high-z gamma-ray bursts by R. Salvaterra (INAF/IASF-MI)

the cosmic dark-ages

why is it interesting? the Universe experienced two fundamental transitions: - transition in the star-formation mode: from massive, metal-free PopIII stars to a normal (Salpeter-like) second generation of stars (PopII) the transition is governed by metals and dust - transition in IGM state/reionization: from a neutral to a ionized intergalactic medium

GRBs in one slide long GRBs are powerful flashes of γ-ray radiation originated by the collapse of a massive stars, likely a Wolf-Rayet star at z>6: 6 spect. + 2 phot. GRBs GRBs provide a complementary (sometime unique) tool to study the high-z Universe prompt afterglow Pros: - high-z events - very bright - inside normal galaxies - power-law continuum - fade away Cons: - fade away - rare - inside galaxies

the Swift satellite launched in Nov. 2004: ~100 burst/yr 1. BAT triggers on GRB and provides the position to within 4 arcmin 2. Spacecraft autonomously slews to GRB position in 20-70 sec. 3. XRT determines position to within ~5 arcsec (usually < 2 arcsec). 4. UVOT images field and transmit finding chart to ground ground-base telescopes ~100 sec [90% detected] as early as 5 min from trigger [50% detected, 30% distance]

the high-z GRB population

GRB 050904 at z=6.29 lightcurve host metallicity ~5% solar Kawai et al. 2005; Totani et al. 2006

GRB 090423 at z=8.2 the most distant object ever detected (so far) spectroscopically confirmed with the 3.5m Italian national telescope Galileo ~14hrs after trigger most distant object at that time TNG Salvaterra et al. 2009; Tanvir et al. 2009

GRB 130606A at z=5.9 host metallicity 3-6% of solar H neutral fraction x HI <0.03 VLT/X-shooter Hartoog et al. 2014; Chornock et al. 2013; Castro-Tirado et al. 2013

what have we learned? 1. they are observable with current facilities even at extreme redshifts 1.a GRB 050904 at z=6.3 was firstly imagined by TAROT (25cm) 1.b GRB 050904 spectrum at 3.4 days but has sufficient S/N to measure metallicity at z=6.3. The first night the afterglow was >2 mag. brighter 1.c GRB 090423 at z=8.2: the z was spectroscopically measured with a medium class telescope (TNG) 1.d GRB 080913B at z=0.94 reached V~5. In principle observable even at z=20 080319B (the naked-eye GRB) brightest events are observable with 1m-class telescopes up to z>16

what have we learned? 2. high-z GRBs are similar to low- and intermediate-z ones Amati relation VLA obs. (Chandra et al. 2010) Salvaterra et al. 2009 Salvaterra et al. 2009

what have we learned? 3. they are 1-2% of the observed GRBs but ~10% of the entire population spectr. GRB spectr.+ phot. GRB models are calibrated on z-distribution of a complete sample of bright GRBs (BAT6) observed high-z GRB a new mission should be foreseen to increase the number of high-z detections: better sensitivity, larger FoV, rapid repointing and NIR telescope. a softer energy band is more efficient only if coupled with a better sensitivity Salvaterra et al. 2012, Ghirlanda et al. 2014

GRBs as a tool... an incomplete list ISM metals and dust reionization (Gallerani et al. 2008; McQuinn et al. 2008; Xu et al. 2011) escape fraction (Chen et al. 2007; Fynbo et al. 2009) identify and study high-z galaxies responsible for the reionization direct detection of PopIII stars (Komissarov & Barkov 2009; Mezsaros & Rees 2010; Toma et al. 2011; Campisi et al. 2011; desouza et al. 2011) indirect PopIII detection (Ma et al. in prep.; Wang et al. 2012) measuring the SFRD at very high-z (Yueksel et al. 2008; Kistler et al. 2010; Ishida et al. 2011; Grieco et al. 2012) probe the intergalactic radiation field (Inoue et al. 2010) constraints on DM (Mesinger et al. 2005, desouza et al. 2013) primordial non-gaussianity (Maio et al. 2012) see also White Paper for the definition of the L2 and L3 missions, arxiv:1306.2325

