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Pasetto A.,Max Planck Institute for Radio Astronomy | Kraus A.,Max Planck Institute for Radio Astronomy | Mack K.-H.,National institute for astrophysics | Bruni G.,Max Planck Institute for Radio Astronomy | And 2 more authors.
Astronomy and Astrophysics | Year: 2016

Context. We characterised and studied, in the radio band, a sample of candidates for high rotation measure (RM). These point-like objects show a strong depolarisation at 21 cm. This feature suggests the presence of a very dense medium surrounding them combined with a strong magnetic field. Aims. This work aims at selecting and studying a sample of radio sources with high RM, thus studying their physical conditions and their status with respect to their surrounding medium. We want to understand whether any connection is present between the AGN-hosting galaxy medium with some evolutionary track and/or some restarting phase of the AGN itself. Methods. Multifrequency single-dish observations were performed with the 100-m Effelsberg telescope to define the initial sample, to characterise the spectral energy distribution (SED) of the final sample (30 targets), and to determine their RM in the 11 to 2 cm wavelength range. Results. From the observations, the SED and the polarisation information, i.e. the fractional polarisation and the polarisation angle, have been determined. Three different object types were revealed from the SED analysis: older, GPS-like, and mixed. For each of the targets, the rotation measure was found and the depolarisation modelled. No significant correlation is found between the depolarisation behaviours and the SEDs, while a correlation is found between sources with mixed SED (with an old component at low frequency and compact components at high frequencies) and high values of the rotation measure (with values in the rest frame greater than 1000 rad/m2). Conclusions. This work helps us to define and identify a sample of sources with high RM. From the analysis we can conclude that the sources showing a restarting phase at high frequency (with a mixed SED) are characterised by a really dense and/or a magnetised medium that strongly rotates the polarisation angle at the different frequencies, leading to a high RM. © 2016 ESO. Source

Gonzalez-Martin O.,Institute Radioastronomia y Astrofisica IRyA UNAM | Hernandez-Garcia L.,Institute Astrofisica Of Andalucia | Masegosa J.,Institute Astrofisica Of Andalucia | Marquez I.,Institute Astrofisica Of Andalucia | And 8 more authors.
Astronomy and Astrophysics | Year: 2016

Context. Obscured active galactic nuclei (AGNs) are thought to be very common in the Universe. Observations and surveys have shown that the number of sources increases for near galaxies and at the low-luminosity regime (the so-called LLAGNs). Furthermore, many AGNs show changes in their obscuration properties at X-rays that may suggest a configuration of clouds very close to the accretion disk. However, these variations could also be due to changes in the intrinsic continuum of the source. It is therefore important to study nearby AGN to better understand the locus and distribution of clouds in the neighbourhood of the nucleus. Aims. We aim to study the nuclear obscuration of LLAGN NGC 835 and its extended emission using mid-infrared observations. Methods. We present sub-arcsecond-resolution mid-infrared 11.5 μm imaging of the LLAGN galaxy NGC 835 obtained with the instrument CanariCam in the Gran Telescopio CANARIAS (GTC), archival Spitzer/IRS spectroscopy, and archival Chandra data observed in 2000, 2008, and 2013. Results. The GTC/CanariCam 11.5 μm image reveals faint extended emission out to ~6 arcsec. We obtained a nuclear flux of F(11.5 μm) ~ 18 mJy, whereas the extended emission accounts for 90% of the total flux within the 6 arcsec. This means that the low angular resolution (~4 arcsec) IRS spectrum is dominated by this extended emission and not by the AGN. This is clearly seen in the Spitzer/IRS spectrum, which resembles that of star-forming galaxies. Although the extended soft X-ray emission shows some resemblance with that of the mid-infrared, the knots seen at X-rays are mostly located in the inner side of this mid-infrared emission. The nuclear X-ray spectrum of the source has undergone a spectral change between 2000/2008 and 2013. We argue that this variation is most probably due to changes in the hydrogen column density from ~8 × 1023 cm-2 to ~3 × 1023 cm-2. NGC 835 therefore is one of the few LLAGN, together with NGC 1052, in which changes in the absorber can be claimed. © 2016 ESO. Source

