Issue |
A&A
Volume 515, June 2010
|
|
---|---|---|
Article Number | A38 | |
Number of page(s) | 9 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/200913915 | |
Published online | 08 June 2010 |
VLBI detection of the HST-1 feature in the M 87 jet at 2 cm
C. S. Chang1 - E. Ros1,2 - Y. Y. Kovalev3,1 - M. L. Lister4
1 - Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121
Bonn, Germany
2 - Departament d'Astronomia i Astrofísica, Universitat de València,
46100 Burjassot, Spain
3 - Astro Space Centre of Lebedev Physical Institute, Profsoyuznaya
84/32, 117997 Moscow, Russia
4 - Department of Physics, Purdue University, 525 Northwestern Avenue,
West Lafayette, IN 47907, USA
Received 18 December 2009 / Accepted 12 February 2010
Abstract
Context. A bright feature 80 pc away from
the core in the powerful jet of M 87 shows highly unusual
properties. Earlier radio, optical, and X-ray observations have shown
that this feature, labeled HST-1, is superluminal, and is possibly
connected with the TeV flare detected by HESS in 2005. It has been
claimed that this feature might have a blazar nature because of these
properties.
Aims. To examine whether HST-1 has a blazar-like
nature, we analyzed 2 cm
VLBA archival data from dedicated full-track observations and the
2 cm survey/MOJAVE VLBI monitoring programs performed between
2000 and 2009.
Methods. We studied the subparsec scale structure of
M 87 jet by using wide-field imaging techniques, after
checking different weighting of the interferometric visibilities as a
function of distance. The HST-1 region was imaged at milliarcsecond
resolution.
Results. We present the first 2 cm VLBI
detection of HST-1 in observations performed between early 2003 and
early 2007, and analyze its evolution with time. Using the detections
of HST-1, we find that the projected apparent speed is .
A comparison of the VLA and VLBA flux densities of this feature
indicates that it is mostly resolved on milliarcsecond scales. This
feature is optically thin (
for S
)
between
2 cm
and
20 cm.
Conclusions. We do not find evidence that HST-1 has
a blazar nature.
Key words: radio continuum: galaxies - techniques: high angular resolution - techniques: interferometric - galaxies: active - galaxies: jets
1 Introduction
Active galactic nuclei (AGN) are among the most energetic phenomena in the Universe, which have been comprehensively studied since their initial discovery by Seyfert (1943). Although there is evidence that a supermassive black hole (SMBH) is the engine launching the powerful jet (Urry & Padovani 1995), the precise mechanism remains unknown.
One of the most widely studied AGN is M 87 (also
known as Virgo A), a nearby elliptical galaxy located in the
Virgo cluster. It hosts a very powerful one-sided jet emerging from the
central region, and was the first extragalactic jet to be discovered (Curtis 1918). The synchrotron
emission nature of the M 87 jet was first suggested by Baade (1956). Observations imply
that M 87 contains
within a 0.25
radius, which is indicative of a SMBH in its nucleus (Harms et al. 1994).
Because of its proximity (16.4 Mpc, z=0.00436,
1 mas = 0.08 pc,
1 mas yr-1 = 0.26 c,
Jordán et al. 2005),
M 87 is an ideal candidate for studying AGN phenomena, and has
been monitored at many different wavelengths over several decades. In
the observed one-sided jet of M 87 (Shklovsky
1964), superluminal motion was reported from Hubble
Space Telescope (HST) observations within
the innermost 6
of the jet with apparent speeds of 4 c to
6 c (Biretta
et al. 1999). Discrepant speeds were reported from
VLBA observations at 7 mm of values ranging between
0.25 c and 0.4 c
(Ly et al. 2007), and a
value of 2 c or even larger (Acciari et al. 2009).
VLBA
2 cm
observations detected apparent speeds of <0.05 c
from 1994 to 2007 (Kovalev
et al. 2007; Lister et al. 2009b).
Therefore, the kinematical properties of the jet in M 87
remain an important topic of discussion.
Table 1:
Journal of VLBA 2 cm
observations of M 87a.
In 1999, HST observations revealed a
bright knot in the jet located 1
(projected distance of 0.08 kpc) away from the core. This
feature, named HST-1, is active in the radio, optical, and X-ray
regimes. VLBA
20
cm observations showed that HST-1 has substructure and appears to
contain superluminal components moving at speeds up to 4 c
(Cheung et al. 2007).
