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?


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

8

from IR lines (Reach & Rho, 2000)

Thermal X-rays


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

30 arcmin

Maser-emitting SNRsYoung SNR

Fermi detects Maser-emitting SNRs

• 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


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?

Conclusions

Undetected in TeV

but plenty of CO

and Masers!

CO 1-0 map from Lee et al. (2009)CO 1-0 map from Lee et al. (2009)

smoothed to Veritas resolution


IC 443

W51 C

Cas A

W44 (100-200 hr?)

3C 391 (25-60 hr?)

more IC 443

maser -emitting remnants - stanford university

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