Keywords

Radio images of the Galactic Center supermassive black hole, Sagittarius A* (Sgr A*), are dominated by interstellar scattering. Previous studies of Sgr A* have adopted an anisotropic Gaussian model for both the intrinsic source and the scattering, and they have extrapolated the scattering using a purely λ² scaling to estimate intrinsic properties. However, physically motivated source and scattering models break all three of these assumptions. They also predict that refractive scattering effects will be significant, which have been ignored in standard model fitting procedures. We analyze radio observations of Sgr A* using a physically motivated scattering model, and we develop a prescription to incorporate refractive scattering uncertainties when model fitting. We show that an anisotropic Gaussian scattering kernel is an excellent approximation for Sgr A* at wavelengths longer than 1 cm, with an angular size of $(1.380\pm 0.013){\lambda }_{\mathrm{cm}}^{2}\,\mathrm{mas}$ along the major axis, $(0.703\pm 0.013){\lambda }_{\mathrm{cm}}^{2}\,\mathrm{mas}$ along the minor axis, and a position angle of $81\buildrel{\circ}\over{.} 9\pm 0\buildrel{\circ}\over{.} 2$. We estimate that the turbulent dissipation scale is at least 600 km, with tentative support for r_in = 800 ± 200 km, suggesting that the ion Larmor radius defines the dissipation scale. We find that the power-law index for density fluctuations in the scattering material is β < 3.47, shallower than expected for a Kolmogorov spectrum (β = 11/3), and we estimate $\beta ={3.38}_{-0.04}^{+0.08}$ in the case of r_in = 800 km. We find that the intrinsic structure of Sgr A* is nearly isotropic over wavelengths from 1.3 mm to 1.3 cm, with a size that is roughly proportional to wavelength: ${\theta }_{\mathrm{src}}\sim (0.4\,\mathrm{mas})\times {\lambda }_{\mathrm{cm}}$. We discuss implications for models of Sgr A*, for theories of interstellar turbulence, and for imaging Sgr A* with the Event Horizon Telescope.

Recent general relativistic magnetohydrodynamic (MHD) simulations have suggested that relativistic jets from active galactic nuclei are powered by the rotational energy of central black holes through magnetic fields. Some mechanisms for the extraction of rotational energy of black holes have been proposed, such as the Penrose process, the Blandford–Znajek mechanism, the MHD/magnetic Penrose process, and superradiance. Among them, the Blandford–Znajek mechanism is the most promising as an important process of the central engines of the high-energy phenomena. However, a distinctive dynamic process of this mechanism has not yet been demonstrated intuitively, whereas extraction via the Penrose process has a simple interpretation. In this study, we performed numerical simulations of 1D force-free magnetodynamics (FFMD) with respect to the emergence of the Blandford–Znajek mechanism at the equatorial plane of the rapidly spinning black hole. A radial monopole-like magnetic field is used as an initial condition. We found a numerical solution of the steady-state region spreading from the ergosphere to the infinity point like a "tsunami." We can intuitively understand that this tsunami is caused and sustained by the degeneracy of the electromagnetic field of the perfect conducting plasma in the ergosphere. However, this degeneracy is not mathematically guaranteed in the time-dependent equations of FFMD. The degeneracy is given by an appropriate initial condition and is sustained by the condition at the "stretched horizon" of the black hole.

The gravitational interaction among bodies orbiting in a spherical potential leads to the rapid relaxation of the orbital planes' distribution, a process called vector resonant relaxation. We examine the statistical equilibrium of this process for a system of bodies with similar semimajor axes and eccentricities. We extend the previous model of Roupas et al. by accounting for the multipole moments beyond the quadrupole, which dominate the interaction for radially overlapping orbits. Nevertheless, we find no qualitative differences between the behavior of the system with respect to the model restricted to the quadrupole interaction. The equilibrium distribution resembles a counterrotating disk at low temperature and a spherical structure at high temperature. The system exhibits a first-order phase transition between the disk and the spherical phase in the canonical ensemble if the total angular momentum is below a critical value. We find that the phase transition erases the high-order multipoles, i.e., small-scale structure in angular momentum space, most efficiently. The system admits a maximum entropy and a maximum energy, which lead to the existence of negative temperature equilibria.

