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. We strongly recommend receiving LIGO/Virgo alerts in the VOEvent XML format.

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 (i.e., 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 source 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 A retraction indicates that the candidate has been withdrawn because it is probably not astrophysical.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
• Localization: inferred sky position and (CBC candidates only) distance
• Inference: inferred source classification and properties (CBC candidates only)

All types of GCN Notices except for Preliminary notices are accompanied by human-readable GCN Circulars, which restates all of the above information as well as a data quality assessment.

Notice Contents

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

Root
IVORN	ivo://nasa.gsfc.gcn/LVC#[{T,M}]SYYMMDDabc-{1,2,3}-{Preliminary,Initial,Update,Preliminary-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}
Notice Type	Numerical equivalent of GCN Notice type: {150,151,152}
FAR	Estimated false alarm rate in Hz
Sky Map	URL of HEALPix FITS localization file
Group	CBC	Burst
Pipeline	{Gstlal,MBTAOnline,PyCBC,SPIIR}	{cWB,oLIB}
CentralFreq	N/A	Central frequency in Hz
Duration		Duration of burst in s
Fluence		Gravitational-wave fluence in erg cm\(^{-2}\)
BNS, NSBH, BBH, Noise	Probability that the source is a BNS, NSBH, NSBH 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 nonzero 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.

Localization

The 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 localization file stored in GraceDb. The localization is saved in a FITS file as a HEALPix all-sky image. See our sample code for instructions on working with localization files.

Inference

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

Classification: Four numbers, summing to unity, giving probability that the source belongs to the following four categories:

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

The figure below shows the extent of the three astrophysical categories (BNS, NSBH, and BBH) in terms of the component masses \(m_1\) 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, or BBH merger):

• HasNS: The mass of one or more of the binary’s two companion compact objects is consistent with a neutron star.
• HasRemnant: A nonzero 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 procedures 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 localization estimates may shift after they have been mitigated, but does not mean that they will. This is to be considered as advisory information.

Localization Ellipse

Generally, GW localizations are irregularly shaped. However, for particularly accurately localized events, the localization region can be well described by an ellipse. For well-localized events, the GCN Circulars will include a 90% containment ellipse in the format of a DS9 region string (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis), for instance:

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.

