Alert Contents¶

Public LIGO/Virgo alerts are distributing using NASA’s Gamma-ray Coordinates Network (GCN). There are two types of alerts:

GCN Notices are machine-readable packets. They are available as VOEvent XML and several other formats. See the Sample Code section for instructions on receiving GCNs in VOEvent format.

Warning

We recommend receiving LIGO/Virgo alerts in the VOEvent XML format using one of GCN’s anonymous VOEvent brokers. VOEvent over anonymous VTP is the only GCN format and distribution method that is fully supported by LIGO/Virgo.

The VOEvent XML alerts are official data products of LIGO/Virgo. GCN produces `several other legacy formats`_ from them, in particular a text-based “full format” and binary format. LIGO/Virgo performs only limited quality control of the legacy formats.

GCN Circulars are short human-readable astronomical bulletins. They are written in a certain well-established format and style. You can subscribe to GCN Circulars to receive and post them by email, or you can view them in the public GCN Circulars archive.

Notice Types¶

For each event, there are up to four kinds of GCN Notices:

A Preliminary GCN Notice is issued automatically within minutes after a gravitational-wave candidate is detected. The candidate must have passed some automated data quality checks, but it may later be retracted after human vetting. There is no accompanying GCN Circular at this stage.

An Initial GCN Notice is issued after human vetting (see Candidate Vetting). If the signal does not pass human vetting (e.g., it is a glitch), then instead of an initial alert there will be a retraction. The initial alert is also accompanied by a GCN Circular, which should be considered as the first formal publication of the candidate and can be cited as such.

An Update GCN Notice is issued whenever further analysis leads to improved estimates of the sky localization, significance, or classification. There may be multiple updates for a given event, and updates may be issued hours, days, or even weeks after the event.

Lastly, a Retraction GCN Notice is issued if the candidate is rejected as a result of vetting by human instrument scientists and data analysts. A retraction indicates that the candidate has been withdrawn because it is probably not astrophysical.

All types of GCN Notices except for Retraction notices contain the following information, which are described in further detail below:

Name: a unique identifier for the candidate
Significance: estimated false alarm rate (FAR)
Sky localization: inferred sky position and (CBC candidates only) distance
Inference: (CBC candidates only) inferred source classification and properties

Of the above fields, Retraction notices provide only the name.

Initial and Update notices are accompanied by human-readable GCN Circulars, which restate all of the above information and also may include a data quality assessment.

Notice Contents¶

The table below is a representation of the contents of a LIGO/Virgo GCN Notice.

Root
IVORN	`ivo://gwnet/LVC#[{T,M}]SYYMMDDabc-{1,2,3}-{Preliminary,Initial,Update,Retraction}`
Role	`{observation,test}`
Who
Date	Time sent (UTC, ISO-8601), e.g. `2018-11-01T22:34:49`
Author	`LIGO Scientific Collaboration and Virgo Collaboration`
WhereWhen	Time of signal (UTC, ISO-8601), e.g. `2018-11-01T22:22:46.654437`
What
GraceID	GraceDB ID: `[{T,M}]SYYMMDDabc`. Example: `MS181101abc`
Packet Type	GCN Notice type: `{Preliminary,Initial,Update,Retraction}`
Notice Type	Numerical equivalent of GCN Notice type: `{150,151,152,164}`
FAR	Estimated FAR in Hz
Sky Map	Versioned URL of HEALPix FITS sky localization file in the format `https://gracedb.ligo.org/api/superevents/[{T,M}]SYYMMDDabc/files/{bayestar,LALInference,cWB}.fits.gz,[0-8]`. Example: `https://gracedb.ligo.org/api/superevents/S190901ap/files/bayestar.fits.gz,0`
Group	`CBC`	`Burst`
Pipeline	`{gstlal,MBTAOnline,pycbc,spiir}`	`{CWB,oLIB}`
CentralFreq	N/A	Central frequency in Hz
Duration		Duration of burst in s
BNS, NSBH, BBH, MassGap, Noise	Probability that the source is a BNS, NSBH, BBH, or MassGap merger, or terrestrial (i.e, noise) respectively	N/A
HasNS, HasRemnant	Probability, under the assumption that the source is not noise, that at least one of the compact objects was a neutron star, and that the system ejected a non-zero amount of neutron star matter, respectively

