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SWIFT: Selected results after
the first three years of the
mission
L. Angelo Antonelli
INAF-Oss. Astron. Roma
&
ASI Science Data Center
MAGIC Meeting on GRB - La Palma, 3 Dec. 2007
Outline
1. SWIFT: an Overall Description
2. Gamma Ray Bursts: A Brief Introduction
3. Selected Results from 3 years of the SWIFT
Mission
• The unexpected behaviour of the Early
Afterglow
• Short GRBs
• GRBs as cosmic beacons
• The GRB-Supernova connection
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
SWIFT
NASA MIDEX Mission
selected in 1999
Primary science is to study gamma-
ray bursts throughout the Universe
International hardware participation
from UK and Italy
Launched on November 20, 2004
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
SWIFT
• Burst Alert Telescope (BAT)
– New CdZnTe detectors
BAT XRT UVOT – Sensitive gamma ray imager 15-150
keV
– Precision 2-3 arcmin
– Field of view: 1/6 of the sky
• X-Ray Telescope (XRT)
– GRB positions within 3”
– Imaging 0.2-10 keV
– Sensitivity 2x10–14 cgs
– CCD spectroscopy
• (UVOT) UV/Optical Telescope
– 30 cm Optical/UV telescope
– Sub-arcsec imaging
– Grism spectroscopy
– 20th mag sensitivity (1000 sec)
– Finding chart for other observers
• Spacecraft
– Autonomous re-pointing, 70 - 120 sec
– Onboard and ground triggers
– Low-orbit => Low- background
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Gamma Ray Bursts: basic facts
GRBs are brief, sudden, intense flash
of gamma-ray radiation.
Discovered by the Vela satellites at
the end of ‘60s
Isotropically distributed (CGRO in
the ‘90s) Duration: 0.1< t < 100 s
Frequency: 10 keV – few MeV
Fluence: 10–6 erg cm–2 s–1
Flux: 10–7 erg cm–2 s–1
GRBs duration
distribution is double
peaked. Short
GRBs are harder
than long GRBs.
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Afterglows
Predicted by models and discovered
in 1997 by BeppoSAX
Afterglows are Long-lasting (days
to months) counterparts of GRBs
observed in the X-ray, optical and
radio band.
Decaying light curvePower law:
F (t ) t –
Very bright soon after GRB: R15–18, FX 10–10 erg s–1 cm–2
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Afterglows before SWIFT
Afterglows => redshift => distance & energetics
Cosmological events: <z> = 1
GRBs energies: 1051 – 1054 erg => 1051 erg if collimated
Very rare in the Universe (~1/100 of SNe)
LONG GRBs SHORT GRBs
Association with Binary compact object
core-collapse SNe binary mergers?
Star-forming host galaxies
Connection with cosmic star
formation Dec. 3,
La Palma, L. A. Antonelli: SWIFT Highlights
The Gap
X-RAY OBSERVATION MANDATORY! ==>Almost all
GRBs have a X-ray afterglow
Fast observations ==>X-ray afterglow rapidly fading
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
The SWIFT Bursts
Launch: 2004 November 20 (36 months operations)
From Dec 2004 up to Nov 2007:
Total Long Short
GRBs 265 238 27
X-ray AG 255 (96%) 233 22
Optical AG 141 (53%) 130 11
Redshift 74 59 16
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Some prompt light curve with BAT
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
GRB 041223
•Discovered by BAT at 14:06:18 on 23 December 2004
•XRT was in midst of thermal tests, taking data in PC mode
•Slewed to GRB 4.6 hrs after burst as ToO
•Observed on 3 consecutive orbits for total of about one hour on-target
XRT position:
RA(J2000) = 06:40:47.5
Dec (J2000) = -37:04:22.5
SWIFT J064047.5-370423
Offset from BAT position:
50 arcseconds
Offset from optical transient:
2.1 arcseconds
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Filling the Gap:
the unexplored world of the
Early Afterglow
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
The Early Afterglow
Filling the gap between the
prompt and afterglow phases
5 orders of magnitude!
t 6
The afterglow smoothly Flare
joins to the prompt emission
t 0.7
t 1.2
There is a steep decay
after the GRB
t 2.3
Jet Break
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
XRT detects early steep decays
Tagliaferri et al. 2005, Nature
436, 985
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
BAT+XRT (0.2-10. keV) light curves (I)
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
BAT+XRT (0.2-10. keV) light curves (II)
GRB050315 BAT+XRT GRB050319 BAT+XRT
Barthelmy et al. 2005, ApJ 635, L133;
Cusumano et al. 2006, ApJ 639, 316;
Vaughan et al. 2006, ApJ 638, 920
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Transformed GRB light
curves
Composite X-ray light curves for 40 GRBs
Prompt (<Tp)
PL Decay
О Flares
О Emission “Hump”
O’Brien et al. 2006, ApJ 647, 1213
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Prompt and afterglow X-ray light curves: a
common functional form?