Preliminary

Initial

Update

Retraction

<?xml version="1.0" ?>
<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/gcn_sender#MS181101ab-1-Preliminary">
    <Who>
        <Date>2018-11-01T22:34:49</Date>
        <Author>
            <contactName>LIGO Scientific Collaboration and Virgo Collaboration</contactName>
        </Author>
    </Who>
    <What>
        <Param name="internal" dataType="int" value="0">
            <Description>Indicates whether this event should be distributed to LSC/Virgo members only</Description>
        </Param>
        <Param name="Packet_Type" dataType="int" value="150">
            <Description>The Notice Type number is assigned/used within GCN, eg type=150 is an LVC_PRELIMINARY notice</Description>
        </Param>
        <Param name="Pkt_Ser_Num" dataType="string" value="1"/>
        <Param name="GraceID" dataType="string" value="MS181101ab" ucd="meta.id">
            <Description>Identifier in GraceDB</Description>
        </Param>
        <Param name="AlertType" dataType="string" value="Preliminary" ucd="meta.version">
            <Description>VOEvent alert type</Description>
        </Param>
        <Param name="Retraction" dataType="int" value="0">
            <Description>Set to 1 if the event is retracted.</Description>
        </Param>
        <Param name="HardwareInj" dataType="int" value="0" ucd="meta.number">
            <Description>Indicates that this event is a hardware injection if 1, no if 0</Description>
        </Param>
        <Param name="OpenAlert" dataType="int" value="1" ucd="meta.number">
            <Description>Indicates that this event is an open alert if 1, no if 0</Description>
        </Param>
        <Param name="EventPage" dataType="string" value="https://example.org/superevents/MS181101abc/view/" ucd="meta.ref.url">
            <Description>Web page for evolving status of this GW candidate</Description>
        </Param>
        <Param name="Instruments" dataType="string" value="H1,L1" ucd="meta.code">
            <Description>List of instruments used in analysis to identify this event</Description>
        </Param>
        <Param name="FAR" dataType="float" value="9.11069936486e-14" ucd="arith.rate;stat.falsealarm" unit="Hz">
            <Description>False alarm rate for GW candidates with this strength or greater</Description>
        </Param>
        <Param name="Group" dataType="string" value="CBC" ucd="meta.code">
            <Description>Data analysis working group</Description>
        </Param>
        <Param name="Pipeline" dataType="string" value="gstlal" ucd="meta.code">
            <Description>Low-latency data analysis pipeline</Description>
        </Param>
        <Param name="Search" dataType="string" value="MDC" ucd="meta.code">
            <Description>Specific low-latency search</Description>
        </Param>
        <Group type="GW_SKYMAP" name="bayestar">
            <Param name="skymap_fits" dataType="string" value="https://emfollow.docs.ligo.org/userguide/_static/bayestar.fits.gz" ucd="meta.ref.url">
                <Description>Sky Map FITS</Description>
            </Param>
            <Param name="skymap_png" dataType="string" value="https://emfollow.docs.ligo.org/userguide/_static/bayestar.png" ucd="meta.ref.url">
                <Description>Sky Map image</Description>
            </Param>
        </Group>
        <Group type="Classification">
            <Description>
                Source classification: binary neutron star (BNS), neutron star-black hole (NSBH), binary black hole (BBH), or terrestrial (noise)
            </Description>
            <Param name="BNS" dataType="float" value="0.95" ucd="stat.probability">
                <Description>Probability that the source is a binary neutron star merger</Description>
            </Param>
            <Param name="NSBH" dataType="float" value="0.01" ucd="stat.probability">
                <Description>Probability that the source is a neutron star - black hole merger</Description>
            </Param>
            <Param name="BBH" dataType="float" value="0.03" ucd="stat.probability">
                <Description>Probability that the source is a binary black hole merger</Description>
            </Param>
            <Param name="Terrestrial" dataType="float" value="0.01" ucd="stat.probability">
                <Description>Probability that the source is terrestrial (i.e., a background noise fluctuation or a glitch)</Description>
            </Param>
        </Group>
        <Group type="Properties">
            <Description>
                Qualitative properties of the source, conditioned on the assumption that the signal is an astrophysical compact binary merger
            </Description>
            <Param name="HasNS" dataType="float" value="0.95" ucd="stat.probability">
                <Description>Probability that at least one object in the binary has a mass that is less than 3 solar masses</Description>
            </Param>
            <Param name="HasRemnant" dataType="float" value="0.91" ucd="stat.probability">
                <Description>Probability that a nonzero mass was ejected outside the central remnant object</Description>
            </Param>
        </Group>
    </What>
    <WhereWhen>
        <ObsDataLocation>
            <ObservatoryLocation id="LIGO Virgo"/>
            <ObservationLocation>
                <AstroCoordSystem id="UTC-FK5-GEO"/>
                <AstroCoords coord_system_id="UTC-FK5-GEO">
                    <Time>
                        <TimeInstant>
                            <ISOTime>2018-11-01T22:22:46.654437</ISOTime>
                        </TimeInstant>
                    </Time>
                </AstroCoords>
            </ObservationLocation>
        </ObsDataLocation>
    </WhereWhen>
    <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>
    </How>
    <Description>Report of a candidate gravitational wave event</Description>
</voe:VOEvent>

