes produced by stellar evolution. If the PI mass gap lower boundary exists between $40-65\,M_\odot$, we firmly establish GW190521 as an outlier to this stellar-mass BBH population [p-value $< 0.01\%$]. If the lower boundary of the PI mass gap is a sharp cutoff at $40\,M_\odot$, we find two BBH events --- GW190521 [p-value $< 0.01\%$] and GW190706_222641 [p-value $< 1\%$] --- to be outliers of this population. Excluding these two events, we find the remaining detected BBH events to be consistent with a population model which includes a build up of black holes before the PI mass gap but inconsistent with a model which does not include this feature.   

Live

Gravitational waves from binary black holes have the potential to yield information on both their masses and spins. While the component masses are usually individually resolvable, a measurement of the component spins is generally elusive. This is partially a consequence of asking about the spins of the most and least massive objects in each binary, a question which becomes ill-defined for equal-mass systems. In this talk, I propose to ask a different question of the data: what are the spins of the most- and least-spinning objects in the binary? Using both simulated systems and the current gravitational-wave events detected by the LIGO-Virgo Collaboration, I will show that this can significantly improve estimates of the individual spins--especially for binary systems with comparable masses--and yield interesting constraints at the population level.   

Live

For black holes in binaries, the misalignment of their individual spins and the binary's orbital angular momentum, $\bf{L}$, causes the orbital plane to precess. The change of direction of $\bf{L}$ induces modulations in the gravitational waveform emitted by the binary. By working in the post-Newtonian regime, where the precession timescale is well-separated from the radiation reaction timescale, we define five new parameters that encapsulate the motion of the orbital angular momentum, $\bf{L}$, about the total angular momentum of the system, $\bf{J}$. Notably, our parameters separate the motion of $\bf{L}$ into its precession and its nutation. We explore the behaviour of our five parameters for isotropic distributions of black hole binaries and review the taxonomy of spin precession using our new framework. One of our results is that the amplitude of nutation is small: the maximum angle between $\bf{L}$ and $\bf{J}$ across our study is $1.4^{\circ}$. We also find that this angle is largest for binaries with moderate mass ratios ($m_2$/$m_1$ $ \approx 0.6$, where $m_1$ ($m_2$) is the mass of the heavier (lighter) black hole) and maximally spinning black holes.   

Live

We present the first search for gravitational waves from the coalescence of stellar mass and sub-solar mass black holes with masses between 20 - 100 $M\odot$ and 0.01 - 1 $M\odot$, respectively. The observation of a single sub-solar mass black hole would establish the existence of primordial black holes and a possible component of dark matter. We'll present results from our search of the public LIGO data and discuss how these observations can inform the potential dark matter contribution from primordial black holes.   

Live

The LIGO-VIRGO Collaboration looks for gravitational waves originating from a broad spectrum of celestial sources. Included among these sources are neutron stars exhibiting non-axisymmetric rotation, which produce long-lived, well-defined, and roughly monochromatic gravitational waves, or continuous waves. Various search methods are used by the LVC when looking for continuous waves based on what prior information is known about the source. Here we describe the use of the Weave implementation of a semi-coherent templated search method using the F-statistic to look for continuous waves in O3a data.   

Live

The recent observation of GW190412, the first high-mass ratio binary black-hole (BBH) merger, by the LIGO-Virgo Collaboration (LVC) provides a unique opportunity to probe the impact of subdominant harmonics and precession effects encoded in a gravitational wave signal. We present refined estimates of source parameters for GW190412 using NRSur7dq4, a recently developed numerical relativity waveform surrogate model that includes all spin-weighted spherical harmonic modes less than 4 as well as the full physical effects of precession. We compare our results with two different variants of phenomenological precessing BBH waveform models, IMRPhenomPv3HM and IMRPhenomXPHM, as well as to the LVC results. Our results are broadly in agreement with IMRPhenomXPHM results and the reported LVC analysis compiled with the SEOBNRv4PHM waveform model, but in tension with IMRPhenomPv3HM. In this talk, I will summarize our refined parameter estimates and the disagreements between models. We also quantify the impact of various modeling assumptions, such the omission of subdominant modes and asymmetric modes (excited during precession), on the interpretation of GW190412. Ongoing parameter estimation efforts with numerical relativity surrogate models will also be summarized.   

Live

In the past few years, there have been attempts to improve the accuracy of waveforms in describing the ringdown signal of a binary black hole merger. Some recent attempts involve including higher, positive-frequency overtones in the waveform to achieve better results. In arXiv:2010.08602, the role of negative-frequency modes, called mirror modes, has been emphasized in obtaining better constraints for remnant parameters at times earlier than the peak of the signal. In this work, we perform data analysis on the ringdown signals of two gravitational wave events, GW150914 and GW190521, and show that the remnant parameters are better estimated at earlier times on the inclusion of mirror modes.