J/A+A/673/A159 Spectral analysis of M87 jet (Ro+, 2023)
Spectral analysis of a parsec-scale jet in M87:
Observational constraint on the magnetic field strengths in the jet.
Ro H., Kino M., Sohn B.W., Hada K., Park J., Nakamura M., Cui Y., Yi K.,
Chung A., Hodgson J., Kawashima T., An T., Trippe S., Algaba J.-C.,
Kim J.-Y., Sawada-Satoh S., Wajima K., Shen Z., Cheng X., Cho I.,
Jiang W., Jung T., Lee J.-W., Niinuma K., Oh J., Tazaki F., Zhao G.-Y.,
Akiyama K., Honma M., Lee J.A., Lu R., Zhang Y., Asada K., Cui L.,
Hagiwara Y., Hirota T., Kawaguchi N., Koyama S., Lee S.-S., Oh S.-J.,
Sugiyama K., Takamura M., Wang X., Hwang J.-Y., Jung D.-K., Kim H.-R.,
Kim J.-S., Kobayashi H., Oh C.-S., Oyama T., Roh D.-G., Yeom J.-H.
<Astron. Astrophys. 673, A159 (2023)>
=2023A&A...673A.159R 2023A&A...673A.159R (SIMBAD/NED BibCode)
ADC_Keywords: Active gal. nuclei; Galaxies, radio ; Radio sources ; VLBI
Keywords: galaxies: active - galaxies: individual: M87 - galaxies: jets -
radio continuum: galaxies - relativistic processes -
techniques: interferometric
Abstract:
Because of its proximity and the large size of its black hole, M87 is
one of the best targets for studying the launching mechanism of active
galactic nucleus jets. Currently, magnetic fields are considered to be
an essential factor in the launching and accelerating of the jet.
However, current observational estimates of the magnetic field
strength of the M87 jet are limited to the innermost part of the jet
(≲100rs) or to HST-1 (∼105rs). No attempt has yet been made
to measure the magnetic field strength in between.
We aim to infer the magnetic field strength of the M87 jet out to a
distance of several thousand rs by tracking the distance-dependent
changes in the synchrotron spectrum of the jet from high-resolution
very long baseline interferometry observations.
In order to obtain high-quality spectral index maps,
quasi-simultaneous observations at 22 and 43GHz were conducted using
the KVN and VERA Array (KaVA) and the Very Long Baseline Arra (VLBA).
We compared the spectral index distributions obtained from the
observations with a model and placed limits on the magnetic field
strengths as a function of distance.
The overall spectral morphology is broadly consistent over the course
of these observations. The observed synchrotron spectrum rapidly
steepens from alpha22-43GHz~-0.7 at ∼2mas to alpha22-43GHz~-2.5 at
∼6mas. In the KaVA observations, the spectral index remains unchanged
until ∼10mas, but this trend is unclear in the VLBA observations. A
spectral index model in which nonthermal electron injections inside
the jet decrease with distance can adequately reproduce the observed
trend. This suggests the magnetic field strength of the jet at a
distance of 2-10mas (∼900rs∼4500rs in the deprojected
distance) has a range of B=(0.3-1.0G)(z/2mas)-0.73. Extrapolating to
the Event Horizon Telescope scale yields consistent results,
suggesting that the majority of the magnetic flux of the jet near the
black hole is preserved out to ∼4500rs without significant
dissipation.
Description:
VLBI CLEAN images of M87 in FITS format. This is a multi-epoch (Eight
epochs for KaVA and five epochs for VLBA) and multi-frequency (22 and
43GHz) data set for reconstructing the spectral index profiles. When
creating the FITS images, data within the (u, v) range of 33-170
mega-lambda has been used for KaVA, and 25-685 mega-lambda for VLBA.
All images have been restored to the same beam size of 1.2masx1.2mas
(for details, see Sect 3.1 of the paper).
Objects:
-----------------------------------------
RA (2000) DE Designation(s)
-----------------------------------------
12 30 49.42 +12 23 28.0 M87 = NGC 4486
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 107 26 List of fits images
fits/* . 26 Individual fits images
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 23 I4 --- Nx Number of pixels along X-axis
25- 28 I4 --- Ny Number of pixels along Y-axis
30- 39 A10 "datime" Obs.date Observation date
41- 47 F7.4 GHz Freq Observed frequency
49- 53 I5 Kibyte size Size of FITS file
55- 67 A13 --- FileName Name of FITS file, in subdirectory fits
69-107 A39 --- Title Title of the FITS file
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Acknowledgements:
Hyunwook Ro, hwro(at)yonsei.ac.kr
(End) Patricia Vannier [CDS] 15-Mar-2023