TABLE OF CONTENTS:
The Optical Gravitational Lensing Experiment (OGLE) instrument
detects gravitational lensing events of stars and planets passing in front of the stars
in the Galactic Bulge (BLG) or the Large/Small Magellanic Clouds (LMC/SMC).
The OGLE telescope is located at Las Companas Observatory, Chile.
The GCN system has been modified to incorporate the distribution
of these events so that follow-up observations can be made of the lensing event lightcurve.
OGLE detects about 200 lensing events per month (during the ~9 months/yr when the BLG is a night-time object).
Most of the time (80%), these Notices are issued before the actual maximum in lensing lightcurve.
You have the opportunity to catch the main part of the event -- where most of the information on the lensing object is derived.
Below is a typical OGLE lightcurve for a lensing event
(OGLE-2011-BLG-0891).
The left panel is an expansion of the right panel around the cusp. The cusp is roughly 10 days wide.
In this case the cusp is not fully sampled and would have benefitted from data points from other follow-up observers.
(For this particular event, the OGLE message was issued ~9.8 days before the cusp.)
Sequence of Data Flow and Processing
OGLE telescope observes a field (many fields per night);
the images are processed and compared to previous observations;
looking for a field star that is brightening.
If the shape of the lightcurve is proper (for a lensing event, and not some variable star),
then a lensing event message is sent to GCN.
GCN puts it into the standard format and content
and sends it to those sites requesting this notice type and that meets their filtering criteria.
All the GCN/OGLE Notices are also archived within the GCN website in the Table of OGLE Events.
Notice Types, Content, and Purpose
There is only 1 GCN/OGLE Notice type. It contains information about a gravitational lensing event.
The Notices contain an RA,Dec location of the lensed star.
Of particular interest to the follow-up observer is that the notice contains a prediction
when the cusp (max brightness) in the lightcurve will happen.
It also contains a measure of the duration of the lensing event (the width of the cusp).
Measurements around the cusp are most important.
The maximum expected magnitude of the cusp plus the baseline mag are also given.
The 4 figures below show the distribution baseline magnitudes of the lensed stars
and the distribution of predicted delta_magnitude decreases.
Both are shown in histogram form and percentile distribution.
(Mag decreases greater than 10 are most likely due to spurious data points and sparse fitting.)
Note that 50% of these events have a delta_mag decrease of 2 or more magnitudes.
The automated OGLE analysis software finds the events and sends the message to GCN.
Most of the time (about 80%), the detections are made on the rising edge of the lensing event.
(And most of the 20% that are post-max_magnitude are still on the steep edge of the lightcurve.)
The distribution notification lead-time is shown in the figure below. (Positive values
are pre-cusp notifications; negative values are after the peak of the lightcurve cusp.)
Samples of the E-mail, Cell/Pager, Short-form Cell/Pager, and Subject-only
distribution methods of the GCN/OGLE Notice type is included below.
The format is very similar to the other spacecraft-instrument sources
of GRB and Transient locations -- the GCN-standard "TOKEN: value" format.
(Also available in XML format via email and sockets.)
The socket packet contents and format are similar to the other mission-specific packet types
and are described in detail in the socket packet definition document.
This document also has explanations of the various fields in the packet
(their content, values, and implications for use),
and those same fields are manifested in the email and cell/pager formats.
(Therefore, it is useful for email-based recipients to read the
socket packet definition document
to better understand the contents of the email/cell/pager notifications are.)
The OGLE Notice is type=139.
Sites can elect to receive the OGLE Notice type.
The standard filtering applies to all the existing distribution methods/media.
Of particular note, the "Intensity" filter is applied to the Peak_Magnitude. This is better than applying it
to the Delta_magnitude, because there is comparable variation in the baseline magnitudes
that these deltas ride on top of.
The location error for the stars is better than 1 arcsec.
There is no ERROR (Token:value) field in the Notices because the value is always constant.
