How do Merger Environments Affect Gravitational Waves?

Title: First constraints on compact binary environments from LIGO-Virgo data

Authors: Giada Caneva Santoro, Soumen Roy, Rodrigo Vicente, Maria Haney, Ornella Juliana Piccinni, Walter Del Pozzo, and Mario Martinez

First Author’s Institution: Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology

Status: Accepted for The Physical Review Letters [closed access]

Once a gravitational wave (GW) is detected, computers are triggered to run models in an attempt to characterize the parameters and location of the merger that generated the GW. The models currently used to characterize a merger often assume that a merger happens in a vacuum, ignoring any effects from the environment surrounding the merger. The authors of today’s bite look for the presence of existing environmental effects in GWTC-1 (the Gravitational-Wave Transient Catalog of Compact Binary Mergers Observed by LIGO and Virgo during the First and Second Observing Runs) and discuss if the inclusion of environmental effects (like accretion or dynamical friction) can change the gravitational waveforms enough to alter the estimations of the binary component parameters.

Know thy Merger, Know thy Gravitational Wave

There are a few reasons to care about the environment that a binary black hole (BBH) merger takes place in:

  1. The environments around black holes can tell us a lot about how they form, evolve, and eventually merge. This can be helpful to know when modeling and characterizing black holes.
  2. BBH mergers can have observable electromagnetic counterparts. For example, GW170817 where Chandra X-Ray Observatory was able to observe the first X-Ray counterpart to a GW event in 2017) which can be affected by the merger environment.
  3. The environment a merger takes place in can alter the gravitational waveform recorded by interferometers by LIGO, Virgo, and KAGRA, which can interfere with the parameter estimations for the merger.

The authors of today’s paper seek to better understand the ways in which the environment of a merger can alter the gravitational waveform (i.e. reason 3 in the above list) by studying three potential environmental effects: dynamical friction (DF; the merger interacting with the surrounding matter causing it to lose kinetic energy and momentum–also called gravitational drag), Bondi-Hoyle-Lyttleton accretion (BHLA; how a massive spherical object can accrete material from a uniform environment), and collisionless accretion (CA; the accretion of collisionless plasma onto the binary system or individual components in the binary). They add each of these environmental effects to existing merger models and compare the waveforms they produce to merger events in GWTC-1 to look for the presence of an environment.

The top of the graphic shows the three phases of the merger: inspiral, merger, and ringdown. The bottom of the graphic shows the gravitational waveform. The beginning of the waveform has little structure but closer to the merger a rhythmic signal appears that becomes larger and closer together until the merger occurs. After the merger the signal returns to its original form with very little structure.
Figure 1. Figure from Bailes et al. 2021, adapted from Abbott et al. 2016, showing the detected gravitational wave for GW150914 from LIGO Hanford (red) and Livingston (blue) on the bottom and the phases of the merger corresponding to the GW on the top.

Chasing the Inspiral

While it’s possible for the environment to affect the estimated merger parameters, the authors did not find any evidence for environments around any of the events in GWTC-1. This is not totally surprising though! The environments for these mergers are most easily detected during the inspiral phase which LIGO is not highly sensitive to as seen in Figure 1. The authors also model the observability of merger environments with future detectors like the Einstein Telescope (ET) or the B-DECi-hertz Interferometer Gravitational wave Observatory (B-DECIGO) which will both be more sensitive to the low frequency inspiral. They find that ET will be sensitive to environmental effects of DF and BHLA in relatively diffuse environment (average density of 10-3 g/cm3) while B-DECIGO will be sensitive to even sparser environments (average density of 10-12 g/cm3).

The plot is split four ways in the y-axis one for each of the recovered parameters. Each plot has 6 simulated environment densities spanning from 0 - 147.8 g/cm^3. At each density value on the plot each recovered parameter has two probability distributions reflected over the vertical axis aligning with the injected environmental density. On the right is the distribution for vacuum and on the left is the distribution for the DF effect at the recovered environmental density.  For all of the parameters, the vacuum model struggles to estimate the injected parameter value at environment densities higher than 1.5 g/cm^3 while the DF effect model consistently recovers the injection parameter value at all environment densities.
Figure 2: Figure 3 in the paper. The recovered chirp mass (Mc), mass ratio (q), and effective spin (Xeff) for a GW170817-like waveform deformed with a DF effect for various environmental densities (shown on the x-axis). The posterior distributions shown in brown reflect the parameter probability when using a model in vacuum and in blue show a model with an added DF effect for an estimated environmental density. The red ticks on each distribution show the injected parameter value.

Parameter Estimation Gone Wrong

The authors show an example of how the environment around a merger can influence parameter estimation  by simulating GW170817 with synthetic data and adding in environmental effects of dynamical friction (DF) in Figure 2. They show that for certain environment densities greater than 1.5 g/cm3, the environment does alter the merger parameter estimations, with the vacuum model overestimating the chirp mass and effective spin, and underestimating the mass ratio. While it seems unlikely that environments of mergers will be detected with LIGO in the near future, it’s clear that when future detectors like ET, B-DECIGO, and the Laser Interferometer Space Antenna (LISA) come online adding environments to merger models will be integral to the success of accurately estimating merger parameters.

Astrobite edited by Tori Bonidie

Featured image credit: LIGO

About Erica Sawczynec

I am a third year graduate student at the University of Texas at Austin working on NIR spectroscopy instrumentation. When I'm not in the lab I manage the archive for IGRINS (RRISA) and use the data products to study molecular hydrogen emission in circumstellar disks. Outside of work you can find me reading sci-fi and fantasy novels, baking bread, hanging out with my cat, or over on Twitter @EricaSawczynec.

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