These observations suggest that HST-1 is a collimated shock in the AGN
jet. Furthermore, multiwavelength observations were used to show that
HST-1 could be related to the origin of the TeV emission observed in
M 87 in 2005 by the HESS telescope (Aharonian
et al. 2006). Comparing data taken in the near
ultraviolet by Madrid (2009),
soft X-rays (Chandra), and VLA
2 cm
observations (Harris
et al. 2006; Cheung et al. 2007), the
light curves of HST-1 reached a maximum in 2005, while the resolved
core showed no correlation with the TeV flare. Therefore, the TeV
emission from M 87 was proposed to originate in HST-1 (Harris et al. 2008).
Based on those findings, Harris
et al. (2008) suggest that HST-1 has a blazar
nature. However, Acciari
et al. (2009) reported rapid TeV flares from
M 87 in February 2008, which probably originated in the core,
instead of HST-1, which remained in a low state during the flares in
2008. The Fermi Large Area Telescope (LAT) team
reported the detection of M 87 at gamma-ray energies (Abdo et al. 2009).
The AGN standard model considers the blazar behavior to
originate in the vicinity of the SMBH. However, HST-1 is 80 pc
away from the core. If the HST-1 blazar hypothesis were true, this
would pose a challenge to current AGN models. In this paper, we examine
this hypothesis using 2 cm
VLBI wide-field imaging of the HST-1 feature. We describe our VLBI
observations and the corresponding data reduction in Sect. 2, the results
are presented in Sect. 3,
and the discussion is presented in Sect. 4. Finally,
a short summary is presented in Sect. 5. Throughout
this paper, we use the term ``core'' as the apparent origin of AGN jets
that commonly appear as the brightest features in VLBI radio images of
blazars (Lobanov
1998; Marscher
2008). We adopt a cosmology with
,
and
(Komatsu et al. 2009).
2 VLBI observations and data analysis
M 87 has been monitored at 2 cm with the VLBA
since 1994 by the 2 cm Survey/MOJAVE programs
(Kellermann et al. 2004;
Lister
et al. 2009a). We reanalyzed 12 epochs of these
monitoring program data obtained after late 2001, and three observing
sets of targeted observations on M 87 in 2000 (see
Table 1).
The 2 cm Survey/MOJAVE epochs from 2001 to 2009 were snapshot
observations with a total integration time from 20 minutes to one hour,
and were broken up into about six-minute-long scans to ensure wider (u,v)
coverage; the three epochs in 2000 were full-track observations with
eight-hour integration times and included a single VLA antenna (Y1).
The VLBA data were processed by following the standard procedures in the AIPS cookbook, as described in detail by Kovalev et al. (2007) and Lister et al. (2009a). The data were fringe-fitted before the imaging process. The data from MOJAVE project epochs after 2007.61 were processed using the pulse calibration signals to align the phases instead of fringe fitting, because the positions of the sources and the VLBA antennas are well determined (Petrov et al. 2009,2008), and the station clocks are known to be stable. However, for our wide-field imaging purposes, fringe fitting improved the accuracy of the measured phase and rate across the observed band, and therefore the quality of the imaging of the extended jet of M 87. It also ensures the homogeneity of all the datasets.
To accurately image HST-1 with VLBI, we need to consider two
issues. First, HST-1 lies 800
2 cm
beamwidth away from the brightest feature (the VLBA core). For this
reason, time or frequency data averaging would cause time and bandwidth
smearing in the HST-1 region. Second, based on earlier VLBA
20 cm
and VLA
2 cm
observations of HST-1 (Cheung
et al. 2007), we expect the total 2 cm flux
density to be at the milliJansky level, which is much weaker than the
total flux density of the inner jet (
2.5 Jy). To detect HST-1, we need to
image the inner jet region and its extended structure (over tens of
milliarcseconds), otherwise, the sidelobes from the core will cover the
HST-1 emission. To reach this goal, we applied natural weighting and (u,v)-tapering
to the whole dataset.
First, we averaged the data 16-sec in time and averaged all
channels in each IF in frequency, and obtained the inner jet initial
model by applying phase and amplitude self-calibration using DIFMAP.
The time-smearing effect of 16 seconds becomes significant at a
distance of 200 mas from the field center, which is beyond the
inner jet scale, and does not affect our data analysis at this stage.
