Context. Short millimeter observations of radio-loud active galactic nuclei (AGN) offer an excellent opportunity to study the physics of their synchrotron-emitting relativistic jets from where the ...bulk of radio and millimeter emission is radiated. On one hand, AGN jets and their emission cores are significantly less affected by Faraday rotation and depolarization than at longer wavelengths. On the other hand, the millimeter emission of AGN is dominated by the compact innermost regions in the jets, where the jet cannot be seen at longer wavelengths due to synchrotron opacity. Aims. We present the first simultaneous dual frequency 86 GHz and 229 GHz polarimetric survey of all four Stokes parameters for a large sample of 211 radio-loud active galactic nuclei, designed to be flux limited at 1 Jy at 86 GHz. Methods. Most of the observations were made in mid-August 2010 using the XPOL polarimeter on the IRAM 30 m millimetric radio telescope. Results. Linear polarization detections above a 3σ median level of ~ 1.0% are reported for 183 sources at 86 GHz and for 23 sources at 229 GHz, where the median 3σ level is ~ 6.0%. We show a clear excess of the linear polarization degree that is detected at 229 GHz with regard to that at 86 GHz by a factor of ~ 1.6. This implies a progressively better ordered magnetic field for blazar jet regions that are located progressively upstream in the jet. We show that the linear polarization angle at 86 and 229 GHz and the jet structural position angle for both quasars and BL Lacs do not show a clear preference to align in either parallel or perpendicular directions. Our variability study with regard to the 86 GHz data from our previous survey points out a large degree of variation. In particular, we report total flux and linear polarization changes in time scales of years by median factors of ~ 1.5 in total flux and ~ 1.7 in linear polarization degree (with maximum variations by factors up to 6.3 and ~ 5, respectively). Moreover, 86% of sources show linear polarization angles evenly distributed with regard to our previous measurements.
We report results from a deep polarization imaging of the nearby radio galaxy 3C 84 (NGC 1275). The source was observed with the Global Millimeter VLBI Array (GMVA) at 86 GHz at an ultrahigh angular ...resolution of 50 μas (corresponding to ∼200Rs). We also add complementary multiwavelength data from the Very Long Baseline Array (VLBA; 15 and 43 GHz) and from the Atacama Large Millimeter/submillimeter Array (ALMA; 97.5, 233.0 and 343.5 GHz). At 86 GHz, we measured a fractional linear polarization of ∼2% in the VLBI core region. The polarization morphology suggests that the emission is associated with an underlying limb-brightened jet. The fractional linear polarization is lower at 43 and 15 GHz (∼0.3−0.7% and <0.1%, respectively). This suggests an increasing linear polarization degree toward shorter wavelengths on VLBI scales. We also obtain a large rotation measure (RM) of ∼105–6 rad m2 in the core at ≳43 GHz. Moreover, the VLBA 43 GHz observations show a variable RM in the VLBI core region during a small flare in 2015. Faraday depolarization and Faraday conversion in an inhomogeneous and mildly relativistic plasma could explain the observed linear polarization characteristics and the previously measured frequency dependence of the circular polarization. Our Faraday depolarization modeling suggests that the RM most likely originates from an external screen with a highly uniform RM distribution. To explain the large RM value, the uniform RM distribution and the RM variability, we suggest that the Faraday rotation is caused by a boundary layer in a transversely stratified jet. Based on the RM and the synchrotron spectrum of the core, we provide an estimate for the magnetic field strength and the electron density of the jet plasma.
We use a combination of high-resolution very long baseline interferometry (VLBI) radio and multiwavelength flux density and polarization observations to constrain the physics of the dissipation ...mechanism powering the broadband flares in 3C 279 during an episode of extreme flaring activity in 2013-2014. Six bright flares superimposed on a long-term outburst are detected at γ-ray energies. Four of the flares have optical and radio counterparts. The two modes of flaring activity (faster flares sitting on top of a long-term outburst) present at radio, optical, and γ-ray frequencies are missing in X-rays. X-ray counterparts are only observed for two flares. The first three flares are accompanied by ejection of a new VLBI component (NC2), suggesting the 43 GHz VLBI core as the site of energy dissipation. Another new component, NC3, is ejected after the last three flares, which suggests that the emission is produced upstream from the core (closer to the black hole). The study therefore indicates multiple sites of energy dissipation in the source. An anticorrelation is detected between the optical percentage polarization (PP) and optical/γ-ray flux variations, while the PP has a positive correlation with optical/γ-ray spectral indices. Given that the mean polarization is inversely proportional to the number of cells in the emission region, the PP versus optical/γ-ray anticorrelation could be due to more active cells during the outburst than at other times. In addition to the turbulent component, our analysis suggests the presence of a combined turbulent and ordered magnetic field, with the ordered component transverse to the jet axis.
VLBA polarimetric monitoring of 3C 111 Beuchert, T.; Kadler, M.; Perucho, M. ...
