Afterglow Population Studies from Swift Follow-up of Fermi-LAT - PowerPoint PPT Presentation

Afterglow Population Studies from Swift Follow-up of Fermi-LAT GRBs J. L. Racusin (NASA/GSFC) S. R. Oates (MSSL-UCL) P. Schady (MPE), J. McEnery, V. Vasileiou, E. Troja, N. Gehrels (NASA/GSFC) “Deciphering the Ancient Universe”, Kyoto, April 19-23, 2010

The Fermi-Swift Era of GRB Science • Fermi – GBM (8 keV - 40 MeV) - detect ~250 GRBs/year (~450 total) – LAT (20 MeV - 300 GeV) - detects ~10 GRBs/ year (17 total, <10% of GBM GRBs observed) • New features observed (delayed onset of LAT emission, extra power-law spectral component, temporally extended emission) – see also Kouveliotou talk & Ohno talk GRB 090902B: Abdo et al. (2009) • Swift-BAT (15 keV - 150 keV) - detects ~100 GRBs/ year y r a n • 65 simultaneous BAT/GBM triggers (19 w/ i m i l e r redshifts, as of Dec 2009) P • only 1 BAT/GBM/LAT detection to date (GRB 090510)

Extended Emission GRB 090902B: Abdo et al. (2009) • >50% of LAT GRBs have >100 MeV emission that lasts significantly longer than prompt emission decaying as a power-law -> Extended Emission • Possible interpretations? – Afterglow • Ghisellini et al. 2009 • Kumar & Barniol Duran 2009 – Jet Photosphere • Toma et al. 2010 GRB 090510: De Pasquale et al. (2010) – Hadronic Models GRB 080916C: • Asano et al. 2009 Abdo et al. (2009) – Leptonic Models • IC, SSC 3 3

The Fermi-Swift Era of GRB Science • Afterglows - GRBs observed by both Swift & Fermi can have spectral coverage over as much as 10 orders of magnitude (not including ground based NIR/radio) – currently only a simultaneous BAT/LAT trigger (e.g. GRB 090510) can have early observations – 8 detected in XRT follow-up (10 observed after > 12 hours) – 7 detected in UVOT follow-up (10 observed after > 12 hours) – all afterglows detected by XRT have led to ground based redshift measurements Preliminary 4

Population Studies • XRT Swift afterglow sample – Sample and characterization techniques from Racusin et al., 2009, ApJ, and Racusin PhD Thesis • UVOT Swift afterglow sample – Sample and normalization technique from Oates et al., 2009, MNRAS, and Oates PhD Thesis • Compare Swift follow-up of LAT GRBs to large well studied BAT GRB sample in order to learn about special properties of LAT bursts Sample Statistics Sample Statistics – Temporal properties XRT UVOT – Luminosity BAT 148 49 – Energetics GBM/BAT 18 11 • Only using GRBs with redshifts LAT/GBM 8 5 • BAT/GBM bursts through end of 2009 • LAT bursts include all detected bursts (including last week’s GRB 100414A) 5

LAT/GBM/BAT GRB Afterglows Swift -XRT X-ray afterglows clustered in Luminosity (except SHB GRB 090510) XRT afterglows analyzed in methods described in Racusin et al. (2009) Swift -UVOT Preliminary UV/optical less clustered, tending toward bright (except SHB) *not yet corrected for host galaxy extinction Preliminary UVOT afterglows analyzed in methods described in Oates et al. (2009) 6

Long vs Short Bursts Swift -UVOT Swift -XRT long short 7

LAT/GBM/BAT X-ray Afterglows Swift -XRT 8

LAT/GBM/BAT Optical Afterglows Swift -UVOT 9

Redshift • No significant differences in redshift distributions 10

Energetics • On average LAT E iso > GBM E iso > BAT E iso • No jet breaks in Swift observations of LAT X-ray or optical afterglows – maybe GRB 090510 or just short burst X-afterglow fast falling morphology – maybe GRB 090328A is all post-JB (McBreen et al., 2010, arXiv:1003.3885) • LAT GRB collimation corrected energies ≳ 10 52 ergs! – not even including extra spectral power-law component – see also Cenko et al., 2010, arXiv:1004:2900 11

Energetics Long bursts Includes Jet Break Short bursts Pre-Jet Break? Includes Jet Break Pre-Jet Break?

X-ray vs γ -ray - Efficiency? • LAT GRBs are most energetic, but not most X-ray luminous – Why are the LAT GRBs clustered in X-ray luminosity? – Different efficiencies? • Why do LAT bursts have later jet breaks than typical Swift bursts? 13

Conclusions • Even with very small number statistics (6-7 LAT GRBs), quantifiable similarities and differences between the LAT/GBM/BAT GRBs – LAT GRBs - brightest end of luminosity function, or a different population? • LAT has detected some of the most energetic (gamma-ray) prompt emission of GRBs over the last 20 years – Where are these GRBs in the Swift sample? • Larger fraction are bright in X-ray/optical for LAT than BAT – Due to simply larger initial energies? – Related to > 100 MeV extended emission? • X-ray afterglows of LAT bursts are brighter than average, but not at the brightest end of Swift sample – Does this suggest brightest BAT bursts could have been bright in LAT too? (maybe not causally connected) – All of those brighter than the brightest LAT X-ray afterglow occurred before Fermi launched (include many notable well- studied Swift GRBs) 14