197 research outputs found
Discovery of a wandering radio jet base after a large X-ray flare in the blazar Markarian 421
We investigate the location of the radio jet bases ("radio cores") of blazars
in radio images, and their stationarity by means of dense very long baseline
interferometry (VLBI) observations. In order to measure the position of a radio
core, we conducted 12 epoch astrometric observation of the blazar Markarian 421
with the VLBI Exploration of Radio Astrometry at 22 GHz immediately after a
large X-ray flare, which occurred in the middle of 2011 September. For the
first time, we find that the radio core is not stationary but rather changes
its location toward 0.5 mas downstream. This angular scale corresponds to the
de-projected length of a scale of Schwarzschild radii (Rs) at the
distance of Markarian~421. This radio-core wandering may be a new type of
manifestation associated with the phenomena of large X-ray flares.Comment: 6 pages, 4 figures, 1 table, has been published in ApJ Letter
X-ray and Radio Follow-up Observations of High-Redshift Blazar Candidates in the Fermi-LAT Unassociated Source Population
We report on the results of X-ray and radio follow-up observations of two GeV
gamma-ray sources 2FGL J0923.5+1508 and 2FGL J1502.1+5548, selected as
candidates for high-redshift blazars from unassociated sources in the {\it
Fermi} Large Area Telescope Second Source Catalog. We utilize the Suzaku
satellite and the VLBI Exploration of Radio Astrometry (VERA) telescopes for
X-ray and radio observations, respectively. For 2FGL J0923.5+1508, a possible
radio counterpart NVSS J092357+150518 is found at 1.4 GHz from an existing
catalog, but we do not detect any X-ray emission from it and derive a flux
upper limit 1.37 10 erg cm
s. Radio observations at 6.7 GHz also result in an upper limit of
19 mJy, implying a steep radio spectrum that is not
expected for a blazar. On the other hand, we detect X-rays from NVSS
J150229+555204, the potential 1.4 GHz radio counterpart of 2FGL J1502.1+5548.
The X-ray spectrum can be fitted with an absorbed power-law model with a photon
index =1.8 and the unabsorbed flux is =4.3 10 erg cm s. Moreover,
we detect unresolved radio emission at 6.7 GHz with flux =30.1
mJy, indicating a compact, flat-spectrum radio source. If NVSS J150229+555204
is indeed associated with 2FGL J1502.1+5548, we find that its multiwavelength
spectrum is consistent with a blazar at redshift .Comment: 24 pages, 7 figures, 6 tables, accepted for publication in Ap
Rotating and infalling motion around the high-mass young stellar object Cepheus A-HW2 observed with the methanol maser at 6.7 GHz
We have measured the internal proper motions of the 6.7 GHz methanol masers
associated with Cepheus A (Cep A) HW2 using Very Long Baseline Interferometery
(VLBI) observations. We conducted three epochs of VLBI monitoring observations
of the 6.7 GHz methanol masers in Cep A-HW2 with the Japanese VLBI Network
(JVN) over the period between 2006-2008. In 2006, we were able to use
phase-referencing to measure the absolute coordinates of the maser emission
with an accuracy of a few milliarcseconds. We compared the maser distribution
with other molecular line observations that trace the rotating disk. We
measured the internal proper motions for 29 methanol maser spots, of which 19
were identified at all three epochs and the remaining ten at only two epochs.
