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1. X-Ray Reflection Signatures of Supermassive Black Hole Binaries

   https://doi.org/10.3847/1538-4357/adea75  

   Malewicz, Julie; Ballantyne, David R; Bogdanović, Tamara; Brenneman, Laura; Dauser, Thomas (August 2025, The Astrophysical Journal)

   Abstract We investigate the presence of supermassive black hole (SMBH) binary signatures and the feasibility of identifying them through X-ray reflection spectra. The X-ray emitting region is modeled as a set of two mini-disks bound to the individual SMBHs separated by 100GM/c²and the spectra calculated as a function of the mass, mass ratio, and total accretion rate of the binary. The X-ray reflection features are strongly influenced by the accretion-inversion phenomenon expected in SMBH binaries, which results in a wide range of ionization conditions in the two mini-disks. These are imprinted in the resulting composite spectra and the double-peaked and time-variable relativistic Fe Kαline profiles. To test whether these features can be used as evidence for the presence of an SMBH binary, we fit mock 100 ks observations with a single active galactic nucleus (AGN) model. For a 10⁹M_⊙binary targeted by pulsar timing arrays (PTAs), atz= 0.1, the single AGN model clearly fails to fit the data, while atz= 1, the fit is acceptable but unable to converge on the SMBH spin. For a 10⁶M_⊙binary, a progenitor of a Laser Interferometer Space Antenna (LISA) source, spectral fitting is only possible atz= 0.1, with the outcomes similar to the PTA binary atz= 1. We also find that PTA binaries can be expected to show a distinct X-ray spectral variability in multiepoch observations, whereas for LISA precursors, orbital averaging results in the loss of spectral variability signatures. 

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2. NOEMA Detection of Circumnuclear Molecular Gas in X-Ray Weak Dual Active Galactic Nuclei: No Evidence for Heavy Obscuration

   https://doi.org/10.3847/1538-4357/acaaf9  

   Hou, Meicun; Li, Zhiyuan; Liu, Xin; Li, Zongnan; Li, Ruancun; Wang, Ran; Wang, Jing; Ho, Luis C. (January 2023, The Astrophysical Journal)

   Abstract Dual active galactic nuclei (AGNs), which are the manifestation of two actively accreting supermassive black holes (SMBHs) hosted by a pair of merging galaxies, are a unique laboratory for studying the physics of SMBH feeding and feedback during an indispensable stage of galaxy evolution. In this work, we present NOEMA CO(2–1) observations of seven kiloparsec-scale dual-AGN candidates drawn from a recent Chandra survey of low redshift, optically classified AGN pairs. These systems are selected because they show unexpectedly low 2–10 keV X-ray luminosities for their small physical separations signifying an intermediate-to-late stage of merger. Circumnuclear molecular gas traced by the CO(2–1) emission is significantly detected in six of the seven pairs and 10 of the 14 nuclei, with an estimated mass ranging between (0.2–21) × 10 9 M ⊙ . The primary nuclei, i.e., the ones with the higher stellar velocity dispersion, tend to have a higher molecular gas mass than the secondary. Most CO-detected nuclei show a compact morphology, with a velocity field consistent with a kiloparsec-scale rotating structure. The inferred hydrogen column densities range between 5 × 10 21 –2 × 10 23 cm −2 , but mostly at a few times 10 22 cm −2 , in broad agreement with those derived from X-ray spectral analysis. Together with the relatively weak mid-infrared emission, the moderate column density argues against the prevalence of heavily obscured, intrinsically luminous AGNs in these seven systems, but favors a feedback scenario in which AGN activity triggered by a recent pericentric passage of the galaxy pair can expel circumnuclear gas and suppress further SMBH accretion. 

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3. A hard X-ray view of luminous and ultra-luminous infrared galaxies in GOALS – I. AGN obscuration along the merger sequence

