Transcript Transparencies - Rencontres de Moriond

Temporal and spectral evolution of Gamma-Ray
Bursts.
Effects of the surrounding environment.
Alessandra Galli1,2 & Luigi Piro2
1Department
of physics, University “La Sapienza”,
Rome, Italy
2IASF,
CNR-INAF, Rome, Italy
XXXXth Rencontres de Moriond, Very High Energy Phenomena in the
Universe 12-19 March, 2005
The observations suggest that GRBs are produced by
massive stars (metal emission and absorption lines, GRB
localization near the centre of the host galaxy, GRB-Supernova
connection)
Theoretical models
The collapsar model (Woosley 1993, Ramirez-Ruiz et al. 2002,
Waxman & Meszaros 2003) predicts that the GRB is produced by
a massive star that loses mass in the form of a wind.
The medium around the central engine has a wind density
profile: n~r-2
Instead, GRB afterglow studies show that in the most of the events
the afterglow light curves are explained in a uniform interstellar
medium (Panaiterscu & Kumar 2001, 2002, Yost et al. 2003)
Can we explain light curves and at the same time solve
the discrepancy ?
The
case
GRB011121
General
of
characteristics
Long duration event, 75 s in the GRBM
and 120 s in the WFC (Piro et al. 2004)
Presence of a fainter event (precursor)
starting about 30 s before the main event
(Piro et al. 2004)
X-ray late burst or reburst occuring about
240 s after the main pulse (Piro et al. 2004)
z=0.36 (Infante et al. 2001)
Piro et al. 2004
Piro et al. (2004) found that in this event:
Precursor spectrum softer than the main pulse
Reburst spectrum softer than the preceding main pulse and consistent
with the afterglow spectrum at 1 day
X-ray and optical data compatible only with a fireball expanding in a
wind
Is possible explain the reburst? What is its origin?
Results
The light curve from the decay part of the reburst to the afterglow is well
REPRODUCED BY A POWERLAW IF THE ORIGIN OF THE TIME t0 IS SHIFTED
TO THE ONSET OF THE REBURST: t0=(240±10) s.
The calculated light curve fits the data only if the fireball interacts with a
WIND PROFILE ENVIRONMENT
THE FIRST EVIDENCE SUGGEST THAT THE REBURST IS THE BEGINNING
OF THE AFTERGLOW EMISSION (talk of Piro L.)
E53=0.28, 130, A*=0.003, e=0.01, B=0.5, p=2.5, t0=250 s
The case of
XRF011030
General
characteristics
No counterpart in the GRBM (Gandolfi
2001)
Duration of about 1100 s
Preceded by a fainter event about 250
s
long
X-ray source without optical
counterpart (Harrison et al. 2001)
0.6 ≤ z ≤ 3.5 (Bloom et al. 2003)
Temporal
Analysis
The light curve of XRF011030 is characterized by the presence of a
precursor preceding the main pulse, and by the presence of a X-ray late
burst or reburst, a sudden emission occuring about 1300 s after the main
pulse and 200 s long. Instead a break occurs between 104 and 106 s after
the main pulse.
Spectral
Analysis
Total event spectrum, preceding
faint event and reburst included:
powerlaw with photon index .84+0.17-0.16
Preceding faint event: a powerlaw steeper than the main event (Tab.1).
Other possibility: a black body model (in agreement with collapsar model
expectation, Tab.2).
Reburst spectrum: softer than the main pulse; its spectrum is consistent
with the one of the afterglow (powerlaw, Tab. 1).
-Energy range: 2-26 keV
Duration
Precursor 35÷280 s
Burts part 1 280÷500 s
Burst part 2 500÷1000 s
Reburst 1300÷1490 s
Afterglow (*)
Photon index   
2.61+0.76-0.61
1.78+0.17 -0.16
1.63+0.33 -0.30
2.10+0.83 -0.64
1.72±0.20
0,81
1,24
1,48
1,51
Tab.1: Fit with the powerlaw
model
(*) Results of analysis performed by D’Alessio & Piro
(2005)
Duration
KT [keV] 
Precursor 35÷280 s 0.90+0.19-0.15
Total event 280÷1490 s
0,96
2.3!!
Tab.2: Fit with the black body
model
Results
This event has characteristics similar to those of GRB011121
We decided to apply the same interpretation on XRF011030
We shifted the origin of the time t0 to the onset of the reburst, and we found that
considering a fireball expanding in a wind the calculated light curve fits the data,
while for an ism it does not.
ISM: DOES NOT WORK
E53=0.0315, 0=45, A*=0.005, e=0.03, eB=0.0089,
p=2.2, t0=1000 s.
