Search

The European Space Agency is studying two large-class missions bound to operate in the decade of the 30s, and aiming at investigating the most energetic and violent phenomena in the Universe. $Athena$ is poised to study the physical conditions of baryons locked in large-scale structures from the epoch of their formation, as well as to yield an accurate census of accreting super-massive black holes down to the epoch of reionization; LISA will extend the hunt for Gravitational Wave (GW) events to the hitherto unexplored mHz regime. We discuss in this paper the science that their concurrent operation could yield, and present possible $Athena$ observational strategies. We focus on Super-Massive (M$\lesssim10^7\rm M_{\odot}$) Black Hole Mergers (SMBHMs), potentially accessible to $Athena$ up to $z\sim2$. The simultaneous measurement of their electro-magnetic (EM) and GW signals may enable unique experiments in the domains of astrophysics, fundamental physics, and cosmography, such as the magneto-hydrodynamics of fluid flows in a rapidly variable space-time, the formation of coronae and jets in Active Galactic Nuclei, and the measurement of the speed of GW, among others. Key to achieve these breakthrough results will be the LISA capability of locating a SMBHM event with an error box comparable to, or better than the field-of-view of the $Athena$ Wide Field Imager ($\simeq0.4\,$deg$^2$) and $Athena$ capability to slew fast to detect the source during the inspiral phase and the post-merger phase. Together, the two observatories will open in principle the exciting possibility of truly concurrent EM and GW studies of the SMBHMs, Comment: 17 pages, 8 figures. Accepted for publication in MNRAS

Present understandings of accretion disks based on X-ray observations are reviewed. The main focus is on observational properties of accretion disks around black holes in X-ray binaries and their phenomenological interpretations. Since there are several excellent reviews on black hole binaries already, we here pick up a limited number of important works which introduced key observational clues which widen and/or deepen the understanding of the accretion disks, recalling histories of X-ray observations of binary X-ray sources with a number of X-ray astronomy satellites successively launched from the United States, European countries, and Japan. We at certain points compare some observational evidence from black hole binaries with similar phenomena observed from binaries with a weakly magnetized neutron star, to clarify the basic nature of accretion disks. We also try to understand accretion environments in active galactic nuclei by applying what we have learned from the black hole binaries to them.

We present two further classical novae, V906 Car and V5668 Sgr, which show jets and accretion disc spectral signatures in their H α complexes throughout the first 1000 d following their eruptions. From extensive densely time-sampled spectroscopy, we measure the appearance of the first high-velocity absorption component in V906 Car, and the duration of the commencement of the main H α emission. We constrain the time taken for V5668 Sgr to transition to the nebular phase using [N ii] 6584 Å. We find these timings to be consistent with the jet and accretion disc model for explaining optical spectral line profile changes in classical novae, and discuss the implications of this model for enrichment of the interstellar medium.

We investigate the radiation fields around super-Eddington accretion flow, in which multiple inverse-Compton scattering plays a principal role, by using newly developed code describing Boltzmann radiation transfer in the Schwarzschild space-time. We apply this code to post-processing spectral calculations based on general relativistic, radiation magnetohydrodynamic simulation data to obtain X-ray spectra seen from various viewing angles. The radiation fields are distinctively separated into a funnel region with an opening angle of ∼30°, which is full with hot (gas temperature of T > 108 K), tenuous, and high-velocity plasmas, and surrounding cooler (T ∼ 107 K) and optically thick outflow regions. Accordingly, there is a clear tendency that the smaller the viewing angle, the harder the spectra. In particular, hard photons with several tens of keV are observable only by observers at viewing angles less than ∼30°, consistent with past spectral studies based on simulations. Further, we investigate how the spectra are varied by a flare occurring in the innermost region, finding that the variation amplitude grows as the photon energy increases and that the harder photons emerge more quickly than softer photons. The observational implications on long-term spectral variability of ultra-luminous X-ray sources are briefly discussed.

We report on the Insight-HXMT observations of the new black hole X-ray binary MAXI J1820+070 during its 2018 outburst. Detailed spectral analysis via the continuum fitting method shows an evolution of the inferred spin during its high soft sate. Moreover, the hardness ratio, the non-thermal luminosity and the reflection fraction also undergo an evolution, exactly coincident to the period when the inferred spin transition takes place. The unphysical evolution of the spin is attributed to the evolution of the inner disc, which is caused by the collapse of a hot corona due to condensation mechanism or may be related to the deceleration of a jet-like corona. The studies of the inner disc radius and the relation between the disc luminosity and the inner disc radius suggest that, only at a particular epoch, did the inner edge of the disc reach the innermost stable circular orbit and the spin measurement is reliable. We then constrain the spin of MAXI J1820+070 to be a*=0.2^{+0.2}_{-0.3}. Such a slowly spinning black hole possessing a strong jet suggests that its jet activity is driven mainly by the accretion disc rather than by the black hole spin., Comment: 14 pages, 13 figures, 5 tables, accepted for publication in MNRAS

