GammaGamma--Ray Observations of Ray Observations of
SLAC Summer Institute, August 4th - August 15, 2008
Olaf Reimer Hansen Experimental Physics Labs & Kavli Institute for Particle
Astrophysics and Cosmology, Stanford University
GRB 080514B has been localized jointly by SuperAGILE
and IPN
(GCN 7715)
and shows a significant gamma ray emission (GCN 7716). Follow-up by Swift (GCN 7719 and 7750)
provided the afterglow in X-rays. Many telescopes participated in the observation of the optical afterglow: Watcher (GCN 7718), GRON (GCN 7722), KPNO (GCN 7725) and NOT (GCN 7734).
SuperAGILE
–
Mars Odyssey annulus
SuperAGILE
1-D
AGILE first gamma-ray detection of a GRB: GRB 080514B (Mereghetti et al., to be submitted)
Gamma-ray Observations of Supernova Remnants
Esposito et al. 1996
W28 W44
γ Cyg IC443
269.6269.8270.0270.2270.4 R.A.
-23.8
-23.6
-23.4
-23.2
-23.0
Decl
inati
on
284.00284.25284.50284.75 R.A.
1.00
1.25
1.50
1.75
2.00
Decl
inati
on
94.294.494.6 R.A.
22.4
22.6
22.8
Dec
linati
on
304.0304.5305.0305.5306.0 R.A.
40.0
40.5
41.0
41.5
Dec
linati
on
EGRET sources and Supernova RemnantsA mixed blessing:
• Spatial coincidences of UNIDs
and cataloged (radio-)SNRs
• No detections in cases like SN1006, Tycho
& Kepler
• multifrequency
support, that several synchrotron nebulae in SNR harbourmagnetospherically
active neutron stars (i.e. CTA1)
•
GeV-measured gamma-ray source positions do not correlate well
with X-
ray brightrim/shell features (although hampered by angular resolution)
• GeV-cutoffs
already significant in EGRET-spectra
→ serious consideration of neutron star origin at GeV
SNR-associations
CTA1 W28
Zhang & Cheng 98
γCyg IC443
Cheng & Zhang 98
EGRET sources and Supernova Remnants –
the X-ray view on the associations
Cas
A : central X-ray point source (Chakrabarty
et al. 2001), unpulsed
IC443 : X-ray point source + PWN outside EGRET contour (Olbert
et al. 2001),
hard point source (Keohane
et al. 1997, Bykov& Bocchino
2001)
γCyg: complete identification of EGRET GeV-contour (Brazier et al. 1996) →
RX J2020.2+4026
However: Becker et al. 2005
CTA1: complete identification of EGRET GeV-contour (Brazier et al. 1998) →
RX J0070.0+7302
W44: association with PSR 1853+01 + PWN (Harrus
et al. 1997)
W28: associaton
with PSR 1801-23 ?
2EG J2020+4026 / RX J2020.3+4026SNR G78.2+2.1
RIGHT ASCENSION20 25
h m20 15
h m20 20
h m
40 00'o
30'
DEC
LIN
ATIO
N
41 00'o
30'
HD 193322
HD 229119
RX J2020.3+4026
2EG J0008+73 / RX J0007.0+7302G119.5+10.2 (CTA1)
DEC
LIN
ATIO
N
73 00'o
74 00'o
30'
30'
72 00'o
RIGHT ASCENSION00 15
h m23 55
h m00 05
h m
RX J0007.0+7302
AGN
21°45'
06h22m 20 18 16
22°00'
15'
15'
30'
45'
23°00'
RIGHT ASCENSION (2000)
DEC
LIN
ATIO
N
Nevertheless: statistical significant correlations with galactic
objects found Montmerle
et al. 1979 COS-B, SNRs, OBs → "SNOB"Sturner
& Dermer
1995 EGRET, SNRs
→ significant positional correlationEsposito et al. 1996 EGRET, SNRs
(X-ray) → 14 associationsRomero et al. 1999 EGRET, SNRs
(radio), OB, WR → 22 associations
•
overwhelming statistical evidence for SNR correlation•
significant evidence for OB association correlation•
marginal support for WRs
and/or Of stars
More than a single population of galactic γ-ray sources present in the EGRET data.
Torres, Romero et al. 2003
“The spectrum is a good match to that predicted by pion
decay, and cannot be explained by other mechanisms.”
(Enomoto
et al. 2002, Nature)
IC interpretation in conflict with data !
