42 research outputs found
A Compact Extreme Scattering Event Cloud Towards AO 0235+164
We present observations of a rare, rapid, high amplitude Extreme Scattering
Event toward the compact BL-Lac AO 0235+164 at 6.65 GHz. The ESE cloud is
compact; we estimate its diameter between 0.09 and 0.9 AU, and is at a distance
of less than 3.6 kpc. Limits on the angular extent of the ESE cloud imply a
minimum cloud electron density of ~ 4 x 10^3 cm^-3. Based on the amplitude and
timescale of the ESE observed here, we suggest that at least one of the
transients reported by Bower et al. (2007) may be attributed to ESEs.Comment: 11 pages, 2 figure
Analysis of an Interplanetary Coronal Mass Ejection by a spacecraft radio signal: A case study
Tracking radio communication signals from planetary spacecraft with ground-based telescopes offers the possibility to study the electron density and the interplanetary scintillation of the solar wind. Observations of the telemetry link of planetary spacecraft have been conducted regularly with ground antennae from the European Very Long Baseline Interferometry Network, aiming to study the propagation of radio signals in the solar wind at different solar elongations and distances from the Sun. We have analyzed the Mars Express spacecraft radio signal phase fluctuations while, based on a 3-D heliosphere plasma simulation, an interplanetary coronal mass ejection (ICME) crossed the radio path during one of our observations on 6 April 2015. Our measurements showed that the phase scintillation indices increased by a factor of 4 during the passage of the ICME. The method presented here confirms that the phase scintillation technique based on spacecraft signals provides information of the properties and propagation of the ICMEs in the heliosphere
Compact Intraday Variable Radio Cores: New Observational Approaches
The evidence for refractive interstellar scintillation (RISS) being the main
cause for rapid intraday variations (Intraday Variability, IDV) in Quasars and
BL Lacs has recently become stronger. If IDV is still a complex composition of
extrinsic and source intrinsic effects, the intrinsic part of the IDV pattern
should show up in the millimeter and sub-millimeter regime due to the frequency
dependence of RISS. Hence, observations at higher frequencies are essential in
order to exclude RISS as the sole cause of IDV. Here we report on our new
attempt to search for rapid variations at much higher frequencies. In addition,
the possibility of a direct detection of the postulated scattering screen in
front of IDV sources will be discussed. Our recent line observations towards a
few IDV sources lead to the first detection of a high latitude molecular cloud
in front of an intraday variable radio core.Comment: 4 pages, 1 figure, Proceedings of the 6th European VLBI Network
Symposium, Bonn 25th-28th June 200
The Size of IDV Jet Cores
Radio variability on timescales from a few hours to several days in
extragalactic flat-spectrum radio sources is generally classified as intra-day
variability (IDV). The origin of this short term variability is still
controversial and both extrinsic and intrinsic mechanisms must be considered
and may both contribute to the observed variations. The measured linear and
circular polarization of IDV sources constrains the low energy end of the
electron population. Any population of cold electrons within sources at or
above the equipartition temperature of 10^11 K depolarizes the emission and can
be ruled out. Intrinsic shock models are shown to either violate the large
fraction of sources displaying IDV or they do not relax the light travel time
argument for intrinsic variations. From structure function analysis, we further
conclude that interstellar scintillation also leads to tight size estimates
unless a very local cloud in the ISM is responsible for IDV.Comment: 8 pages, 3 figures, to appear in: `AGN Variability Across the
Electromagnetic Spectrum', accepted for publication in PAS
Intraday Variability in Northern Hemisphere Radio Sources
We summarize results from flux density monitoring campaigns performed with
the 100 meter radio-telescope at Effelsberg and the VLA during the past 15 yrs.