It is widely believed that super-Eddington accretion flow can produce powerful outflow, but where does this originate and how much mass and energy are carried away in which directions? To answer these questions, we perform a new large-box, two-dimensional radiation hydrodynamic simulation, paying special attention lest the results should depend on the adopted initial and boundary conditions. We achieve a quasi-steady state at an unprecedentedly large range, r = 2–600rS (with rS being the Schwarzschild radius), from the black hole. The accretion rate onto the central 10 M⊙ black hole is $\dot{M}_{\rm BH} \sim 180 L_{\rm Edd}/c^{2}$, whereas the mass outflow rate is ${\dot{M}}_{\rm outflow} \sim 24 L_{\rm Edd}/c^2$ (where LEdd and c are the Eddington luminosity and the speed of light, respectively). The ratio ${\dot{M}}_{\rm outflow}/{\dot{M}}_{\rm BH} \sim 0.14$ is much less than previously reported. By careful inspection we find that most of the outflowing gas reaching the outer boundary originates from the region at R ≲ 140rS, while gas at 140–230rS forms failed outflow. Therefore, significant outflow occurs inside the trapping radius ∼450rS. The mechanical energy flux (or mass flux) reaches its maximum in the direction of ∼15° (∼80°) from the rotation axis. The total mechanical luminosity is Lmec ∼ 0.16LEdd, while the isotropic X-ray luminosity varies from $L_{\rm X}^{\rm ISO}\sim 2.9 L_{\rm Edd}$ (for a face-on observer) to ∼2.1LEdd (for a nearly edge-on observer). The power ratio is $L_{\rm mec}/L_{\rm X}^{\rm ISO}\sim 0.05$–0.08, in good agreement with observations of ultra-luminous X-ray sources surrounded by optical nebulae.

The classical nova YZ Reticuli was discovered in 2020 July. Shortly after this, we commenced a sustained, highly time-sampled coverage of its subsequent rapid evolution with time-resolved spectroscopy from the Global Jet Watch observatories. Its H-alpha complex exhibited qualitatively different spectral signatures in the following weeks and months. We find that these H-alpha complexes are well described by the same five Gaussian emission components throughout the six months following eruption. These five components appear to constitute two pairs of lines, from jet outflows and an accretion disc, together with an additional central component. The correlated, symmetric patterns that these jet/accretion disc pairs exhibit suggest precession, probably in response to the large perturbation caused by the nova eruption. The jet and accretion disc signatures persist from the first 10 d after brightening – evidence that the accretion disc survived the disruption. We also compare another classical nova (V6568 Sgr) that erupted in 2020 July whose H-alpha complex can be described analogously, but with faster line-of-sight jet speeds exceeding 4000 km s−1. We suggest that classical novae with higher mass white dwarfs bridge the gap between recurrent novae and classical novae such as YZ Reticuli.

We present our analysis of the Suzaku data of U Geminorum (U Gem) from 2012 both in quiescence and outburst. Unlike SS Cygni (SS Cyg), the hard X-ray flux of U Gem is known to increase at times of optical outburst. A sophisticated spectral model and reliable distance estimate now reveal that this can be attributed to the fact that the mass accretion rate onto the white dwarf (WD) does not exceed the critical rate that causes the optically thin to thick transition of the boundary layer. From comparison of the X-ray and optical light curves, the X-ray outburst peak seems to be retarded by 2.1 ± 0.5 d, although there remains uncertainty in the X-ray peak identification, due to short data coverage. The larger delay than SS Cyg (0.9–1.4 d) also supports the lower accretion rate in U Gem. A fluorescent iron 6.4 keV emission line bears significant information about the geometry of the X-ray-emitting hot plasma and the accretion disk (AD) that reflects the hard X-ray emission. Our reflection simulation has shown that the optically thick AD is truncated at a distance of 1.20–1.25 times the white dwarf radius (RWD) in quiescence, and the accreting matter in the disk turns into the optically thin hard-X-ray-emitting plasma at this radius. In outburst, on the other hand, our spectral analysis favors the picture that the optically thick disk reaches the WD surface, although disk truncation can take place in the region of