SNR as prime candidate sources for Galactic Cosmic Rays
SN1006 as seen by ASCA(Koyama et al. 1995)
TeV electrons –
YES! →
But what about the hadrons?
Evidence for hadronic particle acceleration in SNRs?
π0 decay interpretation in conflict with data, too !(Reimer & Pohl 2002)
The steepness of previously measured spectrum not confirmed by HESS(Aharonian et al. 2004, 2006)
And then there came H.E.S.S. – SNR seen by ground-based Cherenkov telescopes
Supernova Remnants seen –
primariy
particle distribution > 100 TeV
RX J1713.7-3946as seen by H.E.S.S.
RA SNR seen by6:17 IC 443 MAGIC, VERITAS8:52 RX J0852-4622 CANGAROO, HESS14:42 RCW 86 HESS17:13 RX J1713.7-3946 CANGAROO, HESS18:00 W28 HESS23:23 Cas A HEGRA, MAGIC, VERITAS
Particles accelerated in shock
Particles are confined to source region by pre-existingor dynamically generatedmagnetic fields
On the scale of kyrs,fields decay / are damped and particlesdiffuse out of the source
X-ray / gamma ray correlation
Contour lines: ASCA X-raysY. Uchiyama et al. 2002
H.E.S.S.electrons
X-rays ~ IC γ-rays
protons + 10-4 electrons/protonBerezkho & Völk (2006)
+ gas→ πo → γ-rays ~ X-rays
+ gas→
B2 ~ ρLucek & BellMNRAS 2000
Porter et al. (2006)Katz & Waxman (2007)Plaga (2007)…
B ~ 10 μG
B ~ 100 μG
Key issue:Key issue:StrenghtStrenght of of magnetic magnetic
fieldfield
–
Close correlation with X-rays [+electrons]–
Spectral shape [+protons]–
IC interpretation implies (too) low B-field [+protons]–
No tight correlation with molecular material [+electrons]•
Not yet clear…–
Need data at lower energies to be sure, e.g. GLAST
protons
electrons
Where are we now with RXJ1713.7-3946? Archetypal SNR
Who will settle this quest?
GeV-Imaging of RXJ1713?
H.E.S.S.
GLAST
Assumptions on 3EGJ1714 made, underlying: 5 year exposure, E > 3
GeV(Funk et al. 2006)
Simulated GeV-Spectrum of RXJ1713? Yes (in b/w perspective)
If hadronic, do we see enough SNR, and at the right places?
→ GLAST
GLAST
The remaining freedom in the interpretation of VHE data will be constraint byE < 100 GeV data –
and sensitive X-ray data (Uchiyama et al. 2007)
(a)
3EGJ1714 will be refined/disentangled from RXJ1713.→ Molecular Cloud interaction → improved (CfA & Nanten) CO surveys
(b) GeV emission from RXJ1713 will be detected or an u.l. will be truly sensitive → sanity check
for the leptonic models/hadronic models → SNR ACCELERATION SITE FOR HADRONS OR NOT ?
(c) Nature of 1WGA J1713.4-3949 ? Compact object? Progenitor??
1xx GeV to ~100 TeV – ground-based Cherenkov telescopes
Non-morphological resolved SNR-detections:
Cas
A: HEGRA, MAGIC, VERITASW28: HESSIC443: MAGIC, VERITAS
↓
shellsize
< instumental
resolution: unresolved
↓The “composite”
SNR/PWN: e.g. G0.0+0.1, HESS J1813, …
Energy
Energy Flux π0 decay
Inverse Compton
Synchrotron
Radio IR/Optical X-rays γ-rays VHE γ-rays
IC on target: Synch. (+CMB)
Radio
X-ray
γ-ray
Synchrotron
Synchrotron:
Ex
(keV) = 4 (B/1mG)(Ee
/10TeV)2
IC (on CMB):
Eγ
(TeV) ~ (0.05Ee
)2
Neutral pion
decay:
⟨Eγγ
⟩
~ 0.15 Ep
10 keV
X-ray → 10 TeV e-
1 TeV
γ-ray → 20 TeV e-
→
6 TeV p
Gamma-Rays from Pulsar Wind Nebulae
Gamma-Rays from Pulsar Wind Nebulae
•
Many known X-ray PWN now identified as TeV
emitters and almost all of the highest spin-down power radio pulsars have associated TeV
emission –
Efficient particle accelerators•
May be easier to detect in TeV
than keV
?–
Integration over pulsar lifetime for TeV
electrons (less cooling)–
TeV
instruments sensitive to more extended objects –
no confusion with thermal emission–
Many of our unidentified sources may be PWN
The PWN Population
H.E.S.S. sources near energetic pulsars
435 pulsars in HESS survey region*
randomcoinc.
