We review results from the POLAMI program, which monitors the polarization properties of 36 blazars at the IRAM 30 m telescope. We found that the variability of the degree of linear polarization is ...faster and of higher amplitude at 1 mm than at 3 mm and that the linear polarization is also more variable than the total flux. The linear polarization angle is highly variable in all sources with excursions > 180°; and for the case of the polarization angle, also the 1 mm variations appear to be faster than those at 3 mm. These results are fully compatible with recent multi-zone turbulent jet models, and they definitively rule out the popular single-zone models for blazars. They also further confirm that the short-wavelength (inner) emitting regions have better ordered magnetic fields than the long-wavelength ones (further downstream). Moreover, the POLAMI program has shown statistical evidence that, for most of the monitored sources, circular polarization emission is displayed the majority of the time. The circular polarization detection rate and the maximum degree of circular polarization found are comparable with previous surveys at much longer wavelengths, thus, opening a new window for circular polarization and jet composition studies in the mm range. The process generating circular polarization must not be strongly wavelength-dependent. The widespread presence of circular polarization in the POLAMI sample is likely due to Faraday conversion of the linearly polarized synchrotron radiation in the helical magnetic field of the jets. The peculiar behavior of circular polarization in 3C 66A, which we consider a hallmark of circular polarization generation by Faraday conversion in helical fields, is discussed.
ABSTRACT We perform a multi-wavelength polarimetric study of the quasar CTA 102 during an extraordinarily bright γ-ray outburst detected by the Fermi Large Area Telescope in 2012 September-October ...when the source reached a flux of F>100 MeV = 5.2 0.4 × 10−6 photons cm−2 s−1. At the same time, the source displayed an unprecedented optical and near-infrared (near-IR) outburst. We study the evolution of the parsec-scale jet with ultra-high angular resolution through a sequence of 80 total and polarized intensity Very Long Baseline Array images at 43 GHz, covering the observing period from 2007 June to 2014 June. We find that the γ-ray outburst is coincident with flares at all the other frequencies and is related to the passage of a new superluminal knot through the radio core. The powerful γ-ray emission is associated with a change in direction of the jet, which became oriented more closely to our line of sight (θ ∼ 1 2) during the ejection of the knot and the γ-ray outburst. During the flare, the optical polarized emission displays intra-day variability and a clear clockwise rotation of electric vector position angles (EVPAs), which we associate with the path followed by the knot as it moves along helical magnetic field lines, although a random walk of the EVPA caused by a turbulent magnetic field cannot be ruled out. We locate the γ-ray outburst a short distance downstream of the radio core, parsecs from the black hole. This suggests that synchrotron self-Compton scattering of NIR to ultraviolet photons is the probable mechanism for the γ-ray production.
We present the discovery of the first molecular hot core associated with an intermediate-mass protostar in the Cep A HW2 region. The hot condensation was detected from single-dish and interferometric ...observations of several high-excitation rotational lines (from 100 to 880 K above the ground state) of SO sub(2) in the ground vibrational state and of HC sub(3)N in the vibrationally excited states u sub(7) = 1 and u sub(7) = 2. The kinetic temperature derived from both molecules is 6160 K. The high angular resolution observations (1."25 x 0."99) of the SO sub(2) J = 28 sub(7,21) 1 29 sub(6,24) line (488 K above the ground state) show that the hot gas is concentrated in a compact condensation with a size of 60."6 (6430 AU), located 0."4 (300 AU) east from the radio jet HW2. The total SO sub(2) column density in the hot condensation is 610 super(18) cm super(-2), with an H sub(2) column density ranging from 610 super(23) to 6 x 10 super(24) cm super(-2). The H sub(2) density and the SO sub(2) fractional abundance must be larger than 10 super(7) cm super(-3) and 2 x 10 super(-7), respectively. The most likely alternatives for the nature of the hot and very dense condensation are discussed. From the large column densities of hot gas, the detection of the HC sub(3)N vibrationally excited lines, and the large SO sub(2) abundance, we favor the interpretation of a hot core heated by an intermediate-mass protostar of 10 super(3) L sub( ). This indicates that the Cep A HW2 region contains a cluster of very young stars.