When the inner jet model was reasonably accurate, we applied the
obtained CLEAN-components to the un-averaged data for the first
amplitude-and-phase self-calibration using AIPS. Natural weighting and
tapering with a Gaussian factor of 0.3 at a radius of 200 M
in the (u,v)-plane was applied to
the whole dataset, and the resultant beam sizes are shown in
Table 1.
If we remove tapering and apply uniform weighting, the inner jet is
more clearly resolved but a lower flux density is recovered. Therefore,
the image noise level would be too high to detect the HST-1 feature.
To identify the extended component more clearly, and since
full-resolution, uniform-weighting imaging did not improve the
detection of this feature, we chose the approach of tapering and
natural weighting. Furthermore, we downgraded the resolution to a
larger beam size of
mas in PA 0
.
This beam size was chosen specifically to allow comparison with the
VLBA
20 cm
observations of Cheung
et al. (2007). In the following sections, we label
the smaller beams beam A, noting that the beam sizes differ
slightly between epochs. These are listed in Table 1.
Beam B refers to the beam size of
mas.
3 Results
We completed the wide-field imaging of 15 epochs of M 87 VLBA 2 cm
obtained between 2000 and 2009. Each epoch was imaged using beam A and
beam B (see Sect. 2),
and for each beam size, we applied two image cleaning fields, one for
the M 87 inner jet region, and another for the HST-1 region,
which was phase-shifted by -788.5 mas in right ascension and
348.9 mas in declination. We performed deep cleaning
iterations until the image rms reached the expected thermal noise
value. The rms values of the final images are listed in Table 1. Three
epochs in the year 2000 are full-track observations with an on-source
time of 476 minutes, and have the lowest rms.
![]() |
Figure 1:
Images of the central 220 mas of the M 87 jet (beam
A, resulting from a (u,v)-taper
of Gaussian 0.3 at 200 M |
Open with DEXTER |
![]() |
Figure 2:
Downgraded resolution images of the central 220 mas of the
M 87 jet with beam B (
|
Open with DEXTER |
![]() |
Figure 3:
Total VLBA flux density of M87 at |
Open with DEXTER |
3.1 Imaging of the inner jet of M 87
With tapering and natural weighting of the data, the extended inner jet
was imaged over a region 100-200 mas in size, which varied according to
the different sensitivity of each epoch. The inner jet images are shown
in Fig. 1
(beam A) and Fig. 2
(beam B). The inner jet structure has an average total flux
density of 2.5 Jy,
exhibiting overall flux density changes up to 1 Jy during 2000
to 2009, as shown in Fig. 3. We
estimated the error in flux densities to be 5%, based on the typical
amplitude calibration accuracy of the VLBA (Kovalev
et al. 2005; Ulvestad
& Wrobel 2009). In February 2008, very-high-energy
multiple flares were detected by the multiwavelength campaign of HESS,
MAGIC, VERITAS, while VLBA
7 mm observations
detected a flare at the VLBA core simultaneously (Acciari et al. 2009).
We also detected the 2008 flare from the core in our VLBA
2 cm
data, and therefore confirmed this result (see Fig. 3).
The structure of the inner jet changed during the period of our observations. However, the study of the inner jet is beyond the scope of this paper (see e.g., Kovalev et al. 2007; Walker et al. 2008; Lister et al. 2009b, for a detailed discussion of the structure and kinematics of the inner jet).
3.2 Imaging of the HST-1 region
We detected HST-1in six of the 15 epochs that we analyzed from 2003 to
early 2007 (detection limit: 5). As discussed in
Sect. 4,
this feature was too faint in most epochs, and the image noise was too
high for its detection. Figure 4 shows
hybrid maps of this region. The HST-1 images produced using beam A show
that the brightest component has a size of
10 mas down to the lowest contours,
while the images produced using beam B reveal an extended structure
50 mas
in size, which is comparable to its size in VLBA
20 cm observations (Cheung et al. 2007). The
feature has a total flux density that varies between 4 and
24 mJy (Fig. 5). The peak
surface brightness varies from 1 to 4 mJy beam-1
(beam A) and 2 to 10 mJy beam-1
(beam B), as shown in Fig. 6. Epochs in
which no detection is achieved are marked as upper limits (inverted
triangles) in Figs. 5
and 6.
