# Glossary¶

- BBH
Binary black hole, a binary system composed of two black holes. See BH.

- BH
Black hole.

- BNS
Binary neutron star, a binary system composed of two neutron stars. See NS.

- burst
In the context of gravitational waves, a signal candidate that is detected without a template and without prior knowledge of the waveform. Examples of potential sources of gravitational-wave bursts include high mass BBH mergers, core-collapse supernovae, and cosmic string cusps.

- chirp mass
In a binary system, the chirp mass is a symmetric combination of the primary and secondary component masses \(m_1\) and \(m_2\) that parameterizes the leading-order time or frequency evolution of the gravitational-wave signal. It is usually denoted by a script “M” symbol, \(\mathcal{M}\), and is defined as \(\mathcal{M} = (m_1 m_2)^{3/5} (m_1 + m_2)^{-1/5}\).

- CBC
Compact binary coalescence.

- FAR
False alarm rate, a statistic that is used to describe the significance of a gravitational-wave event. It is defined as the rate of accidental events due to detector noise or glitches, in the absence of any astrophysical sources, that are as loud as or louder than the event in question.

- FITS
Flexible Image Transport System, a format for astronomical tables, images, and multidimensional data sets. See NASA’s FITS Support Office (https://fits.gsfc.nasa.gov) for specifications, software, and documentation.

- GCN
The Gamma-ray Coordinates Network (https://gcn.gsfc.nasa.gov), a portal for discoveries and observations of astronomical transients. Historically, GCN has served high-energy satellites but now also other electromagnetic wavelengths and also gravitational-wave, cosmic ray, and neutrino facilities.

- GCN Circular
A human-readable astronomical bulletin distributed through GCN.

- GCN Notice
A machine-readable alert distributed through GCN.

- GraceDB
Gravitational Wave Candidate Event Database (https://gracedb.ligo.org), the official public marshal portal for LIGO/Virgo candidates.

- GRB
Gamma-ray burst.

- HEALPix
Hierarchical Equal Area isoLatitude Pixelation, a scheme for indexing positions on the unit sphere. See https://healpix.sourceforge.io.

- HEN
High Energy Neutrino, particularly in the context of multi-messenger GW+HEN follow-up.

- KAGRA
Kamioka Gravitational Wave Detector (see KAGRA home page), an underground gravitational-wave detector in the Kamioka mine in Japan.

- LHO
LIGO Hanford Observatory (see LHO observatory home page), site of a 4 km gravitational-wave detector in Hanford, Washington, USA.

- LLO
LIGO Livingston Observatory (see LLO observatory home page), site of a 4 km gravitational-wave detector in Livingston, Louisiana, USA.

- MassGap
Compact binary systems with at least one compact object whose mass is in the hypothetical “mass gap” between neutron stars and black holes, defined here as 3-5 solar masses.

- MCMC
Markov chain Monte Carlo. A numerical algorithm for sampling complex, multidimensional probability distributions, or for integrating functions of many variables. Used extensively in gravitational-wave parameter estimation.

- MOC
Multi-Order Coverage map, a format to describe the coverage of an arbitrary region on the unit sphere. A MOC consists of a list of HEALPix cells at different depths. For the specification, see the HiPS IVOA Recommendation.

- NS
Neutron star.

- NSBH
Neutron star black hole, a binary system composed of one neutron star and one black hole. See NS, BH.

- O1
Advanced LIGO and Advanced Virgo’s first observing run.

- O2
Advanced LIGO and Advanced Virgo’s second observing run.

- O3
Advanced LIGO and Advanced Virgo’s third observing run.

- primary
When referring to the two component compact objects or the masses of the two component compact objects in a binary, the primary is the more massive one, i.e., \(m_1 \geq m_2\). See secondary.

- range
A figure of merit to describe the sensitivity of a gravitational-wave detector to a given source population at cosmologically significant distances. It is defined as the radius \(R\) of a Euclidean sphere with the volume equal to the sensitive volume \(V_z\). It may be written as:

\[R = \left(\frac{3 V_z}{4 \pi}\right)^{1/3}.\]- secondary
When referring to the two component compact objects or the masses of the two component compact objects in a binary, the secondary is the less massive one, i.e., \(m_2 \leq m_1\). See primary.

- sensitive volume
A figure of merit for the sensitivity of a gravitational-wave detector or a network of detectors. It is defined as the space-time volume surveyed per unit detector time, and may be expressed as (cf. 1):

\[V_\mathrm{z} = \frac{ \int_{z < z^*(\Theta)} p(\Theta) \frac{dV_C}{dz} \frac{dz}{1 + z} }{\int p(\Theta) d\Theta}.\]Here, \(\Theta\) is the set of parameters that describe the gravitational-wave signal (merger time, sky location, orbital elements, masses, and spins) and \(p(\Theta)\) is the redshift-independent population model for those parameters. The term \(\frac{dV_C}{dz}\) is differential comoving volume per unit redshift. The function \(z^*(\Theta)\) is the

*threshold redshift*, or the redshift at which a binary with parameters \(\Theta\) is just at the limit of detection. The factor of \({1 + z}\) in the denominator accounts for time dilation from the source frame to the detector frame.If a population of sources occurs at a fixed rate per unit comoving volume per unit proper time \(\dot{n}\), then the rate of observed events in the detector frame is \(\dot{n} V_z\).

- SN
Supernova.

- SNR
Signal-to-noise ratio, here applied to gravitational-wave signals. It is defined the square root of the integral over frequency of the power spectral density of the gravitational-wave signal over the integral over frequency of the average power spectral density of the noise.

- Terrestrial
Classification for signals in gravitational-wave detectors that are of instrumental or environmental origin. Terrestrial signals are not astrophysical and not due to gravitational waves. Some examples of sources of terrestrial signals are statistical noise fluctuations, detector glitches, and ground motion.

- Virgo
Virgo Observatory (see Virgo observatory home page), site of a 3 km gravitational-wave detector in Cascina, Italy.

- VOEvent
An XML format for describing astronomical transients. For the specification, see the official VOEvent IVOA Recommendation.

- VTP
VOEvent Transport Protocol, a simple TCP-based protocol for sending and receiving VOEvents, used by GCN. For the specification, see the official VTP IVOA recommendation.

- 1
Chen, H.-Y., Holz, D. E., et al. 2017,

*Distance measures in gravitational-wave astrophysics and cosmology*. arXiv:1709.08079