<?xml version="1.0" ?>
<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/gcn_sender#MS181101ab-2-Initial">
    <Who>
        <Date>2018-11-01T22:36:25</Date>
        <Author>
            <contactName>LIGO Scientific Collaboration and Virgo Collaboration</contactName>
        </Author>
    </Who>
    <What>
        <Param name="internal" dataType="int" value="0">
            <Description>Indicates whether this event should be distributed to LSC/Virgo members only</Description>
        </Param>
        <Param name="Packet_Type" dataType="int" value="151">
            <Description>The Notice Type number is assigned/used within GCN, eg type=151 is an LVC_INITIAL notice</Description>
        </Param>
        <Param name="Pkt_Ser_Num" dataType="string" value="2"/>
        <Param name="GraceID" dataType="string" value="MS181101ab" ucd="meta.id">
            <Description>Identifier in GraceDB</Description>
        </Param>
        <Param name="AlertType" dataType="string" value="Initial" ucd="meta.version">
            <Description>VOEvent alert type</Description>
        </Param>
        <Param name="Retraction" dataType="int" value="0">
            <Description>Set to 1 if the event is retracted.</Description>
        </Param>
        <Param name="HardwareInj" dataType="int" value="0" ucd="meta.number">
            <Description>Indicates that this event is a hardware injection if 1, no if 0</Description>
        </Param>
        <Param name="OpenAlert" dataType="int" value="1" ucd="meta.number">
            <Description>Indicates that this event is an open alert if 1, no if 0</Description>
        </Param>
        <Param name="EventPage" dataType="string" value="https://example.org/superevents/MS181101abc/view/" ucd="meta.ref.url">
            <Description>Web page for evolving status of this GW candidate</Description>
        </Param>
        <Param name="Instruments" dataType="string" value="H1,L1" ucd="meta.code">
            <Description>List of instruments used in analysis to identify this event</Description>
        </Param>
        <Param name="FAR" dataType="float" value="9.11069936486e-14" ucd="arith.rate;stat.falsealarm" unit="Hz">
            <Description>False alarm rate for GW candidates with this strength or greater</Description>
        </Param>
        <Param name="Group" dataType="string" value="CBC" ucd="meta.code">
            <Description>Data analysis working group</Description>
        </Param>
        <Param name="Pipeline" dataType="string" value="gstlal" ucd="meta.code">
            <Description>Low-latency data analysis pipeline</Description>
        </Param>
        <Param name="Search" dataType="string" value="MDC" ucd="meta.code">
            <Description>Specific low-latency search</Description>
        </Param>
        <Group type="GW_SKYMAP" name="bayestar">
            <Param name="skymap_fits" dataType="string" value="https://emfollow.docs.ligo.org/userguide/_static/bayestar.fits.gz" ucd="meta.ref.url">
                <Description>Sky Map FITS</Description>
            </Param>
            <Param name="skymap_png" dataType="string" value="https://emfollow.docs.ligo.org/userguide/_static/bayestar.png" ucd="meta.ref.url">
                <Description>Sky Map image</Description>
            </Param>
        </Group>
        <Group type="Classification">
            <Description>
                Source classification: binary neutron star (BNS), neutron star-black hole (NSBH), binary black hole (BBH), or terrestrial (noise)
            </Description>
            <Param name="BNS" dataType="float" value="0.95" ucd="stat.probability">
                <Description>Probability that the source is a binary neutron star merger</Description>
            </Param>
            <Param name="NSBH" dataType="float" value="0.01" ucd="stat.probability">
                <Description>Probability that the source is a neutron star - black hole merger</Description>
            </Param>
            <Param name="BBH" dataType="float" value="0.03" ucd="stat.probability">
                <Description>Probability that the source is a binary black hole merger</Description>
            </Param>
            <Param name="Terrestrial" dataType="float" value="0.01" ucd="stat.probability">
                <Description>Probability that the source is terrestrial (i.e., a background noise fluctuation or a glitch)</Description>
            </Param>
        </Group>
        <Group type="Properties">
            <Description>
                Qualitative properties of the source, conditioned on the assumption that the signal is an astrophysical compact binary merger
            </Description>
            <Param name="HasNS" dataType="float" value="0.95" ucd="stat.probability">
                <Description>Probability that at least one object in the binary has a mass that is less than 3 solar masses</Description>
            </Param>
            <Param name="HasRemnant" dataType="float" value="0.91" ucd="stat.probability">
                <Description>Probability that a nonzero mass was ejected outside the central remnant object</Description>
            </Param>
        </Group>
    </What>
    <WhereWhen>
        <ObsDataLocation>
            <ObservatoryLocation id="LIGO Virgo"/>
            <ObservationLocation>
                <AstroCoordSystem id="UTC-FK5-GEO"/>
                <AstroCoords coord_system_id="UTC-FK5-GEO">
                    <Time>
                        <TimeInstant>
                            <ISOTime>2018-11-01T22:22:46.654437</ISOTime>
                        </TimeInstant>
                    </Time>
                </AstroCoords>
            </ObservationLocation>
        </ObsDataLocation>
    </WhereWhen>
    <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>
    </How>
    <Description>Report of a candidate gravitational wave event</Description>
</voe:VOEvent>