Name¶

The name of an event is its GraceDB ID, a uniquely assigned identifier such as MS181101abc. A GraceDB ID has three parts:

Prefix: S for normal candidates and MS or TS for mock or test events respectively. The S stands for superevent.
Date: The six-digit UTC date of the event consisting of a two-digit year, month, and day of month.
Suffix: A lowercase alphabetic string that is incremented automatically (a, b, …, z, aa, ab, … az, aaa, etc.) whenever a candidate on a given date is added to GraceDB.

Significance¶

The significance of the event is quantified by its false alarm rate (FAR): the expected rate of events from the pipeline that produced the preferred event with equal or greater significance in the absence of any astrophysical signals.

Sky Localization¶

The sky localization consists of the posterior probability distribution of the source’s sky position and (for CBC events only) luminosity distance. The GCN Notice and Circular will provide a URL for the sky localization file stored in GraceDB. The sky localization is saved in a FITS file as a HEALPix [1] all-sky image. See our sample code for instructions on working with sky localization files.

The sky map URL will generally be of the form https://gracedb.ligo.org/api/superevents/sid/files/method.fits.gz,v, where sid is the superevent ID, method is the sky localization algorithm (usually bayestar, LALInference, or cWB), and v is an integer that uniquely identifies different versions of the localization. The version number is automatically assigned by GraceDB, starting from 0, and increments for each file of the same name. For example, the first FITS file with the name bayestar.fits.gz becomes bayestar.fits.gz,0, then the next one is bayestar.fits.gz,1, and so on. The filename without the version suffix, such as bayestar.fits.gz, always points to the most recent version.

Important

We generally provide localizations in two HEALPix formats, distinguished by file extension:

*.fits.gz

A subset of the standard HEALPix-in-FITS format (see semi-official specifications from the HEALPix team and from the gamma-ray community) that is recognized by a wide variety of astronomical imaging programs including DS9 and Aladin. It uses HEALPix implicit indexing and the NESTED numbering scheme. This is the primary and preferred format, and the only format that is explicitly listed in the GCN Notices and Circulars. See the section Working with Sky Maps for details.

*.multiorder.fits

A new variant of the HEALPix format that is designed to overcome limitations of the *.fits.gz format for well-localized events from three-detector operations and future gravitational-wave facilities (see rationale in LIGO-G1800186). It uses HEALPix explicit indexing and the NUNIQ numbering scheme, which is closely related to multi-order coverage (MOC) maps in Aladin. This is the internal format that is used by the LIGO/Virgo low-latency alert pipeline. This is an experimental format, and it is currently recommended only for advanced users. See the section Multi-Order Sky Maps (For Advanced Users) for details.

Both formats always use celestial (equatorial, J2000) coordinates.

Inference¶

The inference section is present for CBC events only. It has two parts:

Classification: Five numbers, summing to unity, giving probability that the source belongs to the following five mutually exclusive categories:

BNS merger
NSBH merger
BBH merger
MassGap merger
Terrestrial (i.e., a chance background fluctuation or a glitch)

The figure below shows the extent of the three astrophysical categories (BNS, NSBH, BBH, and MassGap) in terms of the component masses \(m_1\) and \(m_2\).

Note

By convention, the component masses are defined such that \(m_1 \geq m_2\), so that the primary compact object in the binary (i.e., component 1), is always more massive than the secondary compact object (i.e., component 2).

In the mass diagram below, the upper diagonal region \(m_1 < m_2\) is lightly shaded in order to indicate that the definitions of four mass classes (BNS, NSBH, BBH, MassGap) are symmetric in \(m_1\) and \(m_2\).

