probing the faraday screen in the nuclear region of 3c 84 · 2018-08-22 · probing the faraday...
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Probing the Faraday screen in the nuclear region of 3C 84
Minchul Kam & Sascha TrippeSeoul National University
2018 Radio telescope user’s meeting | Aug. 1617. 2018
HST 1.73’ view of NGC 1275
z ~ 0.018d ~ 75 Mpc
VLBA 22GHz
http://pc.astro.brandeis.edu/images/3c84.html
VLBA 22 GHz
http://pc.astro.brandeis.edu/images/3c84.html
VLBA 43 GHz3
mas
= 1
pa r
sec
● 3C 84 – central region of NGC 1275
https://www.bu.edu/blazars/VLBA_GLAST/0316.html
● 3C 84 is an interesting target !
1) very close (z~0.018, d~75 Mpc) → 1 pc scale structure of the central region
is resolved!
core : bright, upstream region
where the jet begins
hotspot : the localbrightest region in the
bowshocklike structure
2) low polarization
→ depolarization by Faraday rotation?
1 pc
● Polarization angle (EVPA) is rotated by Bfield.
Bfieldpolarized emission
RM ∝∫ n Blos dl
φ 1=φ 0+λ12 RM
φ 2=φ 0+λ22 RM
Δφ=λ2 RM
polarization distribution at different frequencies → spatially resolved information of electron density & magnetic field
φ 2−φ 1=(λ12−λ2
2)RM
● Data information
1. Very Long Baseline Array (VLBA) – 10 antennas with baselines up to 8000 km Period : Jun. 2014 ~ Sep. 2017 (35 epochs)
Freq : 43.008 / 43.087 / 43.151 / 43.215 GHz
2. Korea VLBI Network (KVN) – 3 antennas with baselines up to 480 km The KVN Large Program (PAGaN) Freq : 22 / 43 / 86 / 129 GHz
Images of VLBA 43 GHz data (Dec. 2016 ~ Sep. 2017)
Images of KVN 86 GHz (Dec. 2016 ~ Dec. 2017)
● VLBA 43.008 / 43.088 / 43.151 / 43.215 GHz (Jan. 2017)
● VLBA 43.008 / 43.088 / 43.151 / 43.215 GHz (Jan. 2017)
● The RM at the hotspot
2015
|RM|∼4.4×105 rad /m2
● VLBA 43.008 / 43.088 / 43.151 / 43.215 GHz (Jun. 2017)
● VLBA 43.008 / 43.088 / 43.151 / 43.215 GHz (Jun. 2017)
● The RM at the core
|RM|∼6.6×105 rad /m2
● Point I The core RM is lower than the expectation.
RM core=6.6×105 rad /m2RM hsp=4.4×105 rad /m2
The same order of the RMs even though the distance from the SMBH are different !
RM ∝∫ne Bφ dl
● Point II Detection of the negative core RM.
Only positive core RM at 220 & 340 GHz
a. SMA (220 & 340 GHz) 8 radio telescopes in Hawaii
b. CARMA (220 GHz) 23 radio telescopes in California
Plambeck+ 2014
Though SMA & CARMA cannot resolve the core,
they assumed that most of the emission at
220 & 340 GHz originates from the core region.
● Point II Detection of the negative core RM.
Plambeck+ 2014
We detected both positive & negative RM at 43 GHz inconsistent with →the RM at 220 & 340 GHz
● Estimation of m% of hotspot at 220 GHz
m=m0|sin RM λ2
RM λ2 | m=m0|sin 2 RM λ2
2 RM λ2 |
I. external screen + regular Bfield (with RM gradient)
II. internal screen + regular Bfield
22 GHz : 24.4 Jy43 GHz : 13.5 Jy86 GHz : 4.8 Jy129 GHz : 3.1 Jy230 GHz : 1.6 Jy
43 GHz : 0.3 %86 GHz : 2.2 %230 GHz : 9.0 %
43 GHz : 0.3 %86 GHz : 2.2 %230 GHz : 16.4 %
hotspot flux
m% (86 220 GHz) increases at least 4 times. →Flux (86 220 GHz) decrease to 1/3. →
The polarized emission at 220 GHz might be dominated by the hotspot emission.
(x4) (x8)(x1/3)
● The RM at the hotspot
Most of the RMs at the hotspot are POSITIVE !
Considering all the RMs at 220 & 340 GHz are POSITIVE, this supports the possibility that the polarized emission at 220 GHz might bedominated by the hotspot emission.
● Faraday screen I : Emitting region itself (internal Faraday rotation)
Burn 1966
Except the case that emitting region is slab with zero random component of Bfield, EVPA rotation is saturated at low frequencies.
→ RM will increase at higher frequency where the EVPA rotation is less saturated.
u∝λ
Faraday screen : slab Faraday screen : sphere
, μ : random magnetic field component
● Faraday screen II : Hot accretion flow (external Faraday rotation)
Li+ 2016
hot accretion flow geometrically thick & optically thin low accretion rate turbulent
If polarized emission from the core passes through this accretion flow, (1) nondetection or underestimation of the core RM, (2) both positive & negative RM can be explained.
→ RM will not increase at higher frequency.
● To probe the Faraday screen..
Case I : internal to the jet – RM will increase at higher frequency.
Case II : external to the jet – RM will not increase at higher frequency.
→Multifrequency polarimetric observations are necessary !
● KVN observation at frequencies higher than 43 GHz
We proposed multifrequency KVN observation at 86 90 94 & 129 138 142 GHz.
→ The first attempt to obtain the core RM at this high frequency range.