In a recent publication, we reported results of a multi-frequency VLBI
campaign of observations of the Active Galactic Nucleus (AGN) in galaxy M\,81,
phase-referenced to the supernova SN\,1993J. We were able to extract precise
information on the relative astrometry of the AGN radio emission at different
epochs and frequencies. We found strong evidence of precession in the AGN jet
(i.e., a systematic evolution in the jet inclination at each frequency) coupled
to changes in the overall flux density at the different frequencies. In these
proceedings, we summarise the main contents of our previous publication and we
report on (preliminary) new results from our follow-up VLBI observations, now
phase-referenced to the young supernova SN2008iz. We also briefly discuss how
these results match the picture of our previously-reported precession model.Comment: 6 pages, 3 figures. Proceedings of EVN meeting 201

Alberdi, A.

Brunthaler, A.

Marcaide, J. M.

Marti-Vidal, I.

English

arXiv

I. Marti-vidal

J. M. Marcaide

A. Alberdi

A. Brunthaler

CiteSeerX

Jet precession in the active nucleus of M81. Ongoing VLBI monitoring.

In a recent publication, we reported results of a multi-frequency VLBI campaign of observations of the Active Galactic Nucleus (AGN) in galaxy M\,81, phase-referenced to the supernova SN\,1993J. We were able to extract precise information on the relative astrometry of the AGN radio emission at different epochs and frequencies. We found strong evidence of precession in the AGN jet (i.e., a systematic evolution in the jet inclination at each frequency) coupled to changes in the overall flux density at the different frequencies. In these proceedings, we summarise the main contents of our previous publication and we report on (preliminary) new results from our follow-up VLBI observations, now phase-referenced to the young supernova SN2008iz. We also briefly discuss how these results match the picture of our previously-reported precession model

