Swift Identification of Dark GRBs

1. SwiftIdentification of Dark GRBs ApJ Letters, 617, L21-L24 (2004) Palli Jakobsson Jens Hjorth Darach Watson Kristian Pedersen Johan P. U. Fynbo Gulli Björnsson Javier Gorosabel 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 Lyα 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) In many cases, GRBs have been considered dark if no OA was detected, irrespective of how inefficient the search was. This definition has resulted in the community generally accepting a dark burst fraction of around 60%-70%

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 • optical follow-up started within 6 hours Out of 14 such bursts, at least 12 of them had an OA  the ”true” dark burst fraction closer to 10% 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 fittedwith • 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 βOX

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

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

12. The Fopt vs. FX Diagram p = 2 νc > 1018 Hz 7/62  11%

13. astro.hi.is/~pallja/dark.html p = 2 νc > 1018 Hz 7/62  11%

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