We present the results of a new sensitive survey of neutral hydrogen above and below the Galactic Center with the Green Bank Telescope. The observations extend up to Galactic latitude $| b| \lt 10^\circ$ with an effective angular resolution of 95 and an average rms brightness temperature noise of 40 mK in a 1 $\mathrm{km}\,{{\rm{s}}}^{-1}$ channel. The survey reveals the existence of a population of anomalous high-velocity clouds extending up to heights of about 1.5 kpc from the Galactic plane and showing no signature of Galactic rotation. These clouds have local standard of rest velocities $| {V}_{\mathrm{LSR}}| \lesssim 360$ $\mathrm{km}\,{{\rm{s}}}^{-1}$, and assuming a Galactic Center origin, they have sizes of a few tens of parsec and neutral hydrogen masses spanning $10\mbox{--}{10}^{5}\,{M}_{\odot }$. Accounting for selection effects, the cloud population is symmetric in longitude, latitude, and V_LSR. We model the cloud kinematics in terms of an outflow expanding from the Galactic Center and find the population consistent with being material moving with radial velocity ${V}_{{\rm{w}}}\simeq 330\,\mathrm{km}\,{{\rm{s}}}^{-1}$ distributed throughout a bicone with opening angle $\alpha \gt 140^\circ$. This simple model implies an outflow luminosity ${L}_{{\rm{w}}}\gt 3\times {10}^{40}$ erg s⁻¹ over the past 10 Myr, consistent with star formation feedback in the inner region of the Milky Way, with a cold gas mass-loss rate $\lesssim 0.1\,{{M}_{\odot }\mathrm{yr}}^{-1}$. These clouds may represent the cold gas component accelerated in the nuclear wind driven by our Galaxy, although some of the derived properties challenge current theoretical models of the entrainment process.

The Fermi bubbles are among the most important findings of the Fermi Gamma-ray Space Telescope; however, their origin is still elusive. One of the unique features of the bubbles is that their gamma-ray spectrum, including a high-energy cutoff at ∼110 GeV and the overall shape of the spectrum, is nearly spatially uniform. The high-energy spectral cutoff is suggestive of a leptonic origin due to synchrotron and inverse-Compton cooling of cosmic-ray (CR) electrons; however, even for a leptonic model, it is not obvious why the spectrum should be spatially uniform. In this work, we investigate the bubble formation in the leptonic active galactic nucleus (AGN) jet scenario using a new CRSPEC module in FLASH that allows us to track the evolution of a CR spectrum during the simulations. We show that the high-energy cutoff is caused by fast electron cooling near the Galactic center (GC) when the jets were launched. Afterwards, the dynamical timescale becomes the shortest among all relevant timescales, and therefore the spectrum is essentially advected with only mild cooling losses. This could explain why the bubble spectrum is nearly spatially uniform: the CRs from different parts of the bubbles as seen today all share the same origin near the GC at an early stage of the bubble expansion. We find that the predicted CR spatial and spectral distribution can simultaneously match the normalization, spectral shape, and high-energy cutoff of the observed gamma-ray spectrum and their spatial uniformity, suggesting that past AGN jet activity is a likely mechanism for the formation of the Fermi bubbles.