GRBs as a tool... an incomplete list ISM metals and dust reionization (Gallerani et al. 2008; McQuinn et al. 2008; Xu et al. 2011) escape fraction (Chen et al. 2007; Fynbo et al. 2009) identify and study high-z galaxies responsible for the reionization direct detection of PopIII stars (Komissarov & Barkov 2009; Mezsaros & Rees 2010; Toma et al. 2011; Campisi et al. 2011; desouza et al. 2011) indirect PopIII detection (Ma et al. in prep.;wang et al. 2012) measuring the SFRD at very high-z (Yueksel et al. 2008; Kistler et al. 2010; Ishida et al. 2011; Grieco et al. 2012) probe the intergalactic radiation field (Inoue et al. 2010) constraints on DM (Mesinger et al. 2005, desouza et al. 2013) primordial non-gaussianity (Maio et al. 2012) see also White Paper for the definition of the L2 and L3 missions, arxiv:1306.2325

identify and study the high-z galaxy population

searching for high-z hosts the knowledge of position and redshift allow very deep search for the GRB host for GRB 090423 deep HST/WPC3 (m J >30.3), Spitzer and ALMA observations provide a strong limit on host brightness and SFR Tanvir et al. 2012; Basa et al. 2012; Berger et al. 2014

simulated high-z GRB hosts 30 Mpc we use the state-of-the art of numerical simulation of structure formation at high-z including all relevant physical process (e.g. chemical, mechanical and radiative feedback) 10 Mpc the simulation provides a good description of the galaxy LF at all redshift without any fine-tuning of the parameters 5 Mpc Maio et al. 2009, 2010, 2011 Salvaterra et al. 2013; Salvaterra et al. 2011

simulated high-z GRB hosts the probability to host a GRB is proportional to the young stellar mass content t age <10 7 yr fraction of UV photons production z=6 z=8 simulations are consistent with the non detection of high-z GRB host with HST/VLT Salvaterra et al. 2013

high-z host physical properties typical GRB hosts at z=6-10: SFR~0.01-0.3 Msun/yr, stellar masses M * ~10 6-10 8 Msun, ssfr~3-10 Gyr -1 and Z~0.01-0.1 Zsun Salvaterra et al. 2013

PopIII GRBs

PopIII GRBs see e.g. Fryer et al. (2001), Heger et al. 2003, Suwa et al. 2007, Komissarov & Barkow (2010), Meszaros & Rees (2010), Suwa & Ioka (2011), Toma et al. (2011), Nagakura et al. 2012... shock breakout is possible even in a massive, metal-free PopIII stars with a large H envelope thanks to the longlived powerful accretion of the envelope Eiso ~ 10 55 erg T90 ~1000-10000 sec Liso ~ 10 52 erg/s both low-mass (40 Msun) and high-mass (400 Msun) PopIII stars can produce a GRB if the accretion-to-jet conversion efficiency and the opening angle satisfy Suwa & Ioka 2011, Nagakura et al. 2012

PopIII GRB radio afterglows extremely bright radio afterglows are expected (Toma et al. 2011; Inoue et al. 2007) PopIII GRB PopIII radio afterglows peaking at high flux levels at late time could be the signature of high-z, energetic PopIII event (Ghirlanda et al. 2013b) high-z PopII GRBs simulated PopII GRBs [PSYCHE]

PopIII GRB rate a limit to the global PopIII rate can be obtain by assuming that all PopIII GRBs are so bright to be detected with current satellites but none has been detected so far all GRBs pointing us PopII PopIII ratio PopIII GRBs are <10% at z=6 (<50% at z=10) of the detectable population <1 PopIII GRBs every 500 PopIII stars (for M III =100 Msun and θ jet ~5 o ) [vs 1 PopII/I GRB every 300 SNIb/c (Ghirlanda et al. 2013a)] Campisi et al. 2011

indirect search for PopIII stars using the simulations described before we compute the expected rate of PopII GRB exploding in a gas enriched by PopIII (PISN) stars: GRB II III GRB II III intrinsic fraction GRB III 1/10 at z=10 GRB II III(z>6)= 0.06 yr -1 sr -1 ~0.5 in 8 yrs of Swift while the intrinsic rate of GRB II III exceeds by 1 dex that of GRB III their detection with Swift is as probable as that of PopIII GRBs (assuming that GRB III are all detectable ) Ma et al. 2014 in prep.

conclusions three spectroscopically confirmed z>6 GRBs + one photometric detectable at extreme high-z and similar to low-/intermediate-z ones 1-2% of all GRBs detected by Swift should be at z>6 high-z GRBs are a fundamental tool to study the high-z Universe and the fundamental transition expected to occur in the cosmic dark-ages GRB hosts are the signpost of galaxies responsible of the re-ionization process missed even in deepest HST/WFC3 observations simulated hosts: typically SFR=0.01-0.3 Msun/yr, M * =10 6-8 Msun, ssfr=3-10 Gyr -1 and Z=0.01-0.1 Zsun in lines with available constraints GRBs are one of the most promising way to detect directly or indirectly the first stars

EXTraS Exploring the X-ray Transient and variable Sky starting date: 2014, January, 1 duration: 36 months Consortium: - INAF (cooridinator, PI: A. De Luca) - IUSS Pavia - CNR-IMATI Genova - University of Leicester (UK) - MPE Garching (Ge) - Erlangen Center for Astrophysics (Ge) www.extras-fp7.eu

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