In 2003.09, HST-1 is marginally detected with both beam sizes;
the brightness peak is 1 mJy beam
,
and the total flux density is 3.7 mJy. During 2004.61-2005.85,
significant detections of HST-1 are achieved by beam A and beam B for 4
epochs, where the peak component remains dominant, and the total flux
density fluctuation is
30%.
In 2007, the structure of HST-1 becomes more extended and complex,
reaching a total flux density of 10 mJy and developing two
distinct peaks (peak a and peak b in Fig. 4).
For the epochs in which there were no detections, we derived
upper limits to the total VLBI flux density of HST-1, based on the rms
noise values of each epoch. The maximum amount of flux density hidden
beneath the noise would be the rms value times the size of the HST-1
emission region (in units of beam size). Based on our HST-1 detection,
we assume that the size of the HST-1 emission region is about 20 times
the beam size, and we derived accordingly the upper limit for each
epoch. We also derived the upper limit to the brightness temperature in
this region to be K
at
2 cm.
Table 2:
HST-1 20-
2 cm
spectral indexa.
3.3 Spectral properties of HST-1
Figure 7
superimposes two images of HST-1 at 20 cm (Cheung et al. 2007 and
Cheung, priv. comm.) and
2 cm
(beam A) for two adjacent epochs. The images were coaligned using the
peak of the inner jet as a common reference point. This figure
indicates that our observations resolve the HST-1 peak component. We
use those epochs to derive the HST-1 spectral properties by producing a
rendition of the
2 cm
image using the same restoring beam as the
20 cm image, namely,
mas
at a position angle of 0
(beam B; Fig. 4,
right panel). Table 2 lists
the epochs of
20 cm
and
2 cm
observations and the corresponding spectral index
,
where
.
The uncertainties in the spectral indices are formal errors, estimated
using standard error propagation methods. When interpreting these
values, one has to keep in mind that the resolution of
20 cm
and
2 cm
are very different. First, the frequencies differ by one magnitude,
which produces the small formal error bars of the spectral index.
Second, the incomplete (u, v)
coverage on short VLBA baselines might explain the missing flux density
in the HST-1 region at
2 cm
(see Sect. 4.4
for a discussion). The average value of the derived spectral index is
-0.78.
![]() |
Figure 4:
Images of the HST-1 region restored with beam A ( left panel)
and beam B ( right panel). The distance between
epochs is proportional to the relative time interval, and the images
are plotted with the same size scale. The contour levels increase by
successive factors of |
Open with DEXTER |
3.4 HST-1 kinematics
To study the kinematics of HST-1, we fitted the peak of HST-1 and the
M 87 core in the image plane with a Gaussian component using
IMFIT in AIPS. As shown in Fig. 4, it is
difficult to identify moving components in this region because the
detections are weak. It is therefore difficult to derive an accurate
apparent speed for the HST-1 subcomponents. For this reason, we use the
relative position between the peak of HST-1 and the M 87 core
to estimate the apparent speed of HST-1. Among the 6 epochs of
detection, HST-1 was almost resolved in epoch 2007.10, and appeared to
have a double-peak morphology in beam B image (Fig. 4). By
considering that the HST-1 detection in 2007.10 was weak, and the
double-peak structure might be caused by convolution, we excluded this
epoch from the kinematic analysis (see Sect. 4.2
for a discussion). As shown in Fig. 8, we
plotted the peak positions of HST-1 with respect to the M 87
core against time, and we estimated the position errors as the FWHM of
the fitted Gaussian component. By applying a linear regression to the
peak positions, we obtained a value for the projected apparent speed of
the HST-1 feature of mas yr-1,
which corresponds to
.
![]() |
Figure 5:
Total VLBI |
Open with DEXTER |
![]() |
Figure 6:
Brightness peak value at VLBI |
Open with DEXTER |
4 Discussion
4.1 Variability timescale and HST-1 flaring region at parsec-scales
We derived the variability timescale of the 2005 flare from HST-1
assuming that a single flare would produce a logarithmic rise and fall
in the light curve. By fitting the available light curves from VLBI and
VLA measurements before and after the maximum flux density during the
flare, we derived the corresponding variability timescales and the
upper limit to the characteristic size of the region in which the flare
was produced. We define the logarithmic variability timescale as ,
where S is the flux density and t
is the time interval between observations in units of years (Burbidge et al. 1974).
