3C 273 with NuSTAR: Unveiling the AGN

Abstract

We present results from a 244\,ks \textit{NuSTAR} observation of 3C\,273 obtained during a cross-calibration campaign with the \textit{Chandra}, \textit{INTEGRAL}, \textit{Suzaku}, \textit{Swift}, and \textit{XMM-Newton} observatories. We show that the spectrum, when fit with a power-law model using data from all observatories except \textit{INTEGRAL} over the 1--78\,keV band, leaves significant residuals in the \textit{NuSTAR} data between 30--78\,keV. The \nustar\ 3--78\,keV spectrum is well-described by an exponentially cutoff power-law ($Γ= 1.646 \pm 0.006$, E$_\mathrm{cutoff} = 202_{-34}^{+51}$\,keV) with a weak reflection component from cold, dense material. There is also evidence for a weak ($EW = 23 \pm 11$ eV) neutral iron line. We interpret these features as arising from coronal emission plus reflection off an accretion disk or distant material. Beyond 80\,keV \textit{INTEGRAL} data show clear excess flux relative to an extrapolation of the AGN model fit to \nustar. This high-energy power-law is consistent with the presence of a beamed jet, which begins to dominate over emission from the inner accretion flow at 30-40 keV. Modeling the jet locally (in the \textit{NuSTAR} + \textit{INTEGRAL} band) as a power-law, we find the coronal component is fit by $Γ_\mathrm{AGN} = 1.638 \pm 0.045$, $E_\mathrm{cutoff} = 47 \pm 15$\,keV, and jet photon index by $Γ_\mathrm{jet} = 1.05 \pm 0.4$. We also consider \textit{Fermi}/LAT observations of 3C\,273 and here the broad-band spectrum of the jet can be described by a log-parabolic model, peaking at $\sim 2$\,MeV. Finally, we investigate the spectral variability in the \textit{NuSTAR} band and find an inverse correlation between flux and $Γ$.