The gas cloud G2 falling toward Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, is supposed to provide valuable information on the physics of accretion flows and the environment of the black hole. We observed Sgr A* with four European stations of the Global Millimeter Very Long Baseline Interferometry Array (GMVA) at 86 GHz on 1 October 2013 when parts of G2 had already passed the pericenter. We searched for possible transient asymmetric structure -- such as jets or winds from hot accretion flows -- around Sgr A* caused by accretion of material from G2. The interferometric closure phases remained zero within errors during the observation time. We thus conclude that Sgr A* did not show significant asymmetric (in the observer frame) outflows in late 2013. Using simulations, we constrain the size of the outflows that we could have missed to ~2.5 mas along the major axis, ~0.4 mas along the minor axis of the beam, corresponding to approximately 232 and 35 Schwarzschild radii, respectively; we thus probe spatial scales on which the jets of radio galaxies are suspected to convert magnetic into kinetic energy. As probably less than 0.2 Jy of the flux from Sgr A* can be attributed to accretion from G2, one finds an effective accretion rate eta*Mdot < 1.5*10^9 kg/s ~ 7.7*10^-9 Mearth/yr for material from G2. Exploiting the kinetic jet power--accretion power relation of radio galaxies, one finds that the rate of accretion of matter that ends up in jets is limited to Mdot < 10^17 kg/s ~ 0.5 Mearth/yr, less than about 20% of the mass of G2. Accordingly, G2 appears to be largely stable against loss of angular momentum and subsequent (partial) accretion at least on time scales < 1 year.
A possibility of time-delayed radio brightenings of Sgr A* triggered by the pericenter passage of the G2 cloud is studied by carrying out global three-dimensional magnetohydrodynamic simulations taking into account the radiative cooling of the tidal debris of the G2 cloud. Magnetic fields in the accretion flow are strongly perturbed and re-organized after the passage of G2. We have found that the magnetic energy in the accretion flow increases by a factor 3-4 in 5-10 years after the pericenter passage of G2 by a dynamo mechanism driven by the magneto-rotational instability. Since this B-field …
We consider the long-term tidal heating of a star by the supermassive black hole at the Galactic center, SgrA*. We show that gravitational interaction with background stars leads to a linear growth of the tidal excitation energy with the number of pericenter passages near SgrA*. The accumulated heat deposited by excitation of modes within the star over many pericenter passages can lead to a runaway disruption of the star at a pericenter distance that is 4-5 times farther than the standard tidal disruption radius. The accumulated heating may explain the lack of massive ($\gtrsim 10M_{\odot}$) S…
We study the dynamical evolution of the putative gas clouds G1 and G2 recently discovered in the Galactic center. Following earlier studies suggesting that these two clouds are part of a larger gas streamer, we combine their orbits into a single trajectory. Since the gas clouds experience a drag force from background gas, this trajectory is not exactly Keplerian. By assuming the G1 and G2 clouds trace this trajectory, we fit for the drag force they experience and thus extract information about the accretion flow at a distance of thousands of Schwarzschild radii from the black hole. This range …
Tidal disruption events (TDEs) are among the brightest transients in the optical, ultraviolet, and X-ray sky. These flares are set into motion when a star is torn apart by the tidal field of a massive black hole, triggering a chain of events which is -- so far -- incompletely understood. However, the disruption process has been studied extensively for almost half a century, and unlike the later stages of a TDE, our understanding of the disruption itself is reasonably well converged. In this Chapter, we review both analytical and numerical models for stellar tidal disruption. Starting with rela…
Eccentric compact binaries pose not only a challenge for gravitational wave detectors, but also provide a probe into the nuclear equation of state if one of the objects is a neutron star. At the short pericenter passage, tidal interactions excite f-modes on the star, which in turn emit their own gravitational waves. We derive an analytic waveform for these stellar oscillations within the effective fly-by framework, modeling the emission to leading post-Newtonian order. At this order, the f-mode response can be written in a Fourier decomposition in terms of orbital harmonics, with the amplitude…
