TABLE OF CONTENTS:
The GCN system has been modified to incorporate the distribution of positions of GRBs/Transients detected by the MILAGRO instrument.
The MILAGRO instrument detects air showers initiated by high-energy gamma-rays and cosmic rays. (High energy means above ~100 GeV.)
About 5 times a week, the MILAGRO GRB/Transient detection program will trigger on an increase in the event rate within the instrument. It calculates a direction for the events in the detector, and then transmits these localizations to GCN.
Like all the other sources of GRB information within the GCN system, users can elect to receive (or not) these MILAGRO Notices.
The GCN/MILAGRO Notices are archived within the GCN website in the Table of MILAGRO Notices.
This MILAGRO Notice type is really only suitable for robotic telescopes.
Since it is extremely rare that any given notice is a true GRB (and not a false positive
change fluctuation), it is suitable only for a robotic telescope to chase these positions.
I think there has been only detection of GRB by MILAGRO since starting in 1996 (eg the 970417a).
Notice Type, Content, and Purpose
There is only one notice type for MILAGRO.
They are issued only once per trigger.
They contain the calculated RA,Dec location of the burst/transient candidate,
the date & time of the event, and several fields relating to the "significance" of the event:
The "burst_ra" & "burst_dec" are the RA,Dec coordinates of the trigger/burst
as determined by the MILAGRO software (J2000 epoch).
The "signal" is actually the integrated fluence of the burst over the duration of the trigger.
The units are the number of events detected in the intergration interval.
Background events have NOT been subtracted.
The "background" is the counts seen in a 2-hr interval prior to the trigger.
The "burst_error" is the radius of the circle that will contain on average 90% of the events.
It contains a statistical contribution based on the intensity of the event, and
a systematic contribution. The range of values is 0.5 to 1.0 deg.
The "signif" is the -100*log10(probability_of_false_detection).
It is only the statistical significance of the detection, or put another way,
-log10 of the probability that the source could be a natural fluctuation
within the Poisson background. It is typically a small number (10^-12),
but since there are a large number of trials, this by itself is not an indication
of whether this is real GRB or just a chance fluctuation. The annual_rate (see below)
will give a better handle on this aspect.
Probability and Annual description.
The "duration" is the time interval of the search window of the event_times stream
looking for an increase.
The "ann_rate" is the number of times per year that a fluctuation of this significance
at this duration interval is expected to be seen.
Probability and Annual description.
The "zenith" is the MILAGRO instrument zenith angle of the event
as determined by the MILAGRO software.
The "trigger_id" field contains bit_flags which attempt to identify
the type of transient event that triggered the MILAGRO instrument.
The defined flags are:
Item_name Description
--------- -----------
possible_grb 0=No or 1=Yes, it is a possible GRB
definite_grb 0=No or 1=Yes, it is definitely a GRB
def_not_grb 0=No or 1=Yes, it is definitely not a GRB
The "misc" field contains various bit_flags and the message sequence number.
The defined flags are:
Item_name DescriptionThe first 4 bits are the Notice subtype. Currently, only the 'short' subtype
--------- -----------
short Subtype: 1=Short,2=medium(TBD),3=long(TBD)
seq_num The sequence number of this message
There are three causes of triggers (1) background fluctuations,
(2) GRBs, and (3) other astrophysical transients.
By far the largest fraction of the triggers is due to background fluctutations.
WARNING: Although the string "GRB" is used in many of the TOKEN labels in the email format,
it is not intended to indicate the nature of the event that produced that particular Notice.
These "triggers" are occurring quite often (~5 per week), and with high assurance,
most of them are not GRBs (nor anything else astrophysical in nature).
The sequence of activities for a typical burst follows:
1) The MILAGRO instrument detects a rate increase (a "trigger").
2) The background rate in the MILAGRO instrument is determed using the 2 hours before the trigger.
3) Using the events in the instrument during that trigger time window, the direction is determiend.
4) This location (plus other related information) is sent to the GCN.
5) GCN reformats these incoming messages into the standard GCN format.
6) GCN distributes each notice based on the filter criteria for each site.
Samples of the E-mail, Pager, Short-form Pager, & Subject-only
distribution methods of the GCN/MILAGRO Notice are included below.
The format is very similar to the other spacecraft-instrument sources
of GRB locations -- the GCN-standard "TOKEN: value" format.
