The Early Time Properties of GRBs :

Canonical Afterglow and the Importance of Prolonged Central Engine ActivityAndrea Melandri

Collaborators : C.G.Mundell, S. Kobayashi, D. Bersier, I. A. Steele, C.Guidorzi, A.

Gomboc, R. J. Smith, D. Carter, M. F. Bode

Astrophysics Research Institute, Liverpool JMU, UK

Outline

• Canonical X-ray afterglow

• Early optical light curves: canonical?

• X-ray/Optical analysis

• The role of prolonged central engine activity - afterglow detections - dark bursts

• Beyond the Fireball : GRB 070419A

• Conclusions

Generic X-ray Light curve (Zhang et al. 2006)

~ -3

~ -0.5

~ - 1.2

~ -2102 – 103 s 103 – 104 s

104 – 105 s

Prompt Emission

High latitude emission

Late internal shocks

Standard FS emission

Jet break

Prolonged central engine activityEnergy injection into the FSRS with εB & εe much larger than for FSTime dependent εB & εe Dust scatteringPrecursor fireballCannonballsGeometrical models+ other …….

Typical X-ray Light curve (Nousek et al. 2006, O’Brien et al. 2006)

• The X-ray “canonical” light curve is not

ubiquitous

But….

Oddball cases

e.g. : Nousek et al. 2006, Evans et al. 2008

Melandri et al. 2008

• The same mechanisms should produce the optical radiation. Optical should track X-ray flux, but this is not observed in many

GRBs

Early Optical Light curve

Oates et al. 2009Melandri et al. 2008

At early times

Optical/X-ray analysis

Simplest explanation in the Fireball model :

A : no break in the Opt or in the X-ray band C with respect to the Opt and X-ray bands

B : no break in the Opt, break in the X-ray band Passage of C through the X-ray band

C : break in the Opt, no break in the X-ray band Passage of C through the Opt band

D : break in the Opt and in the X-ray band Cessation of energy injection or jet break

Optical/X-ray analysis

A possible explanation is the additional X-ray emission produced by late-time central engine activity, observed as an enhanced component in late-time afterglow light curves.

Sample : 63 GRBs (24 detections + 39 upper limits):

• We observed achromatic breaks for only 1 GRB.

• The majority of the afterglows in our sample (14/24) are in good agreement (or consistent) with the standard model.

• However, for a significant fraction of our sample (10 cases) the observed data can not be explained with this model, even if modifications (i.e. energy injection or variation of the ambient matter) are assumed.

Melandri et al. 2008

Detections

Optical/X-ray analysis

• The majority of the bursts are “Dark” independent of the selected time.

• 10 : were identified at NIR, demonstrating a small population of bursts in high density environments.

• 29 : we suggest that if late-time central engine activity is responsible for the production of early X-ray afterglow emission then the additional emission will mask the simultaneous (but fainter) FS emission, resulting in a larger observed X-ray flux than expected (while the optical flux is not suppressed).

Dark Bursts

One case…..070419A (Melandri et al. 2009)

-ray

• T90 ~ 110 seconds• FRED profile• Single shot few hundred seconds long• Average -ray fluence (5 x 10-7 erg cm-2)• Peak photon flux (~0.14) at the low end of the distribution for Swift GRBs

X-ray

• Some intrinsic rest-frame absorption• Weak evidence for evolution of NH (~4 x 1021 cm-2) and X (~2.2)• Central engine still active up to 103 seconds, then FS emission takes over (X~1.2)

One case…..070419A (Melandri et al. 2009)Optical

-1.5

0.61.5

0.4

• Refreshed shocks could not be ruled out BUT the energy injection rate should be tuned very carefully + we need a sharp cessation of the injection to get a clear break• Assumption of a finely tuned long-lived central engine (consistent with the same conditions invoked to explain the plateau phase in many X-ray light curves of GRBs)

• Clearly too complex to be explained with the FS model

• Features are too sharp to be explained as density enhancement

• If RS dominating at early times tpeak ~ 450 s, ~ 350 t ~ 1500 s = the passage of m,r m,r ~ 3 x 105 Hz; m,f ~ m,r ~ 4 x 1020 Hz FS should peak t ~ 4 x 106 s !!• If the fireball is magnetized (with RB ~ 106) then m,f could be smaller the ratio between RS and FS luminosity should be ~ 4 x 105 when in reality it is only 2 order of magnitude !!

Conclusions

• 60% of the detected afterglows in our sample are “consistent” with the “standard model”

• Few GRBs are not easily explained in the context of the simple fireball model even when modification are made (i.e. energy injection, variation of the density

matter, magnetized fireball, tail component following the fireball, etc…)

• The Dark Bursts fraction remains high (~50%) despite deep early-time observations

• Enhanced X-ray emission from late time central Enhanced X-ray emission from late time central engine activity plays a big role and may explain “non-engine activity plays a big role and may explain “non-

standard” light curves (like GRB070419A) and Dark standard” light curves (like GRB070419A) and Dark BurstsBursts