Astronomy and astrophysics (Berlin),
02/2018, Volume:
610
Journal Article
Peer reviewed
Open access
Context. While studies of large samples of jets of active galactic nuclei (AGN) are important in order to establish a global picture, dedicated single-source studies are an invaluable tool for ...probing crucial processes within jets on parsec scales. These processes involve in particular the formation and geometry of the jet magnetic field as well as the flow itself. Aims. We aim to better understand the dynamics within relativistic magneto-hydrodynamical flows in the extreme environment and close vicinity of supermassive black holes. Methods. We analyze the peculiar radio galaxy 3C 111, for which long-term polarimetric observations are available. We make use of the high spatial resolution of the VLBA network and the MOJAVE monitoring program, which provides high data quality also for single sources and allows us to study jet dynamics on parsec scales in full polarization with an evenly sampled time-domain. While electric vectors can probe the underlying magnetic field, other properties of the jet such as the variable (polarized) flux density, feature size, and brightness temperature, can give valuable insights into the flow itself. We complement the VLBA data with data from the IRAM 30-m Telescope as well as the SMA. Results. We observe a complex evolution of the polarized jet. The electric vector position angles (EVPAs) of features traveling down the jet perform a large rotation of ≳180∘ across a distance of about 20 pc. As opposed to this smooth swing, the EVPAs are strongly variable within the first parsecs of the jet. We find an overall tendency towards transverse EVPAs across the jet with a local anomaly of aligned vectors in between. The polarized flux density increases rapidly at that distance and eventually saturates towards the outermost observable regions. The transverse extent of the flow suddenly decreases simultaneously to a jump in brightness temperature around where we observe the EVPAs to turn into alignment with the jet flow. Also the gradient of the feature size and particle density with distance steepens significantly at that region. Conclusions. We interpret the propagating polarized features as shocks and the observed local anomalies as the interaction of these shocks with a localized recollimation shock of the underlying flow. Together with a sheared magnetic field, this shock-shock interaction can explain the large rotation of the EVPA. The superimposed variability of the EVPAs close to the core is likely related to a clumpy Faraday screen, which also contributes significantly to the observed EVPA rotation in that region.
Aims. The prime motivation of this project was to design and build a state-of-art mm-wave heterodyne receiver system to enhance the observing throughput of the IRAM 30-m radiotelescope. More ...specifically, the requirements were i) state-of-art noise performance for spectroscopic observations; ii) simultaneous dual polarization and dual-frequency observing; iii) coverage of the atmospheric transmission windows from 83 to 360 GHz; iv) compact footprint and minimal maintenance. Methods. Key elements for low noise performance of heterodyne mixers are the superconducting Niobium junctions, operating at ≃4 K. These junctions are embedded in carefully designed coupling structures; furthermore, since atmospheric radiation is a significant contributor to the system noise budget, all mixers are either sideband separating or sideband rejecting. To achieve low noise, it is also essential to maximize the coupling of the receiver to the astronomical source, and to minimize the coupling to thermal radiation from the ground-based environment; this is achieved through mirror optics that realize a wavelength-independent coupling to the telescope. A flexible configuration of mirrors and frequency selective surfaces permits various combinations of frequency bands, as well as dual-load radiometric calibration. Low noise intermediate frequency amplifiers and bias electronics also play an important role in the system performance. Results. The EMIR receiver in operation at the 30 m telescope offers four frequency bands: B1: 83−117 GHz, B2: 129−174 GHz, B3: 200−267 GHz, and B4: 260−360 GHz. In each band, the two orthogonal polarizations are observed simultaneously. Dual-band combinations B1/2 B1/3, and B2/4 are available. Bands 1 and 4 (also 3 as of Nov.-2011) feature sideband separation. In dual-band configuration, including sideband separation and polarization diplexing, up to eight IF channels are delivered to the spectrometers, totaling up to 64 GHz of signal bandwidth (of which 32 GHz can be transported and processed by spectrometers, status Nov.-2011). The EMIR receiver has been in continuous operation for more than two years and has allowed, through a qualitative jump in performance, observations not possible before, as shown by a few selected examples of astronomical results.
Context. The best example of a massive star with an ionized outflow launched from its photoevaporating disk is MWC349A. The large amount of reported radio-continuum and radio-recombination line (RRL) ...observations toward this galactic UC-HII region offers a unique possibility to build a model of the ionized envelope of this source. Aims. To understand the physical conditions and kinematics of the ionized region of the circumstellar disk and also of the outflow of MWC349A. Methods. We compared the bulk of radio-continuum maps, RRL profiles, and the H30α centroid map published to date with the predictions of our non-LTE 3D radiative transfer model, MOdel for REcombination LInes (MORELI), which we describe here in detail. Results. Our non-LTE 3D radiative transfer model provides new evidence that the UC-HII region of MWC349A is composed of an ionized circumstellar disk rotating in Keplerian fashion around a star of 38 M⊙, and an ionized outflow expanding with a terminal velocity of 60 km s-1 and rotating in the same sense as the disk. The model shows that while maser amplification is the dominant process involved for Hnα RRL emission with quantum numbers n < 41, stimulated emission is relevant for the emission of RRLs with n > 41 up at least the H76α line. Conclusions. For the first time, we present a model of MWC349A which satisfactorily explains the vast amount of reported observational data for a very wide range of frequencies and angular resolutions.
