564 research outputs found
Transport into the south polar vortex in early spring
Estimates of the mean circulation and diffusive transport of ozone and other species into the Antarctic polar vortex during the spring of 1987 are made using data from the Airborne Antarctic Ozone Experiment. Measurements of long-lived tracers of tropospheric origin remained relatively constant at the levels of the maximum rate of decline of ozone during September. At lower levels in the stratosphere some evidence exists to support intrusions of tropospheric or low latitude air. Given the distribution in latitude and height of these tracers measured from the ER-2 aircraft, it can be inferred that the Lagrangian or diabatic mean circulation was zero or downward over Antarctica during the period of the ozone decline. The observation of a decline in ozone therefore requires a photochemical sink for ozone. The magnitude of the required photochemical sink must be sufficient to offset the transport of ozone into the polar region and produce the observed decline. Quasi-isentropic mixing and downward motion are coupled and are difficult to estimate from a single tracer. The full suite of measured tracers and auxiliary information are brought together to provide an estimate of the rate at which air is cycled through the polar vortex during spring. Estimates of large scale transport of potential vorticity and ozone from previous years are generally consistent with the data from the airborne experiment in suggesting a relatively slow rate of mass flow through the polar vortex in the lower stratosphere during September
The Long Term Optical Variability of the BL Lac object S5 0716+714: Evidence for a Precessing Jet
We present the historic light curve of the BL Lac object S5 0716+714,
spanning the time interval from 1953 to 2003, built using Asiago archive plates
and our recent CCD observations, together with literature data. The source
shows an evident long term variability, over which well known short term
variations are superposed. In particular, in the period from 1961 to 1983 the
mean brightness of S5 0716+714 remained significantly fainter than that
observed after 1994. Assuming a constant variation rate of the mean magnitude
we can estimate a value of about 0.11 magnitude/year. The simultaneous
occurrence of decreasing ejection velocities of superluminal moving components
in the jet reported by Bach et al. (2005) suggests that both phenomena are
related to the change of the direction of the jet to the line of sight from
about 5 to 0.7 degrees for an approximately constant bulk Lorentz factor of
about 12. A simple explanation is that of a precessing relativistic jet, which
should presently be close to the smallest orientation angle. One can therefore
expect in the next ten years a decrease of the mean brightness of about 1
magnitude.Comment: to appear on The Astronomical Journal, 17 pages, 7 figures. Fig.2 is
given as a separated jpg fil
Optical and Radio monitoring of S5 1803+74
The optical (BVRI) and radio (8.4 GHz) light curves of S5 1803+784 on a time
span of nearly 6 years are presented and discussed. The optical light curve
showed an overall variation greater than 3 mag, and the largest changes occured
in three strong flares. No periodicity was found in the light curve on time
scales up to a year. The variability in the radio band is very different, and
shows moderate oscillations around an average constant flux density rather than
relevant flares, with a maximum amplitude of 30%, without a simultaneous
correspondence between optical and radio luminosity. The optical spectral
energy distribution was always well fitted by a power law. The spectral index
shows small variations and there is indication of a positive correlation with
the source luminosity. Possible explanations of the source behaviour are
discussed in the framework of current models.Comment: 25 pages, 12 figure
3C 294 revisited: Deep Large Binocular Telescope AO NIR images and optical spectroscopy
Context. High redshift radio galaxies are among the most massive galaxies at
their redshift, are often found at the center of protoclusters of galaxies, and
are expected to evolve into the present day massive central cluster galaxies.
Thus they are a useful tool to explore structure formation in the young
Universe. Aims. 3C~294 is a powerful FR II type radio galaxy at z = 1.786. Past
studies have identified a clumpy structure, possibly indicative of a merging
system, as well as tentative evidence that 3C~294 hosts a dual active galactic
nucleus (AGN). Due to its proximity to a bright star, it has been subject to
various adaptive optics imaging studies. Method. In order to distinguish
between the various scenarios for 3C~294 we performed deep, high-resolution
adaptive optics near-infrared imaging and optical spectroscopy of 3C~294 with
the Large Binocular Telescope. Results. We resolve the 3C~294 system into three
distinct components separated by a few tenths of an arcsecond on our images.
One is compact, the other two are extended, and all appear to be non-stellar.
