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Dust Extinction in GRBs: Investigating Host Galaxy Properties and Metallicity Effects

This study focuses on the impact of host galaxy dust extinction on the spectral energy distribution (SED) of gamma-ray bursts (GRBs). Analyzing data from various GRBs, the research examines the relationship between dust-to-gas ratios, metallicity effects, and extinction laws in GRB host galaxies. Key findings suggest that the dust-to-gas ratio may be influenced by the host galaxy's metallicity levels. The study also explores the variation in extinction laws among different GRBs and investigates the presence of the 2175 Å feature in host galaxies. These insights shed light on the complex interplay between dust extinction, metallicity, and the properties of GRB host galaxies.

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Dust Extinction in GRBs: Investigating Host Galaxy Properties and Metallicity Effects

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  1. Dust Extinction in GRBsPatricia SchadyUCL-MSSLM.J.Page (MSSL-UCL), S.R Oates (MSSL-UCL), M. Still (MSSL-UCL), M.De Pasquale (MSSL-UCL), T. Dwelly (Southampton Univ.), N.P.M. Kuin (MSSL-UCL),S.T. Holland (CRESST/GSFC/USRA), F.E. Marshall (NASA/GSFC), P.W.A.Roming (PSU)

  2. GRB as probes to high-z galaxies SED for GRB 060729 • Mean redshift < z > = 2.2 • Median redshift is 1.95 (Fynbo et al., 2009) • Long GRBs occur in most luminous part of their host galaxy • Very bright afterglow shines through host galaxy, leaving imprint of host galaxy interstellar medium on afterglow • GRB radiation mechanism is synchrotron emission Host dust extinction Flux density (Jy) Host gas absorption (Schady et al., 2009) Frequency (Hz) Optical spectrum for GRB 080607 Host dust extinction Relative Flux (erg/s/Å/cm2) (Prochaska et al., 2009) Wavelength (Å)

  3. Broadband SED analysis SMC LMC Galactic Optical/NIR Data • Results: • Of those GRBs well-constrained, 55% best-fit by SMC model, 27% best-fit by LMC model, 18% best-fit my MW model. • 75% GRBs have a host absorption system detected at 90% confidence with • range: 1.1x1021 - 14.1x1021 cm-2 • mean: <NH,X> = 5.6x1021 cm-2 • 71% GRBs have a host extinction system detected at 90% confidence with • range: 0.06 - 0.82 mag • mean: <AV> = 0.33 mag X-ray Data

  4. Mean GRB extinction law? A  ln[Fabs()/Funabs()] Fabs():GRB spectrum reddened by host galaxy Funabs():intrinsic GRB spectrum intrinsic GRB spectrum Spectral Model: NH(Gal) and AV(Gal) X NH(Host) and AV(Host) X power-law continuum Flux Density (mJy) Observed data GRB spectrum extinguished by host galaxy dust only Frequency (Hz)

  5. Mean GRB extinction law? A/A1600Å • Can reject MW-like shape with 99.9% confidence • Power law-like extinction laws provide best-fit i.e. linear, SMC  (Å)

  6. GRB061121: Grey dust? Light Curve SED (Perley et al., 2009)

  7. Broadband SED analysis SMC LMC Galactic Optical/NIR Data • Results: • Of those GRBs well-constrained, 55% best-fit by SMC model, 27% best-fit by LMC model, 18% best-fit my MW model. • 75% GRBs have a host absorption system detected at 90% confidence with • range: 1.1x1021 - 14.1x1021 cm-2 • mean: <NH,X> = 5.6x1021 cm-2 • 71% GRBs have a host extinction system detected at 90% confidence with • range: 0.06 - 0.82 mag • mean: <AV> = 0.33 mag X-ray Data

  8. GRB Host galaxy AV and NH,X distributions Observed NH,X distribution Number of GRBs Number of GRBs Log [NH,X] (cm-2) Log AV

  9. Host Galaxy Dust-to-Gas Ratios NH,X/AV < SMC NH,X/AV > SMC SMC dust NH,X/AV < LMC Log AV NH,X/AV > LMC LMC dust NH,X/AV < MW NH,X/AV > MW Galactic dust Log [NH,X] (cm-2)

  10. Location of absorbing/extinguishing systems NH,X probes gas close to the GRB • Little observational evidence for grey host dust extinction: • AV probes most dust inside molecular cloud • AV probes dust outside of molecular cloud • (Davide Lazzati’s talk on Wednesday) • Therefore… • NH,X/A V is representative of conditions within host galaxy prior to GRB • Pre-GRB NH,X/A V is larger than measured Observed NH,X distribution Expected NHI distribution Number of GRBs Log [NH,X] (cm-2)

  11. Location of absorbing/extinguishing systems Dust destruction/ Metallicity (Z = 0.04 Z) Photoionisation

  12. Quantifying the Selection Effects Observed NH,X distribution Expected NHI distribution Number of GRBs Log [NH,X] (cm-2) Log AV

  13. Metallicity dependency? High-resolution absorption line spectroscopy of the GRB afterglow show a wide range from about 1% solar to nearly solar (Prochaska et al. 2007) [M/H] -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 5 4 3 2 1 0 Age of Universe (Gyr)

  14. Metallicity dependency? GRB environments low in (UV/optical absorbing) dust because of low metallicity smc lmc SMC spectral fit results mw M(dust)/M(HI+H2) NH,X/AV (1021 cm-2) smc lmc LMC spectral fit results mw (Draine et al., 2007) smc lmc MW spectral fit results mw 12 + log [O/H]gas [M/H]

  15. Summary and Open Questions • 70-80% GRBs show significant absorption and extinction local to the GRB, with • <NH,X> = 3.4x1021 cm-2 • <AV> = 0.3 mag. • 83% of sample best-fit with either SMC or LMC extinction law • 2175 Å not prominent in extra-galactic environments • currently no evidenceto suggest that GRB host galaxy extinction laws are typically gray or that there is a paucity of small dust grains • GRB host galaxies have a range in gas-to-dust ratios that are typically larger than Milky Way and Magellanic Clouds. • the current analysis suggests that this is not primarily as a result of dust destruction • tentative evidence to suggest that GRB host galaxy dust-to-gas ratio may be a function of host galaxy metallicity, as has been observed in other more metal rich galaxies. So…. • why do some GRB host galaxies show clear evidence of the 2175Å whilst most don’t. Just a signal-to-noise question? • is there such a thing as a ‘mean’ GRB host extinction law? • Why do GRBs have such large NH,X/AV ratios? Metallicity effects?

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