OGLE detects about 200 microlensing events per month for the ~9 months a year
that the Galactic Bulge is observable.
Less than 10%(tbr) of these events will turn out to be nonlensing events
(ie cataclysmic variables (CVs), other variable stars; but even these
are astrophysical and are of some interest to some follow-up observers).
There are no test notices for this type.
Here is the official policy from OGLE regarding the use of their data:
"The OGLE EWS photometry is provided for informative purposes only
to facilitate the microlensing community monitoring the on-going events
and follow up observations. Please contact us if you want to use the data for publication.
We may require to be co-authors when our photometry is important for analysis or
refuse its usage if we are currently working or plan to work on the event."
Sites are strongly encouraged to contribute their observations to the OGLE team
because no single site can sufficiently sample the lightcurve to achieve a significant fit.
Guidance to the Follow-up Observer
Here are some rough, very general ideas about making follow-up observations on these lensing events.
These are by no means absolute requirements -- just ballpark values and methods.
Seeing: The original OGLE observations were done with 1-2 arcsec seeing.
Crowded Fields: It is the Galactic Bulge, so these fields will be crowded (see OGLE image below:
OGLE-2011-BLG-0891).
Filter: Any filter can be used. If different than the I filter used by OGLE, then cross-calibration
with other stars in the field will be needed to allow your data point(s) to be added to the OGLE lightcurve.
At the moment (this initial release), there is no capability for an "Update" notice or a "Retraction".
Is it possible that Updates for things like:
(a) improved Max_Date_Time values,
(b) improved Amax values (as more of the rising edge of the cusp is available for a better fit),
(c) notifications of events that have planets involved in the micro-lensing, and
(d) retractions and/or reclassifications (eg lensing to CV)
will be implemented.
For further information on this, please contact
Scott Barthelmy (for GCN issues),
or see the
OGLE webpage (for OGLE-specific questions),
and
these GCN web pages, and
GCN/OGLE Events table.
An example of the OGLE Notice type of the e-mail format is shown below.
The "/////" divider bars are NOT part of the messages.
Do not take the actual values shown in these examples as a real lensing event.
While based on data from the OGLE project,
they have been adjusted to provide a broader representation
of the various combinations of fields and value ranges.
For those sites/people that use demons and/or incoming e-mail filters,
the "Subject" line is: GCN/OGLE_Lensing_Event .
///////////////////////// Full e-mail format ////////////////////////////
TITLE: GCN/OGLE NOTICE
NOTICE_DATE: Tue 12 Jun 11 18:07:20 UT
NOTICE_TYPE: OGLE Lensing Event
SRC_ID_NUM: 201100952
SRC_RA: 270.8691d {+18h 03m 29s} (J2000),
271.0489d {+18h 04m 12s} (current),
270.0836d {+18h 00m 20s} (1950)
SRC_DEC: -27.5094d {-27d 30' 33"} (J2000),
-27.5083d {-27d 30' 29"} (current),
-27.5117d {-27d 30' 41"} (1950)
PEAK_MAG: 17.0 mag (estimated)
DISCOVERY_DATE: 15724 TJD; 163 DOY; 11/06/12
DISCOVERY_TIME: 65240 SOD {18:07:20.00} UT
LEAD_TIME: 9.704 [days]
MAX_DATE: 15734 TJD; 173 DOY; 11/06/22
MAX_TIME: 39657 SOD {11:00:57.59} UT
MAX_UNCERT: 0.180 [days]
CUSP_WIDTH: 26.85 +/- 4.72 [days]
u0: 0.18 +/- 0.04 [RingUnits]
BASE_MAG: 18.88 +/- 0.00 [mag]
AMPLIFICATION: 5.57 +/- 1.24 factor
LC_URL: http://www.astrouw.edu.pl/~ogle/ogle3/ews/blg-0952.html
SUN_POSTN: 111.69d {+07h 26m 46s} +21.94d {+21d 56' 23"}
SUN_DIST: 160.47 [deg] Sun_angle= -10.6 [hr] (East of Sun)
MOON_POSTN: 257.51d {+17h 10m 02s} -23.30d {-23d 18' 04"}
MOON_DIST: 12.93 [deg]
GAL_COORDS: 3.22, -2.67 [deg] galactic lon,lat of the burst
ECL_COORDS: 270.77, -4.07 [deg] ecliptic lon,lat of the burst
COMMENTS: OGLE Gravitational Lensing Event.