The top panels represent the most common
type (80%) in which the second afterglow
component dominates at late time
The middle panels represent the cases in
which the prompt component dominates
also at late time
The bottom panels represent the cases
(very rare) in which there is a break at
late time. Only 8 GRBs require a late
break in the fit and of these only 4 are
good candidates to be jet breaks
Willingale et al. 2007, ApJ 662, 1093
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Sketch of the early GRB X-ray light curve
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
The Flaring Activity
Strong flares in the X-ray light curves: 102 – 104 s
Over the power-law decay
GRB 050502B
Flares
Sometimes large
energy in the flare
Power law
Falcone et al. 2006
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
BAT+XRT 0.2-10 keV light curve
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Where do we set the T0 for the flares?
Now the flare is not so evident any more,
although the steep decay is still there ….
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Fitting a power law (or broken-PL) with (t-t0)-
With =2+
Liang et al. 2006, ApJ 646, 351
Indication that each flare is a distinct episode of
the central engine
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
XRT flares: a systematic analysis
No correlation between the number
of prompt pulses and the number of
flares. However same intensity ratio
between successive pulses and flares
Chincarini et al. 2007, ApJ in press
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
XRT flares: a systematic analysis
Best fit model of X-ray flares similar to X-ray flares fluence on
prompt best-fit model (Band function), average a factor of 10
but with peak in the X-ray band lower than prompt fluence
Falcone et al. 2007, ApJ in press; Butler & Kocevski 2007, ApJ 663, 407
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Triggering on a precursor: 050820A
Osborne et al. 2006
BAT, Konus/Wind & XRT light curves
BAT triggered on a precursor to the main GRB
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Precursor before Swift
Precursors Lazzati 2005
• In a search up to T-200 sec Lazzati
(2005) found 20-25% of long BATSE
GRBs show precursors (defined by
pre-trigger emission that declines
before the main burst)
• Precursor emission is softer than
the GRB average, but is non-thermal
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
GRB060124: triggering on a precursor
Romano et al. 2006, AA 456, 917
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
GRB 060124
• Simult. BAT, XRT & Wind
• Single power law fit
• Epk ~130, 240, 28 keV
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
GRB061121
Precursor about 60 sec before
burst optical and X-ray afterglow
start during the burst active phase
Typical XRT light curves, no flares, no
late break, very steep early phase clearly
Page et al. 2007, ApJ in press linked to the prompt emission
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
GRB061121
Multiwavelength light curves Solid lines are Konus-Wind, BAT and XRT
best fits, dashed lines join radio, optical
and 1 keV points.
Note the clear spectral evolution
Page et al. 2007, ApJ in press
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
No X-ray features
Stacked spectra of GRBs with known
redshift
Cts > 800 bkg subtracted
No flaring activity
Up to July 2007
Emission lines have been
observed by other satellites in
the X-ray afterglows of GRBs.
P 99.99%
Detections were at low
significance typically:
2-4
2 / dof 0.99
Conciatore, Antonelli et al., 2008
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Triggering Robotic Telescopes
tpeak = 153 s
Optical light curve observed by
REM: the on-set of the afterglow
(Molinari et al. 2007)
NIR tpeak = 180 s
NIR
X-ray
X-ray
Peak time <=> external shock onset <=>
fireball deceleration (simple physics)
400
0
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
SHORT Gamma Ray Bursts
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
GRB 050509B: Swift Detection
• BAT: very faint GRB
• XRT: T+62 s detects 11
photons(!)
• No optical, no radio. very faint
limits
– Low energy event and/or low
density medium?
• Giant elliptical galaxy in cluster.
T90=40 ms z=0.22 Host?
• E=1048 ergs
Gehrels et al. 2005
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
GRB 050724: Swift Detection
15-150 keV
• Brightest Swift SHB
250 ms
• Hard spike/soft bump
Barthelmy al. 2005
• X-ray, optical and radio
afterglow detected
15-25 keV
T90=40 ms
• X-ray light curves shows
flaring activity
100 s
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Transition from prompt to
afterglow
Barthelmy et al. 2005
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Afterglow of Short Bursts
GRB 050509B
first short
GRB X-ray
afterglow Flux
very faint!
Gehrels et al. 2005
Time since GRB (s)
GRB 050724 – the bright one: optical + X-ray – z 0.258
La Palma, Dec. 3,
Epoch 1 L. A. Antonelli: SWIFT Highlights
Epoch 2 Difference
GRB051221A
Jet break? => θj~7o
Energy injection with reverse shock? n ~ 10-3 cm-3 & no SN => merger
seen in the radio band => evidence for
continuing activity of central engine
Soderberg et al. 2006
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
GRB060313
Multiple peaks in
both gamma-ray
and hard X-ray
bands => highly
variable outflow
from central
explosion
Strong
spectral
evolution
Roming et al. 2006
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
GRB060313
Afterglow exhibit
complex structure =>
structured jet, variable
microphysics parameters
Situation very similar to
long-GRB however
density estimated to be
n ~ 10-4 cm-3 which is
much less than that
expected for a massive
star
Roming et al. 2006
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Some X-ray light curves of SHBs
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Ratio between X-ray & prompt γ-ray
fluence
These short-GRBs
suggest very low density
environment n<10-5 cm-3
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Cumulative redshift distribution of
long- & short-GRBs with known z
However, there are suggestions that at least 25% of short-GRBs
are at z>0.7 (Berger et al. 2006)
(valid if they did not travel long distances from their HG)
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Gamma Ray Bursts
as
Cosmic Beacons
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
GRB050904: a very high redshift
burst (z=6.29)
Cusumano et al. 2006, Nature 440, 164;
Watson et al. 2006, ApJ 637, L69
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
The Farthest Star in the Universe!