Spin-down energy fluxin ergs/kpc2
preliminary
Systematic studies possible !
ATNF PSRs
vs. TeVCarrigan
et al. 2007
GeV
vs. TeVFunk Reimer Torres Hinton 2008
Implied efficiency Spin-down → TeV
~ 1%
•
PSR J1826-1334–
3×1036
erg/s spin-down power, ~2×104
years old•
5’
X-ray PWN–
G 18.0-0.7 (Gaensler et al 2002)•
1°
TeV
γ-ray source–
HESS J1825-137 (Aharonian et al 2005)
–
Energy dependent morphology•
A first at TeV
energies–
Cooling of electrons away from pulsar? (tcool
∝
1/E)[ 2 keV synchrotron emission comes
from 200 TeV electrons (if B ≈ 10 μG)…, γ-rays come from lower energy electrons ]
HESS
HESS J1825-137
Archetypal (before HESS): Crab
Horns 2006
Now: diversity among the PWNs!• often extended, • displaced from PSR, • energy dependend
morphology change
Vela X
PWN (numerically) most prominent class of identified galactic γ-ray sources
The binary system PSR B1259-63 / SS 2883
Discovery: H.E.S.S.,March
2004
First variable galactic TeV
source.
First in a new source class in HE g-rays.
48 ms Pulsar3.4 y period
Be Star10 M
Periastron 7. March 2004
Complex interaction between pulsar and star during
periastron
PSR B1259-63Johnston et al. 1992
Millisecond pulsar (T=48 ms)Mass of ca. 1.4 solar massesMassive Be-type companion star of ca. 10 solar massesHighly eccentric orbit (T= 3.4year)Closest impact is ~1013 cm or ~20 stellar radii
Electron wind from a pulsar terminates onto the strong Be-star outflowShocked electrons radiate in synchrotron (X-rays) & IC (TeV Gamma-rays)
Pulsar
Massive star Shock front
Very plausible scenario, theoretically predicted.
The PSR B1259-63 field of view
March 04 Apr./May 04Feb. 04
H.E.S.S.
Per
iast
ron
Flux
>38
0 G
eV[c
m-2
s-1
]
X-Ray Binaries as Gamma-Ray Sources
Binary systems of a compact object (neutron star or black hole) and a stellar companion
Matter is flowing over from the stellar
companion onto the compact object.
Angular momentum conservation
=> Formation of an accretion disk
Matter in the accretion disk heats up to ~ 106 K
=> X-ray emission
…more on X-Ray Binaries
Generally identified as radio jets
As in most accretion disk systems, this results in the formation of collimated outflows:
Mildly relativistic jets: Γ
~ 2
X-ray binary spectra typically consist of a
thermal disk component plus a hard power-law.
•
Similarities–
massive star (O, Be) e≠0
–
TeV
emission ~ 1033-34
erg/s
–
Low, ~ stable radio and X-ray emission (periodic radio outbursts in LS I+61 303 and PSR B1259-63)
–
Spectral energy distributions
Gamma-ray binaries
O6.5V
LS 5039
0.2 au
Be
LS I +61 303
0.7 au
26 d
4 d
O9.7I
0.2 au5.6 d
Cyg X-1
BH orPSR ?
BH
BH orPSR ?
© G. Dubusobs.
accretionpowered
Be
PSR B1259-63
10 au
3.5 yr
PSR
wind-powered
MAGICTeVPA 2007
γ-ray binaries: They are orbitally modulated!
H.E.S.S.: LS5039 (Aharonian
et al. 2005)MAGIC: LS I 61°303 (TeVPA
2007)
VERITAS: LS I 61°303 (TeVPA
2007)
γ-ray binaries: They flare!
•
Small FoV
telescopes depend on alerts or luck !
•
NEW: Contemporaneous data will always be there!
allsky
capability/high duty cycle: SWIFT/MAXI/GLAST
vs.
small FoV/low duty cycle but high sensitivity/angular resolution: VHE
→ Analogy to blazars!?
LS I +61o
303: MAGIC
(Albert et al. 2006)
Controversy: microquasar or pulsar ?