Properties: Probabilities that the source has each of the following properties, assuming that it is not noise (e.g., assuming that it is a BNS, NSBH, BBH, or MassGap merger):

HasNS: The mass of one or more of the binary’s two companion compact objects is consistent with a neutron star. Equivalently, the mass of the secondary or less massive compact object is consistent with a neutron star, \(m_2 \leq 3 M_\odot\).
HasRemnant: A non-zero amount of neutron star material remained outside the final remnant compact object (a necessary but not sufficient condition to produce certain kinds of electromagnetic emission such as a short GRB or a kilonova).

All of the quantities in the Classification and Properties sections are model dependent to some extent: the Classification section takes into consideration prior knowledge of astrophysical compact binary merger rates from previous LIGO/Virgo observations, and both the Classification and Properties sections depend on details of neutron star physics (e.g. maximum NS mass, equation of state). See the earlier subsection of the Data Analysis section for implementation details.

Circular Contents¶

The following information will be present in the human-readable GCN Circulars.

Data Quality Assessment¶

Circulars may contain concise descriptions of any instrument or data quality issues that may affect the significance estimates or the GW parameter inferences. Unresolved data quality issues could mean that sky localization estimates may shift after they have been mitigated, but does not mean that they will. This is to be considered as advisory information.

Sky Localization Ellipse¶

Generally, GW sky localizations are irregularly shaped. However, for particularly accurately localized events, the sky localization region can be well described by an ellipse. When the area of the 90% ellipse is less than 1.35 times the area of the smallest possible 90% credible region, the GCN Circular will provide a 90% containment ellipse. For details of the ellipse fitting algorithm, see ligo.skymap.postprocess.ellipse.

The ellipse is described in the format of a DS9 region string. Many tools can read DS9 region strings, including DS9, Aladin, astropy-regions, and pyregion. The region string contains the right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis). Here is an example:

icrs; ellipse(03h08m25s, -45d08m14s, 9d, 3d, 112d)

Not Included in Alerts¶

The alerts will not contain quantitative estimates of intrinsic properties such as masses and spins, nor contain information on the GW strain or reconstructed waveforms. After final analysis, those data products are released through the Gravitational Wave Open Science Center.

Examples¶

Below are some sample VOEvents to illustrate the formatting of the GCN Notices.

