First Result of Urumqi 6cm Polarization Observations:

Xiaohui Sun, Wolfgang Reich

JinLin Han, Patricia Reich, Richard Wielebinski

Partner Group of MPIfR at NAOC

New 6cm Maps of Cygnus Loop

Outline• Introduction to the Cygnus Loop• Previous results at radio band• New 6cm observations and results

– System and observational parameters– New 6cm map of the Cygnus Loop– Spectral index map– Rotation Measure results– Depolarization analysis

• Conclusions

Introduction to the Cygnus Loop• Location:

– little obscuration from the Galactic plane

• Size: ~ – ideal for Urumqi 25m dish at 6cm

• Distance: 540 pc (HST observation)– Very precise

– nearby and hence little confusion by local medium

• Age:~14,000 yr– just transition from adiabatic to radiative phase

• Appearance: (Cavity explosion, interaction with cloud?)– bright NE and WEST shells at radio, optical and X-ray

– bright south shell at radio

74l 8.5b

4 4

Previous Results at Radio Band• Spectral index – need high frequency total intensity data

– integrated flux density• 11cm, 20cm and 35cm data =-0.420.06 (Uyaniker et al. 2004)

• 6cm flux density: 73Jy spectral steepening (Kundu & Becker 1972)

– spatial variation over the SNR and spectral curvature• 408MHz, 1420MHz and 2695MHz datalarge spectral variation

(Green 1990; Leahy & Roger 1998)

• NOT supported by high quality data (Uyaniker et al. 2004)

• Polarization – need high frequency polarization data– rotation measure

• south part: -20 rad/m2 (Uyaniker et al. 2002; Kundu & Becker 1972)

– depolarization at the northern part• NE region, internal depolarization (Leahy et al. 1997)

– Appearance• substantial difference between the south and the north part two-SNR scenario (Uyaniker et al. 2002)

• Spectral index of flux density:

-flux density -frequency

• Spectral index of brightness temperature:

S

S

2

Previous Results at Radio Band• Spectral index – need high frequency total intensity data

– integrated flux density• 11cm, 20cm and 35cm data =-0.420.06 (Uyaniker et al. 2004)

• 6cm flux density: 73Jy spectral steepening (Kundu & Becker 1972)

– spatial variation over the SNR and spectral curvature• 408MHz, 1420MHz and 2695MHz datalarge spectral variation

(Green 1990; Leahy & Roger 1998)

• NOT supported by high quality data (Uyaniker et al. 2004)

• Polarization – need high frequency polarization data– rotation measure

• south part: -20 rad/m2 (Uyaniker et al. 2002; Kundu & Becker 1972)

– depolarization at the northern part• NE region, internal depolarization (Leahy et al. 1997)

– Appearance• substantial difference between the south and the north part two-SNR scenario (Uyaniker et al. 2002)

System and Observational parameters

Frequency: 4.8 GHz

Bandwidth: 600 MHz

Tsys: 22 K-25 K

HPBW: 9.5 arcmin

First Sidelobe: 2%

Instrumental polarization:

peak 0.5%

ring: 2%

Tracking error: <1

Map Center: (20h52m, 3030)Map Size: 4.2 4.8Scan velcity: 2/min

Scan separation: 4Integration time: 2 min/pixel

Scan direction: RA and DEC

Observation Date: 8/2004-12/2004

Coverages I (PI): 5 (6)

Source Flux density (Jy) PC (%) PA ()

3C286 7.5 11.3 33

3C48 5.510.10 4.20.4 108.1 1.5

3C138 3.97 0.07 10.8 0.5 168.8 1.2

Reduction

spike and interference

scanning effects

calibration

combine data

HPBW (CL2): 9.7 arcmin

r.m.s-I: 1 mK TB (0.6)

r.m.s-PI: 0.4 mK TB (0.35)

CL4

CL2

New 6cm Map

CL4 (2048+312): Variable

•extragalactic origin, z=3.18 •angular broadening•time scale: 48 days •variation: 30%

NE1 15%

NE2 20%

C 25%

W 20%

S3 20%S2 25%

S1 20%

New 6cm Map

Effelsberg 11cm and 21cm Map

Effelsberg 11cm and 21cm Map

Spectral index: integrated flux density

•Integrated flux density: 90 9 Jy•20% larger than previous value (miss diffuse component)•Consistent with Uyaniker et al (2004)•No spectral steepening or flattening until 5 GHz

New measurement

Spectral index Map: 6cm/11cm/21cm

• zerolevel (TT-plot)•6cm +2 mK•11cm +8 mK•21cm +23 mK

•limit: ~20 r.m.s•high signal-to-noise ratio•influence of background

•spectral index variation•NE, SW, NW, CL4: -0.4•Increase towards diffuse region, maximal difference ~0.3, support Uyaniker et al. (2004)

•Qualitatively explanationIn diffuse region: magnetic field less compressed, high energy electrons contribute