We estimate the upper limit to the characteristic size of the emission
region from light-travel time
,
where z is the redshift, D is
the luminosity distance in Gpc, and
is the Doppler factor (Marscher
et al. 1979). Table 3 shows the
result of the derived characteristic size of the 2005 flare in HST-1.
![]() |
Figure 7:
VLBA images of the HST-1 region in M 87 at |
Open with DEXTER |
Table 3: HST-1 variability timescale and characteristic size during the rise and fall of the 2005 flare.
Harris et al. (2003)
estimated the Doppler factor
of HST-1 to have a value between 2 and 5, based on decay timescales of Chandra
observations. In the same context, Wang
& Zhou (2009) estimated the Doppler factor of HST-1
to be
by fitting the non-simultaneous spectral energy distribution of M87
using a synchrotron spectrum model. Simulations incorporating MHD
models for the M 87 jet have suggested that the recollimation
shock formed close to the HST-1 position had a relatively low Doppler
factor
1-2
(Gracia et al. 2009).
If we were to assume that
,
the derived characteristic sizes of HST-1 emission region during its
flaring time would be
and
mas.
The size scale of structural changes of HST-1 (see Fig. 4) is in the
range 20-50 mas (1 pc = 12.5 mas), which is within
the derived characteristic size scale. However, if
were lower than 3.5,
derived from VLBA
2 cm
would be smaller than 20 mas, creating a causality problem,
since the largest structural change of HST-1 cannot be greater than the
upper limit to the information propagation time. Therefore,
is required because of the causality argument.
If
were lower than 3.5 in the HST-1 region, we could explain the causality
problem using the self-calibration procedure that we used while imaging
HST-1. We applied self-calibration to the inner jet and HST-1 for all
of the epochs with detections in beam A and beam B. Self-calibration is
more effective for stronger objects. In our case, the marginal
detection in epoch 2003.09 might not permit us to recover extended
emission, in contrast to stronger detections between 2004.61 and
2005.85, in which after self-calibration, we were able to recover more
extended emission.
4.2 Speeds of HST-1
We used the peak position and fitted Gaussian errors to estimate the
projected apparent speed of HST-1
(see Fig. 8),
which is sub-luminal. In the kinematic analysis, we excluded epoch
2007.10 to ensure a robust kinematic result. However, to make our
results more solid, we estimated the upper and lower limit to the HST-1
apparent speed by including epoch 2007.10 as a test. The derived
possible range of the apparent speed is
,
which is still consistent with mildly relativistic jet motion.
![]() |
Figure 8:
The linear fit of HST-1 proper motion shown as the red dashed line.
This plot illustrates the position of HST-1 component peaks with
respect to the M 87 core component as a function of time from
2003.09 to 2005.85. The fitted projected apparent speed of HST-1 |
Open with DEXTER |
To compare our kinematic results with previous findings, we show the
positional evolution on the sky of the HST-1 peak at VLBA 2 cm
with both beam A and beam B (see Fig. 4), and
components C1 and C2 at VLBA
20 cm (Cheung et al. 2007, and
priv. comm.). As illustrated, our derived apparent speed range and
structural evolution in time at
2 cm are consistent
with the
20 cm
results. However, our subluminal speed measurements of HST-1 are
inconsistent with the high apparent superluminal motions reported in Cheung et al. (2007)
from VLBA
20 cm
observations and with the HST (Biretta et al. 1999).
Nevertheless, in the VLBA
20 cm observations (Cheung et al. 2007),
lower speeds were derived from some components in HST-1: c2 has
,
and HST-1d has
.
Our results are consistent with these findings.
4.3 Detection limits
HST-1 was not detected in epoch 2006.45. This epoch had only 22 minute
integration time and the sampling rate was 128 Mbit s
.
Therefore, the rms level (0.35 mJy beam-1)
was not low enough to detect HST-1, which was also fading according to
our light curve (Fig. 5).
The total flux density of HST-1 measured by the VLA at 2 cm
reached its maximum value of
123 mJy in 2005 (Harris
et al. 2006). Our VLBA
2 cm results
recovered a HST-1 total flux density of
23 mJy, which is only 19% of the VLA
measurement. We conclude that the innermost region of HST-1 was
resolved by using the long baselines of the VLBA. The low measured flux
density suggests that HST-1 is very extended.
