483 research outputs found
VLBA monitoring of Mrk 421 at 15 GHz and 24 GHz during 2011
High-resolution radio observations are ideal for constraining the value of
physical parameters in the inner regions of active-galactic-nucleus jets and
complement results on multiwavelength (MWL) observations. This study is part of
a wider multifrequency campaign targeting the nearby TeV blazar Markarian 421
(z=0.031), with observations in the sub-mm (SMA), optical/IR (GASP), UV/X-ray
(Swift, RXTE, MAXI), and gamma rays (Fermi-LAT, MAGIC, VERITAS). We investigate
the jet's morphology and any proper motions, and the time evolution of physical
parameters such as flux densities and spectral index. The aim of our wider
multifrequency campaign is to try to shed light on questions such as the nature
of the radiating particles, the connection between the radio and gamma-ray
emission, the location of the emitting regions and the origin of the flux
variability. We consider data obtained with the Very Long Baseline Array (VLBA)
over twelve epochs (one observation per month from January to December 2011) at
15 GHz and 24 GHz. We investigate the inner jet structure on parsec scales
through the study of model-fit components for each epoch. The structure of Mrk
421 is dominated by a compact (~0.13 mas) and bright component, with a
one-sided jet detected out to ~10 mas. We identify 5-6 components in the jet
that are consistent with being stationary during the 12-month period studied
here. Measurements of the spectral index agree with those of other works: they
are fairly flat in the core region and steepen along the jet length.
Significant flux-density variations are detected for the core component. From
our results, we draw an overall scenario in which we estimate a viewing angle
2{\deg} < theta < 5{\deg} and a different jet velocity for the radio and the
high-energy emission regions, such that the respective Doppler factors are
{\delta}r ~3 and {\delta}h.e. ~14.Comment: 9 pages, 4 figure
Observations of OJ 287 from the Geodetic VLBI Archive of the Washington Correlator
We present 27 geodetic VLBI maps of OJ 287 obtained from the archive of the
Washington correlator. The observations presented here were made between 1990
October and 1996 December. During this period a sequence of six superluminal
components has been identified. We measured the proper motion of these
components to be approximately 0.5 mas/yr, which is about twice as high as that
seen in previous VLBI observations. These results imply a higher component
ejection rate than previously observed, in good agreement with the observed
occurrences of radio outbursts. We have examined a possible connection between
VLBI components and optical flares in the framework of a binary black hole
system.Comment: 9 pages, 5 included figures, emulateapj.sty, accepted to The
Astrophysical Journa
VLBI and space VLBI of TeV gamma-ray sources
We have commenced a program of VLBI observations of extra-galactic TeV sources and candidates. Our main result to date is that, despite the high inferred Doppler factors from higher energy observations, the jet components in Mrk 421 and Mrk 501 display sub-luminal motions
Space VLBI Observations of 3C 279 at 1.6 and 5 GHz
We present the first VLBI Space Observatory Programme (VSOP) observations of
the gamma-ray blazar 3C 279 at 1.6 and 5 GHz. The combination of the VSOP and
VLBA-only images at these two frequencies maps the jet structure on scales from
1 to 100 mas. On small angular scales the structure is dominated by the quasar
core and the bright secondary component `C4' located 3 milliarcseconds from the
core (at this epoch). On larger angular scales the structure is dominated by a
jet extending to the southwest, which at the largest scale seen in these images
connects with the smallest scale structure seen in VLA images. We have
exploited two of the main strengths of VSOP: the ability to obtain
matched-resolution images to ground-based images at higher frequencies and the
ability to measure high brightness temperatures. A spectral index map was made
by combining the VSOP 1.6 GHz image with a matched-resolution VLBA-only image
at 5 GHz from our VSOP observation on the following day. The spectral index map
shows the core to have a highly inverted spectrum, with some areas having a
spectral index approaching the limiting value for synchrotron self-absorbed
radiation of 2.5. Gaussian model fits to the VSOP visibilities revealed high
brightness temperatures (>10^{12} K) that are difficult to measure with
