Circular polarization of AGNs on the parsec VLBI scales
Faraday effects possibly plays the major role in generation of circular polarization observed on Very Long Base Interferometry scales. Multi-frequency circular polarization measurementrs can become the desired breakthrough in understanding the active galactic nuclei jet physics and the only possibil...
Збережено в:
| Опубліковано в: : | Advances in Astronomy and Space Physics |
|---|---|
| Дата: | 2011 |
| Автори: | , , |
| Формат: | Стаття |
| Мова: | English |
| Опубліковано: |
Advances in astronomy and space physics
2011
|
| Онлайн доступ: | https://nasplib.isofts.kiev.ua/handle/123456789/118979 |
| Теги: |
Додати тег
Немає тегів, Будьте першим, хто поставить тег для цього запису!
|
| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Цитувати: | Circular polarization of AGNs on the parsec VLBI scales / V.M. Vitrishchak, I.N. Pashchenko, D.C. Gabuzda // Advances in Astronomy and Space Physics. — 2011. — Т. 1., вип. 1-2. — С. 34-36. — Бібліогр.: 12 назв. — англ. |
Репозитарії
Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
nasplib_isofts_kiev_ua-123456789-118979 |
|---|---|
| record_format |
dspace |
| spelling |
Vitrishchak, V.M. Pashchenko, I.N. Gabuzda, D.C. 2017-06-02T18:32:16Z 2017-06-02T18:32:16Z 2011 Circular polarization of AGNs on the parsec VLBI scales / V.M. Vitrishchak, I.N. Pashchenko, D.C. Gabuzda // Advances in Astronomy and Space Physics. — 2011. — Т. 1., вип. 1-2. — С. 34-36. — Бібліогр.: 12 назв. — англ. 987-966-439-367-3 https://nasplib.isofts.kiev.ua/handle/123456789/118979 Faraday effects possibly plays the major role in generation of circular polarization observed on Very Long Base Interferometry scales. Multi-frequency circular polarization measurementrs can become the desired breakthrough in understanding the active galactic nuclei jet physics and the only possibility to estimate some of their vital parameters. We review the possible mechanisms of circular polarization generation and their connection to the jet parameters. We throw a glimpse on the methods of data reduction and finally discuss our current observational progress and its possible interpretation. The work was supported by Russian Federation President grant MK-2839.2009.2. en Advances in astronomy and space physics Advances in Astronomy and Space Physics Circular polarization of AGNs on the parsec VLBI scales Article published earlier |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| title |
Circular polarization of AGNs on the parsec VLBI scales |
| spellingShingle |
Circular polarization of AGNs on the parsec VLBI scales Vitrishchak, V.M. Pashchenko, I.N. Gabuzda, D.C. |
| title_short |
Circular polarization of AGNs on the parsec VLBI scales |
| title_full |
Circular polarization of AGNs on the parsec VLBI scales |
| title_fullStr |
Circular polarization of AGNs on the parsec VLBI scales |
| title_full_unstemmed |
Circular polarization of AGNs on the parsec VLBI scales |
| title_sort |
circular polarization of agns on the parsec vlbi scales |
| author |
Vitrishchak, V.M. Pashchenko, I.N. Gabuzda, D.C. |
| author_facet |
Vitrishchak, V.M. Pashchenko, I.N. Gabuzda, D.C. |
| publishDate |
2011 |
| language |
English |
| container_title |
Advances in Astronomy and Space Physics |
| publisher |
Advances in astronomy and space physics |
| format |
Article |
| description |
Faraday effects possibly plays the major role in generation of circular polarization observed on Very Long Base Interferometry scales. Multi-frequency circular polarization measurementrs can become the desired breakthrough in understanding the active galactic nuclei jet physics and the only possibility to estimate some of their vital parameters. We review the possible mechanisms of circular polarization generation and their connection to the jet parameters. We throw a glimpse on the methods of data reduction and finally discuss our current observational progress and its possible interpretation.
