562 research outputs found
A VLBA survey of the core shift effect in AGN jets I. Evidence for dominating synchrotron opacity
The effect of a frequency dependent shift of the VLBI core position (known as
the "core shift") was predicted more than three decades ago and has since been
observed in a few sources, but often within a narrow frequency range. This
effect has important astrophysical and astrometric applications. To achieve a
broader understanding of the core shift effect and the physics behind it, we
conducted a dedicated survey with NRAO's Very Long Baseline Array (VLBA). We
used the VLBA to image 20 pre-selected sources simultaneously at nine
frequencies in the 1.4-15.4 GHz range. The core position at each frequency was
measured by referencing it to a bright, optically thin feature in the jet. A
significant core shift has been successfully measured in each of the twenty
sources observed. The median value of the core shift is found to be 1.21 mas if
measured between 1.4 and 15.4 GHz, and 0.24 mas between 5.0 and 15.4 GHz. The
core position, r, as a function of frequency, n, is found to be consistent with
an r n^-1 law. This behavior is predicted by the Blandford & Koenigl model of a
purely synchrotron self-absorbed conical jet in equipartition. No systematic
deviation from unity of the power law index in the r(n) relation has been
convincingly detected. We conclude that neither free-free absorption nor
gradients in pressure and/or density in the jet itself and in the ambient
medium surrounding the jet play a significant role in the sources observed
within the 1.4-15.4 GHz frequency range. These results support the
interpretation of the parsec-scale core as a continuous Blandford-Koenigl type
jet with smooth gradients of physical properties along it.Comment: 31 pages, 6 figures, 5 tables; accepted to Astronomy & Astrophysic
A search for periodicity in the light curves of selected blazars
We present an analysis of multifrequency light curves of the sources 2223-052
(3C 446), 2230+114 (CTA 102), and 2251+158 (3C 454.3), which had shown evidence
of quasi-periodic activity. The analysis made use of data from the University
of Michican Radio Astronomy Observatory (USA) at 4.8, 8, and 14.5 GHz, as well
as the Metsahovi Radio Astronomy Observatory (Finland) at 22 and 37 GHz.
Application of two different methods (the discrete autocorrelation function and
the method of Jurkevich) both revealed evidence for periodicity in the flux
variations of these sources at essentially all frequencies. The periods derived
for at least two of the sources -- 2223-052 and 2251+158-- are in good
agreement with the time interval between the appearance of successive VLBI
components. The derived periods for 2251+158 (P = 12.4 yr and 2223-052 (P = 5.8
yr) coincide with the periods found earlier by other authors based on optical
light curves.Comment: 27 pages, 11 figures, accepted for publication in Astronomy Report
A Region of Violent Star Formation in the Irr Galaxy IC 10: Structure and Kinematics of Ionized and Neutral Gas
We have used observations of the galaxy IC 10 at the 6-m telescope of the
Special Astrophysical Observatory with the SCORPIO focal reducer in the
Fabry-Perot interferometer mode and with the MPFS spectrograph to study the
structure and kinematics of ionized gas in the central region of current
intense star formation. Archive VLA 21-cm observations are used to analyze the
structure and kinematics of neutral gas in this region. High-velocity wings of
the H-alpha and [SII] emission lines were revealed in the inner cavity of the
nebula HL 111 and in other parts of the complex of violent star formation. We
have discovered local expanding neutral-gas shells around the nebulae HL 111
and HL 106.Comment: 22 pages, 10 figures; accepted in Astronomy Report
Gas Emission Spectrum in the Irr Galaxy IC 10
Spectroscopic long-slit observations of the dwarf Irr galaxy IC 10 were
conducted at the 6-m Special Astrophysical Observatory telescope with the
SCORPIO focal reducer. The ionized-gas emission spectra in the regions of
intense current star formation were obtained for a large number of regions in
IC 10. The relative abundances of oxygen, N+, and S+ in about twenty HII
regions and in the synchrotron superbubble were estimated. We found that the
galaxy-averaged oxygen abundance is 12 + log(O/H) = 8.17 +- 0.35 and the
metallicity is Z = 0.18 +- 0.14 Z_sun. Our abundances estimated from the strong
emission lines are found to be more reliable than those obtained by comparing
diagnostic diagrams with photoionization models.Comment: Abridged; accepted in Astronomy Letter
Multiwavelength Observations of the Gamma-Ray Blazar PKS 0528+134 in Quiescence
We present multiwavelength observations of the ultraluminous blazar-type
radio loud quasar PKS 0528+134 in quiescence during the period July to December
2009. Significant flux variability on a time scale of several hours was found
in the optical regime, accompanied by a weak trend of spectral softening with
increasing flux. We suggest that this might be the signature of a contribution
from the accretion disk at the blue end of the optical spectrum. The optical
flux is weakly polarized with rapid variations of the degree and direction of
polarization, while the polarization of the 43 GHz radio core remains steady.