The two established techniques for measuring black hole spin in X-ray binaries often yield conflicting results, which must be resolved before either method may be deemed robust. In practice, black hole spin measurements based on fitting the accretion disc continuum effectively do not marginalize over the colour correction factor $f_{\mathrm{col}}$. This factor parametrizes spectral hardening of the disc continuum by the disc atmosphere, whose true properties are poorly constrained. We incorporate reasonable systematic uncertainties in $f_{\mathrm{col}}$ into the eight (non-maximal) black hole spin measurements vetted by the disc continuum fitting community. In most cases, an $f_{\mathrm{col}}$ uncertainty of $\pm$0.2-0.3 dominates the black hole spin error budget. We go on to demonstrate that plausible departures in $f_{\mathrm{col}}$ values from those adopted by the disc continuum fitting practitioners can bring the discrepant black hole spins into agreement with those from iron line modeling. Systematic uncertainties in $f_{\mathrm{col}}$, such as the effects of strong magnetization, should be better understood before dismissing their potentially dominant impact on the black hole spin error budget., Minor changes in proofs stage; Data and code are available on GitHub: https://github.com/gregsalvesen/bhspinf

Changing look active galactic nuclei (CLAGNs) are rare cases of AGNs, where the continuum flux increases/decreases and the broad emission lines appear/disappear within short time-scales. These extreme changes challenge our understanding of accretion disc dynamics. We present a sample of four new CLAGNs at 0.026 < z < 0.107, which were found by cross-matching the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey with AGNs from the SDSS spectroscopic data base. Our results show that the selection criteria of $\gt \mathopen |0.5\mathclose |$ mag change in SDSS-g band is ineffective at lower redshifts. This is, in part, due to the fact that the g band is probing a different part of the AGNs spectral energy distribution at these redshifts. The bigger issue is that the galaxy continuum dominates the spectrum and thus overwhelms any variation of the AGNs continuum that might contribute to photometric variations. This suggests the need to use different methods for future low-redshift CLAGNs searches.

We construct a time-dependent relativistic accretion model for tidal disruption events (TDEs) with an $\alpha-$viscosity and the pressure dominated by gas pressure. We also include the mass fallback rate $\dot{M}_f$ for both full and partial disruption TDEs, and assume that the infalling debris forms a seed disc in time $t_c$, which evolves due to the mass addition from the infalling debris and the mass loss via accretion onto the black hole. Besides, we derive an explicit form for the disc height that depends on the angular momentum parameter in the disc. We show that the surface density of the disc increases at an initial time due to mass addition, and then decreases as the mass fallback rate decreases, which results in a decrease in the disc mass $M_{\rm d}$ with a late-time evolution of $M_{\rm d} \propto t^{-1.05}$ and $M_{\rm d} \propto t^{-1.38}$ for full and partial disruption TDEs respectively, where $t$ is the time parameter. The bolometric luminosity $L$ shows a rise and decline that follows a power-law at late times given by $L \propto t^{-1.8}$ and $L \propto t^{-2.3}$ for full and partial disruption TDEs respectively. Our obtained luminosity declines faster than the luminosity inferred using $L \propto \dot{M}_f$. We also compute the light curves in various spectral bands., Comment: 22 pages, 19 figures with 37 sub-figures, 3 tables, accepted for publication in Monthly Notices of the Royal Astronomical Society

In the conventional view of type II migration, a giant planet migrates inward in the viscous velocity of the accretion disc in the so-call disc-dominate case. Recent hydrodynamic simulations, however, showed that planets migrate with velocities much faster than the viscous one in massive discs. Such fast migration cannot be explained by the conventional picture. Scardoni et al. (2020) has recently argued this new picture. By carrying out similar hydrodynamic simulations, they found that the migration velocity slows down with time and eventually reaches the prediction by the conventional theory. They interpreted the fast migration as an initial transient one and concluded that the conventional type II migration is realised after the transient phase. We show that the migration velocities obtained by Scardoni et al. (2020) are consistent with the previous simulations even in the transient phase that they proposed. We also find that the transient fast migration proposed by Scardoni et al. (2020) is well described by a new model of Kanagawa et al. (2018). The new model can appropriately describe significant inward migration during the initial transient phase that Scardoni et al. (2020) termed. Hence, we conclude that the time-variation of the transient migration velocity is due to the changes of the orbital radius of the planet and its background surface density during the migration., 4 pages, 1 figure, accepted for publication in MNRAS

It is believed that large-scale horseshoe-like brightness asymmetries found in dozens of transitional protoplanetary discs are caused by anticyclonic vortices. These vortices can play a key role in planet formation, as mm-sized dust -- the building blocks of planets -- can be accumulated inside them. Anticyclonic vortices are formed by the Rossby wave instability, which can be excited at the gap edges opened by a giant planet or at sharp viscosity transitions of accretionally inactive regions. It is known that vortices are prone to stretching and subsequent dissolution due to disc self-gravity for canonical disc masses in the isothermal approximation. To improve the hydrodynamic model of protoplanetary discs, we include the disc thermodynamics in our model. In this paper, we present our results on the evolution of the vortices formed at the outer edge of an accretionally inactive region (dead zone) assuming an ideal equation of state and taking $PdV$ work, disc cooling in the $\beta$-approximation, and disc self-gravity into account. Thermodynamics affects the offset and the mode number (referring to the number of small vortices at the early phase) of the RWI excitation, as well as the strength, shape, and lifetime of the large-scale vortex formed through merging of the initial small vortices. We found that the inclusion of gas thermodynamics results in stronger, however decreased lifetime vortices. Our results suggest that a hypothetical vortex-aided planet formation scenario favours effectively cooling discs., Comment: Accepted in MNRAS, 14 pages,8 figures, 1 table

