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1. 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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2. TD-CARMA: Painless, Accurate, and Scalable Estimates of Gravitational Lens Time Delays with Flexible CARMA Processes

   https://doi.org/10.3847/1538-4357/acbea1  

   Meyer, Antoine D. ; van Dyk, David A. ; Tak, Hyungsuk ; Siemiginowska, Aneta ( June 2023 , The Astrophysical Journal)

   Cosmological parameters encoding our understanding of the expansion history of the universe can be constrained by the accurate estimation of time delays arising in gravitationally lensed systems. We propose TD-CARMA, a Bayesian method to estimate cosmological time delays by modeling observed and irregularly sampled light curves as realizations of a continuous auto-regressive moving average (CARMA) process. Our model accounts for heteroskedastic measurement errors and microlensing, an additional source of independent extrinsic long-term variability in the source brightness. The semiseparable structure of the CARMA covariance matrix allows for fast and scalable likelihood computation using Gaussian process modeling. We obtain a sample from the joint posterior distribution of the model parameters using a nested sampling approach. This allows for “painless” Bayesian computation, dealing with the expected multimodality of the posterior distribution in a straightforward manner and not requiring the specification of starting values or an initial guess for the time delay, unlike existing methods. In addition, the proposed sampling procedure automatically evaluates the Bayesian evidence, allowing us to perform principled Bayesian model selection. TD-CARMA is parsimonious, and typically includes no more than a dozen unknown parameters. We apply TD-CARMA to six doubly lensed quasars HS2209+1914, SDSS J1001+5027, SDSS J1206+4332, SDSS J1515+1511,more »SDSS J1455+1447, and SDSS J1349+1227, estimating their time delays as −21.96 ± 1.448, 120.93 ± 1.015, 111.51 ± 1.452, 210.80 ± 2.18, 45.36 ± 1.93, and 432.05 ± 1.950, respectively. These estimates are consistent with those derived in the relevant literature, but are typically two to four times more precise.

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3. A new method to correct for host star variability in multiepoch observations of exoplanet transmission spectra

   https://doi.org/10.1093/mnras/stac1949  

   Panwar, Vatsal ; Désert, Jean-Michel ; Todorov, Kamen O. ; Bean, Jacob L. ; Stevenson, Kevin B. ; Huitson, C. M. ; Fortney, Jonathan J. ; Bergmann, Marcel ( July 2022 , Monthly Notices of the Royal Astronomical Society)

   Transmission spectra of exoplanets orbiting active stars suffer from wavelength-dependent effects due to stellar photospheric heterogeneity. WASP-19b, an ultra-hot Jupiter (Teq ∼ 2100 K), is one such strongly irradiated gas-giant orbiting an active solar-type star. We present optical (520–900 nm) transmission spectra of WASP-19b obtained across eight epochs, using the Gemini Multi-Object Spectrograph (GMOS) on the Gemini-South telescope. We apply our recently developed Gaussian Processes regression based method to model the transit light-curve systematics and extract the transmission spectrum at each epoch. We find that WASP-19b’s transmission spectrum is affected by stellar variability at individual epochs. We report an observed anticorrelation between the relative slopes and offsets of the spectra across all epochs. This anticorrelation is consistent with the predictions from the forward transmission models, which account for the effect of unocculted stellar spots and faculae measured previously for WASP-19. We introduce a new method to correct for this stellar variability effect at each epoch by using the observed correlation between the transmission spectral slopes and offsets. We compare our stellar variability corrected GMOS transmission spectrum with previous contradicting MOS measurements for WASP-19b and attempt to reconcile them. We also measure the amplitude and timescale of broad-band stellar variability of WASP-19 frommore »TESS photometry, which we find to be consistent with the effect observed in GMOS spectroscopy and ground-based broad-band photometric long-term monitoring. Our results ultimately caution against combining multiepoch optical transmission spectra of exoplanets orbiting active stars before correcting each epoch for stellar variability.

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4. MOA-2019-BLG-008Lb: A New Microlensing Detection of an Object at the Planet/Brown Dwarf Boundary

   https://doi.org/10.3847/1538-3881/ac78ed  

   Bachelet, E. ; Tsapras, Y. ; Gould, Andrew ; Street, R. A. ; Bennett, David P. ; Hundertmark, M. P. ; Bozza, V. ; Bramich, D. M. ; Cassan, A. ; Dominik, M. ; et al ( August 2022 , The Astronomical Journal)

   Abstract We report on the observations, analysis and interpretation of the microlensing event MOA-2019-BLG-008. The observed anomaly in the photometric light curve is best described through a binary lens model. In this model, the source did not cross caustics and no finite-source effects were observed. Therefore, the angular Einstein ring radius θ E cannot be measured from the light curve alone. However, the large event duration, t E ∼ 80 days, allows a precise measurement of the microlensing parallax π E . In addition to the constraints on the angular radius θ * and the apparent brightness I s of the source, we employ the Besançon and GalMod galactic models to estimate the physical properties of the lens. We find excellent agreement between the predictions of the two galactic models: the companion is likely a resident of the brown dwarf desert with a mass M p ∼ 30 M Jup , and the host is a main-sequence dwarf star. The lens lies along the line of sight to the Galactic bulge, at a distance of ≤4 kpc. We estimate that in about 10 yr the lens and source will be separated by ∼55 mas, and it will be possible tomore »confirm the exact nature of the lensing system by using high-resolution imaging from ground- or space-based observatories.« less
5. An Isolated Mass-gap Black Hole or Neutron Star Detected with Astrometric Microlensing

   https://doi.org/10.3847/2041-8213/ac7442  

   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 ( July 2022 , The Astrophysical Journal Letters)

   We present the analysis of five black hole candidates identified from gravitational microlensing surveys. Hubble Space Telescope astrometric data and densely sampled light curves from ground-based microlensing surveys are fit with a single-source, single-lens microlensing model in order to measure the mass and luminosity of each lens and determine if it is a black hole. One of the five targets (OGLE-2011-BLG-0462/MOA-2011-BLG-191 or OB110462 for short) shows a significant >1 mas coherent astrometric shift, little to no lens flux, and has an inferred lens mass of 1.6–4.4M_⊙. This makes OB110462 the first definitive discovery of a compact object through astrometric microlensing and it is most likely either a neutron star or a low-mass black hole. This compact-object lens is relatively nearby (0.70–1.92 kpc) and has a slow transverse motion of <30 km s⁻¹. OB110462 shows significant tension between models well fit to photometry versus astrometry, making it currently difficult to distinguish between a neutron star and a black hole. Additional observations and modeling with more complex system geometries, such as binary sources, are needed to resolve the puzzling nature of this object. For the remaining four candidates, the lens masses are <2M_⊙, and they are unlikely to be black holes;more »two of the four are likely white dwarfs or neutron stars. We compare the full sample of five candidates to theoretical expectations on the number of black holes in the Milky Way (∼10⁸) and find reasonable agreement given the small sample size.

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