New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
GRB X-ray Afterglow Observations with Swift-XRT
Abe Falcone(Pennsylvania State University)
and the Swift Team
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
XRT Collaborators
Penn State:David BurrowsJoanne HillJudith RacusinShiho KobayashiPeter MeszarosJohn NousekJamie KenneaDavid MorrisClaudio PaganiDirk GrupeAbe Falcone
OAB:Guido ChincariniGianpiero TagliaferriSergio CampanaAlberto MorettiPatrizia RomanoDaniele MalesaniStefano CovinoPaolo D’Avanzo
UL:Paul O’BrienAlan WellsJulian OsborneTony AbbeyAndy BeardmoreMike GoadKim PageDick Willingale
GSFC:Neil GehrelsLorella Angelini
UNLV:Bing Zhang
ASDC:Paolo GiommiMilvia Capalbi
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
Outline• XRT instrument & Burst follow-up Performance
(Ultra-brief)• Overall properties of Swift GRB afterglows
(a new multi-phase canonical picture emerges)
• The X-ray flares!
• Other equally interesting topics:
– Short GRB afterglows (see Fox et al. talk and Lamb plenary)
– High redshift afterglow (very briefly)
• Conclusions
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
•BAT First Light: 3 December 2004•XRT First Light: 11 December 2004
•First BAT Burst: 17 December 2004•First XRT Afterglow: 23 December 2004•UVOT First Light: 12 January 2005•Data public since 5 April 2005
The Swift Observatory
XRT First Light: Cas A
20 November 2004
Chandra First Light
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
BAT Bursts
• 92 GRBs detected/imaged since 12/17/05 (47 weeks as of 11/24/05) => 102/yr041217 050318 050412 050505 050607 050716 050801 050819
041219A,B,C 050319 050416A,B 050507 050701 050717 050802 050820A,B
041220 050326 050418 050509A,B 050712 050721 050803 050822
041223 050401 050421 050525 050713A,B 050724 050813 050824
041224 050406 050422 050528 050714B 050726 050814 050826
041226 050410 050502B 050603 050715 050730 050815 050827
041228
050117
050124
050126
050128
050202
050215A
050215B
050219A,B
050223
050306
050315
GRB Fluence
0.1
1
10
100
1000
12/1/
2004
1/1/20
052/1
/2005
3/1/20
054/1
/2005
5/1/20
056/1
/2005
7/1/20
058/1
/2005
9/1/20
0510
/1/20
0511
/1/20
05
Date
GR
B F
luen
ce (1
0^-7
)
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
XRT Detections of BAT GRBs
• Detected 71/77 = 92% with XRT (observed @ T < 200 ks)• Observed four during burst• 80% had prompt slews (< 350 s)• 53/56 = 95% of prompt XRT observations yielded detections
• 82% have fast decline or flare within first ~5 minutes
• 22 have redshift measurements:• Average redshift for LGRBs: 2.5
(compared with about 1 for Beppo-SAX bursts)• Highest redshift: 6.29
• SGRB mean redshift: 0.4
XRT Observation Start Times
02468
10
0 60 80 100
150
250
350
500
2000
1000
050
000
2000
0010
0000
0
Seconds since BAT Trigger
0
1
2
3
4
5
6
Freq
uenc
y
0 1 2 3 4 5 6 7
redshift
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
Swift Lightcurves – the Movie
Paul O’Brien / UL
BAT
XRT
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
The Overall Lightcurve
Zhang et al. 2005
Nousek et al. 2005
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
X-ray Flares: Some Background• Prior to Swift:
– Detection of X-ray flux increases from GRB011211 and GRB 011121 using Beppo-SAX (Piroet al. 2005)
– Conclusions were that flaring was onset of afterglow; not late-time internal shocks
– see also Zand et al. 2003, Galli & Piro 2005
• Source stacking studies of 100's of BATSE bursts; evidence for high energy emission well after nominal prompt emission phase (Connaughton2002)
• Could have been early evidence for flares; or it could have simply been the tail of the extrapolated nominal prompt emission; or it could have been the sum of both
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
How often do Swift GRBs have X-ray Flares?
~ 80% Swift XRT detections were prompt observations (< 300 s)
~1/3 of the prompt observations show flaring (>23 GRBs)
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
Myriad X-ray Flares050219A?050406050421050502B050607050712050713A050714B050716050724?050726050730050801050803050819050820A050822050904050908050916050922b051016b051117a
XRF 050406 GRB 050421 GRB 050502B
GRB 050712GRB 050607GRB 050713A
GRB 050714B GRB 050726GRB 050730
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
Flares: Motivation for Study
• Possible activity of internal GRB engine for long timescales can constrain energy sources and mechanisms
• In at least one case the fluence in an X-ray flare was comparable to initial prompt burst fluence
• Determine which flares are associated with internal engine and which are associated with afterglow/external shock
• Use timescales and other properties to constrain jet dynamics
• Investigate possible XRF-GRB connection
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
GRB 050502b: The Giant flare
Falcone, A. et al. 2005, ApJ, submittedBurrows, D. et al. 2005, Science
~500× increase!Rapid rise/decay!
