Classification of Gamma-Ray Bursts: an observational review Paolo D’Avanzo INAF – Osservatorio Astronomico di Brera

Two flavors of GRBs GRBs are short flashes of gamma rays How much short? Duration (s) BATSE ('90s) Short Long Kouveliotou et al. 1993

Another angle Hardness ratio: Paradigm: Long/soft Short/hard

The third hint Time Hard band Soft band lag The hard band leads the soft one Luminosity Time lag Short GRBs seem to have zero lags

According to the “fireball” model the progenitor is hidden Clues about the progenitor:  We never see two GRB from the same source (the progenitor is destroyed)  Local Environment  Host galaxies proprerties  Occurring rate (number of events per unit time) Different progenitors for long and short GRBs The progenitors of GRBs

Short vs. long GRBs: properties of the prompt emission Different progenitors for long and short GRBs E peak Piranomonte et al. 2008

Short vs. long GRBs: properties of the afterglow emission Short GRBs are fainter: less dense environment? Kann et al. 2010

GRBs are cosmological and occur in galaxies

GRB host galaxies Colors: Blue  Hot  Young stars Red  Cold  Old stars

Long GRB hosts Emission line spectra Nebular emission lines excited by hot, young stars Blue  Hot  Young stars

The GRB/Supernovae connection Blue  Hot  Young stars

The GRB/Supernovae connection += Blue  Hot  Young stars

Long GRBs & SNe Galama et al. 1998; Stanek et al. 2003; Hjorth et al. 2003; Della Valle et al. 2003; Malesani et al. 2004; Soderber et al Pian et al. 2006; Campana et al Della Valle et al …

Long GRBs & SNe  Observed rate of long GRBs:  1 / 10 7 yr / galaxy(assuming isotropic emission)  1 / 10 5 yr / galaxy(correcting for beaming)  1 / 10 2 yr / galaxy(supernovae rate)

Most popular model: Coalescence (merging) of a compact object binary system While orbiting, the two objects emit gravitational waves losing energy: MERGING Such systems are observed in our Galaxy: PSR J The progenitors of short GRBs

Most popular model: Coalescence (merging) of a compact object binary system While orbiting, the two objects emit gravitational waves losing energy: MERGING The progenitors of short GRBs - critical parameter: merging time t m Time between the formation of the system and its coalescence t m  a –4 (a: system separation) ->  10 Myr  t m   10 Gyr - kick velocities: Compact objects are the remnants of core-collapse SNe, that can give a “kick” (  1000 km/s) The system can escape from the host galaxy -> OFFSET! (1  100 kpc) - merging can occur in old and young stellar populations

Most popular model: Coalescence (merging) of a compact object binary system While orbiting, the two objects emit gravitational waves losing energy: MERGING Another possibility: dynamical formation of a double compact object system (e.g. in globular clusters) The progenitors of short GRBs  Observed rate of short GRBs:  5 / 10 7 yr / galaxy (isotropy)  1 / 10 5 yr / galaxy (beaming)

Short/hard GRBs no spectral lag faint afterglows in all type of galaxies (or no host galaxy at all) older stellar population no associated SN merger progenitor model Long/soft GRBs spectral lag in SF galaxies younger stellar population many with associated SN collapsar progenitor model

GRB B X-ray position Host galaxy? the first short GRB X-ray afterglow no optical afterglow near (z=0.22) early-type host candidate offset no Supernova Low chance alignment probability ( ) Short GRB hosts Red  Cold  Old stars (Gehrels et al. 2005; Hjort et al. 2005; Castro-Tirado et al. 2005; Bloom et al. 2006)

The host of GRB B: contrasting properties with respect to long GRB hosts Elliptical galaxy Large luminosity Red colors No emission lines Very low/absent star formation rate Offset with respect to the host center Wavelength (Å) Flux density Exactly what expected in the binary compact object merger scenario Short GRB hosts z=0.22

Is this the rule? GRB st optical afterglow near (z=0.16) late-type, star forming host offset no associated SN (Fox et al. 2005; Hjort et al. 2005; Covino et al. 2006)

Short GRBs optical afterglow near (z=0.26) early-type host offset (Barthelmy et al. 2005; Berger et al. 2005; Malesani et al. 2007) GRB st summary: Short GRB seem to be at lower z, less energetic (1e48-1e50 erg), and occur in different environments with respect to long GRBs

Problems… Many short GRBs show only an X-ray afterglow Poor position Chance alignment probabilities ~10% Possible for one case, not for the sample as a whole Faint optical afterglows: Hard to detect redshifts (and so do measure the energy…) Berger et al. 2007

Short GRBs: Extended Emission (T 90 is not enough to discriminate) Initial hard spike, followed by a softer tail – T90 can be > 2s Is the extended emission a signature of a common progenitor or enivronment? Perley et al. 2009

Short GRBs: Offsets (I) 15.0 kpc Fong et al D’Avanzo et al. 2009

Short GRBs: Offsets (I) 15.0 kpc

Short GRBs: host search G2 z=0.24 G1 z=0.71 & HG -G1: bright, HG candidate, large offset - BUT G1 is NOT the HG - HG (no offset) R = G2: bright, near HG candidate, large offset - BUT G2 is NOT the HG - HG (no offset) R = 27.3 (faintest ever detected for a short GRB) So, host galaxy identification needs some caution… GRB (D’Avanzo et al. 2009) GRB (Piranomonte et al. 2008)

Short GRBs: Offsets (II) -no host galaxy at the optical afterglow position down to R ~ 25.9 (HST > 28 mag) - galaxy z~ featureless AG optical spectrum (?) G2 z=0.11 Perley et al HST > 28.5 mag GRB (Stratta et al. 2007) GRB (Perley et al. 2009)

The short GRB T 90 = 1.25 s z = 2.61 Telescopio Nazionale Galileo t-t 0 = 0.4 dt-t 0 = 16.5 d 1 st spectrum of a SGRB optical afterglow Levesque et al Berger et al. Antonelli et al. 2009

GRB t-t 0 = 35.7 d Antonelli et al Thoene et al. 2011

Short vs. long GRBs: properties of the prompt emission Antonelli et al. 2009

Long GRBs & SNe Galama et al. 1998; Stanek et al. 2003; Hjorth et al. 2003; Della Valle et al. 2003; Malesani et al. 2004; Soderber et al Pian et al. 2006; Campana et al Della Valle et al …

Long GRBs & (no) SNe GRB (T90 = 102 s, maybe faint SN or SGRB+EE) GRB (T90 = 4 s, tail of SGRBs distribution) Della Valle et al. 2006; Fynbo et al. 2006

Intermediate GRBs Statistical evidence for a third class of GRBs in the T90 – HR plot Mukherjee et al. 1998; Horvath et al. 1998, 2002, 2010; de Ugarte Postigo et al. 2010

Short/hard GRBs no spectral lag in all type of galaxies (or no host galaxy at all) older stellar population no associated SN merger progenitor model Long/soft GRBs spectral lag in SF galaxies younger stellar population many with associated SN collapsar progenitor model

Zhang et al. 2009

Conclusions Classifications are useful but classify GRBs is tricky (heterogeneous, poorly understood, observational biases…) Probable existence of subclasses Progenitors might be a solution, but highly limited by present instrumentation (GW, SNe up to z~1…)