TO:  All GCN Sites                                             28 Oct 07
RE:  Bright Star and Galaxy Coincidences


INTRO:
There are two new features in all the small-error-circle position-containing
GCN Notices:
1) If the RA,Dec is within 12 arcmin of a bright star (<6.5 mag),
   then a comment is added to the Notices saying so, and giving the magnitude
   and the distance.
2) If the RA,Dec is within one of the nearby galaxies, then a comment is added
   to the Notices giving the name of the galaxy, its semi-major diameter, and
   its magnitude.


NEARBY BRIGHT STARS:
Code was added to GCN to search the Bright Star Catalog (BSC) to see if the
current burst (or transient) RA,Dec location is within 12 arcmin any of these
bright stars.  If so, then a flag bit is set in the socket packet [Note 1] and
for the full-format email Notices, a COMMENT line is added giving the actual
distance to the star and the star's magnitude.  (There is a minor flaw
to the search in that it reports only the first star found in the catalog
closer than 12 arcmin, not the brightest star closer than 12 arcmin
if there is more than one.)

The BSC was obtained from CSD VizieR (V/50).  It contains 9096 stars with
magnitudes brighter than 6.5 mag.  This covers 2.8% of the sky.

The motivation for this new feature is to give optical follow-up observers
a heads-up when there is a bright star in (or near) their FOV.  (A 12-arcmin radius
seemed a reasonalble value for most telescope FOVs.)


GALAXY COINCIDENCES:
Code was added to GCN to search the New General Catalog (NGCC) to see
if the current burst (or transient) RA,Dec location is within a close galaxy.
If so, then a flag bit is set in the socket packet [Note 1] and
for the full-format email Notices, a COMMENT line is added giving the galaxy's
diameter and the galaxy's magnitude.

The NGC was obtained from CSD VizieR (VII/118) which came from the Sky Publishing Inc.
A subset of the full NGC is used for this purpose.  Only 4860 of the 13226 entries
in the original NGC were galaxies and had a size listed, so only those 4860 galaxies
are used.  Further, only 75% of those have magnitudes specified, but that lack
of a magnitude was not sufficient for exclusion (those galaxies have a value
of "0" reported for the magnitude field in the COMMENT string).

The motivation for this new feature is let people know (in the the initial real-time
Notices) that this burst could possibly be associated with a nearby galaxy.
Since the probability that a given bust RA,Dec location is within the major diameter
circle on the sky for that galaxy is small (it varies for each instrument,
but is always less than 0.05%), then a positional match is almost certainly
a causally connected match.


ACTION ITEM:
There is none.  You don't have to do anything for this new feature.
It is already incorporated (for about 3 months now) into Swift BAT_Position,
XRT_Position, and UVOT_Position Notices, plus all the other Swift lightcurve, 
spectra, and image notice types, plus the INTEGRAL Wakeup/Refined/Offline types,
and in the upcoming AGILE and GLAST notice types.
(Even thought there is nothing to sign up for in this announcement,
your current configuration is listed below as a general convenience. You can,
of course, take this opportunity to review your configuration and request
any changes.)


Note 1: See the http://gcn.gsfc.nasa.gov/sock_pkt_def_doc.html document.

Sincerely,
Scott


Scott Barthelmy                      NASA-GSFC, Code 661, Greenbelt, MD   20771
PHONE:  301-286-3106                 (office)
PHONE:  301-346-3733                 (cell)
FAX:    301-286-1684                 (1st choice,  -1682 2nd choice)
EMAIL:  scott@lheamail.gsfc.nasa.gov
PAGER:  3013463733@cingularme.com
WEB:    http://gcn.gsfc.nasa.gov/gcn