We report on simultaneous observations of the magnetar SGR J1745−2900 at frequencies ν = 2.54–225 GHz using the Nançay 94-m equivalent, Effelsberg 100-m, and IRAM 30-m radio telescopes. We detect SGR ...J1745−2900 up to 225 GHz, the highest radio frequency detection of pulsed emission from a neutron star to date. Strong single pulses are also observed from 4.85 up to 154 GHz. At the millimetre band we see significant flux density and spectral index variabilities on time scales of tens of minutes, plus variability between days at all frequencies. Additionally, SGR J1745−2900 was observed at a different epoch at frequencies ν = 296–472 GHz using the APEX 12-m radio telescope, with no detections. Over the period MJD 56859.83-56862.93 the fitted spectrum yields a spectral index of 〈α〉 = −0.4 ± 0.1 for a reference flux density 〈S
154〉 = 1.1 ± 0.2 mJy (with S
ν ∝ να), a flat spectrum alike those of the other radio-loud magnetars. These results show that strongly magnetized neutron stars can be effective radio emitters at frequencies notably higher to what was previously known and that pulsar searches in the Galactic Centre are possible in the millimetre band.
Since the launch of the Fermi satellite, BL Lacertae has been moderately active at γ-rays and optical frequencies until 2011 May, when the source started a series of strong flares. The exceptional ...optical sampling achieved by the GLAST-AGILE Support Program of the Whole Earth Blazar Telescope in collaboration with the Steward Observatory allows us to perform a detailed comparison with the daily γ-ray observations by Fermi. Discrete correlation analysis between the optical and γ-ray emission reveals correlation with a time lag of 0 ± 1 d, which suggests cospatiality of the corresponding jet emitting regions. A better definition of the time lag is hindered by the daily gaps in the sampling of the extremely fast flux variations. In general, optical flares present more structure and develop on longer time-scales than corresponding γ-ray flares. Observations at X-rays and at millimetre wavelengths reveal a common trend, which suggests that the region producing the mm and X-ray radiation is located downstream from the optical and γ-ray-emitting zone in the jet. The mean optical degree of polarization slightly decreases over the considered period and in general it is higher when the flux is lower. The optical electric vector polarization angle (EVPA) shows a preferred orientation of about 15°, nearly aligned with the radio core EVPA and mean jet direction. Oscillations around it increase during the 2011-2012 outburst. We investigate the effects of a geometrical interpretation of the long-term flux variability on the polarization. A helical magnetic field model predicts an evolution of the mean polarization that is in reasonable agreement with the observations. These can be fully explained by introducing slight variations in the compression factor in a transverse shock waves model.
Context. CMB experiments aiming at a precise measurement of the CMB polarization, such as the Planck satellite, need a strong polarized absolute calibrator on the sky to accurately set the detectors ...polarization angle and the cross-polarization leakage. As the most intense polarized source in the microwave sky at angular scales of few arcminutes, the Crab nebula will be used for this purpose. Aims. Our goal was to measure the Crab nebula polarization characteristics at 90 GHz with unprecedented precision. Methods. The observations were carried out with the IRAM 30 m telescope employing the correlation polarimeter XPOL and using two orthogonally polarized receivers. Results. We processed the Stokes I, Q, and U maps from our observations in order to compute the polarization angle and linear polarization fraction. The first is almost constant in the region of maximum emission in polarization with a mean value of αSky = 152.1±0.3° in equatorial coordinates, and the second is found to reach a maximum of Π = 30% for the most polarized pixels. We find that a CMB experiment having a 5 arcmin circular beam will see a mean polarization angle of αSky = 149.9±0.2° and a mean polarization fraction of Π = 8.8±0.2%.
We report observations, using the IRAM 30 m telescope, of 30 ultracompact and hypercompact HII regions in the lines of HCO+(3-2) and/or HCO+(1-0) and H$30\alpha $ and/or H$39\alpha $. Images are ...presented in both HCO+(3-2) and H$30\alpha $ toward a subset of regions (16 in HCO+(3-2), 14 in H$30\alpha $) with a resolution of 12”. In addition, H13CO+(3-2) observations are reported toward 13 HII regions where HCO+(3-2) displays complex profiles. It is shown that the absorption dips in the HCO+ profiles are due to HCO+ self-absorption, not absorption of the HII free-free emission or warm dust emission surrounding the HII region or two velocity components along the line of sight. It was found that among the sources with self-absorbed profiles, 8 are contracting and 5 are expanding. Mass fluxes are found to be typically a few times 10-3 $M_\odot$ yr-1, implying time scales for massive star formation <105 yrs. HCO+ and H2 column densities are estimated for a subset of the sources from which masses of the dense central cloud cores were estimated. Implications of the derived column densities, masses, flow velocities, and mass fluxes are discussed.
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