[vsnet-grb 174810] Swift-BAT_Sub_Sub_Threshold and _Known types are available
Bacodine
vxw at capella2.gsfc.nasa.gov
Wed May 9 08:54:19 JST 2012
TO: All GCN Notice socket-based recipients
RE: Swift-BAT_Sub_Sub_Threshold and _Known types are available
DT: 08 May 2012
INTRODUCTION:
The GCN system has been modified to incorporate the distribution
of the Swift-BAT "sub_sub_threshold" and "known source" notice types.
BAT routinely makes images over various integration times and then scans
those images looking for peaks in the image domain. These images
are requested by two different activities within BAT: (a) when a rate trigger
has been satisfied (and needs an image check), and (b) every 64-sec, 320-sec,
and full observation (1-43 min) (aka the BAT image_trigger).
When peaks are found, they are compared to the on-board catalog. When there
is a location match, they are called 'known' sources. When there is no match,
they are called unknown sources. For unknown sources, the significance of the peak
in the image can be above threshold (>6.5 sigma, aka a "burst") or below the threshold
(aka a "failed" trigger).
The table below shows the naming convention for the four scenarios
of being above/below the threshold and matching/not-matching sources
in the on-board catalog.
SIGNIF | MATCH NO-MATCH
-------+-------------------------------
<6.5 | Known Sub_Sub_Threshold
>6.5 | Known Burst
This new Sub_Sub_Threshold notice type is taking the "failed" peaks
and turning them into a distributed product (for both the failed rate triggers
and for the failed peaks in the image trigger images). And while we are at it,
Swift is also making available the known-source detections from all those images.
There is no distinction above/below threshold for known-source detections;
if the position of the peak in the image matches the location of a known-source
then it is put into the Known notice type.
MOTIVATION:
The Sub_Sub_Threshold notices can be used:
1) by robotic telescopes to take data on these locations
on the remote chance that an optical counterpart can be detected.
2) to correlate with other existing databases of "trigger" and "event"
information from other instruments (ground- & space-based, above & below their thresholds).
It is hard to understand a use-case scenario for the Known notices,
but they are processed into GCN Notices just in case somebody can make use of them.
OCCURRENCE RATE:
There will be about 1000 Sub_Sub_Threshold notices per day and
about 1000 Known source notices per day.
Since they show up at GCN in groups of ~150 after each Swift telemetry downlink pass,
they are distributed with 2-sec intervals between each socket packet
to limit the load impact on the GCN computer and on the recipients.
TIME DELAY:
Both of these types come from automated ground-based analysis processing (~1 hr)
after every telemetry downlink, so the time delays will be 1-8 hours.
(These failed triggers and image-domain scans are not sent down
the real-time TDRSS channel -- too much bandwidth would be used.)
LOCATION ERROR:
The location uncertainties are in the 1-4 arcmin range
(radius, 90% containment; the range depends on significance),
but the Error field in the Notice will always be listed as 4 arcmin
(just like the current above-threshold "Burst" notices have a fixed
3-arcmin location uncertainty).
TIMING ERROR:
The trigger time listed in these notices can be off by +/- 320 msec
for events that have a trigger integration duration less than 64 msec.
This has to do with the way that the Swift-BAT flight software is optimized
for execution speed for evaluating the shortest of the rate-trigger criteria.
And for all duration triggers, there is an additional +/- 20 msec of uncertainty
in applying the UT Correction Factor. Note that both of these uncertainties
can be reduced by off-line/ground processing, but for these automated notices,
these large uncertainties exist. This is important if you are using
these SubSub triggers for temporal correlations with other data sets.
SIGNIFICANCE LEVELS:
The range in significances for the Sub_Sub_Threshold goes from 3.8 sigma
to 6.5 sigma. The lower limit is established by BAT flight software;
the s/w can not be chasing after 100's of 3-sigma fluctuations in each image.
The upper limit is established by what has been operationally defined
as a successful trigger even, ie a "burst" (or other hard x-ray transient).
CONFIDENCE LEVEL:
Because the threshold have been lowered to 3.8 sigma,
the probability that a given Sub_Sub_Threshold 'peak' is
a real astrophysical source is very low. The real above-threshold events
(>6.5 sigma) are included in this stream (ie one in 3or4 thousand notices)
and their confidence level is >99%. For the Known notices,
the confidence level is very high (>99%).
COMPLETENESS and DUPLICATIONS:
Given the semi-real-time nature of the data stream used to extract
these events, the data set is not complete and there are duplications.
About 1-2% are missing of both the Sub_Sub's and the Known's.
And there are about 5-10% duplications. (A more complete data set
can be obtained by using the ground-processed Image_Report FITS files.
If there is sufficient interest, this improved data set can be implemented
to produce "processed" versions of these "raw" Sub_Sub and Known notices.)
WHO CAN RECEIVE THESE NOTICE TYPES:
These two notice types are restricted to socket-based recipients only;
they are NOT available to email-based recipients. This is because
of the sizable load-factor emailing a large number of these notices
to a (potentially) large number of recipients. Only robotic telescopes
can make use of these in an automated process, and any human-based activity
would have time delays comparable to the 1-8 hour built-in delay.
