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

TL;DR

This work investigates the high-energy properties of giant radio sources (GRSs) by presenting a broadband study of NuSTAR J112829+5831.8, the only GRS serendipitously detected by NuSTAR near Arp 299, combining NuSTAR, XMM-Newton, Chandra, DESI optical spectroscopy, HST imaging, and LOFAR/VLASS/NVSS radio data. It reveals an obscured Type 2 AGN with $N_{\rm H} > 10^{23}$ cm$^{-2}$, a steep radio spectrum $\alpha \approx -0.86$, a low core dominance $C_{\rm D} \approx -2.4$ (implying a large jet viewing angle), and a rest-frame 144 MHz power $P_{144\,\mathrm{MHz}} \approx 1\times10^{27}$ W Hz$^{-1}$, consistent with a powerful FR II GRS. The X-ray and optical indicators of a radiatively efficient accreting nucleus coexisting with megaparsec-scale radio jets reinforce the link between obscured nuclei and extensive radio structures in the GRS population. Overall, J112829+5831.8 exemplifies how multiwavelength data can connect core obscuration and accretion physics with large-scale jet evolution in giant radio sources.

Abstract

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.

Figures (2)

• Figure 1: 3$-$79 keV NuSTAR image of Arp 299 region, generated by combining both focal plane module datasets. Color scaling is adjusted to highlight J1128+5831.
• Figure 2: The LOFAR (green) and VLASS (black) contours overplotted on the 0.5$-$7 keV image of the Arp 299 region using the data taken from the Chandra X-ray observatory. Contour levels are drawn at 3$\times$ rms $\times$ ($-$2,$-$1, 1) and increase in multiples of 2 for LOFAR and in multiples of $\sqrt{2}$ for VLASS data. The X-ray data are in native 0.492 arcseconds per pixel and smoothed using bi-cubic interpolation. The restoring beams of LOFAR and VLASS are shown at the bottom corners.