Swift Identification of Dark GRBs Palli Jakobsson Jens Hjorth Darach Watson Kristian Pedersen Johan...
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Transcript of Swift Identification of Dark GRBs Palli Jakobsson Jens Hjorth Darach Watson Kristian Pedersen Johan...
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