Mirabel 2006
High-Energy Emission Model for Microquasar
Jets
Injection, acceleration of ultrarelativistic
electrons
Qe
(γ,t)
γ
Synchrotron emission
νFν
ν
Compton emission
νFν
ν
γ2γ1
γ-q
Seed photons:
Synchrotron (SSC), Accr. Disk + BLR (EC)
Injection over finite length near the base of the jet.
Additional contribution from γγ
absorption along the jet
Leptonic Models
Relativistic jet outflow with Γ
≈
2
+ Companion star light→ Include abs. by companion star light!
Orbital modulation of VHE γ-rays can be explained by γγ
absorption alone!
Pulsar Wind Nebula emission
Proven mechanism (Crab)Proposed long ago for LS I+61 303 by Maraschi
& Treves (1981)
Low steady emission
Radio pulse ? absorbed in strong stellar wind (tighter orbits in LS 5039, LS I+61 303)
[from G. Dubus]
Modeling in a PWN model
Conclusions: gamma-ray binaries as compact PWN
•
Interpretation as pulsar / stellar wind interaction
explains similarities between VHE emitting binaries.
•
VHE emission occurs close to pulsar/star (γγ
absorption should modulate TeV
flux in LS 5039).
•
Large scale emission can be explained by comet-like shocked material, radio morphology depends on orbit.
AGILE: Micro-QSO observations
•
Cyg X-1•
GRS 1915+105
Cyg X-1
the longest continuous hard X-ray monitoring of Cyg
X-1
Total Observation Time: ~ 4.5 Ms
(1196 Orbits)
1 Month~1.3 Crab Flare
(see also INTEGRAL ATels
#1533,1536)
Cyg
X-1
Del Monte et al., in preparation
SuperAGILE
GRID
SuperAGILE light curve
LE (20-25 keV): Yellow
HE (25-50 keV): Cyan
Γ~ 1.61 +/-
0.13
Low/Hard State
Searching for transitions…
…and gamma-ray emission
GRS 1915+105
GRS 1915+105
15 April 2008
Recent reactivation of the microquasar
GRS 1915+105
GRS 1915+105
(Trushkin
S. et al., ATel
#1509)
gamma-ray U.L.
18-60 keV
gamma-ray map
Galactic gamma-ray transients
•
Cygnus region•
Carina region
•
Crux region
AGILE observes variability and detects new transients on time scales of 1 day at flux levels of 10-6 cm-2s-1 , even in crowded, high diffuse emission Galactic plane regions.NO detectable simultaneous hard X-ray emission (F < 20-30 mCrab, 18-60 keV, 1-day integration)
EGRET: Seen one over mission life time. Never identified.AGILE:
GLAST: Expectations translated into full-scale transient trigger and follow-up program, in place by now.
Gamma-Ray Emission from active and passive Molecular Clouds
~85% of all γ
in galactic diffuse
~15% in sources
The most prominent GeV source is our Milkyway itself!
The diffuse γ-ray emission is observational evidence of CR interactions in the interstellar medium via Bremsstrahlung, IC, π0-decay
CR propagation near sources or in molecular clouds
GLAPROP might give us a prediction based on large-
scale consistency of nuclear reactions, ISRF, gas distributions, CR and γ-ray measurements.
May or may not be correct for a localized diffuse emission problem!
→ CS used, avoiding opacity problems
for 12CO (Aharonian et al. 2006)
High-lat molecular clouds
often coincidences yielding ambiguity
between expanding SNR and molecular cloud
(Gabici
& Aharonian ’07)
1 degree
The TeV Galactic Centre
•
Two bright point-sources in the central part of the Galaxy
Diffuse Emission
•
After subtracting point-sources, diffuse emission is seen extending along the galactic plane
1 degree
HESS
Diffuse Emission
1 degree
CS Line Emission (dense clouds)smoothed to match
H.E.S.S. PSF
HESS
Diffuse Emission
•
Molecular clouds are ‘glowing’
in TeV
γ-rays after being bombarded by cosmic ray protons and nuclei!
•
Energy spectrum harder than local cosmic ray spectrum (proximity to accelerator?)
SNR/cloud interactions?
•
Correlations with available target material–
IC 443 and W 28, Old (>104
yr) SNRs
near mol. Clouds–
Both have associated GeV
sources
pp → π0
→ γγ
?
Have we spoken lately about the role of stellar winds in the quest for Cosmic Ray acceleration?
early 80s: COS-B and the “SNOBs“Montmerle 1979: no 1:1 between SNRs (as a class) and gamma-ray
sources, rather linked with young objects
early…late 90 – EGRET era: more g-ray sources, more coincidences (sic!)
