Enhanced X-ray Emission from the Most Radio-Powerful Quasar in the Universe's First Billion Years

Abstract

We present deep (265 ks) Chandra X-ray observations of PSO J352.4034$-$15.3373, a quasar at z=5.831 that, with a radio-to-optical flux ratio of R>1000, is one of the radio-loudest quasars in the early universe and is the only quasar with observed extended radio jets of kpc-scale at $z\gtrsim 6$. Modeling the X-ray spectrum of the quasar with a power law, we find a best fit of $\Gamma =1.99_{-0.28}^{+0.29}$, leading to an X-ray luminosity of $L_{2-10}=1.26_{-0.33}^{+0.45}\times 10^{45}\mathrm{erg}{\mathrm{s}}^{-1}$ and an X-ray to UV brightness ratio of ${\alpha }_{\mathrm{OX}}=-1.36\pm 0.11$. We identify a diffuse structure 50 kpc ($\sim 8^{\prime \prime }$) to the NW of the quasar along the jet axis that corresponds to a $3\sigma$ enhancement in the angular density of emission and can be ruled out as a background fluctuation with a probability of P=0.9985. While with few detected photons the spectral fit of the structure is uncertain, we find that it has a luminosity of $L_{2-10}\sim 10^{44}\mathrm{erg}{\mathrm{s}}^{-1}$. These observations therefore potentially represent the most distant quasar jet yet seen in X-rays. We find no evidence for excess X-ray emission where the previously-reported radio jets are seen (which have an overall linear extent of $0{.}^{\prime \prime }28$), and a bright X-ray point source located along the jet axis to the SE is revealed by optical and NIR imaging to not be associated with the quasar.

16 pages, 7 Figures. Accepted for publication the Astrophysical Journal

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