<?xml version="1.0" ?>
<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/gcn_sender#MS181101ab-2-Initial">
    <Who>
        <Date>2018-11-01T22:36:25</Date>
        <Author>
            <contactName>LIGO Scientific Collaboration and Virgo Collaboration</contactName>
        </Author>
    </Who>
    <What>
        <Param name="internal" dataType="int" value="0">
            <Description>Indicates whether this event should be distributed to LSC/Virgo members only</Description>
        </Param>
        <Param name="Packet_Type" dataType="int" value="152">
            <Description>The Notice Type number is assigned/used within GCN, eg type=151 is an LVC_INITIAL notice</Description>
        </Param>
        <Param name="Pkt_Ser_Num" dataType="string" value="3"/>
        <Param name="GraceID" dataType="string" value="MS181101ab" ucd="meta.id">
            <Description>Identifier in GraceDB</Description>
        </Param>
        <Param name="AlertType" dataType="string" value="Update" ucd="meta.version">
            <Description>VOEvent alert type</Description>
        </Param>
        <Param name="Retraction" dataType="int" value="0">
            <Description>Set to 1 if the event is retracted.</Description>
        </Param>
        <Param name="HardwareInj" dataType="int" value="0" ucd="meta.number">
            <Description>Indicates that this event is a hardware injection if 1, no if 0</Description>
        </Param>
        <Param name="OpenAlert" dataType="int" value="1" ucd="meta.number">
            <Description>Indicates that this event is an open alert if 1, no if 0</Description>
        </Param>
        <Param name="EventPage" dataType="string" value="https://example.org/superevents/MS181101abc/view/" ucd="meta.ref.url">
            <Description>Web page for evolving status of this GW candidate</Description>
        </Param>
        <Param name="Instruments" dataType="string" value="H1,L1" ucd="meta.code">
            <Description>List of instruments used in analysis to identify this event</Description>
        </Param>
        <Param name="FAR" dataType="float" value="9.11069936486e-14" ucd="arith.rate;stat.falsealarm" unit="Hz">
            <Description>False alarm rate for GW candidates with this strength or greater</Description>
        </Param>
        <Param name="Group" dataType="string" value="CBC" ucd="meta.code">
            <Description>Data analysis working group</Description>
        </Param>
        <Param name="Pipeline" dataType="string" value="gstlal" ucd="meta.code">
            <Description>Low-latency data analysis pipeline</Description>
        </Param>
        <Param name="Search" dataType="string" value="MDC" ucd="meta.code">
            <Description>Specific low-latency search</Description>
        </Param>
        <Group type="GW_SKYMAP" name="bayestar">
            <Param name="skymap_fits" dataType="string" value="https://emfollow.docs.ligo.org/userguide/_static/bayestar.fits.gz" ucd="meta.ref.url">
                <Description>Sky Map FITS</Description>
            </Param>
            <Param name="skymap_png" dataType="string" value="https://emfollow.docs.ligo.org/userguide/_static/bayestar.png" ucd="meta.ref.url">
                <Description>Sky Map image</Description>
            </Param>
        </Group>
        <Group type="Classification">
            <Description>
                Source classification: binary neutron star (BNS), neutron star-black hole (NSBH), binary black hole (BBH), or terrestrial (noise)
            </Description>
            <Param name="BNS" dataType="float" value="0.95" ucd="stat.probability">
                <Description>Probability that the source is a binary neutron star merger</Description>
            </Param>
            <Param name="NSBH" dataType="float" value="0.01" ucd="stat.probability">
                <Description>Probability that the source is a neutron star - black hole merger</Description>
            </Param>
            <Param name="BBH" dataType="float" value="0.03" ucd="stat.probability">
                <Description>Probability that the source is a binary black hole merger</Description>
            </Param>
            <Param name="Terrestrial" dataType="float" value="0.01" ucd="stat.probability">
                <Description>Probability that the source is terrestrial (i.e., a background noise fluctuation or a glitch)</Description>
            </Param>
        </Group>
        <Group type="Properties">
            <Description>
                Qualitative properties of the source, conditioned on the assumption that the signal is an astrophysical compact binary merger
            </Description>
            <Param name="HasNS" dataType="float" value="0.95" ucd="stat.probability">
                <Description>Probability that at least one object in the binary has a mass that is less than 3 solar masses</Description>
            </Param>
            <Param name="HasRemnant" dataType="float" value="0.91" ucd="stat.probability">
                <Description>Probability that a nonzero mass was ejected outside the central remnant object</Description>
            </Param>
        </Group>
    </What>
    <WhereWhen>
        <ObsDataLocation>
            <ObservatoryLocation id="LIGO Virgo"/>
            <ObservationLocation>
                <AstroCoordSystem id="UTC-FK5-GEO"/>
                <AstroCoords coord_system_id="UTC-FK5-GEO">
                    <Time>
                        <TimeInstant>
                            <ISOTime>2018-11-01T22:22:46.654437</ISOTime>
                        </TimeInstant>
                    </Time>
                </AstroCoords>
            </ObservationLocation>
        </ObsDataLocation>
    </WhereWhen>
    <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>
    </How>
    <Description>Report of a candidate gravitational wave event</Description>
</voe:VOEvent>

