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Dust and extinction in GRBs

This article explores the use of GRBs for measuring extinction curves and discusses the importance of dust destruction and its impact on observations. It highlights the diversity of extinction behavior in GRB hosts and emphasizes the need for caution when interpreting AV/NH ratios.

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Dust and extinction in GRBs

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  1. Dust and extinction in GRBs Davide Lazzati NCSU

  2. Why use GRBs? Known luminosity Known Spectrum (better of featureless) F(n) n

  3. Why use GRBs? • Known spectrum (featureless power law) Spectral slope can be derived from temporal decay • Known luminosity… not exactly Since spectrum is known and broadband, the absorbed part can be interpolated

  4. Why use GRBs? F(n) n

  5. Why use GRBs? Proximity effect aka dust destruction

  6. Dust destruction in GRBs dn/da a Two mechanisms: thermal sublimation and X-ray ionization

  7. Thermal sublimation Waxman & Draine 2000 Perna & DL 2002 Cold Grain Hot Grain

  8. Thermal sublimation Waxman & Draine 2000 Perna & DL 2002 Radial dependence: big grains can survive where small grins can not

  9. Thermal sublimation Log(n) Big grains left (gray ext.) Dust unaffected No dust left Log(R)

  10. Thermal sublimation: UV bright burst Log(n) Big grains left (gray ext.) Dust unaffected No dust left Log(R) 10pc

  11. Thermal sublimation: UV weak burst Log(n) Big grains left (gray ext.) Dust unaffected No dust left Log(R) 10pc

  12. Intermediate conclusions: • Thermal sublimation produces a quick dust evolution. In principle observable before the burst peaks (few seconds…) • Once the prompt is over, no more evolution • Does not affect our dust extinction measurement because the shell with modified distribution is thin compared to its radius

  13. X-ray ionization • Ionization produces charged grain • If stress due to E field is too large, grain ejects ion • Much less efficient than thermal sublimation • However, is a fluence process and so can go on for longer • It’s important only if very UV poor burst (not more than 1 per cent in any GRB I modeled)

  14. Intermediate conclusions: • Thermal sublimation produces a quick dust evolution. In principle observable before the burst peaks (few seconds…) • Once the prompt is over, no more evolution • Does not affect our dust extinction measurement because the shell with modified distribution is thin compared to its radius • X-ray photoionization is not a worry. Just in case do not use GRB with afterglow fluence larger than prompt fluence.

  15. Observations • There is a lot of confusion… • Claims range from gray extinction curves to SMC-like ones • Sometimes even different claims from the same dataset!!! • Mainly due to difficulty with the band in which the observations are routinely taken. If no FIR, assumptions have to be made on the extinction law • Let’s see some example…

  16. Observations: GRB050904 (z=6.4) Kann et al. 2007 They conclude that SMC dust fits best

  17. Observations: GRB050904 (z=6.4) Stratta et al. 2007 They reject the SMC model and claim a best fit with a QSO model

  18. Observations: GRB050525A (z=0.61) Heng et al. 2008

  19. Observations: GRB050525A (z=0.61) Heng et al. 2008 - SMC & LMC good - MW can be excluded - Lot of UV extinction (small particles)

  20. Observations: GRB070802 (z=2.45) Eliasdottir et al. 2009 No matter the continuum extinction, there is a bump at 2175 Angstrom

  21. Conclusions • GRBs are in principle good candidates for measuring extnction curves. • Dust destruction is important at very early stages, not at time of optical/NIR observations. • Lot of diversity and lot of debates. • Two only good cases show diverse behavior of bump… • Due to dust destruction beaving different than photoionization of gas, don’t do AV/NH ratios of GRB hosts lightly.

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