The magnitude of proper motions ranged from 0.2 to 7.4 km/s, with an average of
3.1 km/s. Although there are large uncertainties in the observed internal
proper motions of the methanol maser spots in Cep A, they are well fitted by a
disk that includes both rotation and infall velocity components. The derived
rotation and infall velocities at the disk radius of 680 au are 0.5 +- 0.7 and
1.8 +- 0.7 km/s, respectively. Assuming that the modeled disk motion accurately
represents the accretion disk around the Cep A-HW2 high-mass YSO, we estimated
the mass infall rate to be 3 x 10^{-4} n_8 Msun/yr (n_8 is the gas volume
density in units of 10^{8} cm^{-3}). The combination of the estimated mass
infall rate and the magnitude of the fitted infall velocity suggests that Cep
A-HW2 is at an evolutionary phase of active gas accretion from the disk onto
the central high-mass YSO. The infall momentum rate is estimated to be 5 x
10^{-4} n_8 Msun/yr km/s, which is larger than the estimated stellar radiation
pressure of the HW2 object, supporting the hypothesis that this object is in an
active gas accretion phase.Comment: 16 pages, 6 figures, 5 tables, accepted for publication in Astronomy
& Astrophysic
Possible Detection of Apparent Superluminal inward motion in Markarian 421 after the Giant X-ray flare in February, 2010
We report on the VLBI follow-up observations using the Japanese VLBI Network
(JVN) array at 22 GHz for the largest X-ray flare of TeV blazar Mrk 421 that
occurred in mid-February, 2010. The total of five epochs of observations were
performed at intervals of about 20 days between March 7 and May 31, 2010. No
new-born component associated with the flare was seen directly in the total
intensity images obtained by our multi-epoch VLBI observations. However, one
jet component located at ~1 mas north-west from the core was able to be
identified, and its proper motion can be measured as -1.66+/-0.46 mas yr^-1,
which corresponds to an apparent velocity of -3.48+/-0.97 c. Here, this
negative velocity indicates that the jet component was apparently moving toward
the core. As the most plausible explanation, we discuss that the apparent
negative velocity was possibly caused by the ejection of a new component, which
could not be resolved with our observations. In this case, the obtained Doppler
factor of the new component is around 10 to 20, which is consistent with the
ones typically estimated by model fittings of spectral energy distribution for
this source.Comment: 9 pages, 6 figures, 3 tables, accepted for publication in Ap
Radio and gamma-ray follow-up of the exceptionally high activity state of PKS 1510-089 in 2011
We investigate the radio and gamma-ray variability of the flat spectrum radio
quasar PKS 1510-089 in the time range between 2010 November and 2012 January.
In this period the source showed an intense activity, with two major gamma-ray
flares detected in 2011 July and October. During the latter episode both the
gamma-ray and the radio flux density reached their historical peak.
Multiwavelength analysis shows a rotation of about 380 deg of the optical
polarization angle close in time with the rapid and strong gamma-ray flare in
2011 July. An enhancement of the optical emission and an increase of the
fractional polarization both in the optical and in radio bands is observed
about three weeks later, close in time with another gamma-ray outburst. On the
other hand, after 2011 September a huge radio outburst has been detected, first
in the millimeter regime followed with some time delay at centimeter down to
decimeter wavelengths. This radio flare is characterized by a rising and a
decaying stage, in agreement with the formation of a shock and its evolution,
as a consequence of expansion and radiative cooling. If the gamma-ray flare
observed in 2011 October is related to this radio outburst, then this strongly
indicates that the region responsible for the gamma-ray variability is not
within the broad line, but a few parsecs downstream along the jet.Comment: 14 pages, 12 figures, accepted for publication in MNRA
A 22-year Southern Sky Survey for Transient and Variable Radio Sources using the Molonglo Observatory Synthesis Telescope
We describe a 22-year survey for variable and transient radio sources,
performed with archival images taken with the Molonglo Observatory Synthesis
Telescope (MOST). This survey covers 2775 \unit{deg^2} of the sky south of
at an observing frequency of 843 MHz, an angular
resolution of 45 \times 45 \csc | \delta| \unit{arcsec^2} and a sensitivity
of 5 \sigma \geq 14 \unit{mJy beam^{-1}}. We describe a technique to
compensate for image gain error, along with statistical techniques to check and
classify variability in a population of light curves, with applicability to any
image-based radio variability survey. Among radio light curves for almost 30000
sources, we present 53 highly variable sources and 15 transient sources. Only 3
of the transient sources, and none of the variable sources have been previously
identified as transient or variable. Many of our variable sources are suspected
scintillating Active Galactic Nuclei. We have identified three variable sources
and one transient source that are likely to be associated with star forming
galaxies at , but whose implied luminosity is higher than the
most luminous known radio supernova (SN1979C) by an order of magnitude. We also
find a class of variable and transient source with no optical counterparts.Comment: Accepted for publication in MNRAS. 34 pages, 30 figures, 7 table
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