Abstract
We report on the first results of the POLAMI (Polarimetric Monitoring of AGNs with Millimetre Wavelengths) programme, a simultaneous 3.5 and 1.3 mm full-Stokes-polarization monitoring of a ...sample of 36 of the brightest active galactic nuclei in the northern sky with the IRAM 30 m telescope. Through a systematic statistical study of data taken from 2006 October (from 2009 December for the case of the 1.3 mm observations) to 2014 August, we characterize the variability of the total flux density and linear polarization. We find that all sources in the sample are highly variable in total flux density at both 3.5 and 1.3 mm, as well as in spectral index, which (except in particularly prominent flares) is found to be optically thin between these two wavelengths. The total flux-density variability at 1.3 mm is found, in general, to be faster, and to have larger fractional amplitude and flatter power-spectral-density slopes than at 3.5 mm. The polarization degree is on average larger at 1.3 mm than at 3.5 mm, by a factor of 2.6. The variability of linear polarization degree is faster and has higher fractional amplitude than for total flux density, with the typical time-scales during prominent polarization peaks being significantly faster at 1.3 mm than at 3.5 mm. The polarization angle at both 3.5 and 1.3 mm is highly variable. Most of the sources show one or two excursions of >180° on time-scales from a few weeks to about a year during the course of our observations. The 3.5 and 1.3 mm polarization angle evolution follows each other rather well, although the 1.3 mm data show a clear preference to more prominent variability on the short time-scales, i.e. weeks. The data are compatible with multizone models of conical jets involving smaller emission regions for the shortest-wavelength emitting sites. Such smaller emitting regions should also be more efficient in energising particle populations, as implied by the coherent evolution of the spectral index and the total flux density during flaring activity of strong enough sources. The data also favour the integrated emission at 1.3 mm to have better ordered magnetic fields than the one at 3.5 mm.
Abstract
We describe the POLAMI (Polarimetric Monitoring of AGN at Millimetre Wavelengths) programme for the monitoring of all four Stokes parameters of a sample of bright radio-loud active galactic ...nuclei with the IRAM 30-m telescope at 3.5 and 1.3 mm. The programme started in 2006 October and accumulated, until 2014 August, 2300 observations at 3.5 mm, achieving a median time sampling interval of 22 d for the sample of 37 sources. This first paper explains the source selection, mostly blazars, the observing strategy and data calibration and gives the details of the instrumental polarization corrections. The sensitivity (1σ) reached at 3.5 mm is 0.5 per cent (linear polarization degree), 4$_{.}^{\circ}$7 (polarization angle), and 0.23 per cent (circular polarization), while the corresponding values at 1.3 mm are 1.7 per cent, 9$_{.}^{\circ}$9 and 0.72 per cent, respectively. The data quality is demonstrated by the time sequences of our calibrators Mars and Uranus. For the quasar 3C 286, widely used as a linear polarization calibrator, we give improved estimates of its linear polarization, and show for the first time occasional detections of its weak circular polarization, which suggests a small level of variability of the source at millimeter wavelengths.
We analyse the circular polarization data accumulated in the first 7 years of the Polarimetric Monitoring of Active Galactic Nuclei at Millimetre Wavelengths (POLAMI) project introduced in an ...accompanying paper. In the 3-mm wavelength band, we acquired more than 2600 observations, and all but one of our 37 sample sources were detected, most of them several times. For most sources, the observed distribution of the degree of circular polarization is broader than that of unpolarized calibrators, indicating that weak (≲0.5 per cent) circular polarization is present most of the time. Our detection rate and the maximum degree of polarization found, 2.0 per cent, are comparable to previous surveys, all made at much longer wavelengths. We argue that the process generating circular polarization must not be strongly wavelength dependent, and we propose that the widespread presence of circular polarization in our short wavelength sample dominated by blazars is mostly due to Faraday conversion of the linearly polarized synchrotron radiation in the helical magnetic field of the jet. Circular polarization is variable, most notably on time-scales comparable to or shorter than our median sampling interval of ≲1 month. Longer time-scales of about 1 yr are occasionally detected, but severely limited by the weakness of the signal. At variance with some longer wavelength investigations we find that the sign of circular polarization changes in most sources, while only seven sources, including three already known, have a strong preference for one sign. The degrees of circular and linear polarization do not show any systematic correlation. We do find however one particular event where the two polarization degrees vary in synchronism during a time span of 0.9 yr. This paper also describes a novel method for calibrating the sign of circular polarization observations.
