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1. A Binary Origin for the First Isolated Stellar-mass Black Hole Detected with Astrometric Microlensing

   https://doi.org/10.3847/2041-8213/acc076  

   Vigna-Gómez, Alejandro ; Ramirez-Ruiz, Enrico ( March 2023 , The Astrophysical Journal Letters)

   The Milky Way is believed to host hundreds of millions of quiescent stellar-mass black holes (BHs). In the last decade, some of these objects have been potentially uncovered via gravitational microlensing events. All these detections resulted in a degeneracy between the velocity and the mass of the lens. This degeneracy has been lifted, for the first time, with the recent astrometric microlensing detection of OB110462. However, two independent studies reported very different lens masses for this event. Sahu et al. inferred a lens mass of 7.1 ± 1.3M_⊙, consistent with a BH, while Lam et al. inferred 1.6–4.2M_⊙, consistent with either a neutron star or a BH. Here, we study the landscape of isolated BHs formed in the field. In particular, we focus on the mass and center-of-mass speed of four subpopulations: isolated BHs from single-star origin, disrupted BHs of binary-star origin, main-sequence stars with a compact object companion, and double compact object mergers. Our model predicts that most (≳70%) isolated BHs in the Milky Way are of binary origin. However, noninteractions lead to most massive BHs (≳15–20M_⊙) being predominantly of single origin. Under the assumption that OB110462 is a free-floating compact object, we conclude that it is moremore »likely to be a BH originally belonging to a binary system. Our results suggest that low-mass BH microlensing events can be useful to understand binary evolution of massive stars in the Milky Way, while high-mass BH lenses can be useful to probe single stellar evolution.

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2. The Impact of Initial–Final Mass Relations on Black Hole Microlensing

   https://doi.org/10.3847/1538-4357/aca09d  

   Rose, Sam ; Lam, Casey Y. ; Lu, Jessica R. ; Medford, Michael ; Hosek, Jr., Matthew W. ; Abrams, Natasha S. ; Ramey, Emily ; Vasylyev, Sergiy S. ( December 2022 , The Astrophysical Journal)

   Uncertainty in the initial–final mass relation (IFMR) has long been a problem in understanding the final stages of massive star evolution. One of the major challenges of constraining the IFMR is the difficulty of measuring the mass of nonluminous remnant objects (i.e., neutron stars and black holes). Gravitational-wave detectors have opened the possibility of finding large numbers of compact objects in other galaxies, but all in merging binary systems. Gravitational lensing experiments using astrometry and photometry are capable of finding compact objects, both isolated and in binaries, in the Milky Way. In this work we improve the Population Synthesis for Compact object Lensing Events (PopSyCLE)microlensing simulation code in order to explore the possibility of constraining the IFMR using the Milky Way microlensing population. We predict that the Roman Space Telescope’s microlensing survey will likely be able to distinguish different IFMRs based on the differences at the long end of the Einstein crossing time distribution and the small end of the microlensing parallax distribution, assuming the small (π_E≲ 0.02) microlensing parallaxes characteristic of black hole lenses are able to be measured accurately. We emphasize that future microlensing surveys need to be capable of characterizing events with small microlensing parallaxes inmore »order to place the most meaningful constraints on the IFMR.

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3. Supplement: “An Isolated Mass-gap Black Hole or Neutron Star Detected with Astrometric Microlensing” (2022, ApJL, 933, L23)

   https://doi.org/10.3847/1538-4365/ac7441  

   Lam, Casey Y. ; Lu, Jessica R. ; Udalski, Andrzej ; Bond, Ian ; Bennett, David P. ; Skowron, Jan ; Mróz, Przemek ; Poleski, Radek ; Sumi, Takahiro ; Szymański, Michał K. ; et al ( June 2022 , The Astrophysical Journal Supplement Series)

   Abstract This supplement provides supporting material for Lam et al. We briefly summarize past gravitational microlensing searches for black holes (BHs) and present details of the observations, analysis, and modeling of five BH candidates observed with both ground-based photometric microlensing surveys and Hubble Space Telescope astrometry and photometry. We present detailed results for four of the five candidates that show no or low probability for the lens to be a BH. In these cases, the lens masses are <2 M ⊙ , and two of the four are likely white dwarfs or neutron stars. We also present detailed methods for comparing the full sample of five candidates to theoretical expectations of the number of BHs in the Milky Way (∼10 8 ).
4. “Super-kilonovae” from Massive Collapsars as Signatures of Black Hole Birth in the Pair-instability Mass Gap

   https://doi.org/10.3847/1538-4357/ac8d04  

   Siegel, Daniel M. ; Agarwal, Aman ; Barnes, Jennifer ; Metzger, Brian D. ; Renzo, Mathieu ; Villar, V. Ashley ( December 2022 , The Astrophysical Journal)

