Search

Aims. We report on the discovery and follow-up of a peculiar transient, OGLE17aaj, which occurred in the nucleus of a weakly active galaxy. We investigate whether it can be interpreted as a new candidate for a tidal disruption event (TDE). Methods. We present the OGLE-IV light curve that covers the slow 60-day-long rise to maximum along with photometric, spectroscopic, and X-ray follow-up during the first year. Results. OGLE17aaj is a nuclear transient exhibiting some properties similar to previously found TDEs, including a long rise time, lack of colour-temperature evolution, and high black-body temperature. On the other hand, its narrow emission lines and slow post-peak evolution are different from previously observed TDEs. Its spectrum and light-curve evolution is similar to F01004-2237 and AT 2017bgt. Signatures of historical low-level nuclear variability suggest that OGLE17aaj may instead be related to a new type of accretion event in active super-massive black holes. [ABSTRACT FROM AUTHOR]

Context. Over the past few years, on several occasions, large, continuous rotations of the electric vector position angle (EVPA) of linearly polarized optical emission from blazars have been reported. These events are often coincident with high energy γ-ray flares and they have attracted considerable attention, since they could allow us to probe the magnetic field structure in the γ-ray emitting region of the jet. The flat-spectrum radio quasar 3C 279 is one of the most prominent examples showing this behaviour. Aims. Our goal is to study the observed EVPA rotations and to distinguish between a stochastic and a deterministic origin of the polarization variability. Methods. We have combined multiple data sets of R-band photometry and optical polarimetry measurements of 3C 279, yielding exceptionally well-sampled flux density and polarization curves that cover a period of 2008-2012. Several large EVPA rotations are identified in the data. We introduce a quantitative measure for the EVPA curve smoothness, which is then used to test a set of simple random walk polarization variability models against the data. Results. 3C 279 shows different polarization variation characteristics during an optical low-flux state and a flaring state. The polarization variation during the flaring state, especially the smooth ~360° rotation of the EVPA in mid-2011, is not consistent with the tested stochastic processes. Conclusions. We conclude that, during the two different optical flux states, two different processes govern polarization variation, which is possibly a stochastic process during the low-brightness state and a deterministic process during the flaring activity. [ABSTRACT FROM AUTHOR]

Conte×t. This is the third paper in a series devoted to the analysis of the multiwavelength data from a campaign on the nearby (z = 0.03) TeV blazar Mrk 421 during 2011. Aims. We investigate the structure of the high angular resolution polarization, the magnetic topology, the total intensity light curve, the γ-ray flu×, and the photon inde×. We describe how they evolve and how they are connected. Methods. We analyzed data in polarized intensity obtained with the Very Long Baseline Array (VLBA) at twelve epochs (one observation per month from January to December 2011) at 15, 24, and 43 GHz. For the absolute orientation of the electric vector position angles (EVPA) we used the D-terms method; we also confirm its accuracy. We also used γ-ray data from the Fermi Large Area Telescope on weekly time bins throughout 2011. Results. The source shows polarized emission, and its properties vary with time, frequency, and location along the jet. The core mean polarization fraction is generally between 1% and 2%, with a 4% peak at 43 GHz in March; the polarization angle is variable, mainly at 15 GHz, where it changes frequently, and less so at 43 GHz, where it oscillates in the range 114°-173°∘. The jet polarization properties are more stable, with a fractional polarization of around 16% and a polarization angle nearly perpendicular to the jet a×is. The average flu× and photon inde× at γ-ray energies are (17.4 ± 0.5) ×10-8 ph cm-2 s-1 and Γ = 1.77 ± 0.02. The γ-ray light curve shows variability, with a main peak of (38 ± 11) × 10-8 ph cm-2 s-1 at the beginning of March and two later peaks centered on September 8 and November 13. The first γ-ray peak appears to be associated with the peak in the core polarized emission at 43 GHz, as well as with the total intensity light curve. A discrete correlation function analysis yields a correlation coefficient of 0.54 at zero delay, with a significance level >99.7%. Conclusions. With this multifrequency study, we accurately determine the polarization properties of Mrk 421, both in the core and in the jet region. The radio and γ-ray light curves are correlated. The observed EVPA variability at 15 GHz is partly due to opacity and partly to a variable Faraday rotation effect. To e×plain the residual variability of the intrinsic polarization angle and the low degree of polarization in the core region, we invoke a blend of variable cross-polarized subcomponents with different polarization properties within the beam. [ABSTRACT FROM AUTHOR]

ABSTRACT We report results from a one-week multiwavelength campaign to monitor the BL Lacertae object (BL Lac) S5 0716+714 (on 2009 December 9-16). Nine ground-based telescopes at widely separated longitudes and one space-based telescope aboard the Swift satellite collected optical data. Radio data were obtained from the Effelsberg and Urumqi observatories and X-ray data from Swift. In the radio bands, the source shows rapid [∼(0.5-1.5) d] intraday variability with peak amplitudes of up to ∼10 per cent. The variability at 2.8 cm leads by about 1 d the variability at 6 and 11 cm. This time lag and more rapid variations suggest an intrinsic contribution to the source's intraday variability at 2.8 cm, while at 6 and 11 cm, interstellar scintillation (ISS) seems to predominate. Large and quasi-sinusoidal variations of ∼0.8 mag were detected in the V, R and I bands. The X-ray data (0.2-10 keV) do not reveal significant variability on a 4 d time-scale, favouring reprocessed inverse Compton over synchrotron radiation in this band. The characteristic variability time-scales in radio and optical bands are similar. A quasi-periodic variation of 0.9-1.1 d in the optical data may be present, but if so it is marginal and limited to 2.2 cycles. Cross-correlations between radio and optical bands are discussed. The lack of a strong radio-optical correlation indicates different physical causes of variability (ISS at long radio wavelengths, source intrinsic origin in the optical) and is consistent with a high jet opacity and a compact synchrotron component peaking at ≃100 GHz in an ongoing very prominent flux-density outburst. For the campaign period, we construct a quasi-simultaneous spectral energy distribution, including γ-ray data from the Fermi satellite. We obtain lower limits for the relativistic Doppler boosting of δ ≥ 12-26, which for a BL Lac-type object is remarkably high. [ABSTRACT FROM AUTHOR]