The changing look of the neutrino-emitter blazar candidate 5BZQ J1243+4043

TL;DR

At redshift $z = 1.5181 \pm 0.0002$, 5BZQ J1243+4043 is analyzed across three optical epochs to test its changing-look nature and suitability as a neutrino-emitter. The authors extract continuum and emission-line properties, deriving $L_{\rm BLR}$, $L_{\rm Edd}$, and jet power from $P_{1.4\,\mathrm{GHz}}$, and track the accretion regime via the $L_{\rm BLR}/L_{\rm Edd}$ diagnostic. They find that, despite optical-changing-look behavior, the source remains radiatively efficient and HERG-like, with intense external radiation fields across epochs. The results support a multi-messenger scenario in which neutrino production can occur in HERG-like, efficiently accreting blazars, even when optical appearances change.

Abstract

In recent years, changing-look blazars have called the traditional view of BL Lacs-flat spectrum radio quasars into question within the empirical classification of blazars. Based on the intensity of optical lines, they appear to transition between the two classes over time. We focus on the blazar 5BZQ J1243+4043, recently proposed as a promising candidate for the emission of high-energy neutrinos observed by the IceCube Neutrino Observatory and reported as a changing-look blazar in the literature. We study the spectral properties of this blazar, inferring its radiation field and accretion regime across different epochs. This study presents new optical spectroscopy observations of 5BZQ J1243+4043 taken with Gran Telescopio Canarias. We used this new dataset and two optical spectra available from the literature to investigate the continuum and line emissions and pinpoint the physical properties of the source. In particular, we used the emission lines to probe the accretion regime. The newly collected data for 5BZQ J1243+4043 shows broad emission lines, consistent with the spectrum of the first epoch and the redshift z = 1.5181$\pm$0.0002 known from the literature. For the second epoch, the spectrum appears featureless and so, we placed limits on the emission lines and related properties. We observed spectral variability for both the continuum and line emissions among the three spectra. Nonetheless, the accretion properties of the blazar generally remain unvaried, indicating that the intrinsic physics stays the same across the three epochs. In the broader multi-messenger context, this suggests that, despite the changing look in the optical band, the candidate neutrino-emitter blazar 5BZQ J1243+4043 is still characterized by the presence of intense external radiation fields and radiatively efficient accretion, typical of high-excitation radio galaxies, which may foster neutrino production.

Figures (6)

• Figure 1: Optical spectra of 5BZQ J1243$+$4043 from: the NOT spectrum #1 Titov:2013 shown in blue, the LAMOST spectrum #2 PenaHerazo_changinglook in orange, and a new GTC spectrum #3 acquired in this work (MJD $60318.21$) in green. The shaded areas indicate the corresponding flux uncertainties.
• Figure 2: Variations in the emission line fluxes as a function of the underlying continuum level across the three epochs of 5BZQ J1243$+$4043. The values for Mg ii, C iv, and C iii] emission lines are indicated in blue, brown, and yellow, respectively. The values corresponding to spectra #1 and #3 are indicated with a square and a filled circle, respectively, while the arrows show the limits derived for epoch #2. The dotted black line traces the diagonal, corresponding to equality.
• Figure 3: Accretion regime $L_{\rm BLR}/L_{\rm Edd}$ as a function of the radio power at $1.4$ GHz. The three considered epochs (#1 as a blue square, #2 as an orange arrow, #3 as a green dot) of 5BZQ J1243$+$4043 are compared to the blazar samples analyzed in Paper PI (gray). The horizontal dotted and vertical dashed-dotted black lines represent the boundaries for the LERG/HERG-like behavior, respectively. As discussed in Sect. \ref{['sec: discussion']}, despite the line emission variations, the object remains in the radiative-efficient regime.
• Figure 4: Test simulation of a Mg ii as bright and broad as in spectra #1 (blue) and #3 (green) over a continuum as high as in #2 (orange). The shaded areas show the corresponding $1\sigma$ uncertainties.
• Figure 5: Test simulation of a C iii] as bright and broad as in spectra #1 (blue) and #3 (green) over a continuum as high as in #2 (orange). The shaded areas show the corresponding $1\sigma$ uncertainties.