Maser -Emitting Remnants - Stanford University
Transcript of Maser -Emitting Remnants - Stanford University
Maser-Emitting Remnants
John W. HewittNASA/Goddard, Xray Lab
Image: IC 443 in H!
Collaborators: F. Zadeh, M. Wardle, R. Petre, S. Katsuda, U. Hwang
Two classes of GeV/TeV supernova remnants
W44
Young SNRs
• X-ray synchrotron, multi-TeV electrons
• ongoing particle acceleration
• but age-limited?
Cas A ~300 yr
image: X-ray shock, ejecta
Older SNRs near clouds
• th. x-rays ~1 keV, late-Sedov/radiative
• Inefficient accel. of e- at TeV energies
fast losses via synchrotron emission
• large target (enhances luminosity)
W44 ~20,000 yr
red: Radio, green: H2, blue: X-ray
Is CR acceleration impulsive (young) or ongoing (middle-aged)?
Is !-ray emission hadronic or leptonic?
How does environment effect acceleration/containment?
• MASER = Microwave Amplification of Stimulated Emission
• “Pumping” of population inversion
radiative or collisional.
• De-excitation by background photon leads
to exponentially strong emission!
• OH = Hydroxyl
ground state
split into four levels
• Only is SNRs do you see:
Emission at 1720 MHz
Absorption at 1667/5, 1612 MHz
What is a Maser?
red: Radio, green: H2, blue: X-ray
Prototype: W44
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OH (1720 MHz)
OH (1667 MHz)
OH (1665 MHz)
OH (1612 MHz)
IC 443 clump G
• OH(1720 MHz) Maser: unique tracer of slow shocks into dense clouds
discovery paper: Frail et al. (1994)
pump models: Lockett et al. (1999), Wardle (1999)
• Only seen in SNRs: age ~ 4-40 kyrs,
with adjacent Molecular Cloud.
• Shock expanding into multi-phase ISM
Radio/H2 also well-correlated... why?
Shock turns radiative in molecular gas,
can obtain compression > 4
Masers = Interacting SNRs
Interaction with Molecular Clouds
• Theory of maser excitation well established
(Wardle 1999; Lockett et al. 1999)
• Collisionally pumped
• Temperature 50 to 125 K
• Density 104 to 105 cm-3
• bright FIR (Tdust >100 K) destroys inversion
• N(OH) ~ 1-1016 cm-2
• These conditions are easily met when a C-type
(non-dissociative) shock strikes cloud
• Shocks are transverse to our line of site to get
velocity coherence
∴ maser VLSR gives the systemic velocity
=> SNR distance => size, age, ESN
IR lines
from Wardle & Yusef-Zadeh (2002)
Diagram of SNR/MC interaction
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from IR lines (Reach & Rho, 2000)
Thermal X-rays
red: Radio, green: H2, blue: X-ray
Prototype: W44
• OH(1720 MHz) Maser: unique tracer of slow shocks into dense clouds
discovery paper: Frail et al. (1994)
pump models: Lockett et al. (1999), Wardle (1999)
• Only seen in SNRs: age ~ 4-40 kyrs,
with adjacent Molecular Cloud.
• Shock expanding into multi-phase ISM
Radio/H2 also well-correlated... why?
Shock turns radiative in molecular gas,
can obtain compression > 4
Masers = Interacting SNRs
Interaction with Molecular Clouds
• Theory of maser excitation well established
(Wardle 1999; Lockett et al. 1999)
• Collisionally pumped
• Temperature 50 to 125 K
• Density 104 to 105 cm-3
• bright FIR (Tdust >100 K) destroys inversion
• N(OH) ~ 1-1016 cm-2
• These conditions are easily met when a C-type
(non-dissociative) shock strikes cloud
• Shocks are transverse to our line of site to get
velocity coherence
∴ maser VLSR gives the systemic velocity
=> SNR distance => size, age, ESN
IR lines
from Wardle & Yusef-Zadeh (2002)
Diagram of SNR/MC interaction
8
from IR lines (Reach & Rho, 2000)
Thermal X-rays
red: Radio, green: H2, blue: X-ray
Prototype: W44
• Masers well-correlated with
shocked H2 S(5) line emission
• Projected sizes ~1015-16 cm
! shock width
(Reach et al., Hoffman et al.)