The nature of each component is unclear. The two extended components could be a
galaxy with an internal absorption feature, a galaxy merger, or two galaxies at
different redshifts. We can now uniquely associate the radio source of 3C~294
with one of the extended components. Based on our spectroscopy, we determined a
redshift of z = 1.784+-0.001, which is similar to the one previously cited. In
addition we found a previously unreported emission line at 6749.4 \AA\
in our spectra. It is not clear that it originates from 3C~294. It could be the
Ne [IV] doublet lambda 2424/2426 AA at z = 1.783, or belong to the compact
component at a redshift of z ~ 4.56. We thus cannot unambiguously determine
whether 3C~294 hosts a dual AGN or a projected pair of AGNs.Comment: 9 pages, 4 figures, accepted for publication in A&
Optical and radio variability of the BL Lac object AO 0235+16: a possible 5-6 year periodicity
New optical and radio data on the BL Lacertae object AO 0235+16 have been
collected in the last four years by a wide international collaboration, which
confirm the intense activity of this source. The optical data also include the
results of the Whole Earth Blazar Telescope (WEBT) first-light campaign
organized in November 1997. The optical spectrum is observed to basically
steepen when the source gets fainter. We have investigated the existence of
typical variability time scales and of possible correlations between the
optical and radio emissions by means of visual inspection, Discrete Correlation
Function analysis, and Discrete Fourier Transform technique. The major radio
outbursts are found to repeat quasi-regularly with a periodicity of about 5.7
years; this period is also in agreement with the occurrence of some of the
major optical outbursts, but not all of them.Comment: to be published in A&
The Properties of the Radio-Selected 1Jy Sample of BL Lacertae Objects
We present new optical and near-IR spectroscopy as well as new high dynamic
range, arcsecond-resolution VLA radio maps of BL Lacs from the complete
radio-selected "1 Jansky" (1Jy) sample (RBLs) for which such data were not
previously available. Unlike BL Lacs from the complete X-ray-selected Einstein
Medium Sensitivity Survey (EMSS) sample (XBLs), most RBLs possess weak but
moderately luminous emission lines. And whereas nearly all XBLs have extended
power levels consistent with FR-1s, more than half of the RBLs have extended
radio power levels too luminous to be beamed FR-1 radio galaxies. In fact, we
find evidence for and examples of three distinct mechanisms for creating the BL
Lac phenomenon in the 1Jy sample: beamed FR-1s, beamed FR-2s and possibly a few
gravitationally-lensed quasars. The v/v_max determined for the 1Jy sample is
0.614+/-0.047, which is markedly different from the negative evolution seen in
the EMSS and other XBL samples. A correlation between logarithmic X-ray to
radio flux ratio and v/v_max is observed across the EMSS and 1Jy samples, from
negative evolution in the more extreme XBLs to positive evolution in the more
extreme RBLs. There is evidence that the selection criteria chosen by Stickel
et al. eliminates some BL Lac objects from the 1Jy sample, although how many is
unknown. And several objects currently in the sample have exhibited strong
emission lines in one or more epochs, suggesting they should be reclassified as
FSRQs. However these selection effects cannot account for the observed
discrepancy in XBL and RBL properties. From these observational properties we
conclude that RBLs and XBLs cannot be related by viewing angle alone, and that
RBLs are more closely related to FSRQs.Comment: 29 pages, 47 figures, accepted A
On the Location of the Gamma-ray Emission in the 2008 Outburst in the BL Lacertae Object AO 0235+164 through Observations across the Electromagnetic Spectrum
We present observations of a major outburst at centimeter, millimeter,
optical, X-ray, and gamma-ray wavelengths of the BL Lacertae object AO
0235+164. We analyze the timing of multi-waveband variations in the flux and
linear polarization, as well as changes in Very Long Baseline Array (VLBA)
images at 7mm with 0.15 milliarcsecond resolution. The association of the
events at different wavebands is confirmed at high statistical significance by
probability arguments and Monte-Carlo simulations. A series of sharp peaks in
optical linear polarization, as well as a pronounced maximum in the 7 mm
polarization of a superluminal jet knot, indicate rapid fluctuations in the
degree of ordering of the magnetic field. These results lead us to conclude
that the outburst occurred in the jet both in the quasi-stationary "core" and
in the superluminal knot, both parsecs downstream of the supermassive black
hole. We interpret the outburst as a consequence of the propagation of a
disturbance, elongated along the line of sight by light-travel time delays,
that passes through a standing recollimation shock in the core and propagates
down the jet to create the superluminal knot. The multi-wavelength light curves
vary together on long time-scales (months/years), but the correspondence is
poorer on shorter time-scales. This, as well as the variability of the
polarization and the dual location of the outburst, agrees with the
expectations of a multi-zone emission model in which turbulence plays a major