Accretion onto black holes at rates above the Eddington limit has long been discussed in the context of supermassive black hole (SMBH) formation and evolution, providing a possible explanation for the presence of massive quasars at high redshifts (z$\gtrsim$7), as well as having implications for SMBH growth at later epochs. However, it is currently unclear whether such `super-Eddington' accretion occurs in SMBHs at all, how common it is, or whether every SMBH may experience it. In this work, we investigate the observational consequences of a simplistic model for super-Eddington accretion flows -- an optically thick, geometrically thin accretion disc (AD) where the inner-most parts experience severe photon-trapping, which is enhanced with increased accretion rate. The resulting spectral energy distributions (SEDs) show a dramatic lack of rest-frame UV, or even optical, photons. Using a grid of model SEDs spanning a wide range in parameter space (including SMBH mass and accretion rate), we find that large optical quasar surveys (such as SDSS) may be missing most of these luminous systems. We then propose a set of colour selection criteria across optical and infra-red colour spaces designed to select super-Eddington SEDs in both wide-field surveys (e.g., using SDSS, 2MASS and WISE) and deep & narrow-field surveys (e.g., COSMOS). The proposed selection criteria are a necessary first step in establishing the relevance of advection-affected super-Eddington accretion onto SMBHs at early cosmic epochs., 24 pages, 13 figures, accepted by MNRAS 07/01/2020

To study the role of Hall effect on the structure of accretion disc, we have considered a toroidal magnetic field in our paper. To study the vertical structure of the disc, we have written a set of magnetohydrodynamic (MHD) equations in the spherical coordinates (r, θ, ϕ) based on the two assumptions of axisymmetric and steady state. Also, we employed the self-similar solutions in the radial direction to obtain the structure of the disc in the θ-direction. We have solved a set of ordinary differential equations in the θ-coordinate with symmetrical boundary conditions in the equatorial plane. In order to describe the behaviour of Hall effect, we introduced the ΛH parameter that was called the dimensionless Hall Elsasser number. The strength of the Hall effect is measured by the inverse of dimensionless Hall Elsasser number. We have shown that the strong Hall effect decreases the accretion rate or infall velocity and size of inflow part. It has also been found the Hall effect is maximum in the equatorial plane and gets the value close to zero near the boundary, and it has the antidiffusive nature. The results display that the strong Hall effect makes the standard accretion sub-Keplerian disc becomes thinner. Our solutions have shown the Hall effect leads to transport magnetic flux outward in the upper layer of the disc and it produces outflows in the surface of the disc.

MAXI J1820+070 (optical counterpart ASASSN-18ey) is a black hole candidate discovered through its recent very bright outburst. The low extinction column and long duration at high flux allow detailed measurements of the accretion process to be made. In this work, we compare the evolution of X-ray spectral and timing properties through the initial hard state of the outburst. We show that the inner accretion disc, as measured by relativistic reflection, remains steady throughout this period of the outburst. Nevertheless, subtle spectral variability is observed, which is well explained by a change in coronal geometry. However, characteristic features of the temporal variability - low-frequency roll-over and QPO frequency - increase drastically in frequency, as the outburst proceeds. This suggests that the variability timescales are governed by coronal conditions rather than solely by the inner disc radius. We also find a strong correlation between X-ray luminosity and coronal temperature. This can be explained by electron pair production with a changing effective radius and a non-thermal electron fraction of ~20%., Re-submitted to MNRAS after reviewers comments. 14 pages, 14 figures

We introduce a deep machine learning approach to studying quasar microlensing light curves for the first time by analyzing hundreds of thousands of simulated light curves with respect to the accretion disc size and temperature profile. Our results indicate that it is possible to successfully classify very large numbers of diverse light curve data and measure the accretion disc structure. The detailed shape of the accretion disc brightness profile is found to play a negligible role, in agreement with Mortonson et al. (2005). The speed and efficiency of our deep machine learning approach is ideal for quantifying physical properties in a `big-data' problem setup. This proposed approach looks promising for analyzing decade-long light curves for thousands of microlensed quasars, expected to be provided by the Large Synoptic Survey Telescope., 11 pages, 7 figures, accepted for publication in MNRAS