////////////////////////////////////////////////////////////////////////
An example of the (regular) cell/pager format is shown below.
There are no "Subject" lines for these e-mails sent to the cell/pager companies
because the Subject line would use up valuable character counts
from the maximum displayable for the body of the message.
RA,Dec are in Current epoch.
///////////////////////// Cell/Pager format //////////////////////////////// GCN/OGLE Lensing Event RA=270.049d DEC=-27.508d MAX_DATE= 11/06/22 MAX_TIME= 11:00:57.00 UT MAX_UNCERT= 0.180 [days] MAX_MAG=17.0 ////////////////////////////////////////////////////////////////////////////
An example of the short-form cell/pager format is shown below.
There are no "Subject" lines for these e-mails sent to the cell/pager companies,
because the Subject-line would use up valuable character counts from the maximum
displayable for the body of the message. And it was the very limited
display character count of some companies that motivated the short-form cell/pager
method in the first place. RA,Dec are in B1950 epoch.
///////////////////////// Short-Cell/Pager format //////////////////////////// OGLE Lensing Event RA=270.436 DEC=-31.695d LeadT=0.574d [days] //////////////////////////////////////////////////////////////////////////////
There are two variations of the Subject-only format: decimal degrees (Current epoch)
and RA=hh:mm:ss Dec=DDdMMmSSs format (J2000 epoch). The two variations are shown below:
///////////////////////// Subject-only format //////////////////////////// OGLE RA=271.435 DEC=-31.689d ////////////////////////////////////////////////////////////////////////// OGLE RA=+18:05:00 DEC=-31:41:26 //////////////////////////////////////////////////////////////////////////
An example of the VOEvent XML format is shown below.
This can be delivered as the body of a email message, or
as an attachment to the (full-format) E-mail format, or
via the XML_SOCKET method.
<?xml version = '1.0' encoding = 'UTF-8'?>
<voe:VOEvent
ivorn="ivo://nasa.gsfc.gcn/OGLE#Lensing_Event_2011-06-12T17:34:58.00_201100735-308"
role="observation" version="1.1"
xmlns:voe="http://www.ivoa.net/xml/VOEvent/v1.1"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xlink="http://www.w3.org/1999/xlink"
xsi:schemaLocation="http://www.ivoa.net/xml/VOEvent/v1.1 http://www.ivoa.net/xml/VOEvent/VOEvent-v1.1.xsd" >
<Who>
<AuthorIVORN>ivo://nasa.gsfc.tan/gcn</AuthorIVORN>
<Author>
<shortName>OGLE (via VO-GCN)</shortName>
<contactName>Andrzej Udalski</contactName>
<contactPhone>+1-XXX-XXX-XXXX</contactPhone>
<contactEmail>udalski@astrouw.edu.pl</contactEmail>
</Author>
<Date>2011-07-12T17:34:58</Date>
<Description>This VOEvent message was created with GCN VOE version: 1.4 04jun11</Description>
</Who>
<What>
<Param name="Packet_Type" value="137" />
<Param name="Pkt_Ser_Num" value="15" />
<Param name="TrigID" value="201100735" ucd="meta.id" />
<Param name="Event_TJD" value="15724" unit="days" ucd="time" />
