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Edo Berger − Harvard University
Toward the Progenitors of Short-Duration Gamma-Ray Bursts
The Prompt Activity of Gamma-Ray Bursts: their Progenitors, Engines, and Radiation Mechanisms − March 2011
Outline
Kouveliotou et al. 1993
• The discovery of short GRB afterglows & host galaxies
• Redshift distribution & galaxy-scale environments
• Offset distribution & sub-galactic environments
• Evidence for Kicks?• Rates• Prompt emission & afterglows• mini-, macro-, kilo-novae?• Gravitational wavesBerger, E. 2011, New Astronomy Reviews, 51, 1
Short vs. Long GRBs
BATSE − short:long = 1:3
Swift − short:long = 1:10
1. Exclusively in star-forming galaxies
Long GRBs: The Death of Massive Stars
Fruchter et al. 2006; Wainwright, Berger, & Penprase 2007
2. Offsets trace star formation in an exponential disk
Fruchter et al. 2006
Long GRBs: The Death of Massive Stars
Bloom et al. 2002
3. Coincident with the brightest UV regions of
their hosts
Short GRB Progenitor ModelsNS-NS / NS-BH
•Broad delay-time distribution•Diverse environments / redshifts•“Kicks”•“Mini-SN”?•Gravitational wavesMagnetars
•Young systems•Star forming galaxies / nearby?•No “kicks”
Short GRB Progenitor ModelsWD/NS AIC
WD-WD merger
•Delayed magnetar / BH formation•Diverse environments •No “kicks”
Early “Smoking Guns”?
Castro-Tirado et al. 2005; Gehrels et al. 2005; Hjorth et al. 2005; Bloom et al. 2006; Prochaska et al. 2006
Berger et al. 2005
z = 0.257
Unambiguous association with an elliptical galaxy & no accompanying
supernova
⇔
z = 0.226
050509B
Afterglows Galore...
Soderberg et al. 2006
Berger et al. 2007
D’Avanzo et al. 2009
Berger et al. 2009
Fong et al. 2011
... and Host Galaxies Berger et al. 2007;
Berger 2009
Host Demographics
SF
Ell
?
“Host-less”
~1/2 of all short GRBs are located at z > 0.7 ⇒ 〈 age 〈≤ 7 Gyr
Berger et al. 2007; Berger 2009; Fong et al. 2011
z = 0.438
z = 0.922z = 1.130
z = 0.410
z = 0.827
Host Redshifts
z = 0.915
Progenitor Ages:
• P(τ) ∝ τn with n ~ −1*• τ ~ 3-4 Gyr, σ ~ 1 * MW: NS-NS systems have n ~
−1
Nakar 2006; Guetta et al. 2006; Berger et al. 2007
Host Redshifts
Host Galaxies: Star Formation Rates
Short GRB hosts have lower specific star formation rates than long GRB hosts; they
trace the general galaxy population
Berger 2009
Host Galaxies: Metallicities
Short GRB hosts have higher metallicities than long GRB hosts; they trace the general
galaxy population
Berger 2009
Host Galaxies: Stellar Masses
Leibler & Berger 2010
Short GRB hosts have higher stellar masses than long GRB hosts
Do Short GRB progenitors track stellar mass alone?
Stellar Population AgesLeibler & Berger 2010
Short GRB hosts (including star-forming) have older ages than long GRB hosts
τshort,SF ~ 0.3 Gyr
τshort,E ~ 3 Gyr
τlong ~ 60 Myr
Do Short GRBs Track Stellar Mass? SF?
Early-type hosts track stellar mass, but star-forming hosts have lower masses than expected;
star-forming dominate (1:1 expected)
Rell ~ 6×10−12 per M☉
Rsp ~ 2×10−11 per M☉
Leibler & Berger 2010
Sub-galactic Environments
Fong, Berger, & Fox 2010
•Are short GRBs associated with young or old stellar populations within their hosts?
•Is the distribution of offsets indicative of “kicks”?
Sub-galactic Environments: Light Fraction
Short GRBs trace low luminosity regions of their hosts; track optical (mass) better than
UV (SFR)
Fong, Berger, & Fox 2010Fruchter et al. 2006; Kelley et al. 2008
Sub-galactic Environments: Offsets
•Short GRB offsets are ~5x larger than for long GRBs•Good agreement with model predictions for NS-NS binaries
Fong, Berger, & Fox 2010
Is there Evidence for Large Kicks?
Berger 2010
Of 20 short GRBs with optical afterglows, 5 have no
coincident hosts to >26 mag.
• Underlying hosts >26 mag + fainter afterglows (high redshift)• No hosts + fainter afterglows (kicks / low density)
Is there Evidence for Large Kicks?
Berger 2010
high-z: same galaxies ⇒ bimodal redshift distribution
Offsets: low chance probability at ~10” ⇒ 50-100 kpc
z ~ 0.1-0.5
Is there Evidence for Large Kicks?