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.
The MILAGRO_Position Notice is type=58.
Sites can elect to receive the MILAGRO Notice type.
This filtering applies to all the existing distribution
methods/media (sockets, email, pagers, cell).
The uncertainty in the location will depend on:
(a) the burst_intensity,
(b) its position in the FOV, and
(c) background rate.
The typical error is:
0.5 degree (diameter, 90(tbr)% containment) for the MILAGRO_Position Notice,
The systematic uncertainty is 0.2(tbr) degrees.
The error values quoted in the Notice contain both the statistical plus the systematic contributions.
There is no test_notice version of this MILAGRO type.
There are several other mission-based test notices within GCN, and
maybe they can fullfill your test notice needs.
Sites are encouraged to acknowledge MILAGRO and GCN in their publications
based on follow-up observations using these GCN/MILAGRO locations.
For further information on this, please contact
Scott Barthelmy (for GCN issues),
or see the
MILAGRO.
and
these GCN web pages, and
GCN/MILAGRO GRB table.
Acknoledgements:
David Noyes developed the program at MILAGRO that detects the transient, calculates
the position, and sends it to GCN.
Examples of the MILAGRO Notice type of the e-mail formats are shown below.
The "/////" divider bars are NOT part of the messages.
Do not take the actual values shown in these examples as real GRBs.
While based on trial data from the instrument,
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" lines for the all notice types are constant.
The subject-line strings are (respectively):
The 'raw' versions:
GCN/MILAGRO_GRB_POSITION
/////////////////////////MILAGRO e-mail format//////////////////////////////
TITLE: GCN/MILAGRO POSITION NOTICE
NOTICE_DATE: Sun 12 Dec 04 18:31:21 UT
NOTICE_TYPE: Short
TRIGGER_NUM: 255 Seq_num: 1
GRB_RA: 280.25d {+18h 41m 01s} (J2000),
280.31d {+18h 41m 14s} (current),
279.70d {+18h 38m 48s} (1950)
GRB_DEC: +17.40d {+17d 24' 17"} (J2000),
+17.41d {+17d 24' 35"} (current),
+17.36d {+17d 21' 24"} (1950)
GRB_ERROR: 0.5 [deg radius, statistical plus systematic]
GRB_FLUENCE: 28 [events]
BKG: 5.1369 [events]
GRB_SIGNIF: 1.862e-12
ANN_RATE: 21.5
GRB_TIME: 66650.69 SOD {18:30:50.69} UT
GRB_DATE: 13351 TJD; 347 DOY; 04/12/12
GRB_DUR: 4.00 [deg]
GRB_ZEN: 30.50 [deg]
SUN_POSTN: 260.30d {+17h 21m 12s} -23.14d {-23d 08' 18"}
SUN_DIST: 45.02 [deg]
MOON_POSTN: 271.14d {+18h 04m 34s} -27.81d {-27d 48' 21"}
MOON_DIST: 46.08 [deg]
GAL_COORDS: 47.43, 10.08 [deg] galactic lon,lat of the burst
ECL_COORDS: 282.89, 40.39 [deg] ecliptic lon,lat of the burst
COMMENTS: MILAGRO TRANSIENT Coordinates.
COMMENTS: Possible GRB
COMMENTS: For more information see:
COMMENTS: http://umdgrb.umd.edu/cosmic/milagro_grb_main.html
////////////////////////////////////////////////////////////////////////////
Examples of the (regular) pager formats are shown below.
There are no "Subject" lines for these e-mails sent to the pager companies
because the Subject line would use up valuable character counts
from the maximum displayable for the body of the message.
/////////////////////////MILAGRO Pager format////////////////////////////// GCN/MILAGRO RA=218.27d DEC=+59.36d TIME: 23:05:12.83 UT I=95 ERR=0.5deg ///////////////////////////////////////////////////////////////////////////
Examples of the short-form pager format are shown below.
There are no "Subject" lines for these e-mails sent to the 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 pager method in the first place.
[i need to fill in the rest of these examples!!!] /////////////////////////MILAGRO Short-Pager format///////////////////////// ////////////////////////////////////////////////////////////////////////////
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:
[i need to fill in these examples!!!] /////////////////////////MILAGRO Subject-only format///////////////////////// ///////////////////////////////////////////////////////////////////////////// /////////////////////////////////////////////////////////////////////////////