|
| isbn |
987-966-439-367-3 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/118979 |
| citation_txt |
Circular polarization of AGNs on the parsec VLBI scales / V.M. Vitrishchak, I.N. Pashchenko, D.C. Gabuzda // Advances in Astronomy and Space Physics. — 2011. — Т. 1., вип. 1-2. — С. 34-36. — Бібліогр.: 12 назв. — англ. |
| work_keys_str_mv |
AT vitrishchakvm circularpolarizationofagnsontheparsecvlbiscales AT pashchenkoin circularpolarizationofagnsontheparsecvlbiscales AT gabuzdadc circularpolarizationofagnsontheparsecvlbiscales |
| first_indexed |
2025-11-24T06:15:05Z |
| last_indexed |
2025-11-24T06:15:05Z |
| _version_ |
1850844165274861568 |
| fulltext |
Circular polarization of AGNs on the parsec VLBI scales
V. M. Vitrishchak1, I. N. Pashchenko2, D. C. Gabuzda3
1Sternberg Astronomical Institute, Moscow State University, Universitetsky pr., 13, 119992, Moscow, Russia
2Astro Space Center of Lebedev Physical Institute of RAS, Profsoyuznaya st. 84/32, 117997, Moscow, Russia
3University College Cork, College Road, Cork, Ireland
vmv@pisem.net
Faraday e�ects possibly plays the major role in generation of circular polarization observed on Very Long
Base Interferometry scales. Multi-frequency circular polarization measurementrs can become the desired
breakthrough in understanding the active galactic nuclei jet physics and the only possibility to estimate
some of their vital parameters. We review the possible mechanisms of circular polarization generation and
their connection to the jet parameters. We throw a glimpse on the methods of data reduction and �nally
discuss our current observational progress and its possible interpretation.
Introduction
Since the begin of the Very Long Base Interferometry (VLBI) epoch in observations of active galactic
nuclei (AGNs) lots of types of data became available. Mapping with the highest resolution up to tens of
microseconds, simultaneous multi-frequency observations, high temporal resolution techniques, linear polar-
ization and rotation measure studies give us terabytes of information on AGNs. Still, even the key properties
and parameters of these sources remain unknown or uncertain (e. g., jet plasma composition, magnetic �eld
geometry, mechanism responsible for jet generation etc.). These problems are not only technical, but some-
times principal. For example linearly polarized (LP) emission from the source to the observer is experiencing
very signi�cant impact of external e�ects, such as an external Faraday rotation of the polarization plane and
the related e�ect of external depolarization. By observating LP and full intensity alone it is very di�cult or
even impossible to separate the external and internal propagation e�ects [2] and thus to study the physics
of AGN jets. On the contrary, the in�uence of external e�ects on the circular polarization component can
be neglected [1]. Thus, the observed circular polarization is related solely to the internal processes of the
AGN emission. Since the signal circular polarization (CP) is dependent on a number of emission parameters
� composition of matter, properties of the magnetic �eld, the presence in the energy spectrum of the low
energetic relativistic particles, etc. it opens the new possibility of estimating these parameters with the help
of CP observations.
Circular polarization
Circular polarization studies are the latest trend of astrophysical VLBI observations of AGNs. Data
processing in this case is complicated because the degree of CP in AGNs even at linear scales of the order
of parsecs rarely reaches 1%. Despite its low degrees, CP can become a new key for estimating various vital
AGN parameters, which cannot be obtained otherwise. There are two mechanisms of CP generation which
likely dominate in AGN jets [12]:
a) direct synchrotron emission, and
b) conversion from linear polarization (LP), which can have 2 modes:
b.1) conversion in changing perpendicular B-�eld, and
b.2) conversion driven by internal Faraday rotation.
