Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 1
Dust scattering X-ray expanding rings
around GRBs
Sandro MereghettiAndrea Tiengo
Giacomo Vianello
INAF – IASF Milano
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 2
1965: Overbeck points out that celestial point X-ray sources should be surrounded by diffuse emission due to dust scattering (Overbeck 1965, ApJ 141, 864)
1970 1980: it is recognized that this can be an important tool to obtain information on size, spatial distribution and composition of interstellar dust (e.g., Hayakawa 1970, Progr. Theor. Phys.43, 1224)
1983: first detections with Einstein Observatory Rolf 1983 Nature 302, 46 (PSPC, GX 339-4 ) Catura 1983 ApJ 275, 645 (HRI, several GX sources)
1985 now: Many works on galactic sources with all imaging X-ray telescopes Einstein Observatory: Mauche & Gorenstein 1986, ApJ 302, 371 ROSAT: Predehl & Schmitt 1995, A&A 293, 889Chandra: Tan & Draine 2004, ApJ 606, 296; Xiang et al. 2006, ApJ 628, 769
X-ray dust halos: brief history - 1/3
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 3
m] [0.1 a [keV] E
arcmin 10 HALO FO SIZE
For dust grains of radius a :
Halo profile is energy-dependent and is determined by: - grain properties:
- composition (e.g. silicates / carbonaceus)- size distribution
- dust distribution along line of sight
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 4
OBSERVER DUST
X-RAYSOURCE
s
D = x Ds (1-x) Ds
Ds
c
D
x
x S
2
1 DELAY TIME
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 5
1973: Trumper & Schonfelder propose to use dust scattering halos to derive distance of variable sources (Trumper & Schonfelder 1973, A&A 25, 445)
1980: an X-ray ring observed around the dwarf nova SU UMa, but interpreted as ejected material (Cordova & Mason 1980, Nature 287, 25).
Later Norwell et al. show that it was a dust scattering expanding ring. (Norwell et al. 1983, Nat. 303, 49)
1994: application to GRB distance problem(Klose 1994, ApJ 423, L23; A&A 289, L1)
X-ray dust halos: brief history - 2/3
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 6
2003: expanding rings detected with XMM around GRB 031203 (Vaughan et al. 2004, ApJ 603, L3)
2004: Draine & Bond proposal to use variable AGNs behind galaxies to measure their distance – e.g. 10 Ms with Chandra could give M31 distance with 1% precision (Draine & Bond 2004, ApJ 617, 987)
2005: expanding rings found for: GRB 050713A with XMM (Tiengo & Mereghetti 2006, A&A 449, 203)
GRB 050724 with Swift (Vaughan et al. 2006, ApJ 639, 323)
X-ray dust halos: brief history - 3/3
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 7
GRB 031203 EPIC pn
58 ks observation starting at to+ 6 hrs
Two dust layers at D=880 pc and D=1390 pc
EPIC MOS (Vaughan et al 2004)
10’
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 8
GRB 050724 (Vaughan et al. 2006)
Swift XRT observation starting at to+ 343 s
Narrow (<22 pc) dust layer at D=139 pc
343-2243 s 6068-8127 s > 10,000 s
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 9
GRB 050713A
• Discovered by Swift
• XMM Observation starting at to+23 ks
• 30 ks long but only 9 ks useful due to high background
EPIC pn0.5-2 keV
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 10
A new method for detection of faint expanding rings “dynamical images”
(Tiengo & Mereghetti 2006, A&A 449, 203)
GRB 031203
Time since GRB
Ang
ular
dis
tanc
e fr
om G
RB
REBINNING
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 11
For each count detected in the X-ray image compute the quantity:
ii
i Dct
2
2
position GRB from distanceangular
GRB thefrom time
i
it
and plot the distribution of the Di values
n(D)
D
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 12
By fitting the peaks with Lorentzians one gets number of counts in the rings and distance of the dust layer
Can be done in different energy bins to extract halo spectrum
GRB 031203
pc 30240FWHM
counts 1740N
pc 51389D
pc 82FWHM
counts 840N
pc 5870D
2
2702402
2
17141
2101801
1
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 13
GRB 050713A
pc 33FWHM
counts 185N
pc 364D
1812
12090
67
-
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 14
“dynamical image” for GRB 050724 with Swift XRT data
Time since GRB(dis
tanc
e fr
om G
RB
pos
ition
)2
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 15
GRB 0507240.2 – 5 keV
pc 24FWHM
counts 130~N
pc 148D
1913
46
-
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 16
We applied this method to all GRBs observed with XMM-Newton and Swift
• XMM: 15 GRBs with follow-up observations starting at t ~ 20-60 ks and lasting few tens of ks
• No other dust rings detected, besides 031203 and 050713A
• Swift: follow-up data for ~125 GRBs, with different quality/sensitivity
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 17
XMM and Swift GRBs 031203
050724
050713A
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Information on the prompt emission
GRBGRBHALO IeII 1
Uncertainties on: - measure of halo intensity IHALO
- knowledge of scat n D
can be estimated from optical absorption:
2
8.1
056.0
15.0
keVV
keVV
EA
EA
Draine & Bond 2004
Predehl & Schmitt 1995
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Sazonov et al. 2006, Astronomy Letters 32, 297
GRB 031203The estimate of the X-ray “prompt” emission is subject to a large uncertainty
… but even the most conservative value is above the extrapolation of the high energy INTEGRAL spectrum
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 20
Conclusions• Expanding dust rings detected in 3 GRBs
(out of ~140)
• “Dynamical images” and distribution of “D” values offer a sensitive detection method
• In principle halos can provide information on prompt emission, not directly observed (e.g. GRB 031203)
• Very powerful to study dust properties and distribution when both direct and scattered radiation are detected ( extremely accurate distance measurements for dust layers)
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 21
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GRB 050724
• Discovered by Swift
• Short (0.25 s) hard spike (ph.index ~1.7) followed by low flux tail (>200 s) with softer spectrum (ph.index ~ 2.5)
• Fluence 6.3x10-7 erg/cm2 (15-350 keV)
• Optical and radio afterglows
• Off-center of elliptical galaxy at z=0.258
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 23
GRB 031203
• Discovered by INTEGRAL
• FRED profile - 40 s long
• Fluence 2x10-6 erg/cm2 (20-200 keV)
• Photon index ~1.63
• z = 0.106
• Eiso ~ (0.6-1.4)x1050 erg
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 24
XMM-EPIC vs. Swift XRT
Swift
XRT
XMM
EPIC
Half Energy Width
@ 1.5 keV
15” 15”
Field of view 23’ 30’
Effective area
@ 1.5 keV
110 cm2 ~2400 cm2 (1300 pn + 1100 2MOS)
Venezia - June 5, 2006 S.Mereghetti - Swift and GRBs Conference 25
Search in all Swift bursts
• 125 GRBs• Ring around GRB
050724 clearly visible• No other rings
detected
GRB 031203 NH = 6 1021 cm-2
GRB 050713A NH =1.1 1021 cm-2
GRB 050724 NH=1.5 1021 cm-2