4U 1608–52 is a soft X-ray transient. The analysis presented here of a particular part of its X-ray activity uses observations of RXTE/ASM and Swift/BAT. We show a time segment (MJD 54262–MJD 55090) (828 d) in which 4U 1608–52 behaved as a quasi-persistent X-ray source with a series of bumps, with a complicated relation between the evolution of fluxes in the soft (1.5–12 keV) and the hard (15–50 keV) X-ray regions. We ascribe these bumps to a series of propagations of heating and cooling fronts over the inner disk region without any transitions to the true quiescence. 4U 1608–52 oscillated around the boundary between the dominance of the Comptonized component and the dominance of the multicolor accretion disk in its luminosity. Only some of the bumps in this series were accompanied by a transition from the hard to the soft state; if it occurred, it displayed a strong hysteresis effect. The hard-band emission with the dominant Comptonized component was present for most of this active state and showed a cycle of about 40 d. We argue that the cyclic variations of flux come from the inner disk region, not, e.g., from a jet. We also discuss the observed behavior of 4U 1608–52 in the context of other quasi-persistent low-mass X-ray binaries.

We investigate the formation and evolution of misaligned accretion discs around the secondary component of a binary through mass transfer driven by Kozai-Lidov oscillations of the circumprimary disc's eccentricity and inclination. We perform SPH simulations to study the amount of mass transferred to the secondary star as a function of both the disc and binary parameters. For the range of parameters we explore, we find that increasing the disc aspect ratio, viscosity parameter and initial inclination as well as decreasing the binary mass ratio leads to larger amount of mass transfer, up to a maximum of about ten per cent of the initial mass of the primary disc. The circumsecondary disc forms with a high eccentricity and a high inclination and is also able to undergo KL oscillations. The circumsecondary disc oscillations have a shorter period than those in the disc around the primary. We find that some of the material that escapes the Roche-lobe of the two components forms a narrow misaligned circumbinary accretion disc. This study has implications for disc evolution in young binary star systems., Comment: 12 pages, 15 figures

arXiv:2004.10508v1, et al., We report on photometric and spectroscopic observations and analysis of the 2019 superoutburst of TCP J21040470+4631129. This object showed a 9 mag superoutburst with early superhumps and ordinary superhumps, which are the features of WZ Sge-type dwarf novae. Five rebrightenings were observed after the main superoutburst. The spectra during the post-superoutburst stage showed Balmer, He I, and possible sodium doublet features. The mass ratio is derived as 0.0880(9) from the period of the superhump. During the third and fifth rebrightenings, growing superhumps and superoutbursts were observed, which have never been detected during a rebrightening phase among WZ Sge-type dwarf novae with multiple rebrightenings. To induce a superoutburst during the brightening phase, the accretion disk needs to have expanded beyond the 3 : 1 resonance radius of the system again after the main superoutburst. These peculiar phenomena can be explained by the enhanced viscosity and large radius of the accretion disk suggested by the higher luminosity and the presence of late-stage superhumps during the post-superoutburst stage, plus by more mass supply from the cool mass reservoir and/or from the secondary because of the enhanced mass transfer than those of other WZ Sge-type dwarf novae., N. Narita is supported by JSPS KAKENHI Grant Numbers JP18H01265 and JP18H05439, and JST PRESTO Grant Number JPMJPR1775. M. Tamura is supported by MEXT/JSPS KAKENHI Grant Nos. 18H05442, 15H02063, and 22000005. P. A. Dubovsky, T. Medulka, and I. Kudzej acknowledge support by NSF AST-1751874 and the Cottrell scholarship from the Research Corporation for Science Advancement and by the Slovak Research and Development Agency under the contract No. APVV-15-0458. A. Zubareva, A. Belinski, A. Dodin, M. Burlak, N. Ikonnikova, E. Mishin, and S. Potanin acknowledge the support from the Program of development of M.V. Lomonosov Moscow State University (Leading Scientific School “Physics of stars, relativistic objects and galaxies”). E. P. Pavlenko, O. I. Antonyuk, and Ju. V. Bbabina acknowledge support by the RSF grant 19-72-10063.

We studied five XMM-Newton observations of the neutron-star binary 4U 1728$-$34 covering the hard, intermediate and soft spectral states. By jointly fitting the spectra with several reflection models, we obtained an inclination angle of 25$-$53$\deg$ and an iron abundance up to 10 times the solar. From the fits with reflection models, we found that the fluxes of the reflection and the Comptonised components vary inconsistently; since the latter is assumed to be the illuminating source, this result possibly indicates the contribution of the neutron star surface/boundary layer to the disc reflection. As the source evolved from the relatively soft to the intermediate state, the disc inner radius decreased, opposite to the prediction of the standard accretion disc model. We also explore the possible reasons why the supersolar iron abundance is required by the data and found that this high value is probably caused by the absence of the hard photons in the XMM-Newton data., Comment: 14 pages, 6 figures

