Swift Identification of Dark GRBs

Palli JakobssonJens HjorthDarach WatsonKristian Pedersen

Johan P. U. FynboGulli BjörnssonJavier Gorosabel

ApJ Letters, 617, L21-L24 (2004)

Reykjavík19 April 2005

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 adark burst diagram to be used as a quickdiagnostic tool for identifying dark GRBs.

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).

Dark Burst Definition

There is (was) no generally accepted criterionfor when a GRB is (was) considered dark.

A popular working definition was to set abrightness limit at a given time after theGRB, e.g. R > 23 @ 1-2 days. (typical search efforts & reaction times)

This definition has resulted in the communitygenerally accepting a dark burst fraction ofaround 60%-70%

In many cases, GRBs have been considered dark if no OA was detected, irrespective ofhow inefficient the search was.

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

Fynbo et al. (2001): GRB 000630 and Implications for

dark GRBs

~75% of GRBswith upper limitsare consistentwith no detectionif they were similar to a dimburst like GRB 000630.

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 anOA the ”true” dark burst fraction closer to 10%

• optical follow-up started within 6 hours

Lamb et al. (2004); Jakobsson et al. (2005)

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.

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

Definition of βOX

X-ray

Optical

Sari et al. (1998)

Definition of βOX

X-ray

Optical 2.0 < p < 2.5

Sari et al. (1998)

The Fopt vs. FX Diagram

p = 2νc > 1018 Hz

7/62 11%

astro.hi.is/~pallja/dark.html

p = 2νc > 1018 Hz

7/62 11%

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