Identification of the host galaxy of
Swift short GRBs by the Chandra
sub-arcsecond position
T. Sakamoto, N. Gehrels, E. Troja,
J. Norris, S. Barthelmy, J. Racusin,
N. Kawai, A. Fruchter
Swift
Mystery solved for short GRBs?
 70% of short GRBs has afterglow
 Host galaxy: star forming and non-star forming
(e.g. long GRBs: always star forming galaxy)
We though we found a distinct difference in short GRB redshifts
on early 2005….
050509B (z=0.226), 050709 (z=0.1606) and 050724 (z=0.257)
However
Redshift of short GRBs: z = 0.1 - 2 (everywhere)
Swift
Mystery solved for short GRBs?
Problem 1:
None of the redshifts of short GRBs has been directly measured
from the afterglow. Redshifts from the “associated” host galaxy.
GRB 090417B (long GRB)
Relatively large Swift/XRT
error circle (2-10”) might
cause a false ID for the host
galaxy
Need sub-arcsec position for
the secure ID of the host galaxy
Swift
Mystery solved for short GRBs?
Problem 2:
Different population in short GRBs?
Short GRBs with or without the extended emission (E.E.)
No E.E.
No lag
With E.E.
No lag
Swift
Mystery solved for short GRBs?
X-ray afterglow of short GRBs:
Short lived X-ray afterglow (45%)
Long lived X-ray afterglow (55%)
Precise position of short GRBs: 1. secure redshift
2. could distinguish among different populations
Swift
Table of Swift short GRBs
Swift
Chandra ToO for a Swift short GRB
Swift short GRBs: Optical AG: 35%, X-ray AG: 73%
Two unsuccessful previous Chandra ToO for Swift short GRBs:
long delay (>4 days after the trigger)
Trigger criteria:
1. Swift short GRBs localized by Swift/XRT
BAT: duration and spectral lag (TDRSS and/or BAT event data)
XRT: arcsec position from TDRSS event data (U of Leicester)
2. No confirmation of an optical afterglow within 5 hours after a
GRB trigger (Chandra response time within 1-3 days after T0)
Event rates which meet our ToO criteria:
5 (2005), 6 (2006), 5 (2007), 5 (2008) and 3 (2009)
Chandra can detect XA: T0 < 3.5 days (SL) and T0+23 days (LL)
Swift
Summary
 Chandra ToO opportunity for a Swift short GRB in Cycle 13
(Nov/Dec 2011-Nov/Dec 2012) to obtain subarcsec position of
XA
 Trigger criteria 1) Swift short GRBs localized by Swift/XRT and
2) no OA confirmation T0<5 hours
 Chandra response time 1-3 days after the trigger (faster judgment,
therefore faster than previous Swift short GRBs ToO program)
 Announce our ToO to the public (our ToO program is an asset for
a whole GRB community)
 Publish the result to the public ASAP
Swift