242 research outputs found
Optical and Radio Variability of BL Lacertae
We observed the prototype blazar, BL Lacertae, extensively in optical and
radio bands during an active phase in the period 2010--2013 when the source
showed several prominent outbursts. We searched for possible correlations and
time lags between the optical and radio band flux variations using
multifrequency data to learn about the mechanisms producing variability. During
an active phase of BL Lacertae, we searched for possible correlations and time
lags between multifrequency light curves of several optical and radio bands. We
tried to estimate any possible variability timescales and inter-band lags in
these bands. We performed optical observations in B, V, R and I bands from
seven telescopes in Bulgaria, Georgia, Greece and India and obtained radio data
at 36.8, 22.2, 14.5, 8 and 4.8 GHz frequencies from three telescopes in
Ukraine, Finland and USA. Significant cross-correlations between optical and
radio bands are found in our observations with a delay of cm-fluxes with
respect to optical ones of ~250 days. The optical and radio light curves do not
show any significant timescales of variability. BL Lacertae showed many optical
'mini-flares' on short time-scales. Variations on longer term timescales are
mildly chromatic with superposition of many strong optical outbursts. In radio
bands, the amplitude of variability is frequency dependent. Flux variations at
higher radio frequencies lead the lower frequencies by days or weeks.
The optical variations are consistent with being dominated by a geometric
scenario where a region of emitting plasma moves along a helical path in a
relativistic jet. The frequency dependence of the variability amplitude
supports an origin of the observed variations intrinsic to the source.Comment: 10 pages, 9 figures, Accepted for publication in A&
Multiband optical variability of the blazar OJ 287 during its outbursts in 2015 -- 2016
We present recent optical photometric observations of the blazar OJ 287 taken
during September 2015 -- May 2016. Our intense observations of the blazar
started in November 2015 and continued until May 2016 and included detection of
the large optical outburst in December 2016 that was predicted using the binary
black hole model for OJ 287. For our observing campaign, we used a total of 9
ground based optical telescopes of which one is in Japan, one is in India,
three are in Bulgaria, one is in Serbia, one is in Georgia, and two are in the
USA. These observations were carried out in 102 nights with a total of ~ 1000
image frames in BVRI bands, though the majority were in the R band. We detected
a second comparably strong flare in March 2016. In addition, we investigated
multi-band flux variations, colour variations, and spectral changes in the
blazar on diverse timescales as they are useful in understanding the emission
mechanisms. We briefly discuss the possible physical mechanisms most likely
responsible for the observed flux, colour and spectral variability.Comment: 11 pages, 6 figures, 4 tables; Accepted for publication in MNRA
Probing the Inner Jet of the Quasar PKS 1510-089 with Multi-waveband Monitoring during Strong Gamma-ray Activity
We present results from monitoring the multi-waveband flux, linear
polarization, and parsec-scale structure of the quasar PKS 1510-089,
concentrating on eight major gamma-ray flares that occurred during the interval
2009.0-2009.5. The gamma-ray peaks were essentially simultaneous with maxima at
optical wavelengths, although the flux ratio of the two wavebands varied by an
order of magnitude. The optical polarization vector rotated by 720 degrees
during a 5-day period encompassing six of these flares. This culminated in a
very bright, roughly 1 day, optical and gamma-ray flare as a bright knot of
emission passed through the highest-intensity, stationary feature (the "core")
seen in 43 GHz Very Long Baseline Array images. The knot continued to propagate
down the jet at an apparent speed of 22c and emit strongly at gamma-ray
energies as a months-long X-ray/radio outburst intensified. We interpret these
events as the result of the knot following a spiral path through a mainly
toroidal magnetic field pattern in the acceleration and collimation zone of the
jet, after which it passes through a standing shock in the 43 GHz core and then
continues downstream. In this picture, the rapid gamma-ray flares result from
scattering of infrared seed photons from a relatively slow sheath of the jet as
well as from optical synchrotron radiation in the faster spine. The 2006-2009.7
radio and X-ray flux variations are correlated at very high significance; we
conclude that the X-rays are mainly from inverse Compton scattering of infrared
seed photons by 20-40 MeV electrons.Comment: 10 pages of text + 5 figures, to be published in Astrophysical
Journal Letters in 201
Impacts of mixed farms on water quality of Pinhal river sub-basin, Santa Catarina, Brazil.
Brazil is one of the largest producers of food in the world. Agriculture and livestock production are concentrated in certain regions of the country. Livestock has been perceived as a constant threat to the quantity and quality of water resources. The objective of this study was to evaluate the impact of mixed farms on superficial water quality in Pinhal River sub-basin, which is located in Concordia, Santa Catarina State. Eight sampling sites representing different land-uses (LU1: dairy cattle; LU2: without animals; LU3: dairy + pigs + poultry + crops; LU4: pigs + poultry + crops; LU5: dairy + pigs + poultry + crops + human; LU6: dairy + pigs + crops; LU7 and LU8: dairy + pigs) were evaluated. These sampling sites were assessed longitudinally and sampled during the summer, spring, autumn, and winter of 2006 to 2009. LU1 presented the worst water quality with high concentrations of total suspended solids (TSS) and chemical oxygen demand. In this point, cows had access to the river. The highest concentration of nitrate (NO3-N) was found at the estuary of the river. There were higher concentrations of TSS, NO3-N and NO2-N in wet season. At this time, intensive fertilizer application to corn fields is common. Autumn and winter presented the worst water quality with respect to nitrogen concentration. Results showed a strong relationship between the sources of pollution (e.g., cows, pigs and poultries) and water quality. Managing the use of animal manure with optimum chemical fertilizer applications along with riparian fencing may provide important mitigation options for protecting water quality of Pinhal River
The correlated optical and radio variability of BL Lacertae. WEBT data analysis 1994-2005
Since 1997, BL Lacertae has undergone a phase of high optical activity, with
the occurrence of several prominent outbursts. Starting from 1999, the Whole
Earth Blazar Telescope (WEBT) consortium has organized various multifrequency
campaigns on this blazar, collecting tens of thousands of data points. One of
the main issues in the study of this huge dataset has been the search for
correlations between the optical and radio flux variations, and for possible
periodicities in the light curves. The analysis of the data assembled during
the first four campaigns (comprising also archival data to cover the period
1968-2003) revealed a fair optical-radio correlation in 1994-2003, with a delay
of the hard radio events of ~100 days. Moreover, various statistical methods
suggested the existence of a radio periodicity of ~8 years. In 2004 the WEBT
started a new campaign to extend the dataset to the most recent observing
seasons, in order to possibly confirm and better understand the previous
results. In this campaign we have collected and assembled about 11000 new
optical observations from twenty telescopes, plus near-IR and radio data at
various frequencies. Here, we perform a correlation analysis on the long-term
R-band and radio light curves. In general, we confirm the ~100-day delay of the
hard radio events with respect to the optical ones, even if longer (~200-300
days) time lags are also found in particular periods. The radio
quasi-periodicity is confirmed too, but the "period" seems to progressively
lengthen from 7.4 to 9.3 years in the last three cycles. The optical and radio
behaviour in the last forty years suggests a scenario where geometric effects
play a major role. In particular, the alternation of enhanced and suppressed
optical activity (accompanied by hard and soft radio events, respectively) canComment: 6 pages, 4 figure
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 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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