We briefly discuss some of the statistical properties of the rapid variability
from now more than 40 high declination sources, which show Intraday Variability
(IDV). In general, IDV is more pronounced for sources with flat radio spectra
and compact VLBI structures. The frequency dependence of the variability
pattern varies with source and observing time. For 0917+62, we present new VLBI
images, which suggest that the variability pattern is modified by the
occurrence of new jet components. For 0716+71, we show the first detection of
IDV at millimeter wavelengths (32 GHz). For the physical interpretation of the
IDV phenomenon, a complex source and frequency dependent superposition of
interstellar scintillation and source intrinsic variability should be
considered.Comment: 8 pages, 4 figures, to appear in: `AGN Variability Across the
Electromagnetic Spectrum', a conference held at CSIRO, Sydney, 25-29 June
2001. To be published by PAS
Multi-wavelength view of the close-by GRB~190829A sheds light on gamma-ray burst physics
Gamma-ray bursts are produced as a result of cataclysmic events such as the
collapse of a massive star or the merger of two neutron stars. We monitored the
position of the close-by gamma-ray burst GRB~190829A, which originated from a
massive star collapse, through very long baseline interferometry (VLBI)
observations with the EVN and the VLBA, involving a total of 30 telescopes
across 4 continents. We carried out a total of 9 observations between 9 and 117
days after the gamma-ray burst at 5 and 15 GHz, with a typical resolution of
few milliarcseconds (mas). We obtained limits on the source size and expansion
rate. The limits are in agreement with the size evolution entailed by a
detailed modelling of the multi-wavelength light curves with a forward plus
reverse shock model, which agrees with the observations across almost 18 orders
of magnitude in frequency (including the High Energy Stereoscopic System data
at TeV photon energies) and more than 4 orders of magnitude in time. Thanks to
the broad, high-cadence coverage of the afterglow, afterglow degeneracies are
broken to a large extent, allowing us to capture some unique physical insights:
we find a low prompt emission efficiency ; we constrain the
fraction of electrons that are accelerated to relativistic speeds in the
forward shock to be at the 90\% credible level; we find that the
magnetic field energy density in the reverse shock downstream must decay
rapidly after the shock crossing. While our model assumes an on-axis jet, our
VLBI astrometric measurements alone are not sufficiently tight as to exclude
any off-axis viewing angle. On the other hand, we can firmly exclude the line
of sight to have been more than away from the border of the
region that produced the prompt gamma-ray emission based on compactness
arguments.Comment: 35 pages, 30 figures, submitted to Nature Astronomy (first revision).
The tentative evidence for source size evolution in the previous version was
an artifact - the source is consistently unresolved in the updated analysis.
Changes highlighted in boldfac
A seasonal cycle and an abrupt change in the variability characteristics of the intraday variable source S4 0954+65
The BLLac object S4 0954+65 is one of the main targets of the Urumqi
monitoring program targeting IntraDay Variable (IDV) sources. Between August
2005 and December 2009, the source was included in 41 observing sessions,
carried out at a frequency of 4.8 GHz. The time analysis of the collected light
curves, performed by applying both a structure function analysis and a
specifically developed wavelet-based algorithm, discovered an annual cycle in
the variability timescales, suggesting that there is a fundamental contribution
by interstellar scintillation to the IDV pattern of the source. The combined
use of the two analysis methods also revealed that there was a dramatic change
in the variability characteristics of the source between February and March
2008, at the starting time of a strong outburst phase. The analysis' results
suggest that the flaring state of the source coincides with the appearance of
multiple timescales in its light curves, indicating that changes in the
structure of the relativistically moving emitting region may strongly influence
the variability observed on IDV timescales.Comment: 9 pages, 8 figures and 3 tables. Accepted for publication in
Astronomy and Astrophysic
Optical and radio behaviour of the BL Lacertae object 0716+714
Eight optical and four radio observatories have been intensively monitoring
the BL Lac object 0716+714 in the last years: 4854 data points have been
collected in the UBVRI bands since 1994, while radio light curves extend back
to 1978. Many of these data are presented here for the first time. The
long-term trend shown by the optical light curves seems to vary with a
characteristic time scale of about 3.3 years, while a longer period of 5.5-6
years seems to characterize the radio long-term variations. In general, optical
colour indices are only weakly correlated with brightness. The radio flux
behaviour at different frequencies is similar, but the flux variation amplitude
decreases with increasing wavelength. The radio spectral index varies with
brightness (harder when brighter), but the radio fluxes seem to be the sum of
two different-spectrum contributions: a steady base level and a harder-spectrum
variable component. Once the base level is removed, the radio variations appear
as essentially achromatic, similarly to the optical behaviour. Flux variations
at the higher radio frequencies lead the lower-frequency ones with week-month
time scales. The behaviour of the optical and radio light curves is quite
different, the broad radio outbursts not corresponding in time to the faster
optical ones and the cross-correlation analysis indicating only weak
correlation with long time lags. However, minor radio flux enhancements
simultaneous with the major optical flares can be recognized, which may imply
that the mechanism producing the strong flux increases in the optical band also
marginally affects the radio one.Comment: 18 pages, 15 Postscript figures, 5 JPEG figures, accepted for
publication in A&