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1. Constraining a Companion of the Galactic Center Black Hole Sgr A*

   https://doi.org/10.3847/1538-4357/ad09b3  

   Will, Clifford M; Naoz, Smadar; Hees, Aurélien; Tucker, Alexandria; Zhang, Eric; Do, Tuan; Ghez, Andrea (December 2023, The Astrophysical Journal)

   Abstract We use 23 yr of astrometric and radial velocity data on the orbit of the star S0-2 to constrain a hypothetical intermediate-mass black hole orbiting the massive black hole Sgr A* at the Galactic center. The data place upper limits on variations of the orientation of the stellar orbit at levels between 0.°02 and 0.°07 per year. We use a combination of analytic estimates and full numerical integrations of the orbit of S0-2 in the presence of a black hole binary. For a companion intermediate-mass black hole outside the orbit of S0-2 (1020 au), we find that a companion black hole with massm_cbetween 10³and 10⁵M_⊙is excluded, with a boundary behaving as $a_c\sim m_c^{1/3}$ . For a companion witha_c< 1020 au, a black hole with mass between 10³and 10⁵M_⊙is excluded, with $a_c\sim m_c^{-1/2}$ . These bounds arise from quadrupolar perturbations of the orbit of S0-2. Significantly stronger bounds on an inner companion arise from the fact that the location of S0-2 is measured relative to the bright emission of Sgr A* and that separation is perturbed by the “wobble” of Sgr A* about the center of mass between it and the companion. The result is a set of bounds as small as 400M_⊙at 200 au; the numerical simulations suggest a bound from these effects varying as $a_c\sim m_c^{-1}$ . We compare and contrast our results with those from a recent analysis by the GRAVITY collaboration. 

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2. X-Ray Constraints on Wandering Black Holes in Stripped Galaxy Nuclei in the Halo of NGC 5128

   https://doi.org/10.3847/1538-4357/adc7b9  

   Feyan, Samuel L; Urquhart, Ryan; Strader, Jay; Seth, Anil C; Sand, David J; Caldwell, Nelson; Crnojević, Denija; Dumont, Antoine; Voggel, Karina (May 2025, The Astrophysical Journal)

   Abstract A subset of galaxies have dense nuclei, and when these galaxies are accreted and tidally stripped, the nuclei can masquerade as globular clusters in the halos of large galaxies. If these nuclei contain massive central black holes, some may accrete gas and become observable as active galactic nuclei. Previous studies have found that candidate stripped nuclei rarely host luminous X-ray sources, but these studies were typically restricted to both the most massive candidate nuclei and the most luminous X-ray sources. Here we use new and archival Chandra and XMM-Newton data to search for X-ray emission in a near-complete sample of massive globular clusters and candidate stripped nuclei in the nearest accessible elliptical galaxy, NGC 5128. This sample has the unique advantage that the candidate stripped nuclei are identified dynamically via elevated mass-to-light ratios (M/L_V). Our central result is that 5/22 ( $23_{-6}^{+11}$ %) of the candidate stripped nuclei have X-ray sources down to a typical limit ofL_X∼ 5 × 10³⁶erg s⁻¹, a fraction lower than or comparable to that among massive clusters with normalM/L_V(16/41; $39_{-7}^{+8}$ %). Hence we confirm and extend the result that nearly all X-ray sources in stripped nuclei are likely to be X-ray binaries rather than active galactic nuclei. If the candidate stripped nuclei have black holes of typical masses ∼2 × 10⁵M_⊙needed to explain their elevatedM/L_V, then they have typical Eddington ratios of ≲ 2 × 10⁻⁶. This suggests that it will be challenging to conduct an accretion census of wandering black holes around even nearby galaxies. 

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3. Observation of the Galactic Center PeVatron beyond 100 TeV with HAWC

   https://doi.org/10.3847/2041-8213/ad772e  

   Albert, A; Alfaro, R; Alvarez, C; Andrés, A; Arteaga-Velázquez, J C; Avila_Rojas, D; Ayala_Solares, H A; Babu, R; Belmont-Moreno, E; Bernal, A; et al (September 2024, The Astrophysical Journal Letters)