A possible origin of kilohertz quasi-periodic oscillations (QPOs) in low-mass X-ray binaries is proposed. Recent numerical magnetohydrodynamic simulations of accretion disks with turbulent magnetic fields of magneto-rotational instability definitely show the presence of two-armed spiral structure in the quasi-steady state of accretion disks. In such deformed disks, two-armed ($m=2$) c-mode ($n=1$) oscillations are excited by wave–wave resonant instability. Among these excited oscillations, the fundamental in the radial direction ($n_{\rm r}=0$) will be the higher kHz QPO of twin QPOs, and the first overtone ($n_{\rm r}=1$) in the radial direction will be the lower kHz QPO of the twin. A possible cause of the twin high-frequency QPOs in black hole X-ray binaries is also discussed in the final section.

The Galactic Center IRS 13E cluster is a very intriguing IR object located at $\sim0.13$ pc from Sagittarius A$^\ast$ (Sgr A$^\ast$) in projection distance. There are both arguments for and against the hypothesis that a dark mass like an intermediate mass black hole (IMBH) exists in the cluster. Recently we have detected the rotating ionized gas ring around IRS 13E3, which belongs to the cluster, in the H30$\alpha$ recombination line using ALMA. The enclosed mass is derived to be $M_{\mathrm{encl.}}\simeq2\times10^4$ $M_\odot$, which agrees with an IMBH and is barely less than the astrometric upper limit mass of the IMBH around Sgr A$^\ast$. Because the limit mass depends on the true three-deminsional (3D) distance from Sgr A$^\ast$, it is very important to determine it observationally. However, the 3D distance is indefinite because it is hard to determine the line-of-sight (LOS) distance by usual methods. We would attempt to estimate the LOS distance by spectroscopic informations. The CH$_3$OH molecule is easily destroyed by cosmic ray around Sgr A$^{\ast}$. However, we detected a highly excited CH$_3$OH emission line in the ionized gas stream associated with IRS 13E3. This indicates that IRS 13E3 is located at $r\gtrsim 0.4$ pc from Sgr A$^{\ast}$., Comment: 9 pages, 2 figures, 1 table, Accepted for publication in PASJ

We present an optical spectroscopic study of the black hole X-ray transient V4641 Sgr (=SAX J1819.3-2525) covering the 1999, 2002 and 2004 outbursts. The spectra were taken over 22 different epochs during the low luminosity phases that follow the sharp and bright outburst peaks displayed by the system. The data reveal the frequent presence of wind-related features in H (Balmer) and He I emission lines in the form of P-Cygni profiles and strong emission lines with broad wings. The terminal velocity of the wind, as measured in the blue-shifted absorption (P-Cygni) components, is in the range of ~ 900-1600 km/s, while the broad emission line wings (so-called nebular phases) imply outflow velocities of up to ~3000 km/s. We show that, at least for several of the wind detections, the radio jet was active and the system was likely in the hard state. This, together with previous detections reported in the literature, shows that V4641 Sgr is the second source of this class, after V404 Cyg, where the presence of these cold wind outflows has been clearly established. We discuss the similar phenomenology observed in both systems as well as the possible nature of the outflow and its impact on the accretion process., Accepted for publication in MNRAS

The evolution of the C-type low frequency quasi-periodic oscillations (LFQPOs) and associated time lag in transient black hole sources as a function of time can be explained by variation of the Compton cloud size in a Two Component Advective Flow solution (TCAF). A similar study of a persistent source, GRS~1915+105, has not been attempted. We fit the evolution of QPOs with propagatory oscillating shock (POS) solution for two sets of so-called $\chi$-state observations and find that the shock steadily recedes with almost constant velocity when QPO frequency is decreasing and the spectrum is hardening. The shock moves inward with a constant velocity $v_0=473.0$ cm s$^{-1}$ and $v_0=400.0$ cm s$^{-1}$ respectively in these two cases, when the QPO frequency is increasing and the spectrum softens. This behavior is similar to what was observed in XTE~J1550-564 during the 1998 outburst. The time lag measured at the QPO frequency varies in a similar way as the size of the Compton cloud. Most interestingly, in both the cases, the lag switches sign (hard lag to soft lag) at a QPO frequency of $\sim 2.3 - 2.5$ Hz irrespective of the energy of photons. We find, at very low frequencies $< 1$ Hz, the Comptonizing Efficiency (CE) increases with QPO frequency and at higher QPO frequencies the trend is opposite. The time lags become mostly positive at all energies when CE is larger than $\sim 0.85\%$ for both the sources.