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
GRB 050502b: The HUGE flare
• Flare fluence:(1.1 ± 0.05)×10-6 erg cm-2
• This is comparable to, or greater than, BAT prompt emission fluence
• Fast rise/decay (~t9)• Continuous power law
fits underlying afterglow, with no energy injection shift evident10
110
210
310
410
510
−2
10−1
100
101
102
103
Time Since BAT Trigger (sec)
XR
T C
ount
Rat
e (c
ount
s −
1 )
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
GRB 050502b: The HUGE flare
• VERY SHORT timescale variability near peak in hard band
• Hardness ratio increase and subsequent decay during flare
• During flare, Band function or cutoff power laws fit better than simple power law
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
GRB 050502b: The HUGE flare
• During flare, Band function or cutoff power laws fit better than simple power law
• Spectral index is harder during flare than that before and after, and a cutoff is indicated in the X-ray band
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
XRF 050406
• Weak burst, characterized as XRF, but with lightcurvecharacteristics very similar to those of GRBs with X-ray flares
• Lightcurve appears to level off– unresolved
rebrightenings?– energy injection into
afterglow shock?
• Flare δt/t ~ 0.3
102 103 104 105 106
Time since BAT trigger (s)
0.0001
0.0010
0.0100
0.1000
1.0000
10.0000
XR
T C
ou
nt
Rat
e (c
ou
nts
s-1)
XRF 050406A
Romano, P., et al. 2005, submitted
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
XRF 050406: Hardness
• Hardness ratio increase during flare, similar to GRB 050502b
0123
CR
(cp
s)
0123
CR
(cp
s)
0 100 200 300 400Time since BAT trigger (s)
0123
H/S
XRF 050406(S) 0.2-0.7 keV
(H) 0.7-10 keV
Romano, P., et al. 2005, submitted
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
Flares: Internal Vs. External• For many flares, there is no evidence for energy injection
into afterglow external shock at time of flare• Afterglow decay rate is consistent with afterglow having
already begun before flare, particularly for GRB 050502b• Small δt/t (<<1) for many flares makes it difficult for
external shock associated processes, particularly for GRB 050406
• For some flares, spectral index appears to change (harden) during flare, then it goes back to pre-flare value after the flare is over
• Bright flares can be fit better with Band function or cutoff power law, rather than simple power law
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
GRB 050820A – A simultaneous BAT/XRT flare
• Clear indication of energy injection into the forward shock
• A major, but only partially observed flare, with interesting spectrum
• z = 2.6
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
More X-ray Flare GRBs: 050716
courtesy of C. Hurkett
• Some flares occur during the steeper (region III) afterglow region
• Not ALL flares are dramatic sharp peaks
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
GRB 050904: z = 6.29Absorption redshift:
Subaru: 6.29 ± 0.01
Highest redshift GRB ever found.
Highest redshift X-ray emission ever detected.
2nd highest redshift ever measured (highest is 6.4).
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
GRB 050904: z = 6.29
Cusumano et al. 2005, Nature, submitted
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
GRB050724: Third Short GRB Afterglow
t90 = 1 s by BATSE definition. (But longsoft tail.)
30x brighter than GRB 050509B. (6E-7 ergs/cm2)
Slewed in 75 s. Very odd X-ray lightcurve.
Extremely steep decay!
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
Conclusions• Swift-XRT has successfully detected afterglows from >90% of BAT GRBs:
– New picture of early afterglow including a rapid decay that extrapolates back to BAT emission
– another decay phase that implies continued internal engine activity to late (>104 s) times
– relatively higher redshifts (mean ~2.7), including one z = 6.3 burst– First ever short burst afterglow detection
• X-ray flares during GRB afterglows are frequent and varied, with variety of peak times, ∆t/t, and relative magnitudes
– At least one flare shows evidence of variability on ~30 sec timescale, and ∆t/t<<1;==> very hard to model this with afterglow related mechanisms such as reverse shocks, IC emission, density variations,...
– Some flare spectra are fit better with Band function or cutoff power law, rather than simple power laws (note: this is important when calculating NH)
• Evidence suggests continued internal engine activity in at least some flares, probably late ejection episodes spawning internal shocks
• Several bursts exhibit multiple flares==> any model must involve multiple episodes
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
More X-ray Flare GRBs: 050713a
• X-ray flare captured during BAT prompt emission
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
More X-ray Flare GRBs: 050714b
figure courtesy K. Page
Possible continuation of prompt decay, with flare superimposed, followed by afterglow
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
More X-ray Flare GRBs: 050607
• Incredibly fast flare rise: ~t14-t22!
• Early flaring (with fast ground-based response)
• fluence:~1.4×10-6 erg cm-2
C. Pagani et al. 2005, submitted
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
More X-ray Flare GRBs: 050726
Courtesy of A. Moretti
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
More X-ray Flare GRBs: 050712
courtesy D. Grupe
• Rising flux at start of XRT observations
• Followed by flares
•Still looking for a break at t > 106 s
• strong UVOT detection during early XRT flaring time region
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
Rosetta Stone(the burst that has it all!)
0
12
34
GRB 050315Vaughn et al. 2005
t -0.4t -0.7
ν -0.73±0.11
t -5.2
ν -1.9±0.9
New Views of the Universe (Dec 2005) Abe Falcone, Penn State University
GRB050509b: First Short GRB Afterglow
t90 = 0.04 s, Fluence = 2E-8 ergs/cm2
XRT counterpart in first 400 s, fades rapidly. 11 photons total.
Location in cluster at z=0.226, near early-type galaxy.
Possible NS-NS merger?
BAT: t-1.3
XRT: t-1.1
XRT error circle on VLT image. XRT position is 9.8”from a bright elliptical galaxy at z=0.226
Chandra
Gehrels et al. 2005, Nature
100x-1000x fainter than typical AG
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