VLBA polarimetry of the Fermi-detected quasar B0954+556:

a rare “spine and sheath” polarisation structure

Jun Yang (JIVE, Netherlands)Alaxander B. Pushkarev (MPIfR, Germany)

Tiziana. Venturi (Istituto di Radioastronomia, Italy)Wei Zhao, Xiao-Yu Hong, Tao An, Wei-Hua Wang (ShAO, China)

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Outline B0954+556: a bright quasar Radio observations VLBA Imaging Results # “Spine and sheath” polarisation structure # Nondetection of the radio core Possible scenarios for the polarisation structure # Plasma compression by shocks and

interactions # A helical magnetic field wrapped around the jet Summary

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B0954+556 (4C 55.17, J0957+5522)

---Active from radio to Gamma-ray# Bright radio source with Sν > 0.5 Jy in 100 MHz – 43 GHz

# Optically, a high polarized quasar: p=8.7 %, mv=17.5,

z=0.9

(Wills et al. 1992)

# Detected by Chandra and ROSAT at X-ray band.

(Tavecchio et al. 2007, Sambruna 1997)

# Detected by the EGRET at Energy > 100 MeV

(Hartman et al. 1999)

# 24 sigma detection by the Fermi (Abdo et al. 2009)

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The used radio observations

# MERLIN data come from the MERLIN data archive. 　

# In the VLBA polarimetry observations, we used DA193 and OQ208 as the D-term calibrator, DA193 and 3C279 as the EVPA calibrators.

# The standard VLBA polarisation calibration procedure was used in the data reduction.

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Radio imaging results

The “Spine and sheath” polarisation (E) structure

# Spine: E || Jet axis at the centre of the jet# Sheath: E ┴ Jet axis on both sides of the

spine

Magnetic field B ┴ E, if it is optically thin synchrotron emission.

## Spine: B ┴ Jet axis## Sheath: B || Jet axis

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Spine and two-sided sheath are clearly seen at 1.7, 5, & 8.4 GHz.

48º away from the Galactic plane. No any correction of the polarisation position angle.

Sticks

polarisation vectors: E

Length of sticks

polarisation degree

Color of sticks

polarisation position angle

Contours

intensity distribution increased from 3 sigma off-source noise level by a factor of -2, -1, 1, 2, 4, …

1 mas = 7.6 parsec

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Polarization degree distribution

Pseudo color: ploarisation degree distribution; contours: intensitity map.

Polarisation degree (>3%) in the spine is much lower than that (>10%) in the sheath.

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Faraday RM distribution

# The RM distribution shows a hint of transverse RM gradients in the inner spine.

Color map

RM distribution Used polarisation position angle

map 1.7, 5, 8.4 GHz VLBA images. Contours

8.4 GHz intensity map convolved with the beam of 1.7

GHz VLBA image.

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The radio core: not seen# There is no compact component detected at

15 GHz (Rossetti et al. 2005).

# No fringes detected even on the shortest baseline in our fringe-detecting VLBA observations at 43 and 86 GHz. The half-Jy source is completely resolved.

# The radio emission is very stable (<5%) over the last 20 years (Fan et al. 2007)

# A rare case in the Fermi-detected quasars and blazars (Abdo et al. 2009).

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The random B field can be aligned by the compression along the shock and interaction planes.

The scenario was invoked to explain the “spine and sheath” polarisation structure in 1055+018 (Attridge et al. 1999) .

# Spine created by a series of transverse shocks. # Sheath formed by the interaction with the ISM.

Problems:1. Hard to explain the low polarisation degree in

the spine and the thick (~100 pc) sheath in our case.

2. Difficult to explain the extended regions of transverse B field and the transverse Faraday RM gradients in some other cases (Gabuzda 2006).

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A helical magnetic field wrapped around the jet

Schematic illustration of how a 3-D helical B field (blue line) can give rise to the projected 2-D B

distribution (black lines) with B┴ (center) and B|| (edge, Gabuzda 2006) (1) A natural result of the rotating black hole, disc, jet and outflow.

(2)The transverse RM gradient with both signs is a systematic change in the line of sight of component of a helical B field.

*Does not rule out the existence of the shocks and interactions.

However, some new features were found in other sources recently.

(1)A “flip” in the direction of the transverse RM gradients, e.g. B1803+784 (Mahmud 2009a).

(2)Transverse RM gradients sometimes changes their direction with distance from the core (Mahmud 2009b).

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SummaryThe “spine and sheath” polarisation structure was clearly detected in an extended pc-scale jet.The spine has the lower polarisation degree (3%) than the sheath (>10%). Transverse RM gradients detected in the inner spine.No bright and compact core detected in B0954+556, an unusual case in the Fermi-detected blasars and quasars. We prefer a magnetic field wrapped around the jet to shocks/interactions to explain the polarisation structure in the case of B0954+556.