Marti-Vidal, Ivan

Marcaide, Jon

Alberdi, Antxon

Brunthaler, Andreas

Chalmers Publication Library

P
oS(11th EVN Symposium)065
Jet precession in the active nucleus of M81.
Ongoing VLBI monitoring.
I. Marti-Vidal∗
Onsala Space Observatory (Chalmers Univ. of Technology, Sweden)
E-mail: ivan.marti-vidal@chalmers.se
J.M. Marcaide
Dpt. Astronomia i Astrofísica (Univ. Valencia, Spain)
E-mail: j.m.marcaide@uv.es
A. Alberdi
Instituto de Astrofísica de Andalucía, CSIC (Granada, Spain)
E-mail: antxon@iaa.es
A. Brunthaler
Max-Planck-Institut für Radioastronomie (Bonn, Germany)
E-mail: brunthal@mpifr-bonn.mpg.de
In a recent publication, we reported results of a multi-frequency VLBI campaign of observa-
tions of the Active Galactic Nucleus (AGN) in galaxy M81, phase-referenced to the supernova
SN 1993J. We were able to extract precise information on the relative astrometry of the AGN ra-
dio emission at different epochs and frequencies. We found strong evidence of precession in the
AGN jet (i.e., a systematic evolution in the jet inclination at each frequency) coupled to changes
in the overall flux density at the different frequencies. In these proceedings, we summarise the
main contents of our previous publication and we report on (preliminary) new results from our
follow-up VLBI observations, now phase-referenced to the young supernova SN2008iz. We also
briefly discuss how these results match the picture of our previously-reported precession model.
11th European VLBI Network Symposium & Users Meeting,
October 9-12, 2012
Bordeaux, France
∗Speaker.
c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/
P
oS(11th EVN Symposium)065
Monitoring the jet precession in the AGN of M81 I. Marti-Vidal
1. Introduction
The galaxy M81 (NGC3031), located at a distance of 3.63±0.34Mpc (Freedman et al. [9]),
hosts the next closest active galactic nucleus (AGN) after Centaurus A. Its high declination (J2000.0
coordinates α = 09h55m33.173s and δ = 69◦03′55.062′′) also makes the AGN in M81 (hereafter
M81*) a perfect target for global VLBI observations using the most sensitive radio telescopes in
the Northern Hemisphere.
The explosion of the radio-luminous supernova SN1993J in M81, which happened around
28 March 1993 (e.g., Weiler et al. [20]), triggered an intense campaign of VLBI observations in
which M81* was selected as the phase calibrator of the SN1993J visibilities (e.g., Marcaide et al.
[14],[15]; Bartel et al. [1]; Bietenholz et al. [3]; Marcaide et al. [16]; Marti-Vidal et al. [17],[18]).
These observations allowed us to study, as a byproduct, the structure and evolution of M81* at
different frequencies (Marti-Vidal et al. [19]). Since the central position of the radio-emitting
shell of a supernova does not depend on the observing frequency, this is the ideal study case for
multi-frequency astrometry of an AGN jet.
Results for some of the M81* observations at 8.4GHz between years 1993 and 1997 are re-
ported in Bietenholz et al. [2], and results for the astrometry analysis of a subset of observations at
all frequencies between years 1993 and 2002 were reported in Bietenholz et al. [4]. These authors
concluded that the proper motion of M81*, relative to the shell center of SN 1993J, was consis-
tent with zero. The contributions of galactic rotation and/or any peculiar motion of the SN1993J
progenitor were also discarded to contribute to a detectable proper motion of M81* relative to the
supernova. These authors also reported a frequency-dependent shift of the peak of brightness of
M81*.
In Marti-Vidal et al. [19], we reported the analysis of the combined dataset of all the VLBI
observations of SN1993J at frequencies from 1.4 to 8.4GHz, phase-referenced to M81*, that were
publicly available at the time of the analysis. The time sampling of this combined dataset1 is
more dense and extended than those of the results previously reported in Bietenholz et al. [2] and
Bietenholz et al. [4] (see Fig. 2). Hence, any systematic evolution in the source position should be
more clearly revealed in our analysis, as it is, indeed, the case (see Sect. 2).
2. Summary of Previous Results
In Marti-Vidal et al. [19], we confirmed the main results reported in Bietenholz et al. [2] and
Bietenholz et al. [4]; on the one hand, the position of the intensity peak in the AGN jet (namely, its
core) depends on the observing frequency, and is shifted along the jet towards larger distances from
the jet base (i.e., the AGN central engine) as we decrease the observing frequency. On the other
hand, the core region is slightly resolved at all frequencies, and its size increases with decreasing
frequency.
The chromatic shift in the position of the peak intensity of an AGN jet (also known as the
core-shift effect) was serendipitously discovered in the pair of quasars 1038+528 A-B (Marcaide
