1. Swift Identification of Dark GRBs Palli Jakobsson Jens Hjorth Darach Watson Kristian Pedersen Johan P. U. Fynbo Gulli Björnsson Javier Gorosabel ApJ Letters, 617, L21-L24 (2004) Reykjavík 19 April 2005

2. Outline What is a dark burst?? How do you define it?? Does there exist an accepted definition?? What is the fraction of dark bursts?? Dark bursts in the Swift era: Introducing a dark burst diagram to be used as a quick diagnostic tool for identifying dark GRBs.

3. What can make a burst optically dark? Obscuration: failed OA detection due to extinction. Early high-energy radiation could destroy dust. But only within R ~ 50 pc. High redshift: some GRBs will be located beyond z > 5. Here the UV light, strongly extinguished by the Ly α forest, is redshifted into the optical. GRB intrinsically dark, as may happen if a relativistic ejecta is decelerated in a low-density ambient medium (Stratta: XRF 040912).

4. Dark Burst Definition There is (was) no generally accepted criterion for when a GRB is (was) considered dark. A popular working definition was to set a brightness limit at a given time after the GRB, e.g. R > 23 @ 1-2 days. (typical search efforts & reaction times) This definition has resulted in the community generally accepting a dark burst fraction of around 60%-70% In many cases, GRBs have been considered dark if no OA was detected, irrespective of how inefficient the search was.

5. Dark Burst Definition Far-reaching conclusions have been drawn from this 60%-70% fraction, e.g. the fraction of the obscured star formation in the Universe. Do we believe this number???? Absolutely not! It’s utter nonsense

6. Fynbo et al. (2001): GRB 000630 and Implications for dark GRBs ~75% of GRBs with upper limits are consistent with no detection if they were similar to a dim burst like GRB 000630.

7. HETE-2 SXC GRBs HETE-2 Soft X-ray Camera (SXC) bursts which: have an error radius <2 arcmin the error radius distributed within 2 hours Out of 14 such bursts, at least 12 of them had an OA  the ”true” dark burst fraction closer to 10% optical follow-up started within 6 hours Lamb et al. (2004); Jakobsson et al. (2005)

8. More evidence that the dark burst fraction is ~10% De Pasquale et al. (2003): analysed 30 BeppoSAX burst: optically faint bursts are also X-ray faint. But: some bursts fainter in the optical than expected from X-rays. Rol et al. (2005): most GRBs can be fitted with standard fireball models. Only 3 (~10%) were inconsistent with all models, i.e. fainter than the faintest optical expectation from X-rays.

9. To catch a dark burst in the act In the Swift era we need an operational definition of dark bursts: have to be able to identify them quickly. A faint burst does not belong to a separate class, e.g. GRB 980613 (Hjorth et al. 2002), GRB 000630 (Fynbo et al. 2001), GRB 020124 (Berger et al. 2002; Hjorth et al. 2003), GRB 021211 (Fox et al. 2003; Crew et al. 2003). Optical faintness has to be supplemented by another parameter……..we propose β OX

10. Definition of β OX X-ray Optical Sari et al. (1998)

11. Definition of β OX X-ray Optical 2.0 < p < 2.5 Sari et al. (1998)

12. The F opt vs. F X Diagram p = 2 ν c > 10 18 Hz 7/62  11%

13. astro.hi.is/~pallja/dark.html p = 2 ν c > 10 18 Hz 7/62  11%

14. GRB host sub-mm emission GRB sub-mm OA R-mag(11 h) dark?? 000210 Yes > 23.1 Maybe 000418 Yes 20.0 No 010222 Yes 19.2 No 970828 No > 25.0 Yes 990506 No > 23.2 Yes 001025A No > 24.3 Yes