We analyze the multi-frequency behavior of the quasar 3C 454.3 during three prominent gamma -ray outbursts: 2009 Autumn, 2010 Spring, and 2010 Autumn. The data reveal a repeating pattern, including a ...triple flare structure, in the properties of each gamma -ray outburst, which implies similar mechanism(s) and location for all three events. The multi-frequency behavior indicates that the lower frequency events are co-spatial with the gamma -ray outbursts, although the gamma -ray emission varies on the shortest timescales. We determine that the variability from UV to IR wavelengths during an outburst results from a single synchrotron component whose properties do not change significantly over the different outbursts. Despite a general increase in the degree of optical linear polarization during an outburst, the polarization drops significantly at the peak of the gamma -ray event, which suggests that both shocks and turbulent processes are involved. We detect two disturbances (knots) with superluminal apparent speeds in the parsec-scale jet associated with the outbursts in 2009 Autumn and 2010 Autumn. The kinematic properties of the knots can explain the difference in amplitudes of the gamma -ray events, while their millimeter-wave polarization is related to the optical polarization during the outbursts. We interpret the multi-frequency behavior within models involving either a system of standing conical shocks or magnetic reconnection events located in the parsec-scale millimeter-wave core of the jet. We argue that gamma -ray outbursts with variability timescales as short as ~3 hr can occur on parsec scales if flares take place in localized regions such as turbulent cells.
The blazar AO 0235+164 located at redshift $z=0.94$, has displayed interesting and repeating flaring activity in the past, with recent episodes in 2008 and 2015. In 2020, the source brightened again, ...starting a new flaring episode that peaked in 2021. We study the origin and properties of the 2021 flare in relation to previous studies and the historical behavior of the source, in particular the 2008 and 2015 flaring episodes. We analyzed the multiwavelength photo-polarimetric evolution of the source. From Very Long Baseline Array images, we derived the kinematic parameters of new components associated with the 2021 flare. We used this information to constrain a model for the spectral energy distribution of the emission during the flaring period. We propose an analytical geometric model to test whether the observed wobbling of the jet is consistent with precession. We report the appearance of two new components that are ejected in a different direction than previously, confirming the wobbling of the jet. We find that the direction of ejection is consistent with that of a precessing jet. Our derived period independently agrees with the values commonly found in the literature. Modeling of the spectral energy distribution further confirms that the differences between flares can be attributed to geometrical effects.
ABSTRACT
We present the multiwavelength flaring activity of the blazar AO 0235 + 164 during its recent active period from 2013 to 2019. From a discrete correlation function analysis, we find a ...significant ($\gt 95~{{\ \rm per\ cent}}$) correlation between radio and gamma-ray light curves with flares at longer wavelengths following flares at shorter wavelengths. We identify a new jet component in 43 GHz Very Long Baseline Array data that was ejected from the radio core on MJD $57246^{+26}_{-30}$ (2015 August 12), during the peak of the 2015 radio flare. From the analysis of the jet component, we derived a Doppler factor of δvar = 28.5 ± 8.4, a bulk Lorentz factor of $\Gamma =16.8^{+3.6}_{-3.1}$, and an intrinsic viewing angle of $\theta _{\rm v}=1.42^{+1.07}_{-0.52}\textrm {~degrees}$. Investigation of the quasi-simultaneous radio data revealed a partially absorbed spectrum with the turnover frequency varying in the range of 10−70 GHz and the peak flux density varying in the range of 0.7−4 Jy. We find the synchrotron self-absorption magnetic field strength to be $B_{\rm SSA}=15.3^{+12.6}_{-14.0}\textrm {~mG}$ at the peak of the 2015 radio flare, which is comparable to the equipartition magnetic field strength of $B_{\rm EQ}=43.6^{+10.6}_{-10.4}\textrm {~mG}$ calculated for the same epoch. Additional analysis of the radio emission region in the relativistic jet of AO 0235 + 164 suggests that it did not significantly deviate from equipartition during its recent flaring activity.
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