<?xml version='1.0' encoding='UTF-8'?>
<voe:VOEvent xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:voe="http://www.ivoa.net/xml/VOEvent/v2.0" xsi:schemaLocation="http://www.ivoa.net/xml/VOEvent/v2.0 http://www.ivoa.net/xml/VOEvent/VOEvent-v2.0.xsd" version="2.0" role="test" ivorn="ivo://gwnet/LVC#MS181101ab-1-Preliminary">
  <Who>
    <Date>2018-11-01T22:34:49</Date>
    <Author>
      <contactName>LIGO Scientific Collaboration and Virgo Collaboration</contactName>
    </Author>
  </Who>
  <What>
    <Param dataType="int" name="Packet_Type" value="150">
      <Description>The Notice Type number is assigned/used within GCN, eg type=150 is an LVC_PRELIMINARY notice</Description>
    </Param>
    <Param dataType="int" name="internal" value="0">
      <Description>Indicates whether this event should be distributed to LSC/Virgo members only</Description>
    </Param>
    <Param dataType="int" name="Pkt_Ser_Num" value="1">
      <Description>A number that increments by 1 each time a new revision is issued for this event</Description>
    </Param>
    <Param dataType="string" name="GraceID" ucd="meta.id" value="MS181101ab">
      <Description>Identifier in GraceDB</Description>
    </Param>
    <Param dataType="string" name="AlertType" ucd="meta.version" value="Preliminary">
      <Description>VOEvent alert type</Description>
    </Param>
    <Param dataType="int" name="HardwareInj" ucd="meta.number" value="0">
      <Description>Indicates that this event is a hardware injection if 1, no if 0</Description>
    </Param>
    <Param dataType="int" name="OpenAlert" ucd="meta.number" value="1">
      <Description>Indicates that this event is an open alert if 1, no if 0</Description>
    </Param>
    <Param dataType="string" name="EventPage" ucd="meta.ref.url" value="https://example.org/superevents/MS181101ab/view/">
      <Description>Web page for evolving status of this GW candidate</Description>
    </Param>
    <Param dataType="string" name="Instruments" ucd="meta.code" value="H1,L1,V1">
      <Description>List of instruments used in analysis to identify this event</Description>
    </Param>
    <Param dataType="float" name="FAR" ucd="arith.rate;stat.falsealarm" unit="Hz" value="9.11069936486e-14">
      <Description>False alarm rate for GW candidates with this strength or greater</Description>
    </Param>
    <Param dataType="string" name="Group" ucd="meta.code" value="CBC">
      <Description>Data analysis working group</Description>
    </Param>
    <Param dataType="string" name="Pipeline" ucd="meta.code" value="gstlal">
      <Description>Low-latency data analysis pipeline</Description>
    </Param>
    <Param dataType="string" name="Search" ucd="meta.code" value="MDC">
      <Description>Specific low-latency search</Description>
    </Param>
    <Group name="GW_SKYMAP" type="GW_SKYMAP">
      <Param dataType="string" name="skymap_fits" ucd="meta.ref.url" value="https://emfollow.docs.ligo.org/userguide/_static/bayestar.fits.gz,0">
        <Description>Sky Map FITS</Description>
      </Param>
    </Group>
    <Group name="Classification" type="Classification">
      <Param dataType="float" name="BNS" ucd="stat.probability" value="0.95">
        <Description>Probability that the source is a binary neutron star merger (both objects lighter than 3 solar masses)</Description>
      </Param>
      <Param dataType="float" name="NSBH" ucd="stat.probability" value="0.01">
        <Description>Probability that the source is a neutron star-black hole merger (primary heavier than 5 solar masses, secondary lighter than 3 solar masses)</Description>
      </Param>
      <Param dataType="float" name="BBH" ucd="stat.probability" value="0.03">
        <Description>Probability that the source is a binary black hole merger (both objects heavier than 5 solar masses)</Description>
      </Param>
      <Param dataType="float" name="MassGap" ucd="stat.probability" value="0.0">
        <Description>Probability that the source has at least one object between 3 and 5 solar masses</Description>
      </Param>
      <Param dataType="float" name="Terrestrial" ucd="stat.probability" value="0.01">
        <Description>Probability that the source is terrestrial (i.e., a background noise fluctuation or a glitch)</Description>
      </Param>
      <Description>Source classification: binary neutron star (BNS), neutron star-black hole (NSBH), binary black hole (BBH), MassGap, or terrestrial (noise)</Description>
    </Group>
    <Group name="Properties" type="Properties">
      <Param dataType="float" name="HasNS" ucd="stat.probability" value="0.95">
        <Description>Probability that at least one object in the binary has a mass that is less than 3 solar masses</Description>
      </Param>
      <Param dataType="float" name="HasRemnant" ucd="stat.probability" value="0.91">
        <Description>Probability that a nonzero mass was ejected outside the central remnant object</Description>
      </Param>
      <Description>Qualitative properties of the source, conditioned on the assumption that the signal is an astrophysical compact binary merger</Description>
    </Group>
  </What>
  <WhereWhen>
    <ObsDataLocation>
      <ObservatoryLocation id="LIGO Virgo"/>
      <ObservationLocation>
        <AstroCoordSystem id="UTC-FK5-GEO"/>
        <AstroCoords coord_system_id="UTC-FK5-GEO">
          <Time unit="s">
            <TimeInstant>
              <ISOTime>2018-11-01T22:22:46.654437</ISOTime>
            </TimeInstant>
          </Time>
        </AstroCoords>
      </ObservationLocation>
    </ObsDataLocation>
  </WhereWhen>
  <Description>Report of a candidate gravitational wave event</Description>
  <How>
    <Description>Candidate gravitational wave event identified by low-latency analysis</Description>
    <Description>H1: LIGO Hanford 4 km gravitational wave detector</Description>
    <Description>L1: LIGO Livingston 4 km gravitational wave detector</Description>
    <Description>V1: Virgo 3 km gravitational wave detector</Description>
  </How>
</voe:VOEvent>

