# Presentation: 802926

## Early phases of afterglow in Swift GRB050401 and GRB050712

- Massimiliano De Pasquale
- Mullard Space Science Laboratory

## Swift and GRBs

*Swift *satellite is the first spacecraft able to detect GRBs and then observe them in the X-ray and in the optical within a few tens of seconds. It also distributes the position to ground observatories in real-time;
- This mission is therefore covering the poorly investigated epoch minutes after the GRBs, the transition phase within the prompt emission phase and the afterglow;
- The study of early phases of  GRB afterglows may provide clues on GRB progenitors, diagnostics about the nature of energy injection and the evolution of ejecta and their interaction with the circumbust medium.

## GRB 050401: a bright burst.

- Peak Count Rate: 5000 c/sec
- Fluence: 8.6E-6 cgs (15-150 keV)
- Duration: 38 sec
- Detected also by Konus-Wind
- Fluence: 1.9E-5 cgs (20-20000 keV) corresponding to a isotropic -ray Energy release
-             3.5 x 1053 erg

## XRT observation I: a broken X-ray lightcurve

- Fit with an unique powerlaw
- is unsatisfactory
- (= 1060 of 242  d.o.f.)
- Fit with a broken powerlaw
-  (= 264 of 240  d.o.f.)
- 1= -0.62 +/-0.02
- 2= -1.43 +/-0.06
- Tbreak = 4580 +/- 460 sec

## XRT observation II: signature of dense circumbust medium

- Fit of the pre-break WT data:
- Energy Index  -0.9+/-0.03
- NH = (1.7 +/- 0.21) x 1022 @ z = 2.9
- WT post-break data:
-  -0.9+/-0.13
- PC data:
-  -0.75+/-0.15
- No spectral evolution is
- required by data.

## The optical lightcurve

- The optical counterpart of the afterglow was detected 1.5 hours after the BAT
- trigger by ground observatories.
- Spectroscopic redshift of z=2.9 (VLT)
- _Observed by several facilities_

- _Different calibration issues  may well be present_

- If this is not the case, then we observe
- _a non-monotonic behavior in the optical lightcurve, with a possible rebrightening after the break in the X-ray lightcurve_.
- We also note the optical decay slope
-  in the first 10 ksec data, gathered by
- a single facility, is  consistent with the first X-ray decay slope.

## What causes the break in the lightcurve?

| XC | 
=1.37+/-0.08 |
| --- | --- |
| XC
 |  
 |

** ****- ****Jet expansion**
**F≈**** ****t****************, ******** and ******** linked (Sari et al 99)**

-  Continous energy injection by the ‘central engine’ (Zhang & Meszaros 2002, already suggested for GRB010222, Stanek et al. 2001, Bjornsson et al. 2002)  The source continues to emit energy after the main event, following a powerlaw decay: L **≈ t****q**** . **The injection would influence the fireball dynamics and hence the lightcurves as long as q > -1.
- = -0.64, =-0.9   q = -0.4  before the break
- q/2)+q +1
- =-1.43, =-0.9     =>    q = -1      after the break
- The possible optical rebrightening after the break in the X-ray lightcurve might be explained as a reverse shock component.

## Strong X-ray but faint optical afterglow

- GRBs with optical counterpart exhibit a
- correlation between optical and X-ray flux.
- Several GRBs without optical counterpart
- (dark GRBs) show a X-ray emission
- consistent with that expected by assuming
- the validity of the same correlation.
- However, about 10-20% of dark GRBs do
- not have weak X-ray fluxes but present
- tight optical upper limits (De Pasquale
- et al. 2003, Jakobsson et al. 2004) and
- have been classified as ‘truly’ dark GRBs,
- whose optical emission is extincted by some mechanism.
- GRB 050401 is an ‘intermediate’ case: an optical afterglow is detected, but it is very faint compared to the X-ray flux of other GRBs with OT. _Actually, its optical-to-X flux ratio would be typical of truly dark GRBs_.

- De Pasquale et al. 2003

## Optical extinction in GMC ?

-     Two main hypothesis have been put forward to explain dark GRBs:
- Origin at very high redshift (z>5), with the optical emission washed out by the Lyforest;
- Extinction of light by dense, dusty environments like those of star forming regions enbedded in Giant Molecular Clouds.
-  The detected absorption, NH = (1.7 +/- 0.21) x 1022 @ z = 2.9, is the range of GMC. Could this medium be responsible for the optical faintness?

- NH = 1.7 x 1022  => AV = 10.
- At z=2.9, the R-band is redshifted to 180 nm. Assuming a Galactic
- extinction curve of A / AV ~   =>  ~ 30 magnitude of extinction!

- This would be unrealistically bright, thus we can exclude
- this simple explanation.

## Possible solutions

- Dust-to-gas ratio much lower than the Galactic one
- Dust grain size distribution skewed toward large grains
- Exposed to the high energy radiation field of the GRB, dust grains tend to be heated and eveporate up to 10-20 pc from the GRB (Waxman & Draine 2000, Draine & Hao 2002).
- Consequence of exposing dust to intense radiation field may be a grain size distribution flatter than the original one (as the destruction is more efficient on small grains) and a more levelled extinction curve (Perna et al. 2003, Perna & Lazzati 2002)
- A distribution of grain size skewed toward large grains may also be the result of  coagulation of  small grains in dense environments (Maiolino et al. 2001, Kim & Martin 1996).

- Stratta et al 2004,
- see also Galama &
- Wijers 2001

## GRB 050712: a case of rebursting

- Fluence of the prompt 15-350 keV emission: 1.75 E-6 cgs
- Duration: 48 seconds

- After the prompt emission ended,
- we see a clear rise of the c/r in
- the XRT: a late X-ray burst with
- two peaks, 280 and 500 seconds
- after the trigger. UVOT detected
- a source.
- Early lightcurve similar to
- GRB011121 and 0112111
- (Piro, De Pasquale, Soffitta et al. 2005).

## GRB 050712 X-ray lightcurve analysis

- The fit of whole data with a single powerlaw with t0 the BAT trigger
-      is not possible
- An accetable fit is provided by a broken powelaw and choosing the rising of second peak as t0.
-  Fit parameters:
-  =-0.79 +/- 0.02
-  =-1.25 +/- 0.16
-  Tbreak = 100000 +/- 37000 sec
- = 46/27 (single powerlaw: 63/29)

- Possible scenario: “thick” fireball (Sari & Piran 1999).
- The early afterglow emission is mixed with the late GRB one.
- The afterglow decay will be described by a powerlaw only if the time is measured from when the central engine turns off its main emission (GRB duration)
-  Continuous low-rate contribution of the inner engine.

## Conclusions

- We have examined two cases of study of early afterglow (since ~ 1 minute from the BAT trigger) performed by Swift;
- In both cases, we have got indication that the ‘inner engine’ of GRB does not stop abruptly the after the prompt emission, but it is active for thousands of seconds, as evidenced by X-ray lightcurves.
- The case of GRB 050401, in which a fairly constrained absorption is detected, indicates that ‘dark GRBs’ can be obscured by dense circumbust medium, typical of star forming regions, but this medium may be characterized by dust grain properties different from the Galactic ones.