[13]) and has been extensively studied in many sources during the last years (e.g., Kovalev et al.
1Observations from a project lead by N. Bartel at Univ. York in Toronto (Canada) and observations from a project
lead by J.M. Marcaide at Univ. Valencia (Spain)
2
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Monitoring the jet precession in the AGN of M81 I. Marti-Vidal
[11]). This apparent shift in the source position is caused by Synchrotron Self-absorption (SSA)
of the jet plasma, due to the strong magnetic fields and the large particle densities (and energy
gradients) close to the jet base (Blandford & Königl [5]).
An increasing source size with decreasing frequency, coupled to the core-shift effect described
in the previous paragraph, maps into a direct measurement of the opening angle in the jet of M81*,
which of course has to be corrected for geometric projection effects (e.g., Sect. 5.4 in Marti-Vidal
et al. [19]).
Besides the confirmation of the main results reported by other authors, the larger amount of
data available allowed us to to better constrain proper motions and to estimate core-shifts with a
higher precision. We were able to constrain the proper motion of the AGN core (relative to the
frame of SN1993J, located in the same galaxy), with a precision of a few µas per year (depending
on frequency). We were also able to determine the normalised core-shift, Ω (see Lobanov [12]),
from a combined fit of all the available shifts for the different frequency pairs, with an uncertainty
of only 11%. With this precise estimate of the core shift, we could derive the coordinates of the
jet base of M81* with precisions ranging from 20µas (at 8.4 GHz) to 100µas (at 1.7 GHz). Our
results were indeed compatible with the estimates given in Bietenholz et al. [4], who used a com-
pletely different approach to derive the coordinates of the jet base. Last but not least, we could
also estimate the mass of the central black hole, by assuming a strongly-magnetised BH scenario
(Kardashev [10]). The resulting black-hole mass (2× 107 M⊙) is in agreement with other inde-
pendent mass estimates (coming from spectroscopic observations of the central rotating star disk
and from the stellar-velocity dispersion in the bulge) and makes M81* an interesting intermediate
object between SgrA* and the more powerful AGN.
2.1 The precessing jet in M 81*
There are strong signatures of a systematic change in the orientation of the jet, both from
the phase-referencing to SN 1993J (Fig. 1, left) and from the evolving inclination of the core, as
estimated from model-fitting to the visibilities2 (Fig. 1, right).
This change in the orientation of the jet seems to be coupled to long-term changes in the
VLBI flux density at the different frequencies. This is probably due to jet bending effects or to
inhomogeneities in the jet path as it changes direction. In Fig. 3, we show the large flare (4 years
long) reported in Marti-Vidal [19], which happens roughly when the jet increases its position angle
from North to East.
3. New Observations
The serendipitous discovery of supernova SN2008iz in M82 (Brunthaler et al. [6]), targeted
an intense VLBI monitoring at different frequencies, to characterise its structure evolution and
expansion (e.g., Brunthaler et al. [7],[8]).
2The position angle of the core major axis is almost independent of the underlying model used in the fit (disc,
Gaussian, sphere, etc.), as long as the source is only marginally resolved. In such a case, we are only able to sample
the first lobe of the visibility function, which has the shape of a paraboloid with minor axis aligned to the major axis of
the source intensity distribution. Given that any symmetric model of a small source has a main lobe with a paraboloidal
shape, the position angle of the minor axis in visibility space (i.e., the position angle of the major axis of the source)
shall be independent of the specific model used to estimate it.
3
P
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Monitoring the jet precession in the AGN of M81 I. Marti-Vidal
−0.4−0.20.00.20.40.60.8
Relative RA (mas)
−0.4
−0.3
−0.2
−0.1
0.0
0.1
0.2
0.3
R
e
la
ti
v
e
 D
e
c 
(m
a
s)
1.7 GHz
2.3 GHz
5.0 GHz
8.4 GHz
1994 1996 1998 2000 2002 2004 2006
Epoch (year)
1996 1998 2000 2002 2004 2006
Epoch (year)
55
60
65
70
75
Co
re
 p
os
iti
on
 a
ng
le
 (d
eg
)
1.7 GHz
5.0 GHz
1.7 GHz model
5.0 GHz model
Figure 1: Left, position of the peak intensity of M81* for each frequency (different symbols) and epoch
(different colors). The core-shift effect can be appreciated from the gathering of the different symbols
around different coordinates, whereas the jet precession can be appreciated from the gathering of symbols