WIND: DOES WORK
E53=0.0315, =45, A*=0.005, =0.03, =0.0089,
p=2.2, t0=1300 s.
Theoretical ground
The onset of the external shock depends on the thickness of the fireball. A shell is thin
if Δ<(E/nmpc2)1/3Г0-8/3 (Sari & Piran 1999).
In this case the reverse shock ends crossing the shell before the fireball
reaches the deceleration radius r0. The origin of the time t0 is equal to the
beginnig of the prompt emission while the onset of the afterglows coincides
with deceleration time.
Therefore, the results of our data analysis and those of our theoretical analysis
seems to FAVOUR the case of a fireball with a THICK SHELL. The shell is
energized until greater times, and the reburst is produced by the shock of the
latest layers of the shell and the external medium.
We analized also the case of a fireball with a thin shell:
STANDARD MODEL: DOES NOT WORK!
It is based on a sperical fireball with a thin shell.
MODELS WITH A DISCONTINUOUS DENSITY PROFILE: DOES NOT WORK!
It is supported by the work of Chevalier, Li & Fransson 2004. When the fireball
interacts with a discontinuity a greater number of photons are produced. But,
during the deceleration phase the emission is depending poorly on the density
profile of the medium.
The interpretation of the
break
➔ Break of spectral nature
F-(p -1)/2, F t-(3p – 2)/4
F-p/2 ,
F t-(3p - 1)/4
population
obs< c
c < obs
c  t1/2
p = spectral index of electrons
We assumed p=2.2 because it is consistent with the spectral analysis (Tab.1),
but the temporal decay of the calculated light curve is not in agreement with the
two CHANDRA observations: theo and obs(D'Alessio & Piro
2005)
➔ Break of dynamic nature: the jet model
 > -1 same evolution of a fireball with spherical simmetry
 < -1 the sideways spread of the jet becomes important, the flux decreases
faster
F t-p
c< obs, c > obs
(Sari, Piran & Halpern 1999)
With p=2.2 the temporal decay is in agreement with the two CHANDRA
observations.
➔
Conclusion
The light curves of sXRF011030 and GRB011121 can be
described only by a fireball with a thick shell.
➔
➔
➔
If the fireball has a thick shell the onset of afterglow
emission is shifted to the instant of the reburst. This
suggest that the reburst represents the beginning of
afterglow emission.
We confirm the claim by Piro et al. (2004) on GRB011121.
For XRF011030 we found a new case in which the event light
curve is explained by a wind profile environment.
THE STANDARD FIREBALL MODEL
GRB011121
ISM
E53=0.28, , n=1, e=0.03, B=0.05, p=2.5.
WIND
E53=0.28, , A*=0.003, e=0.03, B=0.05, p=2.5.
THE STANDARD FIREBALL MODEL
XRF011030
ISM
E53=0.315, , n=1, e=0.03, B=0.05, p=2.2.
WIND
E53=0.0815, , A*=0.01, e=0.03, B=0.05, p=2.2.
MODELS WITH A DISCONTINUOUS DENSITY PROFILE
XRF011030
REGION 1: WIND, REGION 2: ISM
E53=0.315, , A*=0.01 (region1), n=5 (region 2), rdisc=6·1016 cm,
e=0.03, B=0.05, p=2.2.
The main question in gamma-ray burst research is:
Who is the progenitor ?
The observations of GRB afterglows shown that the bursts with an optical
counterpart are localized in the stars formation regions of far galaxies, and
that the energy released is between 1051 and 1054 erg if the emission has
spherical symmetry.
Two families of progenitor could
produce events of such energy:
1. Merging of two compact
objects (Fryer , Woosley &
Hartmann 1999)
2. Collapse of a massive star
(Woosley 1993, Vietri & Stella
1998)
Observations
Fe emission and absorption lines in the spectrum of the main event and in
those of X-ray afterglow (Piro et al. 1999, Antonelli et al. 2000). In some
events were detected also Mg, Si, S and Ar lines (e.g. Reeves et al. 2002)
The counterparts of the most of the events are localized near the centre of
the host galaxy (Bloom et al. 2002)
The optical light curves of some GRBs have components similar to those
of a supernova. The most famous case are GRB980425 (Galama,
Vreeswijk et al. 1998), GRB030329 (Stanek et al. 2003, Hjorth et al. 2003)
and GRB031203 (Malesani et al. 2004)
The value of the column density of Hydrogen on the line of sight of the
observer is consistent with the typical value of star formation regions in our
galaxy (NH~1021÷22 cm-2)
The observations suggest that GRBs are produced by
massive stars