6cm/11cm- 6cm/21cm

• spectral curvature•>0: flattening •<0: steepening

•Difference map: | |<0.15•south, NW, NE: >0•Middle: <0

•NO spectrum curvaturesupport Uyaniker et al. (2004)

Rotation Measure results: South

• Calculation: linear fitting the PAs at 6cm, 11cm and 21cm (PI > 5r.m.s)

• Results: South part– -21 rad/m2

– Rotation of PA ~4• intrinsic magnetic direction

• magnetic field along the shell

– Consistent with previous results

– Foreground contribution

ne=0.02 cm-3, B||=-3G, D=540 pcRM=-26 rad/m2

20 RM

0.81 eRM n B D

Rotation Measure results: South

• Calculation: linear fitting the PAs at 6cm, 11cm and 21cm (PI > 5r.m.s)

• Results: Southern part– -21 rad/m2

– Rotation of PA ~4• intrinsic magnetic direction

• magnetic field along the shell

– Consistent with previous results

– Foreground contribution

ne=0.02 cm-3, B||=-3G, D=540 pcRM=-26 rad/m2

20 RM

0.81 eRM n B D

Rotation Measure results: North

• Region C:– RM=-28 rad/m2

– Projection?

• Region NE1, NE2 and W:– 21cm totally depolarized– Ambiguity n 362 rad/m2

– Average RM for three possibilities: • N=0: RM=-73 rad/m2

• N=-1: RM= 290 rad/m2

• N=1: RM= -430 rad/m2

Rotation Measure results: North

• Region C:– RM=-28 rad/m2

– Projection?

• Region NE1, NE2 and W:– 21cm totally depolarized– Ambiguity n 362 rad/m2

– Average RM for three possibilities: • N=0: RM=-73 rad/m2

• N=-1: RM= 290 rad/m2

• N=1: RM= -430 rad/m2

Rotation Measure results: North

• Region C:– RM=-28 rad/m2

– Projection?

• Region NE1, NE2 and W:– 21cm totally depolarized– Ambiguity n 362 rad/m2

– Average RM for three possibilities: • N=0: RM=-73 rad/m2

• N=-1: RM= 290 rad/m2

• N=1: RM= -430 rad/m2

Minimum Rotation Measure Case (n=0)

Large Rotation Measure Case (n=1):NE1+NE2

• Magnetic field: energy equipartition 47 G– Flux density: 11 Jy– Spectral index: -0.4– Emission volume: 400 pc3

• Electron density: 1 cm-3 pressure balance– 10-100 cm-3 Optical filament observation

• OVI (Long et al. 1992)• SII (Patnaude et al. 2002)

– Several cm-3 X-ray observation• ASCA (Miyata et al. 1994)• ROSAT (Lu & Aschenbach 2005)

• Depth of the filament: 4.8 pc RM=-128 rad/m2 strict lower limit by a factor of a few

2/ 7 2/ 7minB C V L

Large Rotation Measure Case (n=1)

Depolarization Analysis

• Description of depolarization:• Depolarization mechanism:

– Bandwidth depolarization:

– Internal depolarization:

– External depolarization:

– Beam depolarization• Similar to external depolarization• Depends on the resolution• Polarization angle changes in the scale of beam size

o

i

PCDP

PC

2sinc(2 )DP RM

1 exp( )SDP

S

2 4 22 2RMS i 2RM

2 4exp( 2 )RMDP

Depolarization Analysis

• Bandwidth depolarization NOT important =600 MHz @ 6cm =40 MHz @ 11cm =20 MHz @ 21cm

• Beam depolarization is NOT important– No distortions in polarization angle maps– Depolarization does not depend on the resolution

(both at 10 arcmin and 1 arcmin)

Depolarization Analysis

• Southern part and C filament– Observation: PC6cm~PC11cm~PC21cm

– Explanation: no depolarization

• Northern part– Observation:

• DP21cm~0

• NE1: PC6cm~27% PC11cm~23%

• NE2: PC6cm~28% PC11cm~23%

• W: PC6cm~20% PC11cm~15%

– Minimal RM case (-73 rad/m2): DP6cm=94% DP11cm=50%

– Large RM case (-430 rad/m2): DP6cm~0

– Large RM case (290 rad/m2): DP6cm=33% DP11cm=12%

Remarks on Two-SNR picture

Different properties between the

northern part and the southern

part at:

radio (new observations)

other bands (X-ray, optical, …)

Interaction of the two SNR:

Similar distance

no enhancement in the overlap

(SNRs DEM L316 in LMC

Williams et al. 1997)

Conclusions

• No spectral curvatures in:– Spectrum of integrated flux density– Overall spectral maps

• RM– -21 rad/m2 for southern part foreground– -73 rad/m2 or 290 rad/m2 for the northern part– Internal depolarization at the northern part

• Two-SNR scenario– Different properties in the south and north