The complicated nature of the X-ray emission from the field of the strongly lensed hyperluminous infrared galaxy PJ1053+60 at z=3.549

TL;DR

This paper analyzes XMM‑Newton data for the strongly lensed HyLIRG PJ1053+60 at $z=3.549$, complemented by GNIRS spectroscopy that confirms a foreground AGN at $z_{AGN_{SW}}=1.373$. By combining spectral decomposition with 2D spatial modeling (using SMA dust maps to trace HMXB emission) and RefleX‑based AGN components, the authors find that the observed X‑ray flux likely requires an additional foreground AGN (AGN$_{FG}$) to explain the high luminosity, rather than being produced solely by the HyLIRG’s HMXB population and the SW AGN. The results show degeneracies due to limited spatial resolution and emphasize that higher angular resolution X‑ray observations are necessary to definitively characterize the three‑component system (PJ1053+60, AGN$_{SW}$, AGN$_{FG}$) and to robustly test the $L_X$–SFR relation at high redshift. Overall, the work demonstrates both the potential and current limitations of X‑ray studies of distant, strongly lensed HyLIRGs and highlights the importance of multiwavelength and lens modeling approaches.

Abstract

We present an analysis of XMM-Newton X-ray observations of PJ1053+60, a hyperluminous infrared galaxy (HyLIRG) at z=3.549 that is strongly lensed by a foreground group at z=0.837. We also present GNIRS spectroscopy confirming the presence of an active galactic nucleus (AGN) to the southwest of PJ1053+60 ($AGN_{SW}$) at $z_{SW}$ = 1.373 $\pm$ 0.006. Using this redshift prior, we decompose the X-ray spectrum of PJ1053+60 into $AGN_{SW}$ and high-mass X-ray binary (HMXB) components from the HyLIRG. The HMXB component has an unusually high luminosity, $\sim$ 50 times higher than calibration derived from local galaxies, and a characteristic photon index likely too flat to be caused by high-mass X-ray binaries at rest frame energies above a few keV. Our 2-D spatial decomposition also suggests a similarly high X-ray HMXB luminosity, although the limited spatial resolution prevents meaningful morphological constraints on the component. We conclude that the enhanced X-ray emission may only be explained by the presence of another AGN ($AGN_{FG}$) embedded in the foreground group lensing the PJ1053+60 system. The presence of $AGN_{FG}$ is further supported by the detection of a point-like radio continuum source that coincides with the brightest group galaxy (BGG) of the foreground lens. Our study demonstrates the limited capability of current X-ray observatories while highlighting the need for higher angular resolution observations to definitively characterize the nature of X-ray emission in distant, strongly lensed HyLIRGs.

Figures (9)

• Figure 1: Illustration showing the procedure to produce the $X_{\sc HMXB}$ 2D image model. The original masked SMA image is convolved with the XMM-Newton PSF. The convolved image is then rebinned to match the pixel coordinates of the XMM-Newton image to produce the model, with each pixel value being the expected counts from the HMXB population.
• Figure 2: Images comparing the different centroids used in our testing for a potential AGN in the foreground lens. The top left corner bars indicate $2^{\prime\prime}$ length. (A) VLA image showing $\rm AGN_{\rm FG}$ in the green circle and the magenta cross is the centroid of $\rm AGN_{\rm SW}$. (B) HST F160W image with VLA contour overlaid on the image. (C) X-ray image with different centroids and VLA contour overlaid. The green cross is the centroid of the $\rm AGN_{\rm FG}$ and the red cross is the centroid of the SMA image used in the 2D image modeling.
• Figure 3: Plots comparing X-ray flux and luminosities to 6 GHz flux (A) $F_X-S_{6 GHz}$ relation of the foreground AGN and the Gültekin AGN sample. This shows that $\rm AGN_{\rm FG}$ exhibits similar $F_X$ and $S_{6 GHz}$ to other AGN. (B) Expected $L_X$ from $SFR$ and measured $S_{6 GHz}$ of PJ1053+60 compared to the expected $L_X$ from $SFR$ and measured $S_{6 GHz}$ from star-forming sources. Here, we assume $X_{\sc HMXB}$$\sim 3$ for any sources at $z\geq3$, including PJ1053+60. The color of each marker is the redshift of each source to show that PJ1053+60 exhibits similar trends to other sources at high-z.
• Figure 4: Reduced continuum subtracted GNIRS spectra of the AGN with 2D spectra shown in the top panel. The gray regions indicate wavelengths contaminated with significant atmospheric absorption lines. The top panel shows the 2D image cutout of the spectra. Here three emission lines [Si III], Fe II, and Pa-$\epsilon$ have strong detections with S/N $>$ 3.
• Figure 5: XMM-Newton spectral data with the best fit MCMC RefleX model (blue and purple shaded regions) at a 90% confidence level. The data is split into the two XMM-Newton instruments, MOS and pn. Here, the dot markers are the nonAGN or HMXB emission, the X markers are the reprocessed light of $\rm AGN_{\rm SW}$ and the triangle markers are the continuum like from $\rm AGN_{\rm SW}$Ȧlthough most data points are within 1 or 2 error levels, the model has a high goodness percentage of 86.7% (see Table \ref{['t:spec']}). This suggests that another AGN component from $\rm AGN_{\rm FG}$ is required to provide a better g.o.f.; however, due to the small spatial separation from PJ1053+60 and $\rm AGN_{\rm FG}$ we cannot disentangle the spectra into three components: PJ1053+60, $\rm AGN_{\rm SW}$, and $\rm AGN_{\rm FG}$.