Short GRBs and their Progenitors (in the era of pre-Gravitational Waves)
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Short GRBs and their Progenitors(in the era of pre-Gravitational Waves)
Wen-fai FongHarvard
University
Advisor: Edo Berger
LIGO Open Data Workshop, Livingston, LA 10.27.2011
GRB 070809ACS/F606W
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Outline1. Introduction to GRBs and our program
2. Hubble Space Telescope (HST) host galaxy study
3. “Host-less” population best evidence for mergers to date
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Central engine
AfterglowEK
Prompt γ-ray emission
X-rayOpticalNIRRadio
External forward shockProgenitor
Adapted from Gehrels et al. 2007
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??? LONG GRBs ONLY:
Θj ~ 1-10⁰
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Two classes of GRBs originate from different progenitors
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 2007
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The multi-wavelengthTarget-of-Opportunity afterglow chase
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
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What can we learn from host galaxy studies? LONG GRBs
Wainwright, Berger & Penprase 2007
Bloom et al. 2002
Median offset at ~1 re
• Exclusive association with highly SF galaxies• Offsets consistent with massive stars
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HST host galaxy study Morphologies, offsets, host light
distribution 8 short GRBs with host galaxies and
HST data
Fong, Berger & Fox 2010
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What offsets can we expect?
NS-NS merger / NS-BH merger
Young magnetars Accretion-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)
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Short GRB (with hosts) offsets compared to models and long GRBs
• Short GRBs have significantly larger offsets than long GRBs• Physical offsets consistent with NS-NS merger progenitor (lower limit)
Long GRBs from Bloom et al. 2002
Fong et al. 2010
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GRB070809ACS/F606W
Short GRBs arise from all types of environments…
GRB100625AGemini-South
GRB101219AGemini-South
GRB 100206AGemini-South
Star-forming
Earlytype
?
“Host-less”
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Short GRBs without a home:six “hostless” burstsStratta et al. 2007; Fong et
al. 2010; Berger 2010
GRB 061201 GRB 070809HST/ACS/F606W
1. large offsets?2. high redshift faint
hosts?
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How far out are potential hosts?
host-lessw/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
δR (arcsec)
P(<
δR
)
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Short GRBs offsets consistent with NS-NS mergersFong et al. 2010, Berger 2010
Models from Bloom et al. 1999; Fryer et al. 1999; Belczynski et al. 2006
glob
ular
clu
ster
s
Best agreement with NS-NS models!
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Using results to constrain progenitor model predictions
NS-NS merger / NS-BH merger
Young Magnetars Accretion-induced collapse of WD or
NS
Large physical offsets (due to potential kicks from SN asymmetry)
XOffsets that are consistent with locations of normally/highly star-forming regions
~Modest offsets (no kicks)
Consistent with NS-NS, partial contribution from other populations?
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Swift-GW-ground-based synergyLocalize 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
!!!
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Summary 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
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Recommended Reading
HST host galaxy study:Fong, W., Berger, E., & Fox, D. ApJ, 708: 9, 2010.
Host-less bursts:Berger, E. ApJ, 722: 1946, 2010.
EM counterparts of mergers:Metzger, B. & Berger, E.
arXiv:1108.6056
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Extra 1: Stellar population ages
τshort,SF ~ 0.3 Gyr
τshort,E ~ 3 Gyr
τlong ~ 60 Myr
Leibler & Berger 2010
-Short GRB hosts (including star-forming) have older ages than long GRB hosts.-Timescale for mergers 18
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Extra 2: Short GRB redshift distribution
z = 0.438
z = 0.410
z = 0.827
z = 0.915
Berger et al. 2005; Fox et al. 2005; Berger et al. 2007; Berger 2009; Fong et al. 2011; Berger 2010
z ~ 0.1 400 Mpcz ~ 0.2 900 Mpc
z = 0.160z = 0.257
GRB 050724GRB 050709
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Extra 3: Short GRB stats: BATSE, Fermi & Swift
Swift (since 2004)Long:short 10:1
~60 short bursts (a few arcmin)(10 per year)
BATSE (1991-2000)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 hosts10 w/ confirmed redshifts
6 “host-less” (highly kicked?)
Fermi/LAT (arcmin to degrees)
8 w/ hosts, some coincidentFew confirmed redshifts
1 short GRB optical afterglowsSeveral short GRB detections