• Need line-of-sight geometry
maximizes velocity-coherence
thus, VLSR => kinematic distance
• OH Masers require very specific conditions:
• dense, shocked clumps n=105 cm-3
⇒ molecular clouds ~104-5 Msol
• Tdust ~ 50 K... intense far-IR kills 1720 MHz pumping,
inverts 1667/5 MHz
Diagnostic Probes of H2 Shocks
• (data from Neufeld et al. 2007)
•Masers require NOH = 1016-17 cm-2 but in slow, dense C-type shocks,
all OH converted to H2O (Kaufman & Neufeld 1996)
•Solution: dissociate ~few% of post-shock H2O into OH (Wardle 1999)
indirect ionization could be from X-rays, CRs... maybe weak shock UV lines
Large OH columns . . . via CR ionizations?
Figure from Wardle 1999, ApJ 525L,
post-shockshock
front
post-shockshock
front
for T > 400 K
Enhanced Maser Emission?
•W44: Histogram of Maser flux
Dashed line = RMS noise
clearly, sensitivity limited
•Same for brightest maser in each SNR
⇒ could find more Maser SNRs!
•W44: Maser “spots” = many clumps
of wildly varying brightness
Quick digression: Maser surveys aren’t complete.
from Hewitt et al. (2010)
Enhanced Maser Emission?
•W44: Histogram of Maser flux
Dashed line = RMS noise
clearly, sensitivity limited
•Same for brightest maser in each SNR
⇒ could find more Maser SNRs!
•W44: Maser “spots” = many clumps
of wildly varying brightness
Quick digression: Maser surveys aren’t complete.
As maser “spots” resolved, lose flux
⇒ extended or very faint masersfrom Hewitt, Phd Thesis (2010)
• Confined to the inner Galaxy, near dense gas:
• 24 detected of ~250 searched... 10% of all SNRs have masers
but ~25% have some interaction signature (bright IR, thermal x-rays, broad CO)
• Excellent correlation with GeV/TeV sources, even before Fermi!
Galactic Population of Maser SNRs
from Green et al. (1997)
Longitude-velocity diagram of CO in Milky Way
• For "0-decay origin, F" ~ Mcloud dkpc-2 #CR (Drury et al. 1994)
Directly estimate local CR density:
• Given Mcloud, dkpc : determines the CR ionization rate "CR ! #CR "local
• Maser SNRs have #CR enhanced by 10-50 times the local density
CRs are significant source of ionization! ( H2O $ OH? )
Evidence of accelerated Cosmic Rays
Hewitt et al. (2009)
• in first 3 months 5 Maser SNRs detected (Abdo et al. 2009):
• 1-year data yields 7 new detections of Maser SNRs Abdo et al. (2010), Castro & Slane (2010), Hewitt et al. (in prep)
Fermi-LAT detections of Maser SNRs
All images: Fermi-LAT data at 1-50 GeV (front only events) White contours show 20cm radio continuum. Dark crosses at the positions of SNR masers.
G349.7+0.2 CTB 33 3C 391 G5.4-1.2
CTB 37A IC 443W44W28
30 arcmin30 arcmin
30 arcmin30 arcmin30 arcmin30 arcmin
30 arcmin 30 arcmin
• Using ISIS Nonthermal (John Houck, et al.) . . . simple pion decay, synchrotron, IC, bremsstrahlung (not evolving SNR, particles)
• Get same results as Fermi team:
• Pion decay works: Wp ~ 2x1049 ergs
proton pbreak ~12 GeV/c
• Brems. requires Kep>0.01 w/ large B, n
IC limited by high densities ~100 cm-3
• May need Brems. at ~100 MeV
• Power-Law evolves from ~2 to ~3
(radio %=-0.35 => &=1.7)
• Escaping CRs? Probes CR diffusion.
However, no significant displacement
of GeV/TeV sources!