In synchrotron emission each charged particle gyrating around magnetic �eld line radiates elliptically
polarized electro-magnetic waves. Emission of full ensemble of particles is highly linearly polarized, but
small amount of circular component is also emitted [8]. Direct synchrotron emission of typically observed
CP levels requires strong ordered magnetic �elds in radiating regions (CP ∼ B
1/2
|| , e.g. 10−2 G is needed
34
Advances in Astronomy and Space Physics V. M. Vitrishchak, I. N. Pashchenko, D. C. Gabuzda
to generate 1% CP in the homogeneous source) and only works in normal e−�p+ plasma as electrons and
positrons emit the same CP signal with opposite signs.
In Faraday conversion mechanism, initially linearly polarized emission is being converted into circularly
polarized and vice versa when passing through magnetized media with transverse magnetic �eld component.
We can split our emission into two orthogonal modes � one parallel and one perpendicular to the transverse
(to the wave propagation vector) projection of the magnetic �eld vector. The parallel linear mode is delayed as
it accelerates charged particles which then reemit the waves, while the perpendicular mode is left unchanged
as charged particles cannot freely move perpendicular to magnetic �eld lines and thus are not accelerated
in this direction by the passing emission. The resulting emission on the exit of the medium is generally
elliptically polarized. Faraday conversion (or Cotton-Mutton e�ect) works in electron-positron plasma as
electrons and positrons contribute to the e�ect with the same sign.
Note that if only one of the modes was initially present in the linearly polarized wave (that is only parallel
or perpendicular to the transverse projection of the B-�eld in the medium) no conversion would take place.
As long as synchrotron emission is highly linearly polarized with the polarization plane perpendicular to the
local generating magnetic �eld, intrinsic conversion of linear polarization inside the emitting synchrotron
source itself can only happen if the polarization plane or/and transverse projection of the magnetic �eld are
changing along the wave path. In the �rst case Faraday rotation can do the job, requiring normal electron-
proton plasma and longitudal B-�eld component to be present. In the second case the direction of transverse
B-�eld should be changing inside the source still remaining ordered (there will be no conversion but only
depolarization in totally random unordered �eld).
Due to extremely low radiative transfer coe�cients in interstellar medium [7], CP is immune to propaga-
tion e�ects (which, on the contrary, can totally distort linear polarization on the way from the source to the
observer). This property along with the strong dependence of CP on the intrinsic parameters of the source
(like jet plasma composition, density, particle energy distribution, magnetic �eld properties etc.) makes CP
a good mean of estimating these parameters. Details can be found in [5].
Data reduction
As it was mentioned above, the typical degrees of CP in AGNs on the parsec scales rarely exceed 1%
which makes it a real challenge to detect and measure CP signal from these sources. First results in this
direction were obtained in 1999 [6]. The proposed gain transfer method [3] followed by separate �RR/LL
calibration� [9] is the most fruitful technique of CP calibration so far, giving up to 30% CP detection rate
for AGN objects with the 2σ criteria. Only two major CP parsec-scale AGN surveys exist up to date (see
[4, 9, 10].
Current progress
According to current observational data [4, 9, 10] typical CP degrees in the VLBI core region are usually
less then several tenths of a percent (0.86% in 3C279 is maximum so far). CP degrees on the jet edges
(if observed) reach several percents. If CP is generated directly via synchrotron emission, very strong B-
�elds (∼ 1 G) are required to create such high degrees. In most cases CP is observed in the optically
thick VLBI core. CP peak is sometimes shifted from the full intensity peak towards the supermassive black
hole � to even more optically thick regions. This is consistent with all CP generation mechanisms and
states that intrinsic depolarization is weak and thus highly ordered B-�eld presents in the jets. Internal
Faraday rotation is small or absent (which is more likely). It favors e−�e+ jet plasma composition and thus
enabling only the mechanism of CP generation through Faraday conversion in changing B-�eld. Prolonged
CP structures stretching from optically thick VLBI core to optically thin inner jet are detected for several
sources. It shows that CP generation works within wide range of plasma parameters also favoring e−�e+
jet plasma composition as internal Faraday rotation working only in normal e−�p+ plasma is very sensitive
to the intrinsic source parameters. Anti-symmetric CP structures with high degrees and di�erent signs of
CP on the di�erent edges of jet were detected in some of the sources. Toroidal B-�eld component (typical
for toroidal or spiral B-�eld geometries) should be present to explain such polarization pictures which also
means that (at least) CP generation via conversion in changing B-�eld works for sure.
