Transcript Enigmatic Gamma

GRBs: Recent progress and new mysteries
Outline:
• Summary of main results.
Prompt and afterglow emissions.
(our current understanding or lack there of)
•
• Unsolved problems.
Ohio, September 26, 2007
Gamma-ray bursts are short pulses of radiation that
come from random directions a few times a day.
GRB Duration
Short burst
Long
burst
Swift GRB mission
Swift was launched on Nov 20,
2004. Swift has
accurate localization (~2’)
rapid response (~1 min)
and excellent temporal
coverage for a few days.
the nature of
• Determine
short duration GRBs.
• Find GRBs at high z
transition from -ray
• The
prompt to afterglow emission.
What Have We Learned About GRBs?
Long duration GRBs:
• Energy: ~ 1051 erg (wide dispersion)
• The outflow is collimated: j ~ 3o 30o
• Occur in late type galaxies and associated with star formation.
• At least a few of them have SNa Ic associated with them;
these supernove were more energetic than average Ic.
• Medium is uniform (often) with density a few/cc
• Median redshift for Swift bursts is ~ 2.5;
the lowest Z is 0.033 & the highest is 6.29
Short duration GRBs
Swift has detected 15 and HETE 1 short bursts:
• 5 GRBs are located near early type galaxies whereas
2 are in late type galaxy; the offset varies from 0.3
to 13 Rg & SFR < 0.1 Mo yr-1 for two of the host
galaxies (Nakar , 2007).
• Very stringent limit on any underlying SN for
two GRBs (L<4x1040 erg/s) between 7-20 days
(Fox et al.).
• Low density of the circum-stellar medium;
For 2 of the GRBs n< 10-2 cm-3; Panaitescu (2006)
• Lower Eiso (E) by ~ 103 (10) & median z  0.25.
Associated with older stellar population, possibly
binary n-star (but we lack a firm proof).
Nakar, 2007
Evidence for Relativistic outflow
• Superluminal motion in 030329: Rt ~ 3x1017cm at 25d
 vt =Rt/t=5c  ≈7 at 25 days (Taylor et al. 2004).
•
Diffractive scintillation quenched at 30d for 970508 
R~1017cm  V~R/t~C; Goodman 1997; Frail et al.
• Afterglow modeling gives >4.5 at 1 day for 10 bursts
(Panaitescu & Kumar, 2002).
Synchrotron from FS fits late afterglow data
Panaitescu & Kumar
EGRB ~ 1051 erg; jet
Uniform ISM; nISM ~ 10 cm-3
Early Afterglow Results for Swift Bursts
• Rapidly decaying flux in the x-ray; likely the remnant of
decaying -ray source (before the onset of FS emission).
• Slowly decaying lightcurve in the x-ray.
Sudden increase in flux (flare) during the “afterglow”
(long lived central engine activity)
Nousek et al. 2005
Because of smearing due to curvature dt/t ~ 1 in FS. Many of
the flares have t/t << 1 which suggests late time engine activity.
Some of the basic unanswered questions about GRBs
• How does the central engine operate: accretion, B-Z…?
The central engine is hidden -- opaque to EM signals.
So our best hope is to try to model the
-ray emission and the x-ray flares.
• How is the relativistic jet produced?
• Is the GRB outflow baryonic or magnetic?
Understanding the -ray emission mechanism and
detecting RS emission from GRB-ejecta would help.
Prompt -ray generation mechanism
O’Brien et al., 2006
Factor ~ 103
drop in flux!
• The early time data from Swift shows that -rays
are produced by a distinct - short lived - source.
• We exploit this steep falloff to determine -ray
source distance from the center of explosion.
The fastest decay of LCs (Off-Axis Emission)
(Kumar & Panaitescu 2000)
t =R 2/2

t =R -2/2
1
fn
R
t ~ -2-b
t ~ -1
t
np  t -1
fn  t -2-b
Nousek et al. 2005
Gamma-ray Generation (distance)
• The distance from the center of the explosion where
-rays are produced, R , can be determined from
the early x-ray lightcurve:
ct1 ≈ R/202 ;
ct2 ≈ Rfs/2fs2
Since fs < 0
 R > (t1/t2) Rfs
Rfs = [3ct2Eiso/2mpc2n0]1/4
0: -ray source Lorentz factor
fs: forward shock LF at t2
t1: time when -ray emission ends, t2: time when steep x-ray decline ends.
• For 10 Swift bursts (t2/t1) is between 5 & 25 ; the
mean is ~ 14  same for FRED & non-FREDs.
 -ray source lies within a factor ~10 of FS radius.
• Or
-rays are produced at a distance of
~ 1016 cm from the center of explosion.
This distance is much larger than what was
expected for internal shocks  and of order
the distance suggested for poynting model.
The Internal-External Fireball Model
-rays
Inner
Engine
Relativistic
Internal
Shocks
Outflow
106cm
1013-1015cm
Afterglow
External
Shock
1016-1018cm
Piran et al. 1993; Rees & Meszaros 1994; Paczynski & Xu 1994
Understanding -ray emission
(Kumar et al. 2007)
Synchrotron & IC from a relativistic source
emission can be completely described by 5 parameters:
Constraints
Flux
 spectral index below the peak of spectrum
 frequency at peak of spectrum
 burst/pulse duration