   https://doi.org/10.1093/mnras/stab2052  

   Ricci, C; Privon, G C; Pfeifle, R W; Armus, L; Iwasawa, K; Torres-Albà, N; Satyapal, S; Bauer, F E; Treister, E; Ho, L C; et al (August 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The merger of two or more galaxies can enhance the inflow of material from galactic scales into the close environments of active galactic nuclei (AGNs), obscuring and feeding the supermassive black hole (SMBH). Both recent simulations and observations of AGN in mergers have confirmed that mergers are related to strong nuclear obscuration. However, it is still unclear how AGN obscuration evolves in the last phases of the merger process. We study a sample of 60 luminous and ultra-luminous IR galaxies (U/LIRGs) from the GOALS sample observed by NuSTAR. We find that the fraction of AGNs that are Compton thick (CT; $$N_{\rm H}\ge 10^{24}\rm \, cm^{-2}$$) peaks at $$74_{-19}^{+14}{{\ \rm per\ cent}}$$ at a late merger stage, prior to coalescence, when the nuclei have projected separations (dsep) of 0.4–6 kpc. A similar peak is also observed in the median NH [$$(1.6\pm 0.5)\times 10^{24}\rm \, cm^{-2}$$]. The vast majority ($$85^{+7}_{-9}{{\ \rm per\ cent}}$$) of the AGNs in the final merger stages (dsep ≲ 10 kpc) are heavily obscured ($$N_{\rm H}\ge 10^{23}\rm \, cm^{-2}$$), and the median NH of the accreting SMBHs in our sample is systematically higher than that of local hard X-ray-selected AGN, regardless of the merger stage. This implies that these objects have very obscured nuclear environments, with the $$N_{\rm H}\ge 10^{23}\rm \, cm^{-2}$$ gas almost completely covering the AGN in late mergers. CT AGNs tend to have systematically higher absorption-corrected X-ray luminosities than less obscured sources. This could either be due to an evolutionary effect, with more obscured sources accreting more rapidly because they have more gas available in their surroundings, or to a selection bias. The latter scenario would imply that we are still missing a large fraction of heavily obscured, lower luminosity ($$L_{2-10}\lesssim 10^{43}\rm \, erg\, s^{-1}$$) AGNs in U/LIRGs. 

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4. Dust-obscured Galaxies in the XMM-SERVS Fields: Selection, Multiwavelength Characterization, and Physical Nature

   https://doi.org/10.3847/1538-4357/ad8bc0  

   Yu_喻, Zhibo 知博; Brandt, W N; Zou, Fan; Zhu, Ziyuan; Bauer, Franz E; Cristello, Nathan; Luo, Bin; Ni, Qingling; Vito, Fabio; Xue, Yongquan (December 2024, The Astrophysical Journal)

   Abstract Dust-obscured galaxies (DOGs) are enshrouded by dust and many are believed to host accreting supermassive black holes (SMBHs), which makes them unique objects for probing the coevolution of galaxies and SMBHs. We select and characterize DOGs in the 13 deg²XMM-Spitzer Extragalactic Representative Volume Survey (XMM-SERVS), leveraging the superb multiwavelength data—from X-rays to radio. We select 3738 DOGs atz≈ 1.6–2.1 in XMM-SERVS, while maintaining good data quality without introducing significant bias. This represents the largest DOG sample with thorough multiwavelength source characterization. Spectral energy distribution modeling shows DOGs are a heterogeneous population consisting of both normal galaxies and active galactic nuclei (AGNs). Our DOGs are massive ( $\log M_{\star }/M_{☉}\approx 10.7-11.3$ ), 174 are detected in X-rays, and they are generally radio-quiet systems. X-ray detected DOGs are luminous and are moderately to heavily obscured in X-rays. Stacking analyses for the X-ray undetected DOGs show highly significant average detections. Critically, we compare DOGs with matched galaxy populations. DOGs have similar AGN fractions compared with typical galaxy populations. X-ray detected DOGs have higherM_⋆and higher X-ray obscuration, but they are not more star-forming than typical X-ray AGNs. Our results potentially challenge the relevance of the merger-driven galaxy-SMBH coevolution framework for X-ray detected DOGs. 

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5. Exploring Proxies for the Supermassive Black Hole Mass Function: Implications for Pulsar Timing Arrays

   https://doi.org/10.3847/2041-8213/acd18e  

   Simon, Joseph (May 2023, The Astrophysical Journal Letters)

   Abstract Supermassive black holes (SMBHs) reside at the center of every massive galaxy in the local universe with masses that closely correlate with observations of their host galaxy, implying a connected evolutionary history. The population of binary SMBHs, which form following galaxy mergers, is expected to produce a gravitational-wave background (GWB) detectable by pulsar timing arrays (PTAs). PTAs are starting to see hints of what may be a GWB, and the amplitude of the emerging signal is toward the higher end of model predictions. Simulated populations of binary SMBHs can be constructed from observations of galaxies and are used to make predictions about the nature of the GWB. The greatest source of uncertainty in these observation-based models comes from the inference of the SMBH mass function, which is derived from observed host galaxy properties. In this paper, I undertake a new approach for inferring the SMBH mass function, starting from a velocity dispersion function rather than a galaxy stellar mass function. I argue that this method allows for a more direct inference by relying on a larger suite of individual galaxy observations as well as relying on a more “fundamental” SMBH mass relation. I find that the resulting binary SMBH population contains more massive systems at higher redshifts than previous models. Additionally, I explore the implications for the detection of individually resolvable sources in PTA data. 

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