The Incidence of the Host Galaxy in Microvariability Observations of Quasars

Abstract

The incidence of a host galaxy in aperture photometry of active galactic nuclei is studied by means of actual and simulated CCD observations. Our goal is to evaluate the importance of spurious variations, introduced by seeing Ñuctuations during the observations, in the di †erential light curves used to study optical microvariability. Repeated CCD observations during two consecutive nights were used to obtain time-resolved aperture photometry for the BL Lac object PKS 2316[423, which is located at the center of a conspicuous elliptical galaxy, and for several Ðeld stars. The blazar seems to be variable according to standard variability criteria ; however we show that the observed di †erential magnitude variations are strongly correlated with seeing Ñuctuations during the nights. Moreover, another galaxy within the same CCD Ðeld shows nearly identical variations, clearly indicating that such variations are artifacts of the aperture photometry. Simulated observations of quasars within host galaxies of di †erent morphologies and spanning a wide range of luminosities were also used to evaluate the e †ects of changing seeing conditions. The results show that spurious di †erential magnitude variations due to seeing Ñuctuations are larger for active nuclei within brighter hosts, particularly when small photometric apertures (about the seeing FWHM in radius) are used. According to our results, several recommendations are given to future observers.

Multi-Color Optical Monitoring of MRK 501 From 2010 to 2015

“…If the host galaxy contributions are not subtracted, the broadband spectra are highly distorted and relative optical flux variations are underestimated. FWHM changes do not introduce very strong false variability in objects with a prominent host galaxy component (0.01-0.03 mag; Cellone et al 2000;Nilsson et al 2007). For our observations, an aperture radius of 2×FWHM or 1.5×FWHM was selected, and the scale of the aperture radius is same on the same night.…”

Multi-wavelength Study of Blazars Using Variability as a Tool

“…All CCD frames were subject to data reduction with batch procedures, using standard methods, to correct each raw image for bias signals and flat fielding. The aperture photometry was carried out using the correct size of aperture chosen carefully considering the optimum value of the S/N ratio and the prescription of Cellone et al (2000) to avoid spurious variations.…”

Microvariability in the optical polarization of 3C 279

“…Since the superluminal components are expected to be extremely relativistic even respect to the jet, we adopt a mild value γ j ∼ 2 for the stable, underlying flow (Marscher & Gear 1985;Romero et al 1995). In the particular case of 3C 279, we assume an average value of α = −0.95 for the spectral index at optical wavelengths (de Pater & Perley 1983). Concerning the Lorentz factor, we adopt as an average value the result of the fit of superluminal components with the precessing jet model proposed by Abraham & Carrara (1998).…”

“…control and comparison purposes (see Cellone et al 2000, for a detailed discussion of the procedure). The fluctuations in the lightcurves that could be interpreted as intranight variations are within the estimated errors.…”