Context. We present the results of a multi-year monitoring campaign of the Galactic Center (GC) with the MAGIC telescopes. These observations were primarily motivated by reports that a putative gas cloud (G2) would be passing in close proximity to the super-massive black hole (SMBH), associated with Sagittarius A*, located at the center of our galaxy. This event was expected to give astronomers a unique chance to study the effect of in-falling matter on the broad-band emission of a SMBH. Aims. We search for potential flaring emission of very-high-energy (VHE; $\geq$100 GeV) gamma rays from the…
Recently, in a study the X-ray flaring activity of Sgr A* with Chandra and XMM-Newton public observations from 1999 to 2014 and 2014 Swift data, it has been argued that the "bright and very bright" flaring rate raised from 2014 Aug. 31. Thanks to 482ks of observations performed in 2015 with Chandra, XMM-Newton and Swift, we test the significance of this rise of flaring rate and determine the threshold of unabsorbed flare flux or fluence leading to any flaring-rate change. The mean unabsorbed fluxes of the 107 flares detected in the 1999-2015 observations are consistently computed from the extr…
We explore the possibility that the G2 gas cloud falling in towards \sgra is the mass loss envelope of a young TTauri star. As the star plunges to smaller radius at 1000 to 6000 \kms, a strong bow shock forms where the stellar wind is impacted by the hot X-ray emitting gas in the vicinity of \sgra. For a stellar mass loss rate of $4\times10^{-8}$ \msun per yr and wind velocity 100 \kms, the bow shock will have an emission measure ($EM = n^2 vol$) at a distance $\sim10^{16}$ cm, similar to that inferred from the IR emission lines. The ionization of the dense bow shock gas is potentially provide…
We calculate radio light curves produced by the bow shock that is likely to form in front of the G2 cloud when it penetrates the accretion disk of Sgr A*. The shock acceleration of the radio-emitting electrons is captured self-consistently by means of first-principles particle-in-cell simulations. We show that the radio luminosity is expected to reach maximum in early 2013, roughly a month after the bow shock crosses the orbit pericenter. We estimate the peak radio flux at 1.4 GHz to be 1.4 - 22 Jy depending on the assumed orbit orientation and parameters. We show that the most promising frequ…
A white dwarf (WD) captured into a high-eccentricity orbit around a massive black hole (MBH) may undergo many pericenter passages before tidal disruption. During these passages, the tidal potential of the MBH excites internal oscillations or waves in the WD, and the dissipation of these oscillations can significantly influence the physical properties of the WD prior to its disruption. We calculate the amplitude of the tidally excited gravity (buoyancy) waves in the WD as a function of the pericenter distance and eccentricity for realistic WD models, under the assumption that these outgoing gra…
Recent analyses have shown that close encounters between stars and stellar black holes occur frequently in dense star clusters. Depending upon the distance at closest approach, these interactions can lead to dissipating encounters such as tidal captures and disruptions, or direct physical collisions, all of which may be accompanied by bright electromagnetic transients. In this study, we perform a wide range of hydrodynamic simulations of close encounters between black holes and main-sequence stars that collectively cover the parameter space of interest, and we identify and classify the various…
Tidal disruption events (TDEs) occur when a star gets torn apart by the strong tidal forces of a supermassive black hole, which results in the formation of a debris stream that partly falls back towards the compact object. This gas moves along inclined orbital planes that intersect near pericenter, resulting in a so-called "nozzle shock". We perform the first dedicated study of this interaction, making use of a two-dimensional simulation that follows the transverse gas evolution inside a given section of stream. This numerical approach circumvents the lack of resolution encountered near perice…
