Polarization of the changing-look quasar J1011+5442

If the disappearance of the broad emission lines observed in changing-look quasars were caused by the obscuration of the quasar core through moving dust clouds in the torus, high linear polarization typical of type 2 quasars would be expected. We measured the polarization of the changing-look quasar J1011+5442 in which the broad emission lines have disappeared between 2003 and 2015. We found a polarization degree compatible with null polarization. This measurement suggests that the observed change of look is not due to a change of obscuration hiding the continuum source and the broad line region, and that the quasar is seen close to the system axis. Our results thus support the idea that the vanishing of the broad emission lines in J1011+5442 is due to an intrinsic dimming of the ionizing continuum source that is most likely caused by a rapid decrease in the rate of accretion onto the supermassive black hole.Comment: Accepted for publication in Astronomy and Astrophysics Letter

Low optical polarisation at the core of the optically-thin jet of M87

We study the optical linear and circular polarisation in the optically-thin regime of the core and jet of M87. Observations were acquired two days before the Event Horizon Telescope (EHT) campaign in early April 2017. A high degree ($\sim 20$ per cent) of linear polarisation (P$_{\rm lin}$) is detected in the bright jet knots resolved at $\sim 10\, \rm{arcsec}$ to $23\, \rm{arcsec}$ ($0.8$-$1.8\, \rm{kpc}$) from the centre, whereas the nucleus and inner jet show P$_{\rm lin} \lesssim 5$ per cent. The position angle of the linear polarisation shifts by $\sim 90$ degrees from each knot to the adjacent ones, with the core angle perpendicular to the first knot. The nucleus was in a low level of activity (P$_{\rm lin} \sim 2$-$3$ per cent), and no emission was detected from HST-1. No circular polarisation was detected either in the nucleus or the jet above a $3\sigma$ level of P$_{\rm circ} \leq 1.5$ per cent, discarding the conversion of P$_{\rm lin}$ into P$_{\rm circ}$. A disordered magnetic field configuration or a mix of unresolved knots polarised along axes with different orientations could explain the low P$_{\rm lin}$. The latter implies a smaller size of the core knots, in line with current interferometric observations. Polarimetry with EHT can probe this scenario in the future. A steep increase of both P$_{\rm lin}$ and P$_{\rm circ}$ with increasing frequency is expected for the optically-thin domain, above the turnover point. This work describes the methodology to recover the four Stokes parameters using a $\lambda/4$ wave-plate polarimeter.Comment: Accepted for publication in MNRAS. 10 pages, 8 figure

The outburst of the eruptive young star OO Serpentis between 1995 and 2006

OO Serpentis is a deeply embedded pre-main sequence star that went into outburst in 1995 and gradually faded afterwards. Its eruption resembled the well-known FU Orionis-type or EX Lupi-type outbursts. Since very few such events have ever been documented at infrared wavelengths, our aim is to study the temporal evolution of OO Ser in the infrared. OO Ser was monitored with the Infrared Space Observatory starting 4 months after peak brightness and covering 20 months. In 2004-2006 we again observed OO Ser from the ground and complemented this dataset with archival Spitzer obsevations also from 2004. We analysed these data with special attention to source confusion and constructed light curves at 10 different wavelengths as well as spectral energy distributions. The outburst caused brightening in the whole infrared regime. According to the infrared light curves, OO Ser started a wavelength-independent fading after peak brightness. Later the flux decay became slower but stayed wavelength-independent. The fading is still ongoing, and current fading rates indicate that OO Ser will not return to quiescent state before 2011. The outburst timescale of OO Ser seems to be shorter than that of FUors, but longer than that of EXors. The outburst timescale and the moderate luminosity suggest that OO Ser is different from both FUors and EXors, and shows similarities to the recently erupted young star V1647 Ori. Based on its spectral energy distribution and bolometric temperature, OO Ser seems to be an early class I object, with an age of < 10^5 yr. The object is probably surrounded by an accretion disc and a dense envelope. Due to the shorter outburst timescales, the viscosity in the circumstellar disc of OO Ser is probably an order of magnitude higher than usual for FUors.Comment: 12 pages, 7 figures, accepted for publication in A&

The redshift and broad band spectral energy distribution of NRAO 150

Context. NRAO 150 is one of the brightest radio and mm AGN sources on the northern sky. It has been revealed as an interesting source where to study extreme relativistic jet phenomena. However, its cosmological distance has not been reported so far, because of its optical faintness produced by strong Galactic extinction. Aims. Aiming at measuring the redshift of NRAO 150, and hence to start making possible quantitative studies from the source. Methods. We have conducted spectroscopic and photometric observations of the source in the near-IR, as well as in the optical. Results. All such observations have been successful in detecting the source. The near-IR spectroscopic observations reveal strong H$\alpha$ and H$\beta$ emission lines from which the cosmological redshift of NRAO 150 ($z=1.517\pm0.002$) has been determined for the first time. We classify the source as a flat-spectrum radio-loud quasar, for which we estimate a large super-massive black-hole mass $\sim5\times 10^{9} \mathrm{M_\odot}$. After extinction correction, the new near-IR and optical data have revealed a high-luminosity continuum-emission excess in the optical (peaking at $\sim2000$\,\AA, rest frame) that we attribute to thermal emission from the accretion disk for which we estimate a high accretion rate, $\sim30$\,% of the Eddington limit. Conclusions. Comparison of these source properties, and its broad-band spectral-energy distribution, with those of Fermi blazars allow us to predict that NRAO 150 is among the most powerful blazars, and hence a high luminosity -although not detected yet- $\gamma$-ray emitter.Comment: 8 pages, 4 figure