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1. Strongly Lensed Supermassive Black Hole Binaries as Nanohertz Gravitational-wave Sources

   https://doi.org/10.3847/1538-4357/ace16f  

   Khusid, Nicole M. ; Mingarelli, Chiara M. F. ; Natarajan, Priyamvada ; Casey-Clyde, J. Andrew ; Barnacka, Anna ( September 2023 , The Astrophysical Journal)

   Supermassive black hole binary systems (SMBHBs) should be the most powerful sources of gravitational waves (GWs) in the universe. Once pulsar timing arrays (PTAs) detect the stochastic GW background from their cosmic merger history, searching for individually resolvable binaries will take on new importance. Since these individual SMBHBs are expected to be rare, here we explore how strong gravitational lensing can act as a tool for increasing their detection prospects by magnifying fainter sources and bringing them into view. Unlike for electromagnetic waves, when the geometric optics limit is nearly always valid, for GWs the wave-diffraction-interference effects can become important when the wavelength of the GWs is larger than the Schwarzchild radius of the lens, i.e.,$M_{\mathrm{lens}}\sim {10}^8{\left(\frac{f}{\mathrm{mHz}}\right)}^{-1}{M}_{\odot }$. For the GW frequency range explored in this work, the geometric optics limit holds. We investigate GW signals from SMBHBs that might be detectable with current and future PTAs under the assumption that quasars serve as bright beacons that signal a recent merger. Using the black hole mass function derived from quasars and a physically motivated magnification distribution, we expect to detect a few strongly lensed binary systems out toz≈ 2. Additionally, for a range of fixed magnifications 2 ≤μ≤ 100, strong lensing adds up to ∼30 more detectable binaries for PTAs. Finally, we investigate the possibility of observing both time-delayed electromagnetic signals and GW signals from these strongly lensed binary systems—that will provide us with unprecedented multi-messenger insights into their orbital evolution.

    

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2. Toward Astrometric Constraints on a Supermassive Black Hole Binary in the Early-type Galaxy NGC 4472

   https://doi.org/10.3847/1538-4357/ac680b  

   Wrobel, J. M. ; Lazio, T. J. W. ( May 2022 , The Astrophysical Journal)

   The merger of two galaxies, each hosting a supermassive black hole (SMBH) of mass 10⁶M_⊙or more, could yield a bound SMBH binary. For the early-type galaxy NGC 4472, we study how astrometry with a next-generation Very Large Array could be used to monitor the reflex motion of the primary SMBH of massM_pri, as it is tugged on by the secondary SMBH of mass$M_{\sec }$. Casting the orbit of the putative SMBH binary in terms of its periodP, semimajor axisa_bin, and mass ratio$q=M_{\sec }/M_{\mathrm{pri}}\le 1$, we find the following: (1) Orbits with fiducial periods ofP= 4 yr and 40 yr could be spatially resolved and monitored. (2) For a 95% accuracy of 2μas per monitoring epoch, subparsec values ofa_bincould be accessed over a range of mass ratios notionally encompassing major$\left(q>\frac{1}{4}\right)$and minor$\left(q<\frac{1}{4}\right)$galaxy mergers. (3) If no reflex motion is detected forM_priafter 1 (10) yr of monitoring, an SMBH binary with periodP= 4 (40) yr and mass ratioq> 0.01 (0.003) could be excluded. This would suggest no present-day evidence for a past major merger like that recently simulated, where scouring by aq∼ 1 SMBH binary formed a stellar core with kinematic traits like those of NGC 4472. (4) Astrometric monitoring could independently check the upper limits onqfrom searches for continuous gravitational waves from NGC 4472.

    

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3. Analytical Model of Disk Evaporation and State Transitions in Accreting Black Holes

   https://doi.org/10.3847/1538-4357/ac6d5c  

   Cho 조, Hyerin 혜린 ; Narayan, Ramesh ( June 2022 , The Astrophysical Journal)

   State transitions in black hole X-ray binaries are likely caused by gas evaporation from a thin accretion disk into a hot corona. We present a height-integrated version of this process, which is suitable for analytical and numerical studies. With radiusrscaled to Schwarzschild units and coronal mass accretion rate${\dot{m}}_c$to Eddington units, the results of the model are independent of black hole mass. State transitions should thus be similar in X-ray binaries and an active galactic nucleus. The corona solution consists of two power-law segments separated at a break radiusr_b∼ 10³(α/0.3)⁻², whereαis the viscosity parameter. Gas evaporates from the disk to the corona forr>r_b, and condenses back forr<r_b. Atr_b,${\dot{m}}_c$reaches its maximum,${\dot{m}}_{c,\max }\approx 0.02{(\alpha /0.3)}^3$. If atr≫r_bthe thin disk accretes with${\dot{m}}_d<{\dot{m}}_{c,\max }$, then the disk evaporates fully before reachingr_b, giving the hard state. Otherwise, the disk survives at all radii, giving the thermal state. While the basic model considers only bremsstrahlung cooling and viscous heating, we also discuss a more realistic model that includes Compton cooling and direct coronal heating by energy transport from the disk. Solutions are again independent of black hole mass, andr_bremains unchanged. This model predicts strong coronal winds forr>r_b, and aT∼ 5 × 10⁸K Compton-cooled corona forr<r_b. Two-temperature effects are ignored, but may be important at small radii.

    

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4. 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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5. Coherent gravitational waveforms and memory from cosmic string loops

   https://doi.org/10.1088/1361-6382/aba28b  

   Aurrekoetxea, Josu C. ; Helfer, Thomas ; Lim, Eugene A. ( September 2020 , Classical and Quantum Gravity)

   We construct, for the first time, the time-domain gravitational wave strain waveform from the collapse of a strongly gravitating Abelian Higgs cosmic string loop in full general relativity. We show that the strain exhibits a large memory effect during merger, ending with a burst and the characteristic ringdown as a black hole is formed. Furthermore, we investigate the waveform and energy emitted as a function of string width, loop radius and string tensionGμ. We find that the mass normalized gravitational wave energy displays a strong dependence on the inverse of the string tensionE_GW/M₀∝ 1/Gμ, with$E_{\text{GW}}/M_0\sim \mathcal{O}(1)\%$at the percent level, for the regime whereGμ≳ 10⁻³. Conversely, we show that the efficiency is only weakly dependent on the initial string width and initial loop radii. Using these results, we argue that gravitational wave production is dominated by kinematical instead of geometrical considerations.

    

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