Kaaret & Cottam – correlation EGRET <-> OB associationsYadigaroglu & Romani – OB associations, (open clusters, HIIs, PSRs, SNRs)Romero et al. – associations of individual SNR, OB associations
…but got stuck at 3σ
conficence level (compared to the 5-6σ
for SNRs)
2002 onwards – TeVs scored: HEGRA TeVJ2032+4130
inital detection report (112 h obs, 4.6σ)
final 237 h observations (!), 6.1σ, extended 0.1°
(compared 0.07°
psf)
+ hint of confirmation* from Whipple
in massive Cygnus OB2 association
~2600 OBs estimated
Possibility of -
at least two -
explanations–
Faster diffusion of higher energy hadrons and subsequent interaction in molecular clouds yielding a source a bit separated from the accelerator
-
Particle acceleration by stellar winds itself
–
GRB-remnants in our Galaxy ?!–
photo-de-exitation of PeV CRs after photodesintegration on UV-photons ??!
“highscore”: the deepest MWL follow-up for an UNID VHE source so far (55 ksec
Chandra, 50 ksec
XMM)
2006: H.E.S.S. observed the stellar cluster Westerlund
2 (seen here with SPITZER eyes)
WR 20aWR 20b
Westerlund
2
•
embedded in a giant molecular cloud
• ongoing star formation
•
stellar winds blow cavities around massive Wolf-Rayet
(WR) stars
•
WR 20a itself is known as the most massive binary star in our Galaxy (two stars of ~80 Msolar
in a 3.8 day orbit)
Therein: young, hot & massive stars
-> 8 evolutionary earlier then O7, 2 WRs, and in particular WR20a, the most massive measured
stars in our Galaxy (WN6+WN6 binary)
r=19.3 R o
d ~
51..5
3 R o
Rauw
et al. 2005
orbital period: 3.686 d
The H.E.S.S. observations:
• detection of a new very-high-energy gamma ray source, probably associated with the stellar cluster Westerlund
2
based on 14h data, at Ethres
= 380 GeV, Γ=2.53 ±0.16stat
±0.1syst
• origin of energetic γ-rays unlikely the WR stars itself -> HESS J1023-575 is an
extended
and constant
source
)2 (deg2θ0 0.05 0.1 0.15 0.2 0.25
# ex
cess
eve
nts
-50
0
50
100
150
point source for H.E.S.S.
observed source extensionσ
= 0.18°
±
0.02°WR20a
WR20b
VHE γ-ray image
The “blister”
(Whiteoak
& Uchida 1997):
indicative for rapid expansion into a ambient low-density medium (superbubble?)
Shock acceleration at the boundaries of the blister Analogy with SN-driven expansions with expanding stellar winds.
Outbreak phenomenon from winds of hot and massive star ensembles
(Tenario-
Tagle
1979, Völk
& Forman 1982, Cesarsky
& Montmerle
1983) ?
Contribution to Cosmic Rays ? -> Need to see more := common phenomenon or not!
radio: 843 MHz γ-ray image
WR20aWR20b
Implications of the H.E.S.S. findings:
• intriguing new type of VHE gamma-ray source• archetypal for other young massive clusters ?
• if this association is confirmed and further stellar clusters will be detected in γ-rays (by ground-based γ-telescopes,or GLAST)
-> consider a
new class of extreme particle accelerators in our Galaxy
-> consequences for CRs: Will contribute, fraction unclear!
WR20aWR20b
843 MHz radio image
Milagro Pevatron?
Abdo, et al. ApJ Lett 2007
GeV Sources Emit TeV Gamma-Rays ?
•
Milagro has discovered
3 new sources
& 4 candidates in the Galaxy.•
5/7 of these TeV sources have GeV counterparts.
Only 13 GeV counterparts in this region -
excluding Crab. Probability = 3x10-6
E. Ona-Wilhelmi, et al., ICRC 2007
Milagro TeV “spectrum” of MGRO J1908+06 & HESS J1908+063
Median energy for this angle and α=-2.0 is 50 TeV Cut on A4> 4 & 9 gives median E of 60 and 90 TeV
60 90
Things to come?
•
Abundance of sources, many different types
•
Need more sensitivity, better (spatial / temporal) resolution and better MWL
to probe
underlying physics•
Many highly interesting source types just (?) below current sensitivity–
Starburst galaxies
–
Clusters of galaxies–
GRBs
–
…
One γ-ray eye is always on now…