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1. Globular cluster ultraluminous X-ray sources in the furthest early-type galaxies

   https://doi.org/10.1093/mnras/stac3244  

   Thygesen, Erica ; Sun, Yifan ; Huang, Jeff ; Dage, Kristen C. ; Zepf, Stephen E. ; Kundu, Arunav ; Haggard, Daryl ; Maccarone, Thomas J. ( November 2022 , Monthly Notices of the Royal Astronomical Society)

   Ultraluminous X-ray sources (ULXs) in globular clusters (GCs) are low-mass X-ray binaries that achieve high X-ray luminosities through a currently uncertain accretion mechanism. Using archival Chandra and Hubble Space Telescope observations, we perform a volume-limited search (≲70 Mpc) of 21 of the most massive ($\gt 10^{11.5} \, \mathrm{M}_\odot$) early-type galaxies to identify ULXs hosted by GC candidates. We find a total of 34 ULX candidates above the expected background within five times the effective radius of each galaxy, with 10 of these ($\sim 29.4{{\ \rm per\ cent}}$) potentially hosted by a GC. A comparison of the spatial and luminosity distributions of these new candidate GC ULXs with previously identified GC ULXs shows that they are similar: both samples peak at LX ∼ a few × 1039 erg s−1 and are typically located within a few effective radii of their host galaxies.

    

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2. A Chandra X-Ray Survey of Optically Selected Close Galaxy Pairs: Unexpectedly Low Occupation of Active Galactic Nuclei

   https://doi.org/10.3847/1538-4357/acc5e1  

   He, Lin ; Hou, Meicun ; Li, Zhiyuan ; Feng, Shuai ; Liu, Xin ( May 2023 , The Astrophysical Journal)

   High-resolution X-ray observations offer a unique tool for probing the still-elusive connection between galaxy mergers and active galactic nuclei (AGNs). We present an analysis of nuclear X-ray emission in an optically selected sample of 92 close galaxy pairs (with projected separations ≲20 kpc and line-of-sight velocity offsets <500 km s⁻¹) at low redshift ($\overline{z}\sim 0.07$), based on archival Chandra observations. The parent sample of galaxy pairs is constructed without imposing an optical classification of nuclear activity, thus it is largely free of selection effect for or against the presence of an AGN. Nor is this sample biased for or against gas-rich mergers. An X-ray source is detected in 70 of the 184 nuclei, giving a detection rate of$38{\%}_{-5\%}^{+5\%}$, down to a 0.5–8 keV limiting luminosity of ≲10⁴⁰erg s⁻¹. The detected and undetected nuclei show no systematic difference in their host galaxy properties such as galaxy morphology, stellar mass, and stellar velocity dispersion. When potential contamination from star formation is avoided (i.e.,L_{2−10 keV}> 10⁴¹erg s⁻¹), the detection rate becomes$18{\%}_{-3\%}^{+3\%}$(32/184), which shows no excess compared to the X-ray detection rate of a comparison sample of optically classified single AGNs. The fraction of pairs containing dual AGN is only$2{\%}_{-2\%}^{+2\%}$. Moreover, most nuclei at the smallest projected separations probed by our sample (a few kiloparsecs) have an unexpectedly low apparent X-ray luminosity and Eddington ratio, which cannot be solely explained by circumnuclear obscuration. These findings suggest that close galaxy interaction is not a sufficient condition for triggering a high level of AGN activity.

    

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3. NuSTAR Observes Two Bulgeless Galaxies: No Hard X-Ray AGN Detected in NGC 4178 or J0851+3926

   https://doi.org/10.3847/1538-4357/aca7ff  

   Pfeifle, Ryan W. ; Satyapal, Shobita ; Ricci, Claudio ; Secrest, Nathan J. ; Gliozzi, Mario ; Bohn, Thomas ; Canalizo, Gabriela ; Reefe, Michael A. ( January 2023 , The Astrophysical Journal)

   The discovery over the last several decades of low- and moderate-luminosity active galactic nuclei (AGNs) in disk-dominated galaxies—which show no “classical” bulges—suggests that secular mechanisms represent an important growth pathway for supermassive black holes in these systems. We present new follow-up NuSTAR observations of the optically elusive AGNs in two bulgeless galaxies, NGC 4178 and J0851+3926. Galaxy NGC 4178 was originally reported as hosting an AGN based on the detection of [Nev] mid-infrared emission detected by Spitzer, and based on Chandra X-ray imaging, it has since been argued to host either a heavily obscured AGN or a supernova remnant. Galaxy J0851+3926 was originally identified as an AGN based on its Wide-Field Infrared Survey Explorer mid-IR colors, and follow-up near-infrared spectroscopy previously revealed a hidden broad-line region, offering compelling evidence for an optically elusive AGN. Neither AGN is detected within the new NuSTAR imaging, and we derive upper limits on the hard X-ray 10–24 keV fluxes of <7.41 × 10⁻¹⁴and <9.40 × 10⁻¹⁴erg cm⁻²s⁻¹for the AGNs in NGC 4178 and J0851+3926, respectively. If these nondetections are due to large absorbing columns along the line of sight, the nondetections in NGC 4178 and J0851+3926 could be explained with column densities of log(N_H/cm²) > 24.2 and 24.1, respectively. The nature of the nuclear activity in NGC 4178 remains inconclusive; it is plausible that the [Nev] traces a period of higher activity in the past, but that the AGN is relatively quiescent now. The nondetection in J0851+3926 and multiwavelength properties are consistent with the AGN being heavily obscured.