<?xml version='1.0' encoding='UTF-8'?>
<voe:VOEvent xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:voe="http://www.ivoa.net/xml/VOEvent/v2.0" xsi:schemaLocation="http://www.ivoa.net/xml/VOEvent/v2.0 http://www.ivoa.net/xml/VOEvent/VOEvent-v2.0.xsd" version="2.0" role="test" ivorn="ivo://gwnet/LVC#MS181101ab-2-Initial">
  <Who>
    <Date>2018-11-01T22:36:25</Date>
    <Author>
      <contactName>LIGO Scientific Collaboration and Virgo Collaboration</contactName>
    </Author>
  </Who>
  <What>
    <Param dataType="int" name="Packet_Type" value="151">
      <Description>The Notice Type number is assigned/used within GCN, eg type=151 is an LVC_INITIAL notice</Description>
    </Param>
    <Param dataType="int" name="internal" value="0">
      <Description>Indicates whether this event should be distributed to LSC/Virgo members only</Description>
    </Param>
    <Param dataType="int" name="Pkt_Ser_Num" value="2">
      <Description>A number that increments by 1 each time a new revision is issued for this event</Description>
    </Param>
    <Param dataType="string" name="GraceID" ucd="meta.id" value="MS181101ab">
      <Description>Identifier in GraceDB</Description>
    </Param>
    <Param dataType="string" name="AlertType" ucd="meta.version" value="Initial">
      <Description>VOEvent alert type</Description>
    </Param>
    <Param dataType="int" name="HardwareInj" ucd="meta.number" value="0">
      <Description>Indicates that this event is a hardware injection if 1, no if 0</Description>
    </Param>
    <Param dataType="int" name="OpenAlert" ucd="meta.number" value="1">
      <Description>Indicates that this event is an open alert if 1, no if 0</Description>
    </Param>
    <Param dataType="string" name="EventPage" ucd="meta.ref.url" value="https://example.org/superevents/MS181101ab/view/">
      <Description>Web page for evolving status of this GW candidate</Description>
    </Param>
    <Param dataType="string" name="Instruments" ucd="meta.code" value="H1,L1,V1">
      <Description>List of instruments used in analysis to identify this event</Description>
    </Param>
    <Param dataType="float" name="FAR" ucd="arith.rate;stat.falsealarm" unit="Hz" value="9.11069936486e-14">
      <Description>False alarm rate for GW candidates with this strength or greater</Description>
    </Param>
    <Param dataType="string" name="Group" ucd="meta.code" value="CBC">
      <Description>Data analysis working group</Description>
    </Param>
    <Param dataType="string" name="Pipeline" ucd="meta.code" value="gstlal">
      <Description>Low-latency data analysis pipeline</Description>
    </Param>
    <Param dataType="string" name="Search" ucd="meta.code" value="MDC">
      <Description>Specific low-latency search</Description>
    </Param>
    <Group name="GW_SKYMAP" type="GW_SKYMAP">
      <Param dataType="string" name="skymap_fits" ucd="meta.ref.url" value="https://emfollow.docs.ligo.org/userguide/_static/bayestar.fits.gz,0">
        <Description>Sky Map FITS</Description>
      </Param>
    </Group>
    <Group name="Classification" type="Classification">
      <Param dataType="float" name="BNS" ucd="stat.probability" value="0.95">
        <Description>Probability that the source is a binary neutron star merger (both objects lighter than 3 solar masses)</Description>
      </Param>
      <Param dataType="float" name="NSBH" ucd="stat.probability" value="0.01">
        <Description>Probability that the source is a neutron star-black hole merger (primary heavier than 5 solar masses, secondary lighter than 3 solar masses)</Description>
      </Param>
      <Param dataType="float" name="BBH" ucd="stat.probability" value="0.03">
        <Description>Probability that the source is a binary black hole merger (both objects heavier than 5 solar masses)</Description>
      </Param>
      <Param dataType="float" name="MassGap" ucd="stat.probability" value="0.0">
        <Description>Probability that the source has at least one object between 3 and 5 solar masses</Description>
      </Param>
      <Param dataType="float" name="Terrestrial" ucd="stat.probability" value="0.01">
        <Description>Probability that the source is terrestrial (i.e., a background noise fluctuation or a glitch)</Description>
      </Param>
      <Description>Source classification: binary neutron star (BNS), neutron star-black hole (NSBH), binary black hole (BBH), MassGap, or terrestrial (noise)</Description>
    </Group>
    <Group name="Properties" type="Properties">
      <Param dataType="float" name="HasNS" ucd="stat.probability" value="0.95">
        <Description>Probability that at least one object in the binary has a mass that is less than 3 solar masses</Description>
      </Param>
      <Param dataType="float" name="HasRemnant" ucd="stat.probability" value="0.91">
        <Description>Probability that a nonzero mass was ejected outside the central remnant object</Description>
      </Param>
      <Description>Qualitative properties of the source, conditioned on the assumption that the signal is an astrophysical compact binary merger</Description>
    </Group>
  </What>
  <WhereWhen>
    <ObsDataLocation>
      <ObservatoryLocation id="LIGO Virgo"/>
      <ObservationLocation>
        <AstroCoordSystem id="UTC-FK5-GEO"/>
        <AstroCoords coord_system_id="UTC-FK5-GEO">
          <Time unit="s">
            <TimeInstant>
              <ISOTime>2018-11-01T22:22:46.654437</ISOTime>
            </TimeInstant>
          </Time>
        </AstroCoords>
      </ObservationLocation>
    </ObsDataLocation>
  </WhereWhen>
  <Description>Report of a candidate gravitational wave event</Description>
  <How>
    <Description>Candidate gravitational wave event identified by low-latency analysis</Description>
    <Description>H1: LIGO Hanford 4 km gravitational wave detector</Description>
    <Description>L1: LIGO Livingston 4 km gravitational wave detector</Description>
    <Description>V1: Virgo 3 km gravitational wave detector</Description>
  </How>
  <Citations>
    <EventIVORN cite="supersedes">ivo://gwnet/LVC#MS181101ab-1-Preliminary</EventIVORN>
    <Description>Initial localization is now available</Description>
  </Citations>
</voe:VOEvent>