<?xml version="1.0" ?>
<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/gcn_sender#MS181101ab-4-Preliminary-Retraction">
    <Who>
        <Date>2018-11-01T23:36:23</Date>
        <Author>
            <contactName>LIGO Scientific Collaboration and Virgo Collaboration</contactName>
        </Author>
    </Who>
    <What>
        <Param name="internal" dataType="int" value="0">
            <Description>Indicates whether this event should be distributed to LSC/Virgo members only</Description>
        </Param>
        <Param name="Packet_Type" dataType="int" value="150">
            <Description>The Notice Type number is assigned/used within GCN, eg type=150 is an LVC_PRELIMINARY notice</Description>
        </Param>
        <Param name="Pkt_Ser_Num" dataType="string" value="4"/>
        <Param name="GraceID" dataType="string" value="MS181101ab" ucd="meta.id">
            <Description>Identifier in GraceDB</Description>
        </Param>
        <Param name="AlertType" dataType="string" value="Preliminary" ucd="meta.version">
            <Description>VOEvent alert type</Description>
        </Param>
        <Param name="Retraction" dataType="int" value="1">
            <Description>Set to 1 if the event is retracted.</Description>
        </Param>
        <Param name="HardwareInj" dataType="int" value="0" ucd="meta.number">
            <Description>Indicates that this event is a hardware injection if 1, no if 0</Description>
        </Param>
        <Param name="OpenAlert" dataType="int" value="1" ucd="meta.number">
            <Description>Indicates that this event is an open alert if 1, no if 0</Description>
        </Param>
        <Param name="EventPage" dataType="string" value="https://example.org/superevents/MS181101abc/view/" ucd="meta.ref.url">
            <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>
                        <TimeInstant>
                            <ISOTime>2018-11-01T22:22:46.654437</ISOTime>
                        </TimeInstant>
                    </Time>
                </AstroCoords>
            </ObservationLocation>
        </ObsDataLocation>
    </WhereWhen>
</voe:VOEvent>