![]() |
Figure 9:
HST-1 sky positions ( left) and radial distance as a
function of time ( right) relative to the
M 87 core. VLBA |
Open with DEXTER |
4.4 HST-1 parsec-scale spectrum
We derived the spectral index of the HST-1 region based on VLBA 2 cm
and
20 cm
data (Table 2).
However, one should be cautious with these values. First, the two
datasets have a frequency difference of a factor of 10, which results
in small formal error bars in the derived spectral index. The spectral
index here could infer a trend in the spectrum of HST-1 and the
physical condition of the region, but not the absolute value. Second,
we found that the incomplete (u,v)
coverage on short VLBA baselines has resulted in missing flux in the
HST-1 region at
2 cm.
Therefore, the spectral index value of HST-1 region is a lower limit,
and the results suggest that HST-1 was optically thin during the flare
in 2005. If we were to assume that HST-1 is a flat-spectrum source,
there would have been
90 mJy
missing flux in this region in our 2 cm VLBA measurements.
4.5 A blazar nature of HST-1
The intensity of HST-1 reached a maximum in different wavebands in
2005, and the light curves from the VLA 2 cm, VLBA
20 cm,
NUV, and X-ray observations all show the same tendency (Madrid 2009;
Harris
et al. 2009). Our light curve of HST-1 at VLBA
2 cm
(Fig. 5)
is consistent with the other observations. Although this trend can be
interpreted as evidence that HST-1 is the source of the HESS-detected
TeV flare in 2005 (Aharonian
et al. 2006), its VLBA radio properties that we
derived do not support this idea, e.g. (i) the very low compactness of
its dominant emission; (ii) its low brightness temperature; (iii) its
sub-luminal motion; and (iv) the possibly low optical depth across the
feature at parsec-scales. Those are in contrast to the typical blazar
core features, which tend to have flat or inverted spectral indices in
cm-wave VLBA images. We observed that HST-1 has radio properties
consistent with those commonly seen in jet components. TeV
observatories are unable to resolve the M 87 core and HST-1
separately. To probe the origin of TeV emission, searching for
correlations between variability for different bands is a powerful
tool. This approach was performed successfully for a sample of blazars
by Kovalev et al. (2009),
and for M 87 by Acciari
et al. (2009). Another means of probing the TeV
origin is to apply physical models to multiwavelength observations.
Applying models has helped to uncover radio-TeV connections. For
example, high energy flares could be generated from parsec-scale radio
jets by inverse Compton scattering of the photons and particles being
emitted from the core, and Stawarz
et al. (2006) used this approach to explain the TeV
flare from M87 in 2005. However, Acciari
et al. (2009) observed the VHE flare of M87 in 2008,
and found that there might be correlations with the radio flare
detected by VLBA at 43 GHz from the core. From our results,
although we cannot fully discount, however, that HST-1 was the source
of the TeV flare in 2005 based on our results alone, we do not agree
that HST-1 has a blazar nature, as suggested by Harris
et al. (2008).
5 Summary
By analyzing our VLBA 2 cm
data from 2000.06 to 2009.10, we have detected HST-1 during
2003.09-2007.10, which covered the multi-band flaring period of HST-1.
The total flux density of HST-1 varied within the range 4-24 mJy for
our detections. By comparing the images of VLBA
2 cm and
20 cm,
we measured a steep spectrum with
in this region. The projected apparent speed of HST-1 derived from the
brightness peak position is
,
which implies a subluminal motion at VLBA
2 cm.
Our results have indicated that HST-1 is extremely extended on
parsec scales, and has a steep spectrum. No compact feature with a
brightness temperature higher than K
is present in the
2 cm
VLBA observations of this region of the M 87 jet, which
implies that HST-1 does not have the properties of a standard blazar
core. Combining our findings, we do not find evidence that HST-1 in the
jet of M 87 has a blazar nature.