Two X-ray sources were recently discovered by Irwin et al. in compact companions to elliptical galaxies to show ultra-luminous flares with fast rise (~ minute) and decay (~ hour), and with a peak luminosity ~10^{40-41} erg/s. Together with two other sources found earlier, they constitute a new type of fast transients which cannot be attributed to neutron stars but might be due to intermediate-mass black holes (IMBHs; 10^{2-4} M_sun). The flaring behavior is recurrent for at least two sources. If the flare represents a short period of accretion onto an IMBH during the periastron passage of a do…
Converged simulations of the nozzle shock in tidal disruption events
Monash University（莫纳什大学）；OzGrav: The ARC Centre of Excellence for Gravitational Wave Discovery（OzGrav：ARC引力波发现卓越中心）；University of Warwick（华威大学）；Centre for Exoplanets and Habitability, University of Warwick（华威系外行星与宜居性研究中心）；Department of Physics, University of Warwick（华威大学物理系）；Univ. Grenoble Alpes, CNRS, IPAG（格勒诺布尔阿尔卑斯大学、法国国家科学研究中心、IPAG）
We propose that the remarkable object Swift J1644+57, in which multiple recurring hard X-ray flares were seen over a span of several days, is a system in which a white dwarf was tidally disrupted by an intermediate mass black hole. Disruption of a white dwarf rather than a main sequence star offers a number of advantages in understanding the multiple, and short, timescales seen in the light curve of this system. In particular, the short internal dynamical timescale of a white dwarf offers a more natural way of understanding the short rise times ($\sim 100$ s) observed. The relatively long inte…
We have investigated the linear X-ray filament, G359.944-0.052, previously identified as a likely X-ray counterpart of a parsec-scale jet from the Galactic Center super-massive black hole (SMBH), Sagittarius A* (Sgr A*), using a total of ~5.6 Ms ultra-deep Chandra observations taken from September 1999 to July 2017. This unprecedented dataset enables us to examine flux and spectral variations that might be related to intrinsic properties of the weakly accreting SMBH. We find no flux or spectral variation in G359.944-0.052 after the G2 periapsis passage around early 2014, however, a moderate fl…
Very wide binaries (> 500 AU) are subject to numerous encounters with flying-by stars in the Galactic field and can be perturbated into highly eccentric orbits (e ~ 0.99). For such systems tidal interactions at close pericenter passages can lead to orbit circularization and possibly mass transfer, consequently producing X-Ray binaries without the need for common envelope. We test this scenario for the case of Black Hole Low-Mass X-Ray Binaries (BH LMXBs) by performing a population synthesis from primordial binaries with numerical treatment of random stellar encounters. We test various models f…
Stars may be tidally disrupted if, in a single orbit, they are scattered too close to a supermassive black hole (SMBH). Tidal disruption events are thought to power luminous but short-lived accretion episodes that can light up otherwise quiescent SMBHs in transient flares. Here we explore a more gradual process of tidal stripping where stars approach the tidal disruption radius by stellar evolution while in an eccentric orbit. After the onset of mass transfer, these stars episodically transfer mass to the SMBH every pericenter passage giving rise to low-level flares that repeat on the orbital …
We show that a black-hole binary with an external companion can undergo Lidov-Kozai cycles that cause a close pericenter passage, leading to a rapid merger due to gravitational-wave emission. This scenario occurs most often for systems in which the companion has mass comparable to the reduced mass of the binary and the companion orbit has semi-major axis within a factor of $\sim 10$ of the binary semi-major axis. Using a simple population-synthesis model and 3-body simulations, we estimate the rate of mergers in triple black hole systems in the field to be about six per Gpc$^3$ per year in the…
White dwarfs inspiraling into black holes of mass $\MBH\simgt 10^5M_\odot$ are detectable sources of gravitational waves in the LISA band. In many of these events, the white dwarf begins to lose mass during the main observational phase of the inspiral. The mass loss starts gently and can last for thousands of orbits. The white dwarf matter overflows the Roche lobe through the $L_1$ point at each pericenter passage and the mass loss repeats periodically. The process occurs very close to the black hole and the released gas can accrete, creating a bright source of radiation with luminosity close …