Berger 2010
⇒
Kicks?Berger 2010
Extension to larger offsets provides better agreement with the NS-NS merger models. Not expected in other models.
Rates
ℜSHB > 10 Gpc−3 yr−1
Nakar et al. 2007
Rell ~ 6×10−12 M☉−1
Rsp ~ 2×10−11 M☉−1
Leibler & Berger 2010
ρ* ~ 6×1017 M☉ Gpc−3
⇒
τ−1 > 0.4-1.2 Myr−1
ℜDNS ~ 17−290 Myr−1 (95%)Kalogera et al. 2004
Abdo et al. 2009b
Short GRB Prompt EmissionShort GRB090510 Long GRB09092B
Fermi prompt emission properties similar to long GRBs (GeV delay)
Abdo et al. 2009a
Nousek et al. 2006Barthelmy et al. 2005
Short GRB AfterglowsShort GRB050724 Long GRBs
X-ray afterglows are similar to those of long GRBs
Radio/optical afterglows similar to long GRBs
⇒ Afterglow & engine physics are similar to long GRBs
Berger et al. 2005
Short GRB Afterglows
Berger et al. 2000
Short GRB050724 Long GRB000301C
Berger et al. 2005
Short GRB Afterglows: 050724
Eγ,iso ≈ 4 × 1050 ergEK,iso ≈ 2 × 1051 ergθj > 25 degn ≈ 0.01−0.1 cm−3
εe ≈ εB ≈ 0.03
Barthelmy et al. 2005
•Extended X-ray emission•X-ray flare @ 1 day
Short GRB Afterglows: 051221
θj ≈ 7 degEγ ≈ 1.5 × 1049 ergEK ≈ 0.8 × 1049 ergn ≈ 1.5 × 10−3 cm−3
εe ≈ εB ≈ 0.2
Soderberg et al. 2006; Burrows et al. 2006
•X-ray plateau @ 0.1 days•Radio reverse shock•No supernova
Energy scale of short GRBs is generally lower than for long GRBs
Nakar 2007; Berger 2007; Gehrels et al. 2008; Nysewander et al.
2009
Short GRB Afterglows: Energetics
Short GRB Afterglows: CSM Density
Soderberg et al. 2006
The circumburst densities (~0.1 pc) are lower than for long GRBs
Mini-SN / Macronova / Kilonova?
neutron-rich ejecta:
•tidal tails (Mej < 0.1 M⊙)•AD outflows (Mej ~ 10-
3−10-2 M⊙)
Li & Paczynski 1998; Janka et al. 1999; Lee & Kluzniak 1999; Ruffert & Janka 2001; Rosswog et al. 2004; McLaughlin & Surman 2005; Rosswog 2005; Shibata & Taniguchi 2006; Metzger et al. 2008; Giacomazzo et al. 2009; Lee et al. 2009; Rezzolla et al. 2010
Decompressing neutron-rich ejecta ⇒ R-process (A ~ 100)vej ~ 0.1 cMej ~ 10−3 − 10−1 M⊙
fr.a. ~ 10−6
Lp ~ 1041 erg/stp ~ 1 day}
Courtesy: Brian Metzger
Mini-SN / Macronova / Kilonova?
Nakar & Piran 2011
Mildly-relativistic ejecta (or off-axis jet) will interact with ISM to produce a isotropic radio signal (cf radio SNe)
* at d ~ 300 Mpc
Strongly suppressed if n < 1 and/or β < 1 and/or E <1051
EVLA
Gravitational Waves
LIGO:Blind DNS detections: ~20 Mpc
Advanced LIGO: Blind DNS detections: ~0.2-0.3 GpcTriggered Short GRB: ~0.5-1.3 Gpc Nakar et al. 2006; Berger
et al. 2007
⇒ Detection rate of ~few per year
⇒ ⇒
⇒
Summary I
•The progenitors of short GRBs are not massive stars; they belong to an evolved population with a wide range of ages
•The short GRB rate in star-forming galaxies is elevated relative to that in ellipticals ⇒ A channel that tracks SF?
•If related to the star formation activity, the typical delay time is ~0.3 Gyr; the typical delay time in ellipticals is ~3 Gyr
•The short GRB volumetric rate, combined with the inferred rate per unit stellar mass, indicate a temporal rate of >1 Myr−1 per galaxy (consistent with the MW DNS rate)
Summary II
•The local environments of short GRBs exhibit:-large offsets (in agreement with NS-NS models)
-better correlation with optical (mass) than UV (SF)
-low density (~ 0.001-1 cm−3)
•Short GRBs with optical afterglows and no coincident hosts are likely due to kicks (bimodal redshift distribution?)
•Short and long GRB afterglows are similar, but lower E, n
•Orphan afterglow and mini-SN (optical/radio) detections extremely challenging ⇒ γ-ray triggers are essential!
•It is crucial to have a γ/X-ray mission capable of ~arcmin positions in conjunction with GW detectors.