with similar colors. Right, position angle of the core at 5GHz and 1.7GHz as a function of time. A simple
sinusoidal precession model is over-plotted.
Given that M81* was again selected as phase calibrator, we can use these new observations
to continue our study of the evolving jet in M81*. We show in Fig. 2 the time distribution of all
the epochs used so far in our analysis. These are those previously reported in Bietenholz et al. [4]
(red), Marti-Vidal et al. [19] (red and blue), and the new observations (green), phase-referenced to
SN 2008iz and first reported in these proceedings, that will be discussed in deeper detail elsewhere
(Marti-Vidal et al. in preparation).
1995 2000 2005 2010
Epoch (year)
2
4
6
8
10
Fr
eq
ue
nc
y 
(G
Hz
)
Figure 2: Time sampling of our VLBI monitoring. Red points are epochs also reported in Bietenholz et al.
[4]; blue points are epochs from the project led by J.M. Marcaide; green points are the new observations,
phase-referenced to SN2008iz.
4. New Preliminary Results
We have generated hybrid images of M81* for all the new epochs of observation of SN 2008iz
(i.e., those corresponding to the green points shown in Fig. 2), and have performed model-fitting
to the visibilities in a subset of epochs. The model structure and the fitting algorithm used are
described in Marti-Vidal et al. [19] (see their Sect. 3.1).
4
P
oS(11th EVN Symposium)065
Monitoring the jet precession in the AGN of M81 I. Marti-Vidal
We show in Fig. 3 the preliminary results (overall VLBI flux density of the core at bottom;
core inclination at top) for these new epochs, together with the results reported in Marti-Vidal et al.
[19]. Since we are still working on the relative astrometry between M81* and SN 2008iz, it is not
possible to show information on the core-shift, for the new epochs, in these proceedings. In any
case, it can be seen that the position angle at 5GHz for the new epochs matches well the prediction
by the simple model reported in Marti-Vidal et al. [19]. The difference between the position angles
around year 2001 and year 2011 is obvious.
For clarity reasons, we have over-plotted two dashed lines marking the epochs of years 1997.5
and 2001. As reported in Marti-Vidal et al. [19], the long flare can be related to the increase of
position angle of the core (i.e., when the core turns from North into East direction). In the case of
a periodic precession, we would expect to see a second long flare roughly between years 2005 and
2008. Unfortunately, there is lack of observations just in this time range. We notice, though, that the
core flux densities at 1.7GHz seem to reach a minimum short after year 2010, which corresponds
to the time when the jet reaches its minimum position angle. The interpretation of these results
in the frame of the precessing jet, together with the report of a more complete set of observations
(now being reduced) will be discussed in a future publication.
55
60
65
70
75
Co
re
 p
os
iti
on
 a
ng
le
 (d
eg
)
1995 2000 2005 2010
Epoch (year)
0.05
0.10
0.15
0.20
Co
re
 fl
ux
 d
en
si
ty
 (J
y) 1.7 GHz
2.3 GHz
5.0 GHz
8.4 GHz
Figure 3: Top, position angle of the M81* core vs. time. Bottom, lightcurve of the M81* core, obtained
from the hybrid imaging of all our available VLBI epochs.
5. Summary
We report new multi-frequency VLBI observations of M81*, phase-referenced to the super-
nova SN2008iz in M82, which will allow us to extend the analysis reported in Marti-Vidal et al.
[19]. We also report preliminary results on the flux-density evolution of the jet core at each fre-
quency (8.4, 5.0, and 5.0GHz) and the position angle of the slightly-resolved elongated core. On
the one hand, the new core position angles are in agreement with the extrapolation of the simple
model reported in Marti-Vidal et al. [19]. On the other hand, we lack observations in the time
5
P
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Monitoring the jet precession in the AGN of M81 I. Marti-Vidal
range when a second long flare would have been observed. Continued observations will allow us
to finally confirm and improve, or reject, the periodicity of the precession model for the AGN jet
in galaxy M81.
Acknowledgements
This research has been partially supported by projects AYA2009-13036-C02-01 and AYA2009-
13036-C02-02 of the MICINN and by grant PROMETEO 104/2009 of the Generalitat Valenciana.
A. B. was supported by a Marie Curie Outgoing International Fellowship (FP7) of the European
Union (project number 275596).
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Jet precession in the active nucleus of M81. Ongoing VLBI monitoring

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Jet precession in the active nucleus of M81. Ongoing VLBI monitoring

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