Wen-fai Fong Harvard University Advisor: Edo Berger LIGO Open Data Workshop, Livingston, LA GRB ACS/F606W

1. Introduction to GRBs and our program 2. Hubble Space Telescope (HST) host galaxy study 3. “Host-less” population  best evidence for mergers to date 2

Central engine Afterglow E K Prompt γ -ray emission X-ray Optical NIR Radio External forward shock Progenitor Adapted from Gehrels et al Physics of GRBs: The big picture and open questions Internal shocks Interesting questions: Nature of central engine? How energy dissipated? Structure of outflow? Nature of the progenitor??? 3 LONG GRBs ONLY: Θ j ~ 1-10 ⁰

How can we know more about the progenitors at present? Host galaxy and environmental studies. Long GRBs have a supernova association, short GRBs do not Kouveliotou et al. 1993, Nakar

Swift discovers a burst… …and we chase after the afterglow (and eventually host)! Magellan (Chile) Gemini-North (HI) and South (Chile) Swift satellite EVLA (New Mexico) Chandra Essential post-docs! Ashley Zauderer Ryan Chornock 5

Wainwright, Berger & Penprase 2007 Bloom et al Median offset at ~1 r e Exclusive association with highly SF galaxies Offsets consistent with massive stars

 Morphologies, offsets, host light distribution  8 short GRBs with host galaxies and HST data Fong, Berger & Fox

NS-NS merger / NS-BH merger Young magnetarsAccretion-induced collapse of WD or NS Large physical offsets (due to potential kicks from SN asymmetry) Offsets that are consistent with locations of normally star-forming regions Modest offsets (no kicks) 8

Short GRBs have significantly larger offsets than long GRBs Physical offsets consistent with NS-NS merger progenitor (lower limit) 9 Long GRBs from Bloom et al Fong et al. 2010

GRB ACS/F606W GRB100625A Gemini-South GRB101219A Gemini-South GRB A Gemini-South Star- forming Early type ? “Host-less”

Stratta et al. 2007; Fong et al. 2010; Berger 2010 GRB GRB HST/ACS/F606W 1. large offsets? 2. high redshift faint hosts?

host-less w/hosts What is the probability of chance coincidence? (What is the likelihood of finding an unrelated galaxy?) If these systems are highly kicked, offsets ~10’’ Extension of Berger 2010 δ R (arcsec) P(< δ R )

Fong et al. 2010, Berger 2010 Models from Bloom et al. 1999; Fryer et al. 1999; Belczynski et al globular clusters Best agreement with NS-NS models!

NS-NS merger / NS-BH merger Young MagnetarsAccretion-induced collapse of WD or NS Large physical offsets (due to potential kicks from SN asymmetry) X Offsets that are consistent with locations of normally/highly star- forming regions ~Modest offsets (no kicks) Consistent with NS-NS, partial contribution from other populations? 14

Localize to: 1-4 arcmin within 15 seconds (gamma-ray) 2-3 arcsec within an hour (X-ray) Sub-arcsec within several hours (opt/NIR) Target-of-opportunity programs 15 !!!

 Short GRBs with hosts: Older stellar populations Larger physical offsets (than long GRBs) Larger masses, sizes, metallicities and luminosities  Include short GRBs without hosts: If highly kicked, provide best evidence for NS-NS/NS- BH merger origin to date 16

HST host galaxy study: Fong, W., Berger, E., & Fox, D. ApJ, 708: 9, Host-less bursts: Berger, E. ApJ, 722: 1946, EM counterparts of mergers: Metzger, B. & Berger, E. arXiv:

τ short,SF ~ 0.3 Gyr τ short,E ~ 3 Gyr τ long ~ 60 Myr Leibler & Berger Short GRB hosts (including star-forming) have older ages than long GRB hosts. -Timescale for mergers 18

z = z = z = z = Berger et al. 2005; Fox et al. 2005; Berger et al. 2007; Berger 2009; Fong et al. 2011; Berger 2010 z ~ 0.1  400 Mpc z ~ 0.2  900 Mpc z = z = GRB GRB

Swift (since 2004) Long:short 10:1 ~60 short bursts (a few arcmin) (10 per year) BATSE ( ) Long:short 3:1 70 short bursts per year! No afterglows. ~40 X-ray afterglows (a few arcsec) 21 optical afterglows (sub-arcsecond) 15 w/ coincident hosts 10 w/ confirmed redshifts 6 “host-less” (highly kicked?) Fermi/LAT (arcmin to degrees) 8 w/ hosts, some coincident Few confirmed redshifts 1 short GRB optical afterglows Several short GRB detections 20