<?xml version='1.0' encoding='UTF-8'?>
<voe:VOEvent xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:voe="http://www.ivoa.net/xml/VOEvent/v2.0" xsi:schemaLocation="http://www.ivoa.net/xml/VOEvent/v2.0 http://www.ivoa.net/xml/VOEvent/VOEvent-v2.0.xsd" version="2.0" role="test" ivorn="ivo://gwnet/LVC#MS181101ab-3-Update">
  <Who>
    <Date>2018-11-01T22:36:25</Date>
    <Author>
      <contactName>LIGO Scientific Collaboration and Virgo Collaboration</contactName>
    </Author>
  </Who>
  <What>
    <Param dataType="int" name="Packet_Type" value="152">
      <Description>The Notice Type number is assigned/used within GCN, eg type=152 is an LVC_UPDATE notice</Description>
    </Param>
    <Param dataType="int" name="internal" value="0">
      <Description>Indicates whether this event should be distributed to LSC/Virgo members only</Description>
    </Param>
    <Param dataType="int" name="Pkt_Ser_Num" value="3">
      <Description>A number that increments by 1 each time a new revision is issued for this event</Description>
    </Param>
    <Param dataType="string" name="GraceID" ucd="meta.id" value="MS181101ab">
      <Description>Identifier in GraceDB</Description>
    </Param>
    <Param dataType="string" name="AlertType" ucd="meta.version" value="Update">
      <Description>VOEvent alert type</Description>
    </Param>
    <Param dataType="int" name="HardwareInj" ucd="meta.number" value="0">
      <Description>Indicates that this event is a hardware injection if 1, no if 0</Description>
    </Param>
    <Param dataType="int" name="OpenAlert" ucd="meta.number" value="1">
      <Description>Indicates that this event is an open alert if 1, no if 0</Description>
    </Param>
    <Param dataType="string" name="EventPage" ucd="meta.ref.url" value="https://example.org/superevents/MS181101ab/view/">
      <Description>Web page for evolving status of this GW candidate</Description>
    </Param>
    <Param dataType="string" name="Instruments" ucd="meta.code" value="H1,L1,V1">
      <Description>List of instruments used in analysis to identify this event</Description>
    </Param>
    <Param dataType="float" name="FAR" ucd="arith.rate;stat.falsealarm" unit="Hz" value="9.11069936486e-14">
      <Description>False alarm rate for GW candidates with this strength or greater</Description>
    </Param>
    <Param dataType="string" name="Group" ucd="meta.code" value="CBC">
      <Description>Data analysis working group</Description>
    </Param>
    <Param dataType="string" name="Pipeline" ucd="meta.code" value="gstlal">
      <Description>Low-latency data analysis pipeline</Description>
    </Param>
    <Param dataType="string" name="Search" ucd="meta.code" value="MDC">
      <Description>Specific low-latency search</Description>
    </Param>
    <Group name="GW_SKYMAP" type="GW_SKYMAP">
      <Param dataType="string" name="skymap_fits" ucd="meta.ref.url" value="https://emfollow.docs.ligo.org/userguide/_static/bayestar.fits.gz,0">
        <Description>Sky Map FITS</Description>
      </Param>
    </Group>
    <Group name="Classification" type="Classification">
      <Param dataType="float" name="BNS" ucd="stat.probability" value="0.95">