Optical spectropolarimetry suggests a trend of increasing degree of
polarization with increasing wavelength, providing additional evidence for an
accretion disc contribution towards the blue end of the optical spectrum. We
constructed four SEDs indicating that even in the quiescent state, the
bolometric luminosity of PKS 0528+134 is dominated by its gamma-ray emission. A
leptonic single-zone jet model produced acceptable fits to the SEDs with
contributions to the high-energy emission from synchrotron self-Compton
radiation and Comptonization of direct accretion disk emission. Fit parameters
close to equipartition were obtained. The moderate variability on long time
scales implies the existence of on-going particle acceleration, while the
observed optical polarization variability seems to point towards a turbulent
acceleration process. Turbulent particle acceleration at stationary features
along the jet therefore appears to be a viable possibility for the quiescent
state of PKS 0528+134.Comment: Accepted for Publication in The Astrophysical Journal. -
Acknowledgement adde
INSAT-2A and 2B development mechanisms
The Indian National Satellite (INSAT) 2A and 2B have deployment mechanisms for deploying the solar array, two C/S band antenna reflectors and a coilable lattice boom with sail. The mechanisms have worked flawlessly on both satellites. The configuration details, precautions taken during the design phase, the test philosophy, and some of the critical analysis activities are discussed
Another look at the BL Lacertae flux and spectral variability
The GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope
(WEBT) monitored BL Lacertae in 2008-2009 at radio, near-IR, and optical
frequencies. During this period, high-energy observations were performed by
XMM-Newton, Swift, and Fermi. We analyse these data with particular attention
to the calibration of Swift UV data, and apply a helical jet model to interpret
the source broad-band variability. The GASP-WEBT observations show an optical
flare in 2008 February-March, and oscillations of several tenths of mag on a
few-day time scale afterwards. The radio flux is only mildly variable. The UV
data from both XMM-Newton and Swift seem to confirm a UV excess that is likely
caused by thermal emission from the accretion disc. The X-ray data from
XMM-Newton indicate a strongly concave spectrum, as well as moderate flux
variability on an hour time scale. The Swift X-ray data reveal fast (interday)
flux changes, not correlated with those observed at lower energies. We compare
the spectral energy distribution (SED) corresponding to the 2008 low-brightness
state, which was characterised by a synchrotron dominance, to the 1997 outburst
state, where the inverse-Compton emission was prevailing. A fit with an
inhomogeneous helical jet model suggests that two synchrotron components are at
work with their self inverse-Compton emission. Most likely, they represent the
radiation from two distinct emitting regions in the jet. We show that the
difference between the source SEDs in 2008 and 1997 can be explained in terms
of pure geometrical variations. The outburst state occurred when the
jet-emitting regions were better aligned with the line of sight, producing an
increase of the Doppler beaming factor. Our analysis demonstrates that the jet
geometry can play an extremely important role in the BL Lacertae flux and
spectral variability.Comment: 12 pages, 10 figures, accepted for publication in A&
A Multi-wavelength View of the TeV Blazar Markarian 421: Correlated Variability, Flaring, and Spectral Evolution
We report results from a multi-wavelength monitoring campaign on Mrk 421 over
the period of 2003-2004. The source was observed simultaneously at TeV and
X-ray energies, with supporting observations frequently carried out at optical
and radio wavelengths. The large amount of simultaneous data has allowed us to
examine the variability of Mrk 421 in detail. The variabilities are generally
correlated between the X-ray and gamma-ray bands, although the correlation
appears to be fairly loose. The light curves show the presence of flares with
varying amplitudes on a wide range of timescales both at X-ray and TeV
energies. Of particular interest is the presence of TeV flares that have no
coincident counterparts at longer wavelengths, because the phenomenon seems
