[vsnet-grb 16872] Swift-XRT Position Error Change

[Bacodine]

All GCN Sites,                                     09 Feb 06

XRT POSITION UNCERTAINTY IS NOW INTENSITY-DEPENDANT

The way that the position uncertainty for the XRT_Position & _Image was changed
from being a fixed value of 6 arcsec to an intensity-dependent based value.

The old value of "6 arcsec radius 90% containment" always came out as 5.8 arcsec
on the full-format email notices because of the 100 microdegree quantization
used within the GCN inter-process communications (ie same as for the
socket packets).

The new method uses the GRB_INTEN field in the Image notice to calculate
the uncertainty based on a fit to the previous year's worth of XRT GRB
detections and calibration data.  This translates to a range of 5 to 9 arcsec.
This includes three contributions: statistical, star tracker, and
systematic uncertainties.

Example: 45 counts ==> 8.3 arcsec, radius, 90% containment.


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 at lheamail.gsfc.nasa.gov
PAGER:  3013463733 at mmode.com
WEB:    http://gcn.gsfc.nasa.gov/gcn

///////////////////////////// Your Configuration ////////////////////////////////


Site_name: VSNET
Lon,Lat=  135.75   35.00
Distribution_method: EMAIL  Attachment_format: NONE
Address: vsnet-grb at kusastro.kyoto-u.ac.jp   
Filter:  ALL
Error_limit=   360.100 [deg, diameter]
Delta_T_limit= 999.900 [hr]
Swift_Trigger_ID_filter:   Disabled
Sources & Notice Types/Subtypes:
    RXTE-PCA_Alert_wont:     Enabled
    RXTE-PCA_Alert_will:     Enabled
    RXTE-PCA_nosaw:          Enabled
    RXTE-PCA_saw:            Enabled
    RXTE-ASM:                Enabled
    IPN_POS:                 Enabled
    ALEXIS:                  Enabled
    RXTE-ASM_TRANS:          Enabled
    HETE_S/C_ALERT:          Enabled
    HETE_S/C_UPDATE:         Enabled
    HETE_S/C_LAST:           Enabled
    HETE_GND_ANA:            Enabled
    HETE_Test_notices:       Disabled
    Test_type2_notices:      Disabled
    INTEGRAL_Test_notices:   Disabled
    SWIFT_Test_notices:      Disabled
    INTEGRAL_POINTDIR:       Disabled
    INTEGRAL_SPIACS:         Enabled
    INTEGRAL_WAKEUP:         Enabled
    INTEGRAL_REFINED:        Enabled
    INTEGRAL_OFFLINE:        Enabled
    SWIFT_BAT_GRB_ALERT:     enabled
    SWIFT_BAT_GRB_POS:       enabled
    SWIFT_BAT_GRB_POS_NACK:  enabled
    SWIFT_BAT_GRB_LIGHTCURVE:enabled
    SWIFT_FOM_2OBS:          enabled
    SWIFT_SC_2SLEW:          enabled
    SWIFT_XRT_POS:           enabled
    SWIFT_XRT_POS_NACK:      enabled
    SWIFT_XRT_SPECTRUM:      enabled
    SWIFT_XRT_IMAGE:         enabled
    SWIFT_XRT_LIGHTCURVE:    enabled
    SWIFT_UVOT_IMAGE:        enabled
    SWIFT_UVOT_SRCLIST:      enabled
    SWIFT_UVOT_POS:          enabled
    SWIFT_BAT_GRB_LC_PROC:   enabled
    SWIFT_XRT_SPECTRUM_PROC: enabled
    SWIFT_XRT_IMAGE_PROC:    enabled
    SWIFT_UVOT_IMAGE_PROC:   enabled
    SWIFT_UVOT_SRCLIST_PROC: enabled
    SWIFT_FULL_DATA_INIT:    enabled
    SWIFT_FULL_DATA_UPDATE:  enabled
    SWIFT_POINTDIR:          enabled
    SWIFT_BAT_TRANS:         Disabled


When requesting changes to your configuration,
it helps if you refer to your site_name.