A Serendipitous NuSTAR Detection of a Giant Radio Source Harboring an Obscured Active Galactic Nucleus

Giant radio sources (GRSs) harbor the Universe’s largest structures generated by individual galaxies, with projected source sizes exceeding 700 kpc. These enigmatic objects have been mainly studied at radio frequencies, and their physical properties in the high-energy domain are poorly understood. Here we present the results of a multiwavelength study focused on NuSTAR J112829+5831.8 (J1128+5831), the only known GRS serendipitously detected with the Nuclear Spectroscopic Telescope Array. Being located in proximity to the famous interacting galaxy system, Arp 299, J1128+5831 has been serendipitously observed also by the Chandra X-ray Observatory, Hubble Space Telescope, and XMM-Newton satellites. From radio observations with the Low Frequency Array, the NRAO VLA Sky Survey and the Very Large Array Sky Survey, we have determined that J1128+5831 has an overall steep radio spectrum ($α=-0.86$; $F_ν\proptoν^α$) and a low core dominance ($C_{\rm D}=-2.4$, in log-scale), indicating the source to be viewed at large angles. From the X-ray spectral analysis, we found J1128+5831 to harbor an obscured active galactic nucleus (AGN) with neutral hydrogen column density exceeding $10^{23}$ cm$^{-2}$. Its optical spectrum, taken with the Dark Energy Spectroscopic Instrument, exhibits prominent narrow emission lines but lacks broad components, thus confirming J1128+5831 to be a Type 2 AGN powered by a radiatively efficient accreting system. Overall, the broadband properties of J1128+5831 are consistent with those observed for the general GRS population.

💡 Research Summary

The authors present the first comprehensive multi‑wavelength study of a giant radio source (GRS) that has been serendipitously detected by the Nuclear Spectroscopic Telescope Array (NuSTAR). By cross‑matching the most recent LOFAR‑based GRS catalog (∼11 500 objects) with the NuSTAR 80‑month serendipitous hard‑X‑ray catalog (1 274 sources), they identified a single overlap: NuSTAR J112829+5831.8 (hereafter J1128+5831). The source lies 1.6 arcmin south of the well‑studied interacting galaxy system Arp 299, which means that archival Chandra, XMM‑Newton, Hubble Space Telescope, and several radio observations also cover its position.

Radio imaging from LOFAR (144 MHz), the NRAO VLA Sky Survey (1.4 GHz) and the VLA Sky Survey (3 GHz) reveals a classic FR II morphology with two lobes extending over 2.64 arcmin, corresponding to a projected linear size of ∼864 kpc at the spectroscopic redshift z = 0.4103. The integrated radio spectrum is steep (α = ‑0.86, where Fν ∝ ν^α), and the core is very weak: a stacked VLASS image yields a 3 GHz core flux density of 0.46 ± 0.15 mJy. Using the standard core‑dominance parameter CD = log