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Gamma-ray Burst Afterglow Spectroscopy

Gamma-ray Burst Afterglow Spectroscopy. J. P. U. Fynbo, Niels Bohr Institute / Dark Cosmology Centre. What is a gamma-ray burst?. Brief ( ms - min ) and intense ( ~10 -7 erg cm –2 s –1 ) burst of soft (~ 100 keV ) gamma- ray radiation. Discovered by chance in the 60ies

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Gamma-ray Burst Afterglow Spectroscopy

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  1. Gamma-ray Burst Afterglow Spectroscopy J. P. U. Fynbo, Niels Bohr Institute / Dark CosmologyCentre

  2. What is a gamma-ray burst? Brief (ms - min) and intense (~10-7 erg cm–2s–1) burstofsoft (~100 keV) gamma-rayradiation • Discovered by chance in the 60ies • Rapid variability • Non-thermal spectra • About 1 per day with current sensitivities • The key problem in revealing their nature was getting precise positions.

  3. GRB Afterglows VLT/FORS1: Spectroscopic redshift: z = 1.62 Host: R(AB)>26 Detection of polarization Wijers et al. (1999); Covino et al. (1999); Beuermanet al. (1999); Vreeswijk et al. (2001)

  4. GRB sightline QSO sightline Halo HII regions protodisk Pontzen et al. (2008) HI, HII, H2, metals, dust DM, supermassive BH (Cold) accretion IGM Outflows driven by SNe or AGN

  5. GRB afterglow spectroscopy provides a unique probe of star-forming galaxies • HI • H2 • Metallicities • Extinction curves • UV-photon escape fraction • Unique selection of star-forming galaxies (#massive stars per LF bin)

  6. GRB030323 VLT + HST/ACS z = 3.371 Host: V(AB)=28.0 SFR = 1 M/yr AV<0.5 mag [Fe/H]=-1.5 [S/H]=-1.3 Vreeswijk et al. (2004)

  7. Building a representative sample Sample definitionJakobsson et al. 2006, A&A; Fynbo et al.2009, ApJS) • T90 > 2 sec • XRT localized within 12 hr. • Galactic AV < 0.5. • -70o < declination < +70o • θSun > 55o. March 2005 - September 2008: • 148 bursts • 75% optical/near-IR afterglows • 45% afterglow-based spectroscopic redshifts

  8. Redshifts from optical afterglow spectroscopy High or medium resolution is preferred as much more information is available. But most OAs are too faint for high-res spectroscopy with available spectrographs

  9. Elíasdóttiret al. (2009); Krühler et al. (2008)  FORS spectrum of the afterglow with 2175 Å extinction bump (z=2.45, AV~1mag) X-shooter spectrum revealing emission lines from the host (M. Sparre) 

  10. Afterglow derived extinction curve with 2175 Å extinction bump (z=1.65, AV=0.5 mag) Zafaret al. (2011,2012) X-shooter spectrum of the host  SFR = 30 M/yr Oxygen abundance 40-110% solar Krühleret al. (2012)

  11. GRB080607 Spectroscopy started 20 min post burst – Bloom and Perley R>24 when observable from La Palma (12 hr later) Keck z = 3.04 logNHI=22.7 H2 and CO Forest of metal lines! Solar metallicity AV=3.3 mag 2175Å extinction bump. Bright/massive and dusty host SFR = 10 M/yr Imagine E-ELT GRB070306 Prochaska et al. (2009)

  12. GRB090323 VLT/FORS z = 3.57 logNHI=20.7 Super-solar metallicity AV=0.1-0.2 mag GRB070306 Savaglio et al. (2012)

  13. GRB120815A Spectroscopy started 1.67 hr post burst VLT / X-shooter (40 min exp.) z = 2.358 logNHI=22.1 H2 Low metallicity AV≈0.2 mag No 2175Å extinction bump. Tentative detec. host emisson lines. GRB070306 Krühler, Ledoux, Fynbo, et al. in prep (2012)

  14. GRB07306, z=1.50, VLT, strong reddening, but blue host. GRB070306 Jaunsen et al. (2008) Jaunsen et al. 2008

  15. Metallicities (Examples from X-shooter GTO) D’Elia et al. (2010), Thöneet al. (2012); Sparre et al. in prep.; D’Elia et al., in prep; Savaglio et al (2012)

  16. What we need for the E-ELT HIRES • An instrument that will allow a spectroscopic study of the underrepresented faint, dusty, metal- and likely molecular-rich GRB sightlines. • Specs: ideally 3200-24000 Å, high efficiency (need to reach AB>22), R>10000, AO assisted where possible to gain the depth • The blue/near-UV is important (HI, H2). Rather loose K and the blue. • Decent flux calibration • RRM/ToO observing mode

  17. GRB090926A SFR = <2 M/yr [S/H] = -1.9 D’Eliaet al. (2010)

  18. The mean escape fraction of UV photons from star-forming galaxies: fescape<7% (95% confidence) Chen et al. (2007); Fynbo et al. (2009)

  19. DLA at z=2.354 towards Q2222-0946[Zn/H]=-0.46, SFR=20M/yr

  20. z=2.58 DLA towards Q0918+1636: [Zn/H]=-0.12, H2, CI, SFR≈20M/yr

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