If humans want to do correlative activities, they can
(a) pull the archived copies (see Archive discription below), or
(b) they can request a socket-based entry (using socket_demo
for the GCN-original 160-byte binary packet format, or
xml_socket_demo or voevent_client_demo for the VOEvent XML messages).
(Socket_demo has been updated to handle these two new types, and
sock_xml_demo and voevent_client_demo are type-independant.)
FILTERING:
All the normal GCN filtering functions are applicable, but given the special
nature of these two Notices types, the only functions that are valid are:
type, sky location (celestial, galactic, & ecliptic coord systems), error,
intensity, significance, time-of-day window.
ACTION ITEM:
If, after reviewing the information above/below, you want to receive
these Swift-BAT_SUB_SUB_THRESHOLD or Swift-BAT_KNOWN notices,
please send me a request to have them enabled. You can do this
by forwarding this email (which has your important sitename identifier)
to scott at milkyway.gsfc.nasa.gov .
Or you can go to http://gcn.gsfc.nasa.gov/gcn/config_builder.html
and select "#2 Modify an existing config".
Your current configuration is appended below.
(You can also take this opportunity to make other changes to your configuration.)
MORE INFORMATION:
Can be found at: http://gcn.gsfc.nasa.gov/gcn/swift_sub_sub_threshold.html
Particularly, there are two histograms showing the Significance distribution.
NOT TO BE CONFUSED WITH BAT_SUB_THRESHOLD NOTICE TYPE:
This new "sub_sub_threshold" type should not be confused with the "sub_threshold" type.
The Sub_Threshold type was introduced a few years back when Swift was attempting
to dig a little deeper into distribution of events in Swift-BAT. Back then
an event above 5.6 sigma was allowed to cause Swift to slew to the location
so that the XRT and UVOT could make the same follow-up observations they routinely
do for the >6.5 sigma triggers. A Sub_Threshold that had an XRT or UVOT source,
got elevated to a normal BAT_POSITION notice and all the standard notice types
were created and available for distribution. A Sub_Threshold that did not
have an XRT/UVOT source were just available as the Sub_Threshold series
to thse recipients that requested that series. The time delay for this "sub" series
was 5-20 minutes. That experiment did not work very well and was discontinued
in Swift operations (and therefore not available in the GCN Notice pool).
These new Sub_Sub_Threshold (and Known's) are different in that (a) the threshold
has been lowered even more to ~3.8 sigma, and (b) they are not real-time
(hours instead of minutes).
CONTENT:
Each Sub_Sub_Threshold and Known Notice contains the following fields:
NOTICE_TYPE [=140or141]
REF_NUM
RA [deg, J2000]
DEC [deg, J2000]
ERROR [deg]
THETA,PHI within the BAT FoV [deg]
INTENSITY [image_cnts/sec]
SIGNIFICANCE [sigma]
IMAGE_INTEGRATION_TIME [sec]
EVENT_DATE [TJD]
EVENT_TIME [SOD]
SOLN_STATUS [flag bits]
MISC [flag bits]
CATALOG_NUMBER (only for the Known's)
The details of the socket packet contents can be found at:
http://gcn.gsfc.nasa.gov/gcn/sock_pkt_def_doc.html
REMINDER OF RECENT ADDITIONS TO GCN:
In case you have missed them, here is a brief list of the most recent
additions to the GCN system. You might consider signing up
for these Notice types as well. The full announcements/descriptions
for each of these can be found at: http://gcn.gsfc.nasa.gov/gcn/whats_new.html
VOEvent Direct Server (09 Mar 2012)// Added 3 VOEvent servers to allow for direct distribution of VOEvents
FERMI_LAT_MON/_TRANS (26 Aug 2011) // Flare-ups from known sources monitored by Fermi-LAT & non-GRB transients
FERMI_LAT_GND (02 Aug 2011) // Automated ground processing to get GRB positions
MOA (01 Aug 2011) // Locations of up-coming gravitation lensing events
DOW_TOD (Jun 2011) // Preprogrammed notices at specific days-of-week & time-of-day
FERMI_GBM FIN_POS (Jun 2011) // Human-in-the-loop ground-based analysis
MAXI_UNKNOWN/_KNOWN (Apr 2011) // Unknown transients & Flare-ups from Known sources
INTEGRAL_WEAK (Feb 2011) // Subthreshold detections by INTEGRAL
SWIFT_BAT_MONITOR (Feb 2010) // Flare-ups from known sources monitored by Swift-BAT
Sincerely,
Scott
Scott Barthelmy NASA-GSFC, Code 661, Greenbelt, MD 20771
PHONE: 301-286-3106 (office) CELL: 301-346-3733
EMAIL: scott at lheamail.gsfc.nasa.gov
PAGER: 3013463733 at cingularme.com
WEB: http://gcn.gsfc.nasa.gov/gcn
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