ground-only arrays. An extensive error analysis was performed on the brightness
temperature measurements. Most components did not have measurable brightness
temperature upper limits, but lower limits were measured as high as 5x10^{12}
K. This lower limit is significantly above both the nominal inverse Compton and
equipartition brightness temperature limits. The derived Doppler factor,
Lorentz factor, and angle to the line-of-sight in the case of the equipartition
limit are at the upper end of the range of expected values for EGRET blazars.Comment: 11 pages, 6 figures, emulateapj.sty, To be published in The
Astrophysical Journal, v537, Jul 1, 200
Simultaneous radio-interferometric and high-energy TeV observations of the gamma-ray blazar Mkn 421
The TeV-emitting BL Lac object Mkn 421 was observed with very long baseline
interferometry (VLBI) at three closely-spaced epochs one-month apart in
March-April 1998. The source was also monitored at very-high gamma-ray energies
(TeV measurements) during the same period in an attempt to search for
correlations between TeV variability and the evolution of the radio morphology
on parsec scales. While the VLBI maps show no temporal changes in the Mkn 421
VLBI jet, there is strong evidence of complex variability in both the total and
polarized fluxes of the VLBI core of Mkn 421 and in its spectrum over the
two-month span of our data. The high-energy measurements indicate that the
overall TeV activity of the source was rising during this period, with a
gamma-ray flare detected just three days prior to our second VLBI observing
run. Although no firm correlation can be established, our data suggest that the
two phenomena (TeV activity and VLBI core variability) are connected, with the
VLBI core at 22 GHz being the self-absorbed radio counterpart of synchrotron
self-Compton (SSC) emission at high energies. Based on the size of the VLBI
core, we could derive an upper limit of 0.1 pc (3 x 10**17 cm) for the
projected size of the SSC zone. This determination is the first model-free
estimate of the size of the gamma-ray emitting region in a blazar.Comment: 12 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
The Sub-Luminal Parsec-scale Jet of Mkn 501
We have combined Very Long Baseline Interferometry (VLBI) data from several
programs in order to resolve differences in reported parsec-scale jet speeds
for the TeV gamma-ray source Mkn501. Data from the VLBA 2cm survey, and 8 and
15 GHz data from the Radio Reference Frame Image Database, have been combined
with data from a 5 GHz VSOP Space VLBI observation to determine the apparent
motions of jet components in this source. The combined data set consists of 12
observations between April 1995 and July 1999. Four jet components are detected
at most epochs, all of which are clearly sub-luminal (i.e. with apparent speeds
< c), and two of which appear stationary. The established TeV gamma-ray sources
Mkn 501 and Mkn 421 thus both have sub-luminal parsec-scale jets, in contrast
to the apparently super-luminal jets of the majority of >100 MeV sources
detected by EGRET. No new VLBI component has emerged from the core following
the extended TeV high-state in 1997, again in contrast to the general behavior
of >100 MeV gamma-ray sources.Comment: 7 pages, 2 figures, in press at ApJ
The Fastest Relativistic Jets: VLBA Observations of Blazars with Apparent Speeds Exceeding 25c
We have measured peak apparent speeds of 25.6c+/-7.0c, 25.6c+/-4.4c, and 28.2c+/-6.6c in the jets of 0235+164, 0827+243, and 1406-076, respectively, based on six epochs of high-sensitivity VLBA observations at 22 and 43 GHz during 2002 and 2003 (H0=71 km s-1 Mpc-1, Ωm=0.27, and ΩΛ=0.73). These blazars had been identified as potentially having apparent speeds exceeding 40c in an earlier VLBA survey of EGRET blazars by Jorstad and coworkers. We therefore confirm (with high confidence in 0827+243, and lower confidence in 0235+164 and 1406-076) the presence of highly relativistic pattern speeds in these three jets, although not at the \u3e40c levels reported by Jorstad and coworkers. The lower limit to the bulk Lorentz factor implied by the observed apparent speeds is Γ\u3e~25-30 in these three sources, if the pattern speeds are equal to or slower than the bulk flow speed
Electronic collection system for spacelab mission timeline requirements