role in modulating the synchrotron and inverse Compton fluxes.Comment: Accepted for Publication in the Astrophysical Journal Letters. 7
pages (including 5 figures). Minor corrections with regard to previous
version, as proposed by the refere
The Spectral Energy Distribution of Fermi bright blazars
(Abridged) We have conducted a detailed investigation of the broad-band
spectral properties of the \gamma-ray selected blazars of the Fermi LAT Bright
AGN Sample (LBAS). By combining our accurately estimated Fermi gamma-ray
spectra with Swift, radio, infra-red, optical and other hard X-ray/gamma-ray
data, collected within three months of the LBAS data taking period, we were
able to assemble high-quality and quasi-simultaneous Spectral Energy
Distributions (SED) for 48 LBAS blazars.The SED of these gamma-ray sources is
similar to that of blazars discovered at other wavelengths, clearly showing, in
the usual Log - Log F representation, the typical broad-band
spectral signatures normally attributed to a combination of low-energy
synchrotron radiation followed by inverse Compton emission of one or more
components. We have used these SEDs to characterize the peak intensity of both
the low and the high-energy components. The results have been used to derive
empirical relationships that estimate the position of the two peaks from the
broad-band colors (i.e. the radio to optical and optical to X-ray spectral
slopes) and from the gamma-ray spectral index. Our data show that the
synchrotron peak frequency is positioned between 10 and
10 Hz in broad-lined FSRQs and between and Hz in
featureless BL Lacertae objects.We find that the gamma-ray spectral slope is
strongly correlated with the synchrotron peak energy and with the X-ray
spectral index, as expected at first order in synchrotron - inverse Compton
scenarios. However, simple homogeneous, one-zone, Synchrotron Self Compton
(SSC) models cannot explain most of our SEDs, especially in the case of FSRQs
and low energy peaked (LBL) BL Lacs. (...)Comment: 85 pages, 38 figures, submitted to Ap
Multiwavelength observations of 3C 454.3. III. Eighteen months of AGILE monitoring of the "Crazy Diamond"
We report on 18 months of multiwavelength observations of the blazar 3C 454.3
(Crazy Diamond) carried out in July 2007-January 2009. We show the results of
the AGILE campaigns which took place on May-June 2008, July-August 2008, and
October 2008-January 2009. During the May 2008-January 2009 period, the source
average flux was highly variable, from an average gamma-ray flux F(E>100MeV) >
200E-8 ph/cm2/s in May-June 2008, to F(E>100MeV)~80E-8 ph/cm2/s in October
2008-January 2009. The average gamma-ray spectrum between 100 MeV and 1 GeV can
be fit by a simple power law (Gamma_GRID ~ 2.0 to 2.2). Only 3-sigma upper
limits can be derived in the 20-60 keV energy band with Super-AGILE. During
July-August 2007 and May-June 2008, RXTE measured a flux of F(3-20 keV)=
8.4E-11 erg/cm2/s, and F(3-20 keV)=4.5E-11 erg/cm2/s, respectively and a
constant photon index Gamma_PCA=1.65. Swift/XRT observations were carried out
during all AGILE campaigns, obtaining a F(2-10 keV)=(0.9-7.5)E-11 erg/cm2/s and
a photon index Gamma_XRT=1.33-2.04. BAT measured an average flux of ~5 mCrab.
GASP-WEBT monitored 3C 454.3 during the whole 2007-2008 period from the radio
to the optical. A correlation analysis between the optical and the gamma-ray
fluxes shows a time lag of tau=-0.4 days. An analysis of 15 GHz and 43 GHz VLBI
core radio flux observations shows an increasing trend of the core radio flux,
anti- correlated with the higher frequency data. The modeling SEDs, and the
behavior of the long-term light curves in different energy bands, allow us to
compare the jet properties during different emission states, and to study the
geometrical properties of the jet on a time-span longer than one year.Comment: Accepted for publication in ApJ. Adapted Abstract. 17 pages, 19
Figures, 5 Table
Multi-Epoch Multiwavelength Spectra and Models for Blazar 3C~279
Of the blazars detected by EGRET in GeV gamma rays, 3C 279 is not only the
best-observed by EGRET, but also one of the best-monitored at lower
frequencies. We have assembled eleven spectra, from GHz radio through GeV gamma
rays, from the time intervals of EGRET observations. Although some of the data
have appeared in previous publications, most are new, including data taken
during the high states in early 1999 and early 2000. All of the spectra show
substantial gamma-ray contribution to the total luminosity of the object; in a
high state, the gamma-ray luminosity dominates over that at all other
frequencies by a factor of more than 10. There is no clear pattern of time
correlation; different bands do not always rise and fall together, even in the
optical, X-ray, and gamma-ray bands.
The spectra are modeled using a leptonic jet, with combined synchrotron
self-Compton + external Compton gamma-ray production. Spectral variability of
3C 279 is consistent with variations of the bulk Lorentz factor of the jet,
accompanied by changes in the spectral shape of the electron distribution. Our
modeling results are consistent with the UV spectrum of 3C 279 being dominated
by accretion disk radiation during times of low gamma-ray intensity.Comment: 39 pages including 13 figures; data tables not included (see ApJ web
version or contact author
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