For all the sources observed at several epochs the sign of CP persisted throughout the epochs. For
most of the sources CP degree persisted as well (within the error limits). This states that jet parameters
responsible for CP generation (e.g. ordered B-�eld geometry) are persistent on the timescales of at least
several years. For several objects changes in CP signal correlat with the major changes in the total source
35
Advances in Astronomy and Space Physics V. M. Vitrishchak, I. N. Pashchenko, D. C. Gabuzda
�ux between the epochs. It can related to the CP variability, blazar activity and emerging of a new jet
component. No obvious common CP frequency dependence was found so far. Only few sources have shown
the Vpeak ∼ ν2 dependence (typical for direct CP generation via synchrotron emission), also stating that
conversion mechanism(s) dominate in most of the sources. Various sources show sign changes with frequency
that can also be explained by conversion mechanism. Di�erence in CP degrees was found for quasars and
BL Lacertae type objects; the latter shows the lack of objects with high (> 0.4%) CP degrees which can be
explained by various reasons concerning the physical di�erences between these two classes of objects [11].
Conclusions
Generated CP signal is sensitive to many internal parameters of AGN jets: plasma composition, particle
energy distribution and acceleration mechanism, magnetic �eld properties etc. Together with the other data,
CP can be used to estimate these parameters. The most likely mechanism of CP generation is conversion
from the linear polarization while propagating through the medium with changing transverse B-�eld along
the line of sight. While some important qualitative conclusions can be made even now (like the evidence
of toroidal magnetic �eld component in AGN jets), it is clear that the most promising way lies through
numerical modeling of AGN jets, solving the radiation transfer problem and comparing the results with real
observations. AGN CP database is still in its stone age and needs lots to be done to e�ectively use it in
statistical studies.
Acknowledgement
The work was supported by Russian Federation President grant MK-2839.2009.2.
References
[1] Beckert T., Falcke H. Astron. & Astrophys., V. 388, pp. 1106-1119 (2002)
[2] Burn B. J. Mon. Notic. Roy. Astron. Soc., V. 133, pp. 67-83 (1966)
[3] Homan D. C., Attridge J. M., Wardle J. F. C. Astrophys. J., V. 556, pp. 113-120 (2001)
[4] Homan D. C., Lister M. L. Astron. J., V. 131, pp. 1262-1279 (2006)
[5] Homan D. C., Lister M. L. Astrophys. J., V. 696, pp. 328-347 (2009)
[6] Homan D. C., Wardle J. F. C. Astron. J., V. 118, pp. 1942-1962 (1999)
[7] Jones T. W., O'Dell S. L. Astrophys. J., V. 214, pp. 552-539 (1977)
[8] Sazonov V. N. Soviet Astronomy, V. 13, pp. 396-402 (1969)
[9] Vitrishchak V. M., Gabuzda D. C. Astron. Rep., V. 51, pp. 695-708 (2007)
[10] Vitrishchak V. M., Gabuzda D. C., Rastorgueva E. A. et al. Mon. Notic. Roy. Astron. Soc., V. 391, pp. 124-135 (2008)
[11] Vitrishchak V. M., Pashchenko I. N., Gabuzda D. C. Astron. Rep., V. 54, pp. 269-276 (2010)
[12] Wardle J. F. C., Homan D. C. Astrophys. Space Sci., V. 288, pp. 143-153 (2003)
36
|