- evn2024@mpifr.de
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Contribution
A Distance Measurement for a Blazar TXS 0506+056 Using its Radio Variability and VLBI Images
Speakers
- Mr. Chanwoo SONG
Primary authors
- Mr. Chanwoo SONG (Korea Astronomy and Space Science Institute, Korea University of Science and Technology)
- Prof. Sang-Sung LEE (Korea Astronomy and Space Science Institute, Korea University of Science and Technology)
Content
We present the result of the angular diameter distance measurements for a blazar TXS 0506+056 ($z=0.3365$), a radio-bright active galactic nucleus (AGN) whose jet is aligned with the line of sight. We used the 15 GHz Owens Valley Radio Observatory (OVRO) 40 m single dish (SD) data from MJD 54474 to MJD 59023 (12 years) and the 15 GHz Very Long Baseline Array (VLBA) data from MJD 54838 to MJD 60126 (13 years). The OVRO SD flux density ranges from $0.29\pm0.03$ to $2.44\pm0.03$ Jy and the VLBA core flux density varies from $0.22\pm0.03$ to $1.90\pm0.17$ Jy. We used a variability timescale ($\tau$) and a causality argument of a linear size $R=c\delta\tau/(1+z)$ (taking into account a Doppler factor $\delta$ and a cosmological redshift $z$) to constrain the angular diameter distance ($D_{A}=R/\theta_{R}$) to the source (with its angular size $\theta_{R}$). Using the OVRO data, we estimated a variability timescale of $\tau=296.2_{-1.6}^{+0.4}$ days for the giant flare in 2020 February. We found that the giant flare is dominated by the variability of the VLBA core by fitting circular Gaussian model components to the VLBA images. The rest frame brightness temperature of an emission region ($T_{\mathrm{b}}^{\mathrm{em}}$) and the observed brightness temperature by the receiver ($T_{\mathrm{b}}^{\mathrm{rec}}$) are related as $T_{\mathrm{b}}^{\mathrm{em}}=T_{\mathrm{b}}^{\mathrm{rec}}(1+z)/\delta$. To constrain the Doppler factor $\delta=T_{\mathrm{b}}^{\mathrm{rec}}(1+z)/T_{\mathrm{b}}^{\mathrm{em}}$, we assume that $T_{\mathrm{b}}^{\mathrm{em}}$ is saturated to the intrinsic brightness temperature $T_{\mathrm{b,int}}$ by the inverse Compton catastrophe when the flare of the emission region peaks. To calculate $T_{\mathrm{b}}^{\mathrm{rec}}$, a flux density variation and an angular size of the emission region are required. The angular size $\theta_{R}$ of the emission region (i.e., the VLBA core) is obtained from a Gaussian modelfit parameter $\theta_{\mathrm{FWHM}}$ ($\theta_{R}=0.8\theta_{\mathrm{FWHM}}$), ranging in 0.047-0.228 milli-arcsecond (mas), and its uncertainty is determined to be 2.54-14.0 %. We considered the core's flux density variation is the difference between the maximum and the minimum flux density. With same timescale and core flux density variation, we calculate the angular diameter distance $D_{\mathrm{A}}=R/\theta_{R}$, along core sizes. At $\Lambda$-CDM model, the angular diameter distance of TXS 0506+056 is determined as $D_{\mathrm{A}}=1038.9_{-7.7}^{+7.7}$ Mpc or $D_{\mathrm{A}}=958.7_{-13.5}^{+13.8}$ Mpc at Hubble constant $H_{0}=67.4\pm0.50$ km/s/Mpc or $H_{0}=73.04\pm1.04$ km/s/Mpc, respectively, with a matter density parameter $\Omega_{\mathrm{M}}=0.27$ and a vacuum density parameter $\Omega_{\mathrm{\Lambda}}=0.73$. From MJD 58468 to MJD 58968 near the total flux peak, we found the consistent results with $\Lambda$-CDM within the uncertainty 29.6-59.1 %.