# Glossary¶

- Avro¶
Apache Avro is an open, binary data serialization format. See https://avro.apache.org for more information.

- Base64¶
A common binary encoding scheme supported by many open-source libraries, e.g. the Python standard library.

- 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.

- EOS¶
Equation of state. The equation of state determines the relation between mass and radius, or mass and compactness, or pressure and (mass, energy, or number) density, of neutron stars.

- FAP¶
False alarm probability, a measure of the fraction of noise events incorrectly classified as astrophysical events by a binary classifier.

- FNP¶
False negative probability, a measure of the fraction of astrophysical events incorrectly classified as noise by a binary classifier.

- FAR¶
False alarm rate, a statistic that is used to describe the significance of a gravitational-wave event. See section false alarm rate (FAR) for details. FAR has units of frequency (one over time).

- 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 General Coordinates Network (https://gcn.nasa.gov), a NASA-hosted public portal for discoveries and observations of astronomical transients. GCN hosts one of the Kafka brokers used to distribute LIGO/Virgo/KAGRA alerts. See https://gcn.nasa.gov.

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

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

- GRB¶
Gamma-ray burst.

- GWSkyNet¶
A machine learning classifier for the identification of multi-detector CBC events and noise [1]. See section GWSkyNet classification for details.

- 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.

- JSON¶
JavaScript Object Notation is an open data serialization format. JSON-serialized data look like JavaScript literals. See https://json.org for more information.

- Kafka¶
Apache Kafka is an open-source distributed event streaming platform. See https://kafka.apache.org for more information.

- 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.

- Notice¶
- 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 that is uniformly distributed in volume. 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:

\[R = \left(\frac{3 V_z}{4 \pi}\right)^{1/3}.\]- ROC¶
A receiver operating characteristic (ROC) curve is a plot that illustrates the diagnostic ability of a binary classifier system with variable discrimination threshold. It is a plot of the true positive rate (TPR) against the false positive rate (FPR), at various threshold settings.

- 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. [2]):

\[V_\mathrm{z} = \frac{ \int_{z < z^*(\Theta)} p(\Theta) \frac{dV_C}{dz} \frac{dz}{1 + z} }{\int p(\Theta) d\Theta}.\]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. Detection may be defined, for instance, by a threshold on event SNR or FAR; see Detection Threshold for thresholds used in this guide. 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 as the square root of the integral over frequency of the power spectral density of the gravitational-wave signal divided by the integral over frequency of the average power spectral density of the noise.

- source-frame mass¶
Since observed frequencies of distant sources are subject to redshift by \(f_\mathrm{obs} = (1 + z)^{-1} f_\mathrm{source}\), and gravitational-wave frequency scales inversely with mass, the observer- and source-frame masses are related by \(m_\mathrm{obs} = (1 + z) m_\mathrm{source}\).

- 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.

- SCiMMA¶
Scalable Cyberinfrastructure to support Multi-Messenger Astrophysics. SCiMMA hosts one of the Kafka brokers used to distribute LIGO/Virgo/KAGRA alerts. See https://scimma.org.

- 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.