This paper describes the Functional Objective Requirements Collection System (FORCS) software tool that has been developed for use by Principal Investigators (PI's) and Payload Element Developers (PED's) on their own personal computers to develop on-orbit timelining requirements for their payloads. The FORCS tool can be used either in a totally stand-alone mode, storing the information in a local file on the user's personal computer hard disk or in a remote mode where the user's computer is linked to a host computer containing the integrated database of the timeline requirements for all of the payloads on a mission. There are a number of features incorporated in the FORCS software to assist the user. The user may move freely back and forth between the various forms for inputting the data. Several methods are used to input the information, depending on the type of the information. These methods range from filling in text boxes, using check boxes and radio buttons, to inputting information into a spreadsheet format. There are automated features provided to assist in developing the proper format for the data, ranging from limit checking on some of the parameters to automatic conversion of different formats of time data inputs to the one standard format used for the timeline scheduling software
Erratic Jet Wobbling in the BL Lacertae Object OJ287 Revealed by Sixteen Years of 7mm VLBA Observations
We present the results from an ultra-high-resolution 7mm Very Long Baseline
Array (VLBA) study of the relativistic jet in the BL Lacertae object OJ287 from
1995 to 2011 containing 136 total intensity images. Analysis of the image
sequence reveals a sharp jet-position-angle swing by >100 deg. during
[2004,2006], as viewed in the plane of the sky, that we interpret as the
crossing of the jet from one side of the line of sight to the other during a
softer and longer term swing of the inner jet. Modulating such long term swing,
our images also show for the first time a prominent erratic wobbling behavior
of the innermost ~0.4mas of the jet with fluctuations in position angle of up
to ~40 deg. over time scales ~2yr. This is accompanied by highly superluminal
motions along non-radial trajectories, which reflect the remarkable
non-ballistic nature of the jet plasma on these scales. The erratic nature and
short time scales of the observed behavior rules out scenarios such as binary
black hole systems, accretion disk precession, and interaction with the ambient
medium as possible origins of the phenomenon on the scales probed by our
observations, although such processes may cause longer-term modulation of the
jet direction. We propose that variable asymmetric injection of the jet flow;
perhaps related to turbulence in the accretion disk; coupled with hydrodynamic
instabilities, leads to the non-ballistic dynamics that cause the observed
non-periodic changes in the direction of the inner jet.Comment: Accepted for Publication in The Astrophysical Journal. 11 pages, 6
figures, 4 tables. High resolution images on figure 1 and complete tables 1
and 2 may be provided on reques
Parsec scale polarization properties of the TeV blazar Markarian 421
In this work we present a polarization analysis at radio frequencies of
Markarian 421, one of the closest (z=0.03) TeV blazars. The observations were
obtained, both in total and in polarized intensity, with the Very Long Baseline
Array (VLBA) at 15, 24, and 43 GHz throughout 2011, with one observation per
month (for a total of twelve epochs). We investigate the magnetic field
topology and the polarization structure on parsec scale and their evolution
with time. We detect polarized emission both in the core and in the jet region,
and it varies with frequency, location and time. In the core region we measure
a mean fractional polarization of about 1-2%, with a peak of about 4% in March
at 43 GHz; the polarization angle is almost stable at 43 GHz, but it shows
significant variability in the range 114-173 deg at 15 GHz. In the jet region
the polarization properties show a more stable behavior; the fractional
polarization is about 16% and the polarization angle is nearly perpendicular to
the jet axis. The higher EVPA variability observed at 15 GHz is due both to a
variable Faraday rotation effect and to opacity. The residual variability
observed in the intrinsic polarization angle, together with the low degree of
polarization in the core region, could be explained with the presence of a
blend of variable cross-polarized subcomponents within the beam.Comment: 6 pages, 4 figures, proceedings of the 12th European VLBI Network
Symposium and Users Meeting (7-10 October 2014, Cagliari, Italy
- …