A star approaching a supermassive black hole (SMBH) can be torn apart in a tidal disruption event (TDE). We examine ultra-deep TDEs, a new regime in which the disrupted debris approaches close to the black hole's Schwarzschild radius, and the leading part intersects the trailing part at the first pericenter passage. We calculate the range of penetration factors $β$ vs SMBH masses $M$ that produce these prompt self-intersections using a Newtonian analytic estimate and a general relativistic (GR) geodesic model. We find that significant self-intersection of Solar-type stars requires $β\sim 50 - …
Successive Partial Disruptions with Orbital Precession in a White Dwarf-Black Hole System for Repeating GRB 250702B
Astronomical Institute, Graduate School of Science, Tohoku University（东北大学理学研究科天文学研究所）；Department of Astronomy, School of Science, The University of Tokyo（东京大学理学部天文学科）
The object G2 was recently discovered descending into the gravitational potential of the supermassive black hole (BH) Sgr A*. We test the photoionized cloud scenario, determine the cloud properties, and estimate the emission during the pericenter passage. The incident radiation is computed starting from the individual stars at the locations of G2. The radiative transfer calculations are conducted with CLOUDY code and $2011$ broadband and line luminosities are fitted. The spherically symmetric, tidally distorted, and magnetically arrested cloud shapes are tested with both the interstellar mediu…
How accretion proceeds around the massive black hole in the Galactic center and other highly sub-Eddington accretors remains poorly understood. The orbit of the S2 star in the Galactic center passes through the accretion disk of the massive black hole and any observational signature from such interaction may be used as an accretion probe. Because of its early stellar type, S2 is expected to possess a fairly powerful wind. We show here that the ram pressure of the accretion disk shocks the stellar wind fairly close to the star. The shocked fluid reaches a temperature of ~ 1 keV and cools effici…
We discuss the formation and evolution of systems comprised of a low-mass ($M_\star \lesssim 4 \, \rm M_\odot$) main sequence star, orbiting a $10^5-10^7 \, \rm M_\odot$ supermassive black hole with an orbital period of order $\sim$hours, and a mild eccentricity ($e\approx0.1-0.2$), episodically shedding mass at each pericenter passage. We argue that the resulting mass transfer is likely unstable, with Roche lobe overflow initially driven by gravitational wave emission, but then being accelerated by the star's expansion in response to its mass loss, undergoing a runaway process. We show that s…
Relativistic Tidal Dissipation and the Gravitational-wave Signal of a White Dwarf Orbiting an Intermediate-Mass Black Hole
Peking University（北京大学）；Kavli Institute for Astronomy and Astrophysics, Peking University（北京大学科维理天文与天体物理研究所）；Beijing Institute of Mathematical Sciences and Applications（北京数学与应用理科研究所）
A dense ionized cloud of gas has been recently discovered to be moving directly toward the supermassive black hole, Sgr A*, at the Galactic Center. In June 2013, at the pericenter of its highly eccentric orbit, the cloud will be approximately 3100 Schwarzschild radii from the black hole and will move supersonically through the ambient hot gas with a velocity of v_p ~ 5400 km/s. A bow shock is likely to form in front of the cloud and could accelerate electrons to relativistic energies. We estimate via particle-in-cell simulations the energy distribution of the accelerated electrons and show tha…
Tidal disruption events (TDEs), events in which a star passes very close to a supermassive black hole, are generally imagined as leading either to the star's complete disruption or to its passage directly into the black hole. In the former case it is widely believed that in all cases the bound portion of the debris quickly "circularizes" due to relativistic apsidal precession, i.e., forms a compact accretion disk, and emits a flare of standardized lightcurve and spectrum. We show here that TDEs are more diverse and can be grouped into several distinct categories on the basis of stellar pericen…
