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1. Bridging the Gap: Categorizing Gravitational-wave Events at the Transition between Neutron Stars and Black Holes

   https://doi.org/10.3847/1538-4357/ac5f03  

   Farah, Amanda ; Fishbach, Maya ; Essick, Reed ; Holz, Daniel E. ; Galaudage, Shanika ( May 2022 , The Astrophysical Journal)

   We search for features in the mass distribution of detected compact binary coalescences which signify the transition between neutron stars (NSs) and black holes (BHs). We analyze all gravitational-wave (GW) detections by the LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration (LVK) made through the end of the first half of the third observing run, and find clear evidence for two different populations of compact objects based solely on GW data. We confidently (99.3%) find a steepening relative to a single power law describing NSs and low-mass BHs below${2.4}_{-0.5}^{+0.5}{M}_{\odot }$, which is consistent with many predictions for the maximum NS mass. We find suggestions of the purported lower mass gap between the most massive NSs and the least massive BHs, but are unable to conclusively resolve it with current data. If it exists, we find the lower mass gap’s edges to lie at${2.2}_{-0.5}^{+0.7}{M}_{\odot }$and${6.0}_{-1.4}^{+2.4}{M}_{\odot }$. We reexamine events that have been deemed “exceptional” by the LVK collaborations in the context of these features. We analyze GW190814 self-consistently in the context of the full population of compact binaries, finding support for its secondary to be either a NS or a lower mass gap object, consistent with previous claims. Our models are the first to accommodate this event, which is an outlier with respect to the binary BH population. We find that GW200105 and GW200115 probe the edges of, and may have components within, the lower mass gap. As future data improve global population models, the classification of these events will also improve.

    

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2. The Sloan Digital Sky Survey Reverberation Mapping Project: The Black Hole Mass–Stellar Mass Relations at 0.2 ≲ z ≲ 0.8

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

   Li, Jennifer I-Hsiu ; Shen, Yue ; Ho, Luis C. ; Brandt, W. N. ; Grier, Catherine J. ; Hall, Patrick B. ; Homayouni, Y. ; Koekemoer, Anton M. ; Schneider, Donald P. ; Trump, Jonathan R. ( September 2023 , The Astrophysical Journal)

   We measure the correlation between black hole massM_BHand host stellar massM_*for a sample of 38 broad-line quasars at 0.2 ≲z≲ 0.8 (median redshiftz_med= 0.5). The black hole masses are derived from a dedicated reverberation mapping program for distant quasars, and the stellar masses are derived from two-band optical+IR Hubble Space Telescope imaging. Most of these quasars are well centered within ≲1 kpc from the host galaxy centroid, with only a few cases in merging/disturbed systems showing larger spatial offsets. Our sample spans two orders of magnitude in stellar mass (∼10⁹–10¹¹M_⊙) and black hole mass (∼10⁷–10⁹M_⊙) and reveals a significant correlation between the two quantities. We find a best-fit intrinsic (i.e., selection effects corrected)M_BH–M_*,hostrelation of$\log (M_{\mathrm{BH}}/M_{\odot })={7.01}_{-0.33}^{+0.23}+{1.74}_{-0.64}^{+0.64}\log (M_{*,\mathrm{host}}/{10}^{10}{M}_{\odot })$, with an intrinsic scatter of${0.47}_{-0.17}^{+0.24}$dex. Decomposing our quasar hosts into bulges and disks, there is a similarM_BH–M_*,bulgerelation with slightly larger scatter, likely caused by systematic uncertainties in the bulge–disk decomposition. TheM_BH–M_*,hostrelation atz_med= 0.5 is similar to that in local quiescent galaxies, with negligible evolution over the redshift range probed by our sample. With direct black hole masses from reverberation mapping and the large dynamical range of the sample, selection biases do not appear to affect our conclusions significantly. Our results, along with other samples in the literature, suggest that the locally measured black hole mass–host stellar mass relation is already in place atz∼ 1.

    

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3. Tidal Disruption Event Demographics with the Zwicky Transient Facility: Volumetric Rates, Luminosity Function, and Implications for the Local Black Hole Mass Function

   https://doi.org/10.3847/2041-8213/acf216  

   Yao, Yuhan ; Ravi, Vikram ; Gezari, Suvi ; van Velzen, Sjoert ; Lu, Wenbin ; Schulze, Steve ; Somalwar, Jean J. ; Kulkarni, S. R. ; Hammerstein, Erica ; Nicholl, Matt ; et al ( September 2023 , The Astrophysical Journal Letters)