Pulsars around the massive black hole (MBH) in the Galactic center (GC) are expected to be revealed by upcoming facilities (e.g., the Square Kilometer Array). Under a full relativistic framework with the pulsar approximated as a test particle, we investigate the constraints on the spinning of the MBH by monitoring the timing of surrounding pulsars. For GC pulsars orbiting closely around the MBH (e.g., ≲1000 au), we find that full relativistic treatment in modeling accurately their timing signals can be necessary, as the relativistic signals are orders of magnitude larger than the time-of-arrival measurement accuracies. Although usually there are near degeneracies among MBH spin parameters, the constraints on the spinning of the MBH are still very tight. By continuously monitoring a normal pulsar in orbits with a period of ∼2.6 yr and an eccentricity of 0.3–0.9 under a timing precision of 1–5 ms, within ∼8 yr the spin magnitude and the orientations of the GC MBH can be constrained with $2\sigma$ error of ${10}^{-3}\mbox{--}{10}^{-2}$ and ${10}^{-1}\mbox{--}{10}^{0}$, respectively. Even for pulsars in orbits similar to the detected star S2/S0-2 or S0-102, we find that the spinning of the MBH can still be constrained within 4–8 yr, with the most significant constraints provided near the pericenter passage. If the proper motion of the pulsars with astrometric accuracy of 10 μas can also be collected along with the timing measurement, then the position, velocity, mass, and distance to the solar system of the MBH can be constrained to ∼10 μas, ∼1 μas yr⁻¹, $\sim 10\,{M}_{\odot }$, and ∼1 pc, respectively.

Previous radio observations revealed widespread gas-phase methanol (CH₃OH) in the Central Molecular Zone (CMZ) at the Galactic center (GC), but its origin remains unclear. Here, we report the discovery of CH₃OH ice toward a star in the CMZ, based on a Subaru 3.4–4.0 μm spectrum, aided by NASA/IRTF $L^{\prime}$ imaging and 2–4 μm spectra. The star lies ∼8000 au away in projection from a massive young stellar object (MYSO). Its observed high CH₃OH ice abundance ($17 \% \pm 3 \%$ relative to H₂O ice) suggests that the 3.535 μm CH₃OH ice absorption likely arises in the MYSO's extended envelope. However, it is also possible that CH₃OH ice forms with a higher abundance in dense clouds within the CMZ, compared to within the disk. Either way, our result implies that gas-phase CH₃OH in the CMZ can be largely produced by desorption from icy grains. The high solid CH₃OH abundance confirms the prominent 15.4 μm shoulder absorption observed toward GC MYSOs arises from CO₂ ice mixed with CH₃OH.

We carry out a series of simulations of G2-type clouds interacting with the black hole at the galactic center, to determine why no large changes in the luminosity of Sgr A* were seen, and to determine the nature of G2. We measure the accretion rate from the gas cloud onto Sgr A* for a range of simulation parameters, such as cloud structure, background structure, background density, grid resolution, and accretion radius. For a broad range of parameters, the amount of cloud material accreted is small relative to the amount of background material accreted. The total accretion rate is not significantly effected for at least 30 yr after periapsis. We find that reproducing observations of G2 likely requires two components for the object: an extended, cold gas cloud responsible for the Br-γ emission, and a compact core or dusty stellar object dominating the bolometric luminosity. In simulations, the bolometric and X-ray luminosity have a peak lasting from about one year before to one year after periapsis, a feature not detected in observations. Our simulated Br-γ emission is largely consistent with observations leading up to periapsis, with a slight increase in luminosity and a large increase in the FWHM of the line velocity. All emission from a gaseous component of G2 should fade rapidly after periapsis and be undetectable after one year, due to shock heating and expansion of the cloud. Any remaining emission should be from the compact component of G2.

We examine dense self-gravitating stellar systems dominated by a central potential, such as nuclear star clusters hosting a central supermassive black hole. Different dynamical properties of these systems evolve on vastly different timescales. In particular, the orbital-plane orientations are typically driven into internal thermodynamic equilibrium by vector resonant relaxation before the orbital eccentricities or semimajor axes relax. We show that the statistical mechanics of such systems exhibit a striking resemblance to liquid crystals, with analogous ordered-nematic and disordered-isotropic phases. The ordered phase consists of bodies orbiting in a disk in both directions, with the disk thickness depending on temperature, while the disordered phase corresponds to a nearly isotropic distribution of the orbit normals. We show that below a critical value of the total angular momentum, the system undergoes a first-order phase transition between the ordered and disordered phases. At a critical point, the phase transition becomes second order, while for higher angular momenta there is a smooth crossover. We also find metastable equilibria containing two identical disks with mutual inclinations between 90° and 180°.