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. Proximity effect aka dust destruction
Why use GRBs?
Proximity effect aka dust destruction

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

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