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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

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

3

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

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

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

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

HST host galaxy study Morphologies, offsets, host light

distribution 8 short GRBs with host galaxies and

HST data

Fong, Berger & Fox 2010

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)

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

GRB070809ACS/F606W

Short GRBs arise from all types of environments…

GRB100625AGemini-South

GRB101219AGemini-South

GRB 100206AGemini-South

Star-forming

Earlytype

?

“Host-less”

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?

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

)

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!

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?

14

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

!!!

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

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

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

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

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