Characterization of the western Pictor A hotspot in the hard X-rays with NuSTAR

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2402.06218.pdf (3.98 MB)

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http://arxiv.org/abs/2402.06218

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https://doi.org/10.48550/arXiv.2402.06218
 http://hdl.handle.net/11603/31761

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UMBC Faculty Collection
UMBC Center for Space Sciences and Technology (CSST) / Center for Research and Exploration in Space Sciences & Technology II (CRSST II)
UMBC Physics Department
UMBC Student Collection

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Author/Creator ORCID

https://orcid.org/0000-0002-7676-9962

Date

2024-02-09

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Journal articles
preprints
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Citation of Original Publication

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This item is likely protected under Title 17 of the U.S. Copyright Law. Unless on a Creative Commons license, for uses protected by Copyright Law, contact the copyright holder or the author.This item is likely protected under Title 17 of the U.S. Copyright Law. Unless on a Creative Commons license, for uses protected by Copyright Law, contact the copyright holder or the author.
CC BY-NC-SA 4.0 DEED Attribution-NonCommercial-ShareAlike 4.0 International

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Abstract

The origin of X-ray emission from the resolved kiloparsec-scale jets of many active galactic nuclei (AGN) remains uncertain, particularly where the X-ray emission is separate from the radio-optical synchrotron component. Possible explanations include inverse-Compton upscattering of the Cosmic Microwave Background (IC-CMB) and synchrotron emission from a second electron population, alternatives which imply very different physical conditions within the jet. Until now, X-ray studies of resolved jets have been restricted to the soft X-rays (below ~8 keV), often showing a hard spectral index indicating a spectral peak at higher energies. Here we present \emph{NuSTAR} and \emph{Chandra} observations of the nearby powerful radio galaxy Pictor A, in which we clearly detect the western hot spot at 4' from the host galaxy, the first detection of a kpc-scale jet above 10 keV. The \emph{NuSTAR} 3-20 keV spectrum is best fit by a powerlaw of index Γ = 2.03 ± 0.04. Using a maximum likelihood model, we confirm previous reports of variations in the soft X-ray flux detected by \emph{Chandra} over the 2000 to 2015 period, at a 6.7σ level, higher than the 3.4σ significance level. This rises to >8σ in the common 3-8 keV band using the full 22-year span of \emph{Chandra} and \emph{NuSTAR} observations. The variability of Pictor A rules out IC-CMB as the source of the X-ray emission, and the \emph{NuSTAR} observations are in keeping with a synchrotron origin for the X-ray emission from the western hotspot of Pictor A, with no sign of a cutoff to the flat spectrum.