        <Description>Probability that the source is a binary neutron star merger (both objects lighter than 3 solar masses)</Description>
      </Param>
      <Param dataType="float" name="NSBH" ucd="stat.probability" value="0.01">
        <Description>Probability that the source is a neutron star-black hole merger (primary heavier than 5 solar masses, secondary lighter than 3 solar masses)</Description>
      </Param>
      <Param dataType="float" name="BBH" ucd="stat.probability" value="0.03">
        <Description>Probability that the source is a binary black hole merger (both objects heavier than 5 solar masses)</Description>
      </Param>
      <Param dataType="float" name="MassGap" ucd="stat.probability" value="0.0">
        <Description>Probability that the source has at least one object between 3 and 5 solar masses</Description>
      </Param>
      <Param dataType="float" name="Terrestrial" ucd="stat.probability" value="0.01">
        <Description>Probability that the source is terrestrial (i.e., a background noise fluctuation or a glitch)</Description>
      </Param>
      <Description>Source classification: binary neutron star (BNS), neutron star-black hole (NSBH), binary black hole (BBH), MassGap, or terrestrial (noise)</Description>
    </Group>
    <Group name="Properties" type="Properties">
      <Param dataType="float" name="HasNS" ucd="stat.probability" value="0.95">
        <Description>Probability that at least one object in the binary has a mass that is less than 3 solar masses</Description>
      </Param>
      <Param dataType="float" name="HasRemnant" ucd="stat.probability" value="0.91">
        <Description>Probability that a nonzero mass was ejected outside the central remnant object</Description>
      </Param>
      <Description>Qualitative properties of the source, conditioned on the assumption that the signal is an astrophysical compact binary merger</Description>
    </Group>
  </What>
  <WhereWhen>
    <ObsDataLocation>
      <ObservatoryLocation id="LIGO Virgo"/>
      <ObservationLocation>
        <AstroCoordSystem id="UTC-FK5-GEO"/>
        <AstroCoords coord_system_id="UTC-FK5-GEO">
          <Time unit="s">
            <TimeInstant>
              <ISOTime>2018-11-01T22:22:46.654437</ISOTime>
            </TimeInstant>
          </Time>
        </AstroCoords>
      </ObservationLocation>
    </ObsDataLocation>
  </WhereWhen>
  <Description>Report of a candidate gravitational wave event</Description>
  <How>
    <Description>Candidate gravitational wave event identified by low-latency analysis</Description>
    <Description>H1: LIGO Hanford 4 km gravitational wave detector</Description>
    <Description>L1: LIGO Livingston 4 km gravitational wave detector</Description>
    <Description>V1: Virgo 3 km gravitational wave detector</Description>
  </How>
  <Citations>
    <EventIVORN cite="supersedes">ivo://gwnet/LVC#MS181101ab-2-Initial</EventIVORN>
    <EventIVORN cite="supersedes">ivo://gwnet/LVC#MS181101ab-1-Preliminary</EventIVORN>
    <Description>Updated localization is now available</Description>
  </Citations>
</voe:VOEvent>