   We conduct a systematic tidal disruption event (TDE) demographics analysis using the largest sample of optically selected TDEs. A flux-limited, spectroscopically complete sample of 33 TDEs is constructed using the Zwicky Transient Facility over 3 yr (from 2018 October to 2021 September). We infer the black hole (BH) mass (M_BH) with host galaxy scaling relations, showing that the sampleM_BHranges from 10^5.1M_⊙to 10^8.2M_⊙. We developed a survey efficiency corrected maximum volume method to infer the rates. The rest-frameg-band luminosity function can be well described by a broken power law of$\varphi (L_g)\propto {\left[{\left(L_g/L_{\mathrm{bk}}\right)}^{0.3}+{\left(L_g/L_{\mathrm{bk}}\right)}^{2.6}\right]}^{-1}$, withL_bk= 10^43.1erg s⁻¹. In the BH mass regime of 10^5.3≲ (M_BH/M_⊙) ≲ 10^7.3, the TDE mass function follows$\varphi (M_{\mathrm{BH}})\propto M_{\mathrm{BH}}^{-0.25}$, which favors a flat local BH mass function (${\mathit{dn}}_{\mathrm{BH}}/d\log M_{\mathrm{BH}}\approx \mathrm{constant}$). We confirm the significant rate suppression at the high-mass end (M_BH≳ 10^7.5M_⊙), which is consistent with theoretical predictions considering direct capture of hydrogen-burning stars by the event horizon. At a host galaxy mass ofM_gal∼ 10¹⁰M_⊙, the average optical TDE rate is ≈3.2 × 10⁻⁵galaxy⁻¹yr⁻¹. We constrain the optical TDE rate to be [3.7, 7.4, and 1.6] × 10⁻⁵galaxy⁻¹yr⁻¹in galaxies with red, green, and blue colors.

    

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4. Investigating the Lower Mass Gap with Low-mass X-Ray Binary Population Synthesis

   https://doi.org/10.3847/1538-4357/ace9d9  

   Siegel, Jared C. ; Kiato, Ilia ; Kalogera, Vicky ; Berry, Christopher P. L. ; Maccarone, Thomas J. ; Breivik, Katelyn ; Andrews, Jeff J. ; Bavera, Simone S. ; Dotter, Aaron ; Fragos, Tassos ; et al ( September 2023 , The Astrophysical Journal)

   Mass measurements from low-mass black hole X-ray binaries (LMXBs) and radio pulsars have been used to identify a gap between the most massive neutron stars (NSs) and the least massive black holes (BHs). BH mass measurements in LMXBs are typically only possible for transient systems: outburst periods enable detection via all-sky X-ray monitors, while quiescent periods enable radial velocity measurements of the low-mass donor. We quantitatively study selection biases due to the requirement of transient behavior for BH mass measurements. Using rapid population synthesis simulations (COSMIC), detailed binary stellar-evolution models (MESA), and the disk instability model of transient behavior, we demonstrate that transient LMXB selection effects introduce observational biases, and can suppress mass-gap BHs in the observed sample. However, we find a population of transient LMXBs with mass-gap BHs form through accretion-induced collapse of an NS during the LMXB phase, which is inconsistent with observations. These results are robust against variations of binary evolution prescriptions. The significance of this accretion-induced collapse population depends upon the maximum NS birth mass$M_{\mathrm{NS},\mathrm{birth}-\max }$. To reflect the observed dearth of low-mass BHs,COSMICandMESAmodels favor$M_{\mathrm{NS},\mathrm{birth}-\max }\lesssim 2M_{\odot }$. In the absence of further observational biases against LMXBs with mass-gap BHs, our results indicate the need for additional physics connected to the modeling of LMXB formation and evolution.

    

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5. A Reanalysis of the Isolated Black Hole Candidate OGLE-2011-BLG-0462/MOA-2011-BLG-191

   https://doi.org/10.3847/1538-4357/aced4a  

   Lam, Casey Y. ; Lu, Jessica R. ( September 2023 , The Astrophysical Journal)

   There are expected to be ∼10⁸isolated black holes (BHs) in the Milky Way. OGLE-2011-BLG-0462/MOA-2011-BLG-191 (OB110462) is the only such BH with a mass measurement to date. However, its mass is disputed: Lam et al. measured a lower mass of 1.6–4.4M_⊙, while Sahu et al. and Mróz et al. measured a higher mass of 5.8–8.7M_⊙. We reanalyze OB110462, including new data from the Hubble Space Telescope (HST) and rereduced Optical Gravitational Lensing Experiment (OGLE) photometry. We also rereduce and reanalyze the HST data set with newly available software. We find significantly different (∼1 mas) HST astrometry than Lam et al. in the unmagnified epochs due to the amount of positional bias induced by a bright star ∼0.″4 from OB110462. After modeling the updated photometric and astrometric data sets, we find the lens of OB110462 is a${6.0}_{-1.0}^{+1.2}{M}_{\odot }$BH. Future observations with the Nancy Grace Roman Space Telescope, which will have an astrometric precision comparable or better to HST but a field of view 100× larger, will be able to measure hundreds of isolated BH masses via microlensing. This will enable the measurement of the BH mass distribution and improve understanding of massive stellar evolution and BH formation channels.

    

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