difficult to understand in the context of the proposed emission models for TeV
blazars. We have also found that the TeV flux reached its peak days before the
X-ray flux during a giant flare in 2004. Such a difference in the development
of the flare presents a further challenge to the emission models. Mrk 421
varied much less at optical and radio wavelengths. Surprisingly, the normalized
variability amplitude in optical seems to be comparable to that in radio,
perhaps suggesting the presence of different populations of emitting electrons
in the jet. The spectral energy distribution (SED) of Mrk 421 is seen to vary
with flux, with the two characteristic peaks moving toward higher energies at
higher fluxes. We have failed to fit the measured SEDs with a one-zone SSC
model; introducing additional zones greatly improves the fits. We have derived
constraints on the physical properties of the X-ray/gamma-ray flaring regions
from the observed variability (and SED) of the source. The implications of the
results are discussed. (Abridged)Comment: 32 pages, 12 figures, to appear in Ap
76 T dwarfs from the UKIDSS LAS : benchmarks, kinematics and an updated space density
We report the discovery of 76 new T dwarfs from the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS). Near-infrared broad- and narrow-band photometry and spectroscopy are presented for the new objects, along with Wide-field Infrared Survey Explorer (WISE) and warm-Spitzer photometry. Proper motions for 128 UKIDSS T dwarfs are presented from a new two epoch LAS proper motion catalogue. We use these motions to identify two new benchmark systems: LHS 6176AB, a T8p+M4 pair and HD 118865AB, a T5.5+F8 pair. Using age constraints from the primaries and evolutionary models to constrain the radii, we have estimated their physical properties from their bolometric luminosity. We compare the colours and properties of known benchmark T dwarfs to the latest model atmospheres and draw two principal conclusions. First, it appears that the H - [4.5] and J - W2 colours are more sensitive to metallicity than has previously been recognized, such that differences in metallicity may dominate over differences in T-eff when considering relative properties of cool objects using these colours. Secondly, the previously noted apparent dominance of young objects in the late-T dwarf sample is no longer apparent when using the new model grids and the expanded sample of late-T dwarfs and benchmarks. This is supported by the apparently similar distribution of late-T dwarfs and earlier type T dwarfs on reduced proper motion diagrams that we present. Finally, we present updated space densities for the late-T dwarfs, and compare our values to simulation predictions and those from WISE.Peer reviewe
The brightest gamma-ray flaring blazar in the sky: AGILE and multi-wavelength observations of 3C 454.3 during November 2010
Since 2005, the blazar 3C 454.3 has shown remarkable flaring activity at all
frequencies, and during the last four years it has exhibited more than one
gamma-ray flare per year, becoming the most active gamma-ray blazar in the sky.
We present for the first time the multi-wavelength AGILE, SWIFT, INTEGRAL, and
GASP-WEBT data collected in order to explain the extraordinary gamma-ray flare
of 3C 454.3 which occurred in November 2010. On 2010 November 20 (MJD 55520),
3C 454.3 reached a peak flux (E>100 MeV) of F_gamma(p) = (6.8+-1.0)E-5 ph/cm2/s
on a time scale of about 12 hours, more than a factor of 6 higher than the flux
of the brightest steady gamma-ray source, the Vela pulsar, and more than a
factor of 3 brighter than its previous super-flare on 2009 December 2-3. The
multi-wavelength data make a thorough study of the present event possible: the
comparison with the previous outbursts indicates a close similarity to the one
that occurred in 2009. By comparing the broadband emission before, during, and
after the gamma-ray flare, we find that the radio, optical and X-ray emission
varies within a factor 2-3, whereas the gamma-ray flux by a factor of 10. This
remarkable behavior is modeled by an external Compton component driven by a
substantial local enhancement of soft seed photons.Comment: Accepted for publication in ApJ Letters. 18 Pages, 4 Figures, 1 Tabl
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