Dust Extinction in GRBs Patricia Schady UCL-MSSL M.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)

GRB as probes to high-z galaxies Frequency (Hz) Flux density (Jy) Wavelength (Å) Relative Flux (erg/s/Å/cm 2 ) Optical spectrum for GRB (Prochaska et al., 2009) Host dust extinction SED for GRB (Schady et al., 2009) Host dust extinction Host gas absorption Mean redshift = 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

Broadband SED analysis Optical/NIR Data SMC LMC Galactic X-ray 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: 1.1x x10 21 cm -2 mean: = 5.6x10 21 cm -2 71% GRBs have a host extinction system detected at 90% confidence with range: mag mean: = 0.33 mag

intrinsic GRB spectrum GRB spectrum extinguished by host galaxy dust only Observed data Spectral Model: N H (Gal) and A V (Gal) X N H (Host) and A V (Host) X power-law continuum A  ln[F abs ( )/F unabs ( )] F abs ( ): GRB spectrum reddened by host galaxy F unabs ( ): intrinsic GRB spectrum Frequency (Hz) Flux Density (mJy) Mean GRB extinction law?

A /A 1600Å Model  2 /dofReduced  2 Null- Hypothesis Probability linear21/ SMC21/ LMC24/ MW53/ Calzetti32/  Can reject MW-like shape with 99.9% confidence  Power law-like extinction laws provide best-fit i.e. linear, SMC (Å)

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

Broadband SED analysis Optical/NIR Data SMC LMC Galactic X-ray 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: 1.1x x10 21 cm -2 mean: = 5.6x10 21 cm -2 71% GRBs have a host extinction system detected at 90% confidence with range: mag mean: = 0.33 mag

GRB Host galaxy A V and N H,X distributions Number of GRBs Log A V Log [N H,X ] (cm -2 ) Number of GRBs Observed N H,X distribution

Log [N H,X ] (cm -2 ) Log A V Galactic dust N H,X /A V > MW N H,X /A V > LMC N H,X /A V > SMC LMC dust SMC dust N H,X /A V < MW N H,X /A V < LMC N H,X /A V < SMC Host Galaxy Dust-to-Gas Ratios

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

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

Quantifying the Selection Effects Log [N H,X ] (cm -2 ) Observed N H,X distribution Expected N HI distribution Log A V Number of GRBs

Metallicity dependency? Age of Universe (Gyr) [M/H] (Prochaska et al. 2007) High-resolution absorption line spectroscopy of the GRB afterglow show a wide range from about 1% solar to nearly solar

GRB environments low in (UV/optical absorbing) dust because of low metallicity [M/H] N H,X /A V (10 21 cm -2 ) smc mw lmc smc SMC spectral fit results LMC spectral fit results MW spectral fit results 12 + log [O/H] gas M(dust)/M(HI+H 2 ) (Draine et al., 2007) Metallicity dependency?

Summary and Open Questions  70-80% GRBs show significant absorption and extinction local to the GRB, with  = 3.4x10 21 cm -2  = 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 evidence to 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 N H,X /A V ratios? Metallicity effects?