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 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 waves Berger, 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 Wainwright, EB & Penprase 2007, ApJ Long GRBs: The Death of Massive Stars Fruchter et al. 2006; Wainwright, Berger, & Penprase 2007

2. Offsets trace star formation in an exponential disk Wainwright, EB & Penprase 2007, ApJ Fruchter et al Long GRBs: The Death of Massive Stars Bloom et al Coincident with the brightest UV regions of their hosts

Short GRB Progenitor Models NS-NS / NS-BH Broad delay-time distribution Diverse environments / redshifts “Kicks” “Mini-SN”? Gravitational waves Magnetars Young systems Star forming galaxies / nearby? No “kicks”

Short GRB Progenitor Models WD/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 Berger et al z = Unambiguous association with an elliptical galaxy & no accompanying supernova ⇔ z = B

Afterglows Galore... Soderberg et al Berger et al D’Avanzo et al Berger et al 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 z = z = z = z = z = Host Redshifts z = 0.915

Progenitor Ages: P(τ) ∝ τ n 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 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 Ages Leibler & Berger 2010 Short GRB hosts (including star-forming) have older ages than long GRB hosts τ short,SF ~ 0.3 Gyr τ short,E ~ 3 Gyr τ long ~ 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) R ell ~ 6×10 −12 per M ☉ R sp ~ 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 2010 Fruchter 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” ⇒ kpc z ~ 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 ℜ SHB > 10 Gpc −3 yr −1 Nakar et al R ell ~ 6×10 −12 M ☉ −1 R sp ~ 2×10 −11 M ☉ −1 Leibler & Berger 2010 ρ * ~ 6×10 17 M ☉ Gpc −3 ⇒ τ −1 > Myr −1 ℜ DNS ~ 17−290 Myr −1 (95%) Kalogera et al. 2004

Abdo et al. 2009b Short GRB Prompt Emission Short GRB090510Long GRB09092B Fermi prompt emission properties similar to long GRBs (GeV delay) Abdo et al. 2009a

Nousek et al. 2006Barthelmy et al Short GRB Afterglows Short GRB050724Long 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 Short GRB Afterglows Berger et al Short GRB050724Long GRB000301C

Berger et al Short GRB Afterglows: E γ,iso ≈ 4 × erg E K,iso ≈ 2 × erg θ j > 25 deg n ≈ 0.01−0.1 cm −3 ε e ≈ ε B ≈ 0.03 Barthelmy et al Extended X-ray emission X-ray 1 day

Short GRB Afterglows: θ j ≈ 7 deg E γ ≈ 1.5 × erg E K ≈ 0.8 × erg n ≈ 1.5 × 10 −3 cm −3 ε e ≈ ε B ≈ 0.2 Soderberg et al. 2006; Burrows et al X-ray 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 Short GRB Afterglows: Energetics

Short GRB Afterglows: CSM Density Soderberg et al The circumburst densities (~0.1 pc) are lower than for long GRBs

Mini-SN / Macronova / Kilonova? neutron-rich ejecta: tidal tails (M ej < 0.1 M ⊙ ) AD outflows (M ej ~ −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 Decompressing neutron-rich ejecta ⇒ R-process (A ~ 100) v ej ~ 0.1 c M ej ~ 10 −3 − 10 −1 M ⊙ f r.a. ~ 10 −6 L p ~ erg/s t p ~ 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 <10 51 EVLA

Gravitational Waves LIGO: Blind DNS detections: ~20 Mpc Advanced LIGO: Blind DNS detections: ~ Gpc Triggered Short GRB: ~ Gpc Nakar et al. 2006; Berger et al ⇒ 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 (~ 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.