   The core collapse of rapidly rotating massive ∼ 10M_⊙stars (“collapsars”), and the resulting formation of hyperaccreting black holes, comprise a leading model for the central engines of long-duration gamma-ray bursts (GRBs) and promising sources ofr-process nucleosynthesis. Here, we explore the signatures of collapsars from progenitors with helium cores ≳ 130M_⊙above the pair-instability mass gap. While the rapid collapse to a black hole likely precludes prompt explosions in these systems, we demonstrate that disk outflows can generate a large quantity (up to ≳ 50M_⊙) of ejecta, comprised of ≳ 5–10M_⊙inr-process elements and ∼ 0.1–1M_⊙of⁵⁶Ni, expanding at velocities ∼0.1 c. Radioactive heating of the disk wind ejecta powers an optical/IR transient, with a characteristic luminosity ∼ 10⁴²erg s⁻¹and a spectral peak in the near-IR (due to the high optical/UV opacities of lanthanide elements), similar to kilonovae from neutron star mergers, but with longer durations ≳1 month. These “super-kilonovae” (superKNe) herald the birth of massive black holes ≳ 60M_⊙, which—as a result of disk wind mass loss—can populate the pair-instability mass gap “from above,” and could potentially create the binary components of GW190521. SuperKNe could be discovered via wide-field surveys, such as those planned with the Roman Space Telescope, or via late-timemore »IR follow-up observations of extremely energetic GRBs. Multiband gravitational waves of ∼ 0.1–50 Hz from nonaxisymmetric instabilities in self-gravitating massive collapsar disks are potentially detectable by proposed observatories out to hundreds of Mpc; in contrast to the “chirp” from binary mergers, the collapsar gravitational-wave signal decreases in frequency as the disk radius grows (“sad trombone”).

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5. KMT-2021-BLG-0912Lb: a microlensing super Earth around a K-type star

   https://doi.org/10.1051/0004-6361/202142495  

   Han, Cheongho ; Bond, Ian A. ; Yee, Jennifer C. ; Zang, Weicheng ; Albrow, Michael D. ; Chung, Sun-Ju ; Gould, Andrew P. ; Hwang, Kyu-Ha ; Jung, Youn Kil ; Kim, Doeon ; et al ( February 2022 , Astronomy & Astrophysics)

   Aims. The light curve of the microlensing event KMT-2021-BLG-0912 exhibits a very short anomaly relative to a single-lens single-source form. We investigate the light curve for the purpose of identifying the origin of the anomaly. Methods. We model the light curve under various interpretations. From this, we find four solutions, in which three solutions are found underthe assumption that the lens is composed of two masses (2L1S models), and the other solution is found under the assumption that the source is comprised of binary stars (1L2S model). The 1L2S model is ruled out based on the contradiction that the faint source companion is bigger than its primary, and one of the 2L1S solutions is excluded from the combination of the poorer fit, blending constraint, and lower overall probability, leaving two surviving solutions with the planet/host mass ratios of q ~ 2.8 × 10 −5 and ~ 1.1 × 10 −5 . A subtle central deviation supports the possibility of a tertiary lens component, either a binary companion to the host with a very large or small separation, or a second planet lying near the Einstein ring, but it is difficult to claim a secure detection due to the marginal improvementmore »of the fit, lack of consistency among different data sets, and difficulty in uniquely specifying the nature of the tertiary component. Results. With the observables of the event, it is estimated that the masses of the planet and host are ~ (6.9  M ⊕ , 0.75  M ⊙ ) according to one solution and~(2.8  M ⊕ , 0.80  M ⊙ ) according to the other, indicating that the planet is a super Earth around a K-type star, regardless of the solution. The fact that 16 (including the one reported in this work) out of 19 microlensing planets with M ≲ 10  M ⊕ were detected during the last 6 yr nicely demonstrates the importance of high-cadence global surveys in detecting very low-mass planets.« less