IC 443: Nonthermal Modeling
Clouds
within ~10 pc ~10-20 pc
Torres et al. (2008)
• GeV/TeV detections in IC 443
IC 443: Nonthermal Modeling
WMAP 41GHz Fermi 5-50 GeV
• Include WMAP: 26, 33, 41, 61, 93 GHz
• !-ray break! Ebr,GeV = 10-70 GeV/c
Radio break! Ebr,radio ~ "br,radio-1 ~ B-2
for Ebr,radio = Ebr,GeV requires B=100 #G
Radio Spectrum
Gamma-Ray Spectrum
• Age ~ 3.7±0.3 kyrs (similar to IC 443)
• X-ray/Dust correlated emission
density gradient enhanced to NE
• Central CCO “anti-magnetar”
No PWN, very weak PSR
• “Bright Eastern Knot”
recent cloud interaction, < 2 kyrs
•
Left: CO 1-0 mapping reveals dense cloud
Right: thermal X-rays offset from optical [O III] emission (Hwang et al., Blair et al.)
A non-Maser SNR: Puppis A
Radio 20cm IRAS 24 'm ROSAT X-ray
• Fermi 15.4( detection
Counts Map 2-50 GeV
• Nearby CO clouds
(Paron et al. 2005)
• Spatially extended
X-ray model: 7.6( better than Pt src
A non-Maser SNR: Puppis A
• Electron break tells us nothing about proton break!
Seen in all bright Maser SNRs...
(Plotted spectral fits using pion model)
• What about all detected SNRs?
Break will steepen “apparent” index
As break $ 1 GeV, more steepening
Plot of all Maser SNRs in 1FGL!(plus a few young SNRs)
Expect &=2 from DSA.
• Spectral evidence of CR diffusion?
and why so few/faint TeV Maser SNRs
• Cosmic ray diffusion: D(E) = D0 E) with D0 ~ 1026 to 1028 cm2 s-1, ) ~ 0.5?
Assuming “fast” diffusion, D0 = 1028 cm2 s-1
~1 GeV break develops in ~10-20 kyrs
Explaining the GeV/TeV Break
Are SNRs Cosmic Ray Factories?
• Previously only circumstantial evidence:
• DSA can accel. CRs (Blanford & Eichler 1977)
• SNR kinetic energy -> CR works energetically
(Ginzburg & Syrovatskii 1964)
• Some SNRs have bright radio emissivities.
• Now with Fermi/Veritas, etc.
• Clearly seeing CR yields ~ 1-10% in SNRs
up to ~1-10 TeV range w/o cutoff (but a break)
• No evidence that Maser SNRs are still
accelerating many CRs, just holding them in?
• Need to explain & ~2.7 of Gal. CR protons
same in Maser SNRs, but explained... D~E0.7 ?
• Alternative Theory: Magnetic Scattering (Malkov et al. 2010)
predict *& of exactly 1 (easy to test)
What we know
• Lots of sources! Detected ~1/2 Maser SNRs
• GeV/TeV emission is likely hadronic(~100 MeV may be Bremsstrahlung)
• Wp~1-10% ESN => significant source of Gal. CRs
• Cutoff ~50 GeV makes TeV detections hard (target a few, interesting sources... complement with EVLA)
Things to explain
• Mechanism for Radio / "-ray breaks (at different energies) Detecting old SNRs like $ Cygni is interesting...
• For how long do SNRs accelerate CRs (e- vs p+) ?
What is the total CR yield (1-10%, 50%) ?
• How do we reconcile DSA %~2 with:
1. Galactic CRs with %~2.7 ?
2. Radio spectra flatter than -0.5 (eg, IC443 with -0.36) ?
Contamination from thermal emission?
Conclusions
My Veritas wish list
IC 443
W51 C
Cas A
W44 (100-200 hr?)
3C 391 (25-60 hr?)
more IC 443
Conclusions
My Veritas wish list
IC 443
W51 C
Cas A
W44 (100-200 hr?)
3C 391 (25-60 hr?)
more IC 443
Plenty of CO / Masers to East,
but undetected in TeV?
CO 1-0 map from Lee et al. (2009)
Ejecta-driven shock
(Bykov et al.)
could accel. CRs?