<Param name="Event_SOD" value="63298.00" unit="sec" ucd="time" />
<Param name="Detection_SOD" value="63298.00" unit="sec" ucd="time" />
<Param name="Maximum_TJD" value="15601" unit="days" ucd="time" />
<Param name="Maximum_SOD" value="51321.60" unit="sec" ucd="time" />
<Param name="Max_Time_Uncert" value="1.52" unit="sec" ucd="time" />
<Param name="Max_Magnitude" value="18.36" unit="sigma" ucd="stat.snr" />
<Param name="Current_Mag" value="0.00" unit="sigma" ucd="stat.snr" />
<Param name="Baseline_Mag" value="19.15" unit="sec" ucd="time.interval" />
<Param name="Baseline_Uncert" value="0.00" />
<Param name="Soln_status" value="0x0" />
<Param name="Misc_flags" value="0x0" />
<Group name="Trigger_ID" >
</Group>
<Group name="Misc_Flags" >
</Group>
<Param name="Coords_Type" value="1" unit="dn" />
<Param name="Coords_String" value="source_object" />
<Group name="Obs_Support_Info" >
<Description>The Sun and Moon values are valid at the time the VOEvent XML message was created.</Description>
<Param name="Sun_RA" value="111.67" unit="deg" ucd="pos.eq.ra" />
<Param name="Sun_Dec" value="21.94" unit="deg" ucd="pos.eq.dec" />
<Param name="Sun_Distance" value="154.06" unit="deg" ucd="pos.angDistance" />
<Param name="Sun_Hr_Angle" value="-10.16" unit="hr" />
<Param name="Moon_RA" value="257.17" unit="deg" ucd="pos.eq.ra" />
<Param name="Moon_Dec" value="-23.30" unit="deg" ucd="pos.eq.dec" />
<Param name="MOON_Distance" value="8.89" unit="deg" ucd="pos.angDistance" />
<Param name="Moon_Illum" value="92.51" unit="%" ucd="arith.ratio" />
<Param name="Galactic_Long" value="358.20" unit="deg" ucd="pos.galactic.lon" />
<Param name="Galactic_Lat" value="1.45" unit="deg" ucd="pos.galactic.lat" />
<Param name="Ecliptic_Long" value="264.67" unit="deg" ucd="pos.ecliptic.lon" />
<Param name="Ecliptic_Lat" value="-6.36" unit="deg" ucd="pos.ecliptic.lat" />
</Group>
</What>
<WhereWhen>
<ObsDataLocation xmlns="http://www.ivoa.net/xml/STC/stc-v1.30.xsd">
<ObservatoryLocation xlink:href="ivo://STClib/Observatories#GEOLUN/" xlink:type="simple" id="GEOLUN" />
<ObservationLocation>
<AstroCoordSystem xlink:href="ivo://STClib/CoordSys#UTC-FK5-GEO/" xlink:type="simple" id="FK5-UTC-GEO" />
<AstroCoords coord_system_id="FK5-UTC-GEO">
<Time unit="s">
<TimeInstant>
<ISOTime>2011-06-12T17:34:58.00</ISOTime>
</TimeInstant>
</Time>
<Position2D unit="deg">
<Name1>RA</Name1>
<Name2>Dec</Name2>
<Value2>
<C1 pos_unit="deg">263.9037</C1>
<C2 pos_unit="deg">-29.6911</C2>
</Value2>
<Error2Radius>0.0000</Error2Radius>
</Position2D>
</AstroCoords>
</ObservationLocation>
</ObsDataLocation>
<Description>The RA,Dec coordinates are of the type: source_object.</Description>
</WhereWhen>
<How>
<Description>OGLE: Optical Gravitational Lensing Experiment</Description>
<Reference uri="http://gcn.gsfc.nasa.gov/ogle.html" type="url" />
</How>
<Why importance=1.0>
<Inference probability="1.0">
<Concept>process.variation.burst;em.gamma</Concept>
</Inference>
</Why>
<Description>
</Description>
</voe:VOEvent>