Rodriguez-Kamenetzky A.,Institute Astronomia Teorica y Experimental | Rodriguez-Kamenetzky A.,Institute Radioastronomia y Astrofisica IRyA UNAM | Carrasco-Gonzalez C.,Institute Radioastronomia y Astrofisica IRyA UNAM | Araudo A.,University of Oxford | And 5 more authors.
Astrophysical Journal | Year: 2016

While most protostellar jets present free-free emission at radio wavelengths, synchrotron emission has also been proposed to be present in a handful of these objects. The presence of nonthermal emission has been inferred by negative spectral indices at centimeter wavelengths. In one case (the HH 80-81 jet arising from a massive protostar), its synchrotron nature was confirmed by the detection of linearly polarized radio emission. One of the main consequences of these results is that synchrotron emission implies the presence of relativistic particles among the nonrelativistic material of these jets. Therefore, an acceleration mechanism should be taking place. The most probable scenario is that particles are accelerated when the jets strongly impact against the dense envelope surrounding the protostar. Here we present an analysis of radio observations obtained with the Very Large Array of the triple radio source in the Serpens star-forming region. This object is known to be a radio jet arising from an intermediate-mass protostar. It is also one of the first protostellar jets where the presence of nonthermal emission was proposed. We analyze the dynamics of the jet and the nature of the emission and discuss these issues in the context of the physical parameters of the jet and the particle acceleration phenomenon. © 2016. The American Astronomical Society. All rights reserved. Source

Hull C.L.H.,Harvard - Smithsonian Center for Astrophysics | Girart J.M.,Harvard - Smithsonian Center for Astrophysics | Girart J.M.,Institute Of Ciencies Of Lespai | Kristensen L.E.,Harvard - Smithsonian Center for Astrophysics | And 8 more authors.
Astrophysical Journal Letters | Year: 2016

We report Atacama Large Millimeter/submillimeter Array (ALMA) observations of a one-sided, high-velocity (∼80 km s-1) CO(J = ) jet powered by the intermediate-mass protostellar source Serpens SMM1-a. The highly collimated molecular jet is flanked at the base by a wide-angle cavity; the walls of the cavity can be seen in both 4 cm free-free emission detected by the Very Large Array and 1.3 mm thermal dust emission detected by ALMA. This is the first time that ionization of an outflow cavity has been directly detected via free-free emission in a very young, embedded Class 0 protostellar source that is still powering a molecular jet. The cavity walls are ionized either by UV photons escaping from the accreting protostellar source or by the precessing molecular jet impacting the walls. These observations suggest that ionized outflow cavities may be common in Class 0 protostellar sources, shedding further light on the radiation, outflow, and jet environments in the youngest, most embedded forming stars. © 2016. The American Astronomical Society. All rights reserved. Source

Hernandez-Garcia L.,Institute Astrofisica Of Andalucia | Masegosa J.,Institute Astrofisica Of Andalucia | Gonzalez-Martin O.,Institute Radioastronomia y Astrofisica IRyA UNAM | Marquez I.,Institute Astrofisica Of Andalucia | Perea J.,Institute Astrofisica Of Andalucia
Astrophysical Journal | Year: 2016

X-ray variability is very common in active galactic nuclei (AGNs), but these variations may not occur similarly in different families of AGNs. We aim to disentangle the structure of low-ionization nuclear emission-line regions (LINERs) compared to Seyfert 2s by the study of their spectral properties and X-ray variations. We assembled the X-ray spectral parameters and variability patterns, which were obtained from simultaneous spectral fittings. Major differences are observed in the X-ray luminosities and the Eddington ratios, which are higher in Seyfert 2s. Short-term X-ray variations were not detected, while long-term changes are common in LINERs and Seyfert 2s. Compton-thick sources generally do not show variations, most probably because the AGN is not accesible in the 0.5-10 keV energy band. The changes are mostly related to variations in the nuclear continuum, but other patterns of variability show that variations in the absorbers and at soft energies can be present in a few cases. We conclude that the X-ray variations may occur similarly in LINERs and Seyfert 2s, i.e., they are related to the nuclear continuum, although they might have different accretion mechanisms. Variations at UV frequencies are detected in LINER nuclei but not in Seyfert 2s. This is suggestive of at least some LINERs having an unobstructed view of the inner disk where the UV emission might take place, with UV variations being common in them. This result might be compatible with the disappeareance of the torus and/or the broad-line region in at least some LINERs. © 2016. The American Astronomical Society. All rights reserved.. Source

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