We investigate the central sub-arcsec region of the low-luminosity active galactic nucleus NGC 1052, using a high-angular resolution dataset that covers 10 orders of magnitude in frequency. This allows us to infer the continuum emission within the innermost $\sim {17}\,$pc around the black hole to be of non-thermal, synchrotron origin and to set a limit to the maximum contribution of a standard accretion disc. Assuming the canonical 10 per cent mass-light conversion efficiency for the standard accretion disc, its inferred accretion power would be too low by one order of magnitude to account for the observed continuum luminosity. We thus introduce a truncated accretion disc and derive a truncation radius to mass-light conversion efficiency relation, which we use to reconcile the inferred accretion power with the continuum luminosity. As a result we find that a truncated disc providing the necessary accretion power must be truncated at $r_\text{tr} \gtrsim {26}\, r_\text{g}$, consistent with the inner radius derived from the observations of the Fe K$\alpha$ line in the X-ray spectrum of this nucleus. This is the first time to derive a limit on the truncation radius of the accretion disc from high-angular resolution data only., Comment: 5 pages, 2 figures, accepted MNRAS letter, http://dx.doi.org/10.1093/mnrasl/sly079

We analyzed the effects of a toroidal magnetic field in the formation of several magnetized accretion tori, dubbed as ringed accretion disks (RADs), orbiting around one central Kerr supermassive Black Hole (SMBH) in AGNs, where both corotating and counterotating disks are considered. Constraints on tori formation and emergence of RADs instabilities, accretion onto the central attractor and tori collision emergence, are investigated. The results of this analysis show that the role of the central BH spin-mass ratio, the magnetic field and the relative fluid rotation and tori rotation with respect the central BH, are crucial elements in determining the accretion tori features, providing ultimately evidence of a strict correlation between SMBH spin, fluid rotation and magnetic fields in RADs formation and evolution. More specifically we proved that magnetic field and disks rotation are in fact strongly constrained, as tori formation and evolution in RADs depend on the toroidal magnetic fields parameters. Eventually this analysis identifies specific classes of tori, for restrict ranges of magnetic field parameter, that can be observed around some specific SMBHs identified by their dimensionless spin., 15 pages, 7 multi-panels figures. To appear on MNRAS

The surroundings of massive protostars constitute an accretion disc which has numerically been shown to be subject to fragmentation and responsible for luminous accretion-driven outbursts. Moreover, it is suspected to produce close binary companions which will later strongly influence the star's future evolution in the Hertzsprung-Russel diagram. We present three-dimensional gravitation-radiation-hydrodynamic numerical simulations of 100 Mo pre-stellar cores. We find that accretion discs of young massive stars violently fragment without preventing the (highly variable) accretion of gaseous clumps onto the protostars. While acquiring the characteristics of a nascent low-mass companion, some disc fragments migrate onto the central massive protostar with dynamical properties showing that its final Keplerian orbit is close enough to constitute a close massive proto-binary system, having a young high-mass and a low-mass component. We conclude on the viability of the disc fragmentation channel for the formation of such short-period binaries, and that both processes -close massive binary formation and accretion bursts- may happen at the same time. FU-Orionis-type bursts, such as observed in the young high-mass star S255IR-NIRS3, may not only indicate ongoing disc fragmentation, but also be considered as a tracer for the formation of close massive binaries - progenitors of the subsequent massive spectroscopic binaries - once the high-mass component of the system will enter the main-sequence phase of its evolution. Finally, we investigate the ALMA-observability of the disc fragments., Accepted for publication by MNRAS, 24 pages, 16 figures

We present 44 and 226 GHz observations of the Galactic Centre within 20 arcsec of Sgr A*. Millimetre continuum emission at 226 GHz is detected from eight stars that have previously been identified at near-IR and radio wavelengths.We also detect a 5.8 mJy source at 226 GHz coincident with the magnetar SGR J1745-29 located 2.39 arcsec SE of Sgr A* and identify a new 2.5 arcsec × 1.5 arcsec halo of mm emission centred on Sgr A*. The X-ray emission from this halo has been detected previously and is interpreted in terms of a radiatively inefficient accretion flow. The mm halo surrounds an EW linear feature that appears to arise from Sgr A* and coincides with the diffuse X-ray emission and a minimum in the near-IR extinction. We argue that the millimetre emission is produced by synchrotron emission from relativistic electrons in equipartition with an ~1.5 mG magnetic field. The origin of this is unclear but its coexistence with hot gas supports scenarios in which the gas is produced by the interaction of winds either from the fast moving S-stars, the photoevaporation of low-mass YSO discs or by a jet-driven outflow from Sgr A*. The spatial anti-correlation of the X-ray, radio and mm emission from the halo and the low near-IR extinction provides a compelling evidence of an outflow sweeping up the interstellar material, creating a dust cavity within 2 arcsec of Sgr A*. Finally, the radio and mm counterparts to eight near-IR identified stars within ~10 arcsec of SgrA* provide accurate astrometry to determine the positional shift between the peak emission at 44 and 226 GHz.© 2017 The Authors., This work is partially supported by the grant AST-0807400 from the NSF and by an Outside Studies Program Fellowship awarded to Macquarie University.

