June 21, 2025 Version française 🇫🇷
Active Galactic Nuclei (AGN) refer to a class of galaxies whose centers harbor a supermassive black hole undergoing active accretion of matter. This process releases a considerable amount of energy, radiated over a wide range of wavelengths, from the radio domain to X-rays and γ-rays. The mass of black holes in AGN typically ranges from $10^6$ to $10^{10}$ solar masses.
Within the framework of the unified model of AGN, the observed differences between subclasses (Seyfert 1, Seyfert 2, quasars, blazars, etc.) do not reflect fundamentally different physical properties, but are mainly explained by geometric orientation effects and by the presence of a dust torus that obscures certain regions depending on the observer's line of sight. Thus, the same object may appear as a Seyfert 1 or 2 depending on whether the Broad Line Region (BLR) is visible or not.
source : https://cosmicvarta.in/narrow_broad_seyfert_galaxies_vivek
Seyfert 1 galaxies are characterized by the simultaneous presence of both narrow and broad emission lines in their optical spectra. These broad lines, in particular $H_α$ and $H_β$, originate from gas in rapid orbit within the Broad Line Region (BLR), located in the immediate vicinity of the black hole. The visibility of these lines implies that our line of sight provides direct access to the BLR, unlike in Seyfert 2 galaxies, where this region is obscured by a dust torus.
Telescope: Newtonian Skywatcher PDS 150/750 mounted on a ZWO AM5 harmonic mount.
Spectroscopy: Star’Ex BR spectrograph equipped with a 300 lines/mm grating. $R \approx 900$. 26-micron slit. Science camera: Player One Uranus M Pro (IMX585). Guiding camera: ASI290MM Mini.
Photometry: EvoGuide 50 mm refractor equipped with an ASI533MM Pro camera and a Baader Bessel V filter, used for absolute flux calibration.
Software: CCDCiel, SpecINTI, Python scripts
Each night, the observation protocol consisted of acquiring the spectrum of a reference star from the CALSPEC catalog, used for calibration and for computing the instrument response. We then acquired several spectra of a galaxy, while simultaneously performing photometric observations in the Bessel V band on the same target.
Six active galactic nuclei (AGN) were observed during this campaign: NGC 4151, NGC 4051, NGC 5548, NGC 3516, Mrk 304 and 3C 273. These targets were selected to cover a wide range of supermassive black hole masses, from Seyferts hosting a black hole of “only” about one million solar masses (i.e., approximately $10^6,M_☉$, like NGC 4051) to a very massive quasar with several billion solar masses (around $10^9,M_☉$ for 3C 273). NGC 4151, observed on June 16, 2025, benefited from good observing conditions, with a total exposure time of 5600 s (4 × 1400 s) and a signal-to-noise ratio (SNR) of 90. In contrast, the nights of June 18 and June 20, devoted respectively to 3C 273, NGC 4051, and NGC 5548, were affected by unstable weather conditions, including frequent cloud passages. Although similar exposure times had been initially planned, the SNRs obtained for these targets remain modest, ranging between 20 and 25.
$$
\begin{array}{lcclll}
\text{name} &
\text{obs. date} &
\text{exp. time} &
\text{SNR} &
\text{{$E(B$-$V)$}} &
\\ \hline \\ \textbf{NGC 4151} & \text{2025-06-16} & 4\times1400s & 90 & \text{0.027} &
\\
\textbf{NGC 4051} &
\text{2025-06-20} &
2\times1400s &
20 &
\text{0.013} &
\\
\textbf{NGC 3516} &
\text{2025-06-23} &
4\times1400s &
60 &
\text{0.042} &
\\
\textbf{NGC 5548} &
\text{2025-06-20} &
2\times1400s &
25 &
\text{0.020} &
\\
\textbf{3C 273} &
\text{2025-06-18} &
4\times1400s &
20 &
\text{0.022} &
\\
\textbf{Mrk 304} &
\text{2025-07-05} &
4\times1400s &
15 &
\text{0.108} &
\\
\end{array} $$
The reduction of the spectral data involved several key steps. First, a wavelength calibration was performed using the Balmer lines and telluric lines of the reference star. Next, the heliocentric velocity correction was applied to account for the Earth's motion.
The spectrum was then calibrated in absolute flux ($\mathrm{erg\,s^{-1}\,cm^{-2}\,\AA^{-1}}$) based on the average ($V$) magnitude measured during the observing session and the transmission curve of the Baader Bessel V filter. A Python script was developed for this purpose. This script converts a spectrum in relative flux into a spectrum in absolute flux, calibrated according to a given photometric system (AB or Vega). It uses the filter transmission curve and the observed magnitude in that filter to perform the calibration. The principle is based on computing a synthetic flux through the filter from the spectrum, which is then compared to a reference absolute flux derived from the magnitude and the chosen photometric system. The spectrum is then rescaled (multiplied by a factor) so that its synthetic flux matches the expected flux.
The core method consists of numerically integrating the product of the spectrum and the filter transmission to obtain the synthetic flux. Here, we used the AB system, in which the reference flux is calculated from the standard formula linking magnitude, frequency, and wavelength.
$$ \begin{align*} f_\nu &= 10^{-0.4 (m_{\mathrm{AB}} + 48.6)} \quad \text{[erg/s/cm}^2/\text{Hz]} \\ f_\lambda &= \frac{f_\nu \cdot c}{\lambda_{\mathrm{eff}}^2} \quad \text{[erg/s/cm}^2/\text{\AA]} \end{align*} $$
The spectral shift (redshift $z$) was measured via a simple Gaussian fit of the $H_β$ line. This shift allowed the spectrum to be re-centered in the rest frame to measure the absolute flux of the continuum at 5100 Å. Finally, the broad component of the $H_β$ line was extracted to measure the full width at half maximum (FWHM) by performing a basic fit of the $H_β$ and $[OIII]$ lines using multiple Gaussian profiles. The instrumental broadening affecting the FWHM was then corrected.