    

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4. XMM- Newton -discovered Fast X-ray Transients: host galaxies and limits on contemporaneous detections of optical counterparts

   https://doi.org/10.1093/mnras/stad3924  

   Eappachen, D. ; Jonker, P. G. ; Quirola-Vásquez, J. ; Sánchez, D. Mata ; Inkenhaag, A. ; Levan, A. J. ; Fraser, M. ; Torres, M. A. P. ; Bauer, F. E. ; Chrimes, A. A. ; et al ( December 2023 , Monthly Notices of the Royal Astronomical Society)

   Extragalactic fast X-ray transients (FXTs) are a class of soft (0.3–10 keV) X-ray transients lasting a few hundred seconds to several hours. Several progenitor mechanisms have been suggested to produce FXTs, including supernova shock breakouts, binary neutron star mergers, or tidal disruptions involving an intermediate-mass black hole and a white dwarf. We present detailed host studies, including spectroscopic observations of the host galaxies of seven XMM-Newton-discovered FXTs. The candidate hosts lie at redshifts 0.0928 <z < 0.645 implying peak X-ray luminosities of 1043 erg s−1<LX < 1045 erg s−1 and physical offsets of 1 kpc < rproj < 22 kpc. These observations increase the number of FXTs with a spectroscopic redshift measurement by a factor of 2, although we note that one event is re-identified as a Galactic flare star. We infer host star formation rates and stellar masses by fitting the combined spectroscopic and archival photometric data. We also report on a contemporaneous optical counterpart search to the FXTs in Pan-STARRS and ATLAS by performing forced photometry at the position of the FXTs. We do not find any counterpart in our search. Given our constraints, including peak X-ray luminosities, optical limits, and host properties, we find that XRT 110 621 is consistent with an supernova shock breakout (SN SBO) event. Spectroscopic redshifts of likely host galaxies for four events imply peak X-ray luminosities that are too high to be consistent with SN SBOs, but we are unable to discard either the binary neutron star or white dwarf–intermediate-mass black hole tidal disruption event scenarios for these FXTs.

    

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5. The ROSAT Raster survey in the north ecliptic pole field: X-ray catalogue and optical identifications

   https://doi.org/10.1051/0004-6361/202039476  

   Hasinger, G. ; Freyberg, M. ; Hu, E. M. ; Waters, C. Z. ; Capak, P. ; Moneti, A. ; McCracken, H. J. ( January 2021 , Astronomy & Astrophysics)

   null (Ed.)

   The north ecliptic pole (NEP) is an important region for extragalactic surveys. Deep and wide contiguous surveys are being performed by several space observatories, most currently with the eROSITA telescope. Several more are planned for the near future. We analyse all the ROSAT pointed and survey observations in a region of 40 deg 2 around the NEP, restricting the ROSAT field of view to the inner 30′ radius. We obtain an X-ray catalogue of 805 sources with 0.5−2 keV fluxes > 2.9 × 10 −15 erg cm −2 s −1 , about a factor of three deeper than the ROSAT All-Sky Survey in this field. The sensitivity and angular resolution of our data are comparable to the eROSITA All-Sky Survey expectations. The 50% position error radius of the sample of X-ray sources is ∼10″. We use HEROES optical and near-infrared imaging photometry from the Subaru and Canada/France/Hawaii telescopes together with GALEX, SDSS, Pan-STARRS, and WISE catalogues, as well as images from a new deep and wide Spitzer survey in the field to statistically identify the X-ray sources and to calculate photometric redshifts for the candidate counterparts. In particular, we utilize mid-infrared (mid-IR) colours to identify active galactic nucleus (AGN) X-ray counterparts. Despite the relatively large error circles and often faint counterparts, together with confusion issues and systematic errors, we obtain a rather reliable catalogue of 766 high-quality optical counterparts, corresponding redshifts and optical classifications. The quality of the dataset is sufficient to look at ensemble properties of X-ray source classes. In particular we find a new population of luminous absorbed X-ray AGN at large redshifts, identified through their mid-IR colours. This populous group of AGN was not recognized in previous X-ray surveys, but could be identified in our work due to the unique combination of survey solid angle, X-ray sensitivity, and quality of the multi-wavelength photometry. We also use the WISE and Spitzer photometry to identify a sample of 185 AGN selected purely through their mid-IR colours, most of which are not detected by ROSAT. Their redshifts and upper limits to X-ray luminosity and X-ray–to–optical flux ratios are even higher than for the new class of X-ray selected luminous type 2 AGN (AGN2); they are probably a natural extension of this sample. This unique dataset is important as a reference sample for future deep surveys in the NEP region, in particular for eROSITA and also for Euclid and SPHEREX. We predict that most of the absorbed distant AGN should be readily picked up by eROSITA, but they require sensitive mid-IR imaging to be recognized as optical counterparts. 

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