1. Swift 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

2. 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) 050509B (z=0.226), 050709 (z=0.1606) and 050724 (z=0.257) We though we found a distinct difference in short GRB redshifts on early 2005…. However Redshift of short GRBs: z = 0.1 - 2 (everywhere)

3. Swift Mystery solved for short GRBs? None of the redshifts of short GRBs has been directly measured from the afterglow. Redshifts from the “associated” host galaxy. Problem 1: 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

4. Swift Mystery solved for short GRBs? Different population in short GRBs? Short GRBs with or without the extended emission (E.E.) No E.E. With E.E. No lag Problem 2:

5. 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

6. Swift Table of Swift short GRBs

7. 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) 1. Swift short GRBs localized by Swift/XRT 2. No confirmation of an optical afterglow within 5 hours after a GRB trigger (Chandra response time within 1-3 days after T 0 ) BAT: duration and spectral lag (TDRSS and/or BAT event data) XRT: arcsec position from TDRSS event data (U of Leicester) Chandra can detect XA: T 0 < 3.5 days (SL) and T 0 +23 days (LL) Event rates which meet our ToO criteria: 5 (2005), 6 (2006), 5 (2007), 5 (2008) and 3 (2009) Trigger criteria:

8. 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 T 0 <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