We thank A. Moré, G. Cimò, S. Mühle, M. A. Garrett, R. W. Porcas, R. C. Walker, and C. M. Fromm for valuable comments and inspiring discussions. Special thanks are due to C. C. Cheung for providing the VLBA20 cm images (Cheung et al. 2007), and D. E. Harris for providing VLA
2 cm light curve (Harris et al. 2009). We thank the anonymous referee for useful comments and suggestions. This research was supported by the EU Framework 6 Marie Curie Early Stage Training program under contract number MEST/CT/2005/19669 ``ESTRELA''. C.S. Chang is a member of the International Max Planck Research School for Astronomy and Astrophysics. Part of this project was done while Y.Y.K. was working as a research fellow of the Alexander von Humboldt Foundation. Y.Y.K. was partly supported by the Russian Foundation for Basic Research (project 08-02-00545) and the Alexander von Humboldt return fellowship. This research has made use of data from the 2 cm Survey (Kellermann et al. 2004) and MOJAVE programs that is maintained by the MOJAVE team (Lister et al. 2009a). The MOJAVE project is supported under National Science Foundation grant AST-0807860 and NASA Fermi grant NNX08AV67G. Part of this work made use of archived VLBA and VLA data obtained by K. I. Kellermann, J. Biretta, F. Owen, and W. Junor. The Very Long Baseline Array is operated by the USA National Radio Astronomy Observatory, which is a facility of the USA National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has made use of NASA's Astrophysics Data System.
References
- Abdo, A. A., Ackermann, M., Ajello, M., et al. 2009, ApJ, 707, 55 [NASA ADS] [CrossRef] [Google Scholar]
- Acciari, V. A., Aliu, E., Arlen, T., et al. 2009, Science, 325, 444 [NASA ADS] [CrossRef] [PubMed] [Google Scholar]
- Aharonian, F., Akhperjanian, A. G., Bazer-Bachi, A. R., et al. 2006, Science, 314, 1424 [NASA ADS] [CrossRef] [PubMed] [Google Scholar]
- Baade, W. 1956, ApJ, 123, 550 [NASA ADS] [CrossRef] [Google Scholar]
- Biretta, J. A., Sparks, W. B., & Macchetto, F. 1999, ApJ, 520, 621 [NASA ADS] [CrossRef] [Google Scholar]
- Burbidge, G. R., Jones, T. W., & Odell, S. L. 1974, ApJ, 193, 43 [NASA ADS] [CrossRef] [Google Scholar]
- Cheung, C. C., Harris, D. E., & Stawarz, ▯. 2007, ApJ, 663, L65 [NASA ADS] [CrossRef] [Google Scholar]
- Curtis, H. D. 1918, Publications of Lick Observatory, 13, 31 [Google Scholar]
- Gracia, J., Vlahakis, N., Agudo, I., Tsinganos, K., & Bogovalov, S. V. 2009, ApJ, 695, 503 [NASA ADS] [CrossRef] [Google Scholar]
- Harms, R. J., Ford, H. C., Tsvetanov, Z. I., et al. 1994, ApJ, 435, L35 [NASA ADS] [CrossRef] [Google Scholar]
- Harris, D. E., Biretta, J. A., Junor, W., et al. 2003, ApJ, 586, L41 [NASA ADS] [CrossRef] [Google Scholar]
- Harris, D. E., Cheung, C. C., Biretta, J. A., et al. 2006, ApJ, 640, 211 [NASA ADS] [CrossRef] [Google Scholar]
- Harris, D. E., Cheung, C. C., Stawarz, L., et al. 2008, in Extragalactic Jets: Theory and Observation from Radio to Gamma Ray, ed. T. A. Rector & D. S. De Young, ASP Conf. Ser., 386, 80 [Google Scholar]
- Harris, D. E., Cheung, C. C., Stawarz, ▯., Biretta, J. A., & Perlman, E. S. 2009, ApJ, 699, 305 [NASA ADS] [CrossRef] [Google Scholar]
- Jordán, A., Côté, P., Blakeslee, J. P., et al. 2005, ApJ, 634, 1002 [NASA ADS] [CrossRef] [Google Scholar]
- Kellermann, K. I., Lister, M. L., Homan, D. C., et al. 2004, ApJ, 609, 539 [NASA ADS] [CrossRef] [Google Scholar]
- Komatsu, E., Dunkley, J., Nolta, M. R., et al. 2009, ApJS, 180, 330 [NASA ADS] [CrossRef] [Google Scholar]
- Kovalev, Y. Y., Kellermann, K. I., Lister, M. L., et al. 2005, AJ, 130, 2473 [NASA ADS] [CrossRef] [Google Scholar]
