Variability of the blazar 4C 38.41 (B3 1633+382) from GHz frequencies to GeV energies

Abstract

Context. After years of modest optical activity, the quasar-type blazar 4C 38.41 (B3 1633+382) experienced a large outburst in 2011, which was detected throughout the entire electromagnetic spectrum, renewing interest in this source. Aims. We present the results of low-energy multifrequency monitoring by the GASP project of the WEBT consortium and collaborators, as well as those of spectropolarimetric/spectrophotometric monitoring at the Steward Observatory. We also analyse high-energy observations of the Swift and Fermi satellites. This combined study aims to provide insights into the source broad-band emission and variability properties. Methods. We assemble optical, near-infrared, millimetre, and radio light curves and investigate their features and correlations. In the optical, we also analyse the spectroscopic and polarimetric properties of the source. We then compare the low-energy emission behaviour with that at high energies.Results. In the optical-UV band, several results indicate that there is a contribution from a quasi-stellar-object (QSO) like emission component, in addition to both variable and polarised jet emission. In the optical, the source is redder-when-brighter, at least for R > ∼ 16. The optical spectra display broad emission lines, whose flux is constant in time. The observed degree of polarisation increases with flux and is higher in the red than the blue. The spectral energy distribution reveals a bump peaking around the U band. The unpolarised emission component is likely thermal radiation from the accretion disc that dilutes the jet polarisation. We estimate its brightness to be R QSO ∼ 17.85-18 and derive the intrinsic jet polarisation degree. We find no clear correlation between the optical and radio light curves, while the correlation between the optical and γ-ray flux apparently fades in time, likely because of an increasing optical to γ-ray flux ratio. Conclusions. As suggested for other blazars, the long-term variability of 4C 38.41 can be interpreted in terms of an inhomogeneous bent jet, where different emitting regions can change their alignment with respect to the line of sight, leading to variations in the Doppler factor δ. Under the hypothesis that in the period 2008-2011 all the γ-ray and optical variability on a one-week timescale were due to changes in δ, this would range between ∼ 7 and ∼ 21. If the variability were caused by changes in the viewing angle θ only, then θ would go from ∼ 2.6 • to ∼ 5 • . Variations in the viewing angle would also account for the dependence of the polarisation degree on the source brightness in the framework of a shock-in-jet model.

Quasi-periodic behaviour in the optical and γ-ray light curves of blazars 3C 66A and B2 1633+38

“…Moreover, a shock wave propagating along a helical path in the blazar's jet was proposed to explain the multi-frequency behavior of an outburst in 2011 of the blazar S5 0716+71 (Larionov et al 2013). The strong correlation between continuum flux and polarization degree of B2 1633+38 is consistent with the result found by Raiteri et al (2012) using a much shorter time window. Such behaviour was explained in terms of an inhomogeneous jet in which variability is caused by changes of the emitting region viewing angle, θ = 2.6 o − 5.3 o , coupled with a high Lorentz factor Γ = 31.1 (see also Raiteri et al 2013).…”

“…The long-term optical data used in this analysis have been obtained from several observatories and surveys, listed in Table 1. The GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope 2 (WEBT, Villata et al 2002Villata et al , 2009) provided V and R-band data for the two sources, which were taken during multiwavelength monitoring campaigns of 3C 66A in (Böttcher et al 2005) and 2007-2008(Böttcher et al 2009Abdo et al 2011), and of B2 1633+38 in 2008(Raiteri et al 2012. Additional R-band data on 3C 66A come from the LX-200 0.4m telescope from the St. Petersburg University and the AZT-8 0.7m telescope from the Crimean Astrophysical Observatory (SPB and CrAO, Larionov et al 2013).…”

Relativistic Jets in Gamma-Ray-Emitting Narrow-Line Seyfert 1 Galaxies

“…X-ray variability has been observed in most of the γ-ray-emitting NLSy1, in particular during high γ-ray activity periods (e.g., [38,61,63,65,112]), although not extreme and rapid as observed in radio-quiet NLSy1 (e.g., IRAS 13224−3809; [113]). The contemporaneous high activity level observed in X-rays and γ-rays suggests a co-spatiality of the emitting region, with the same mechanism (i.e., IC scattering) responsible for the radiation emitted in those energy bands, as observed in several FSRQ (e.g., [114][115][116]). At hard X-rays only one γ-ray-emitting NLSy1 is included in the Swift-BAT 105-month catalogue [117], 1H 0323+342, with a photon index of 1.62 ± 0.30.…”

Sharp Polarimetric Eyes: More Trees than Forest?

“…If the model is valid, this is the highest level of optical polarization seen for this class of AGNs. Using SO monitoring data, the blue spectral component has been polarimetrically identified into the ultraviolet out to C IV λ1550 in B2 1633+382 (z = 1.814) [13].…”