<?xml version='1.0' encoding='UTF-8'?>
<voe:VOEvent xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:voe="http://www.ivoa.net/xml/VOEvent/v2.0" xsi:schemaLocation="http://www.ivoa.net/xml/VOEvent/v2.0 http://www.ivoa.net/xml/VOEvent/VOEvent-v2.0.xsd" version="2.0" role="test" ivorn="ivo://gwnet/LVC#MS181101ab-4-Retraction">
  <Who>
    <Date>2018-11-01T23:36:23</Date>
    <Author>
      <contactName>LIGO Scientific Collaboration and Virgo Collaboration</contactName>
    </Author>
  </Who>
  <What>
    <Param dataType="int" name="Packet_Type" value="164">
      <Description>The Notice Type number is assigned/used within GCN, eg type=164 is an LVC_RETRACTION notice</Description>
    </Param>
    <Param dataType="int" name="internal" value="0">
      <Description>Indicates whether this event should be distributed to LSC/Virgo members only</Description>
    </Param>
    <Param dataType="int" name="Pkt_Ser_Num" value="4">
      <Description>A number that increments by 1 each time a new revision is issued for this event</Description>
    </Param>
    <Param dataType="string" name="GraceID" ucd="meta.id" value="MS181101ab">
      <Description>Identifier in GraceDB</Description>
    </Param>
    <Param dataType="string" name="AlertType" ucd="meta.version" value="Retraction">
      <Description>VOEvent alert type</Description>
    </Param>
    <Param dataType="int" name="HardwareInj" ucd="meta.number" value="0">
      <Description>Indicates that this event is a hardware injection if 1, no if 0</Description>
    </Param>
    <Param dataType="int" name="OpenAlert" ucd="meta.number" value="1">
      <Description>Indicates that this event is an open alert if 1, no if 0</Description>
    </Param>
    <Param dataType="string" name="EventPage" ucd="meta.ref.url" value="https://example.org/superevents/MS181101ab/view/">
      <Description>Web page for evolving status of this GW candidate</Description>
    </Param>
  </What>
  <WhereWhen>
    <ObsDataLocation>
      <ObservatoryLocation id="LIGO Virgo"/>
      <ObservationLocation>
        <AstroCoordSystem id="UTC-FK5-GEO"/>
        <AstroCoords coord_system_id="UTC-FK5-GEO">
          <Time unit="s">
            <TimeInstant>
              <ISOTime>2018-11-01T22:22:46.654437</ISOTime>
            </TimeInstant>
          </Time>
        </AstroCoords>
      </ObservationLocation>
    </ObsDataLocation>
  </WhereWhen>
  <Citations>
    <EventIVORN cite="retraction">ivo://gwnet/LVC#MS181101ab-3-Update</EventIVORN>
    <EventIVORN cite="retraction">ivo://gwnet/LVC#MS181101ab-2-Initial</EventIVORN>
    <EventIVORN cite="retraction">ivo://gwnet/LVC#MS181101ab-1-Preliminary</EventIVORN>
    <Description>Determined to not be a viable GW event candidate</Description>
  </Citations>
</voe:VOEvent>

[1]	Górski, K.M., Hivon, E., Banday, A.J., et al. 2005, Astrophys. J., 622, 759. doi:10.1086/427976

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