- Kovalev, Y. Y., Lister, M. L., Homan, D. C., & Kellermann, K. I. 2007, ApJ, 668, L27 [NASA ADS] [CrossRef] [Google Scholar]
- Kovalev, Y. Y., Aller, H. D., Aller, M. F., et al. 2009, ApJ, 696, L17 [NASA ADS] [CrossRef] [Google Scholar]
- Lister, M. L., Aller, H. D., Aller, M. F., et al. 2009a, AJ, 137, 3718 [NASA ADS] [CrossRef] [Google Scholar]
- Lister, M. L., Cohen, M. H., Homan, D. C., et al. 2009b, AJ, 138, 1874 [NASA ADS] [CrossRef] [Google Scholar]
- Lobanov, A. P. 1998, A&A, 330, 79 [NASA ADS] [Google Scholar]
- Ly, C., Walker, R. C., & Junor, W. 2007, ApJ, 660, 200 [NASA ADS] [CrossRef] [Google Scholar]
- Madrid, J. P. 2009, AJ, 137, 3864 [NASA ADS] [CrossRef] [Google Scholar]
- Marscher, A. P. 2008, in Extragalactic Jets: Theory and Observation from Radio to Gamma Ray, ed. T. A. Rector & D. S. De Young, ASP Conf. Ser., 386, 437 [Google Scholar]
- Marscher, A. P., Marshall, F. E., Mushotzky, R. F., et al. 1979, ApJ, 233, 498 [NASA ADS] [CrossRef] [Google Scholar]
- Petrov, L., Kovalev, Y. Y., Fomalont, E. B., & Gordon, D. 2008, AJ, 136, 580 [NASA ADS] [CrossRef] [Google Scholar]
- Petrov, L., Gordon, D., Gipson, J., et al. 2009, J. Geod., 83, 859 [NASA ADS] [CrossRef] [Google Scholar]
- Seyfert, C. K. 1943, ApJ, 97, 28 [NASA ADS] [CrossRef] [Google Scholar]
- Shklovsky, I. S. 1964, Soviet Astron., 7, 748 [NASA ADS] [Google Scholar]
- Stawarz, ▯., Aharonian, F., Kataoka, J., et al. 2006, MNRAS, 370, 981 [NASA ADS] [CrossRef] [Google Scholar]
- Ulvestad, J. S., & Wrobel, J. M. 2009, VLBA status summary, http://www.vlba.nrao.edu/astro/obstatus/current/obssum.html, NRAO [Google Scholar]
- Urry, C. M., & Padovani, P. 1995, PASP, 107, 803 [NASA ADS] [CrossRef] [Google Scholar]
- Walker, R. C., Ly, C., Junor, W., & Hardee, P. J. 2008, J. Phys. Conf. Ser., 131, 012053 [NASA ADS] [CrossRef] [Google Scholar]
- Wang, C., & Zhou, H. 2009, MNRAS, 395, 301 [NASA ADS] [CrossRef] [Google Scholar]
Footnotes
- ... programs
- http://www.physics.purdue.edu/MOJAVE/
All Tables
Table 1:
Journal of VLBA 2 cm
observations of M 87a.
Table 2:
HST-1 20-
2 cm
spectral indexa.
Table 3: HST-1 variability timescale and characteristic size during the rise and fall of the 2005 flare.
All Figures
![]() |
Figure 1:
Images of the central 220 mas of the M 87 jet (beam
A, resulting from a (u,v)-taper
of Gaussian 0.3 at 200 M |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Downgraded resolution images of the central 220 mas of the
M 87 jet with beam B (
|
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Total VLBA flux density of M87 at |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Images of the HST-1 region restored with beam A ( left panel)
and beam B ( right panel). The distance between
epochs is proportional to the relative time interval, and the images
are plotted with the same size scale. The contour levels increase by
successive factors of |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
Total VLBI |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Brightness peak value at VLBI |
Open with DEXTER | |
In the text |
![]() |
Figure 7:
VLBA images of the HST-1 region in M 87 at |
Open with DEXTER | |
In the text |
![]() |
Figure 8:
The linear fit of HST-1 proper motion shown as the red dashed line.
This plot illustrates the position of HST-1 component peaks with
respect to the M 87 core component as a function of time from
2003.09 to 2005.85. The fitted projected apparent speed of HST-1 |
Open with DEXTER | |
In the text |
![]() |
Figure 9:
HST-1 sky positions ( left) and radial distance as a
function of time ( right) relative to the
M 87 core. VLBA |
Open with DEXTER | |
In the text |
Copyright ESO 2010
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.