5 unknowns and 3 constraints gives
2-D solution surface.
-rays produced via the synchrotron process?
(Ep =100kev; flux=1mJy; t=1s; low energy spectral index -ve)
Synchrotron solution is also ruled out when fnn+ve
• Synchrotron peak frequency = 100 kev
 Bi2 = 1013
• Electron cooling:
c/i ~ 10-17 i3 /tGRB(1+Y)
Compton Y ~ eci
 c/i ~ 10-9 [ i/(tGRB e)]1/2
Therefore, c/i <<1  fnn-1/2
-rays produced via the SSC process?
(Ep =100kev; flux=1mJy; t=1s)
SSC gives consistent solutions. It predicts bright, prompt,
optical which we see in a few cases: 041219 ~ 14 mag.
GRB jet that produces -rays baryonic
• Is the
or poynting outflow?
• For baryonic outflow we should see RS emission.
Reverse shock emission?
One of the things Swift was going to do is find many
more bright optical flashes like GRB 990123  where
are they?
Roming et al. 2006
Ejecta
Forward shock
ISM
Reverse shock
•
Ejecta
ISM
New puzzles posed by Swift data
(Problem when you have good data!)
Flares lasting
for hours - short
and long GRBs
Chromatic plateau
In x-ray LCs
RS emission?
Do we have the
FS AG right?
Jet breaks?
A sudden drop in x-ray flux in a few cases!
Troja et al. astro-ph/0702220
Summary of Results
1. Long duration GRBs are associated with collapse
of a massive star (at least in several cases!).
2. The short GRBs have much less energy and are
associated with old stellar population.
3. The rapid fall off of the early x-ray afterglow suggests
that -ray emission is produced by a short lived
source; we find that it is most likely SSC at a distance
of ~ 1016 cm  baryonic jets have a few problems.
4. X-ray lightcurves show flares on time scale of minutes
to a day suggesting that the central engine of GRBs
can be active for a period of order ~ 1 day.
Unsolved Problems
1. The nature of the central engine is not understood.
2. Is the energy from the explosion carried outward
by magnetic field, e±, or baryonic material?
3. No firm evidence for r-2 density structure (except
perhaps in 1 or 2 cases). And very low density found
in several cases is puzzling.
4. Collisionless shocks, particle acceleration, magnetic
field generation etc. poorly understood.
Superluminal motion in GRB 030329
(Taylor et al., 2004, 609, L1)
≈7
v=3c
v=5c
v 
b sin 
1 b cos 
 ≈ 50

Solid line: Spherical outflow in a uniform ISM; E52/n0 =1
Dashed line: jet model with tj =10 days & E52/n0 =20.
Nakar, 2007
Host of short-GRBs)
Tagliafferi et al. 2005
t-0.72
t-2.4
Break in the LC at
2.6 days implies:
j ~ 3o
E ~ 4x1051 erg
Determining Jet Angle from Break in LC
(Rhoads 1999, Sari et al. 1999, Kumar & Panaitescu 2000)
At late time: -1 ≥ 
At early time: -1 ≤ 

R 
1 1
R
Area visible to observer = (R/)2
Area visible to observer = (R)2  (R/)2 ()2  (R/)2 t -3/4
t
t ~ -1
t ~ -2
~ -2
t ~ -1
Future Missions
GLAST, due for launch in 2008,
will cover 10 Kev – 300 Gev, and
detect > 200 GRBs yr-1.
AGILE (an Italian mission)
30 Mev – 30 Gev & 10 – 40 kev
is expected to launch in 2005.
• ICECUBE, ANTARES will explore
Neutrino emission from GRBs: 10 Gev – 105Tev.
• Gravitational waves from GRBs?
GRB 021004 (HETE II: Shirasaki et al.)
Temporal fluctuations
Absorption lines at different velocities
(spectrum at ~ 1 day -- McDonald HET)
Bersier et al. 2002, astro-ph/0211130
Nakar & Piran, 2003, ApJ 598, 400
Schaefer et al. 2002
(similar velocity features are also
seen in 050505; Berger et al.)
Long GRBs - collapse of massive stars
(Woosley and Paczynski)
GRB 030329/SN2003dh
SN 2003dh/
GRB 030329:
z=0.166
(afterglow-subtracted)
SN 1998bw:
local, energetic,
core-collapsed
Type Ic
Stanek et al.,
Chornock et al.
Eracleous et al.,
Hjorth et al.,
Kawabata et al.
Detectability of Bursts at high Z
 The peak flux for GRB 050904 was ~ 3x10-8 erg cm-2 s-1
(BAT sensitivity, 15-150 kev, is 0.25 photons cm-2 s-1
or 1.2x10-8 erg cm-2 s-1 for fn  n-1/2)
So Swift can detect bursts like 050904 to Z~10.
 Price et al. (2005) claim that 8 out of 9 Swift bursts
(at z>1) could be detected at z=6.3 and 3 of these
could be detected at z~20.
Detectability of Afterglows at high Z
 10 min after GRB 050904 the 0.2-10 kev flux was
~ 10-9 erg cm-2 s-1 and the luminosity (isotropic
equivalent) was ~ 1050 erg s-1 (the flux at earlier
time scaled as t-2).
Swift/XRT detection limit is 10-13 erg cm-2 s-1
for 100s integration time.
 At 1 hr the J-band flux was 17th-mag and the luminosity
(isotropic equivalent) was ~ 1047 erg s-1
 Negative k-correction helps: f (t)  n- t-b
n
(~1 and b~1-3 at early times)