   Abstract We report an observation of ultrahigh-energy (UHE) gamma rays from the Galactic center (GC) region, using 7 yr of data collected by the High-Altitude Water Cherenkov (HAWC) Observatory. The HAWC data are best described as a point-like source (HAWC J1746-2856) with a power-law spectrum ( $\mathit{d}N/\mathit{d}E=\varphi {\left(E/26\mathrm{TeV}\right)}^{\gamma }$ ), whereγ= −2.88 ± 0.15_stat− 0.1_sysandϕ= 1.5 × 10⁻¹⁵(TeV cm²s)⁻¹ $\pm {0.3}_{\mathrm{stat}}{}_{-{0.13}_{\mathrm{sys}}}^{+{0.08}_{\mathrm{sys}}}$ extending from 6 to 114 TeV. We find no evidence of a spectral cutoff up to 100 TeV using HAWC data. Two known point-like gamma-ray sources are spatially coincident with the HAWC gamma-ray excess: Sgr A* (HESS J1745-290) and the Arc (HESS J1746-285). We subtract the known flux contribution of these point sources from the measured flux of HAWC J1746-2856 to exclude their contamination and show that the excess observed by HAWC remains significant (>5σ), with the spectrum extending to >100 TeV. Our result supports that these detected UHE gamma rays can originate via hadronic interaction of PeV cosmic-ray protons with the dense ambient gas and confirms the presence of a proton PeVatron at the GC. 

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4. A Panchromatic Study of the X-Ray Binary Population in NGC 300 on Subgalactic Scales

   https://doi.org/10.3847/1538-4357/ad46d9  

   Binder, Breanna A; Williams, Rosalie; Payne, Jacob; Eracleous, Michael; Belles, Alexander; Williams, Benjamin F (July 2024, The Astrophysical Journal)

   Abstract The population-wide properties and demographics of extragalactic X-ray binaries (XRBs) correlate with the star formation rates (SFRs), stellar masses (M_⋆), and environmental factors (such as metallicity,Z) of their host galaxy. Although there is evidence that XRB scaling relations (L_X/SFR for high-mass XRBs (HMXBs) andL_X/M_⋆for low-mass XRBs) may depend on metallicity and stellar age across large samples of XRB-hosting galaxies, disentangling the effects of metallicity and stellar age from stochastic effects, particularly on subgalactic scales, remains a challenge. We use archival X-ray through IR observations of the nearby galaxy NGC 300 to self-consistently model the broadband spectral energy distribution and examine radial trends in its XRB population. We measure a current (<100 Myr) SFR of 0.18 ± 0.08 M_⊙yr⁻¹and stellar massM_⋆= $\left({2.15}_{-0.14}^{+0.26}\right)\times {10}^9$ M_⊙. Although we measure a metallicity gradient and radially resolved star formation histories that are consistent with the literature, there is a clear excess in the number of X-ray sources below ∼10³⁷erg s⁻¹that are likely a mix of variable XRBs and additional background active galactic nuclei. When we compare the subgalacticL_X/SFR ratios as a function ofZto the galaxy-integratedL_X-SFR-Zrelationships from the literature, we find that only the regions hosting the youngest (≲30 Myr) HMXBs agree with predictions, hinting at time evolution of theL_X–SFR–Zrelationship. 

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5. AGN STORM 2. VIII. Investigating the Narrow Absorption Lines in Mrk 817 Using HST-COS Observations

   https://doi.org/10.3847/1538-4357/ad5ff4  

   Dehghanian, Maryam; Arav, Nahum; Kriss, Gerard A; Mehdipour, Missagh; Byun, Doyee; Walker, Gwen; Sharma, Mayank; Barth, Aaron J; Bentz, Misty C; Boizelle, Benjamin D; et al (September 2024, The Astrophysical Journal)

   Abstract We observed the Seyfert 1 galaxy Mrk 817 during an intensive multiwavelength reverberation mapping campaign for 16 months. Here, we examine the behavior of narrow UV absorption lines seen in the Hubble Space Telescope/Cosmic Origins Spectrograph spectra, both during the campaign and in other epochs extending over 14 yr. We conclude that, while the narrow absorption outflow system (at −3750 km s⁻¹with FWHM = 177 km s⁻¹) responds to the variations of the UV continuum as modified by the X-ray obscurer, its total column density (logN_H= 19.5 ${}_{-0.13}^{+0.61}$ cm⁻²) did not change across all epochs. The adjusted ionization parameter (scaled with respect to the variations in the hydrogen-ionizing continuum flux) is logU_H= −1.0 ${}_{-0.3}^{+0.1}$ . The outflow is located at a distance smaller than 38 pc from the central source, which implies a hydrogen density ofn_H> 3000 cm⁻³. The absorption outflow system only covers the continuum emission source and not the broad emission line region, which suggests that its transverse size is small (< 10¹⁶cm), with potential cloud geometries ranging from spherical to elongated along the line of sight. 

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