Tagliaferri et al. 2005
The record: z 6.29
Ly dropout suppressing
optical emission
Flux (erg cm–2 s–1 Å–1)
Spectroscopic confirmation!
Kawai et al. 2006
La Palma, Dec. 3,
Rest wavelenght (Å) L. A. Antonelli: SWIFT Highlights
Optical-NIR observations
• 40-50% of the Swift GRBs
have no optical counterpart or
in any case the optical
counterpart is very weak
(absorption? intrinsically
optically weak? high
redshift?)
• The average redshift is quite
high <z>~2.5 to be compared
with a value of <z>~1 expected
before the launch of Swift.
Due to the higher sensitivity
and harder energy band of
BAT with respect to
BeppoSAX WFC and HETE II
and also to faster reaction in GRBs are thus ideal
the otical-NIR follow-up
(e.g. Fiore et al. 2007, AA 470. 515)
probes of the high-
redshift Universe
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
GRBs as Cosmic Beacons
GRB 050505 z 4.2748
Bright afterglows
allow
high-quality
spectroscopy Flux (erg cm–2 s–1 Å–1)
Ly
Penprase et al. 2005, Chen et al. 2005
Rest wavelenght (Å)
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
The Early Universe Composition
Dust composition/evolution: the case of GRB 050904 @z=6.3
A large X-ray absorption and UV dust extinction is observed.
Haislip WFCAM-UKIRT QSO@6.2 extinction curve
~0.5 days Stratta et al., 2007 0.5 day A3000=0.89+\-0.16
Ly corr. = 3.02 1 day A3000=1.33+\-0.29
3 days A3000=0.46+\-0.28
Tagliaferri FORS-VLT
~1 day
Ly corr. = 1.27
NH~1023 cm-2 => AV/NH~50
times
Haislip GMOS-Gemini lower than Galactic!!
~3 days
Ly corr. = 2.38 @z~6 no dust from AGB stars.
Only sources are CCSNe (and
AGNs)
Much less dust and much smaller A V/NH
Less dust => less extinction @z>5 => high-z afterglows
easier to detect => Swift GRB sample with redshifts not
strongly biased against high-z objects.
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
SWIFT
&
The GRB-Supernova Connection
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
GRB-SN conne
GB980425: in the
BeppoSAX error box:
SN1998bw (Pian et al99,Kulkarni
et al, Galama et al al 98).
Exploded within 1 day
from the GRB.
Chance P=10-4
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
GRB 021211
GRB -SN Connection
(Della Valle et al. 2003)
(Zeh et al. 2003)
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
The GRB-Supernova Connection
GRB 060218 (the second closest GRB) z 0.03352
Campana et al. 2005
THERMAL SPECTRUM
SHOCK BREAKOUT
SUPERNOVA
La Palma, Dec. 3, Time SWIFT GRB
L. A. Antonelli: since Highlights
SN 2006aj – another hypernova
Detailed spectroscopic
monitoring
Broad-lined “hypernova”
Pian et al. 2005, Sollerman et al. 2005
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
GRB060614: no SN down to a very stringent
limit!
A very unsual burst:
a long without a SN
or a short masked as
a long GRB?
Eiso~8.4x1050 ergs
Della Valle et al. 2006, Nature 444, 1047 Gehrels et al. 2006 Nature 444, 1044
Fynbo et al. 2006, Nature 444, 1050 Mangano et al. 2007, AA 470, 105
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Conclusions (I)
• Complex X-ray afterglow light curve in the early
phases
• Many burst show X-ray flares, some very bright
=> still due to central engine activity, that last
much longer than previously thought?
• The flat part can have has much fluence as the
prompt => due to energy injection?
• X-ray spectra are definitely featureless
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Conclusions (II):
• SHB closer on average than long bursts <z>=0.37
• No SN associated to SHB
• Short hard bursts occur in both spiral and elliptical
galaxies (cf SN Ia)
• Short burst environment is cleaner than in long
burst
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
Conclusions (III):
• GRBs can be used as cosmic beacons probing
high redshifts Universe.
• SN e can still be associated to GRBs but …
NOT Always! associated to SHB
La Palma, Dec. 3, L. A. Antonelli: SWIFT Highlights