Several studies of nearby active galactic nuclei (AGN) have shown that the soft X-ray emission presents a size and morphology that resembles that of the narrow-line region (NLR) traced by [O III]. Since the NLR is mainly constituted by gas photoionised by the AGN, it seems logical to assume that this is also the primary source of the soft X-ray emission. However, these results are based on individual sources or small samples, particularly focused on type-2 Seyfert galaxies. Very little has been said concerning other types of AGN. The purpose of this work is to compare the circumnuclear morphologies of soft X-ray and [O III] images to test whether they match in different optical classes of AGN. Our sample is composed of 27 AGN: nine type-1 Seyferts, 10 type-2 Seyferts, and eight low ionisation nuclear emission-line regions (LINERs). We find a good match in 100% of the type-2 Seyferts in our sample. This correspondence is less frequent in type-1 Seyferts (22%) and it is not seen in LINERs. The good resemblance in type-2 Seyferts constitutes an evidence for a common physical origin. We argue that the lack of correspondence in type-1 Seyferts might be due to the line of sight perpendicular to the accretion disk. Based on the morphologies of the eight LINERs in our sample, we discard a common origin for the soft X-ray and [O III] emissions in these objects. Regarding the X-ray properties, both high column density and hard X-ray luminosity are associated with matched morphologies., Accepted by MNRAS. 20 pages, 9 figures

We have compiled a comprehensive library of all X-ray observations of Magellanic pulsars carried out by XMM-Newton, Chandra, and RXTE in the period 1997–2014. In this work, we use the data from 53 high-mass Be/X-ray binaries in the Small Magellanic Cloud to demonstrate that the distribution of spin-period derivatives vs. spin periods of spinning-down pulsars is not at all different than that of the accreting spinning-up pulsars. The inescapable conclusion is that the up and down samples were drawn from the same continuous parent population, therefore Be/X-ray pulsars that are spinning down over periods spanning 18 years are in fact accreting from retrograde disks. The presence of prograde and retrograde disks in roughly equal numbers supports a new evolutionary scenario for Be/X-ray pulsars in their spin period-period derivative diagram.

The post-outburst rebrightening phenomenon in dwarf novae and X-ray novae is still one of the most challenging subjects for theories of accretion disks. It has been widely recognized that post-outburst rebrightenings are a key feature of WZ Sge-type dwarf novae, which predominantly have short ($\lesssim$0.06 d) orbital periods. I found four post-outburst rebrightenings in ASASSN-14ho during its 2014 outburst, whose orbital period has recently measured to be exceptionally long [0.24315(10) d]. Using the formal solution of the radial velocity study in the literature, I discuss the possibility that this object can be an SU UMa-type dwarf nova near the stability border of the 3:1 resonance despite its exceptionally long orbital period. Such objects are considered to be produced if mass transfer occurs after the secondary has undergone significant nuclear evolution and they may be hidden in a significant number among dwarf novae showing multiple post-outburst rebrightenings., Comment: 4 pages, 1 figure, accepted for publication in PASJ

We present the results of a two year optical continuum photometric reverberation mapping campaign carried out on the nucleus of the Seyfert-1 galaxy Mrk509. Specially designed narrow-band filters were used in order to mitigate the line and pseudo-continuum contamination of the signal from the broad line region, while allowing for high-accuracy flux-calibration over a large field of view. We obtained light curves with a sub-day time sampling and typical flux uncertainties of $1\%$. The high photometric precision allowed us to measure inter-band continuum time delays of up to $\sim 2$ days across the optical range. The time delays are consistent with the relation $\tau \propto \lambda^{4/3}$ predicted for an optically thick and geometrically thin accretion disk model. The size of the disk is, however, a factor of 1.8 larger than predictions based on the standard thin-disk theory. We argue that, for the particular case of Mrk509, a larger black hole mass due to the unknown geometry scaling factor can reconcile the difference between the observations and theory., Comment: 20 pages, 11 figures, published on Monthly Notices of the Royal Astronomical Society

In order to explore various aspects of radiative shocks, we examine standing radiative shock waves in spherical accretion flows onto a central gravitating body under the equilibrium diffusion approximation. In contrast to the usual one-dimensional shock, in radiative shocks a radiative precursor appears in the pre-shock region before the shock front, due to the radiative diffusion effect. Furthermore, in spherical flows around a central object the gravitational potential varies in this radiative precursor, and a curvature effect also exists. We first formulate such radiative shocks in spherical flows, derive the overall jump conditions, and solve the structure of the radiative precursor for both the gas and radiation pressure dominated cases. Since the jump conditions contain the coordinates of both ends of the radiative precursor, we must obtain both the solution and the endpoints of the precursor simultaneously. We find that the gravitational effect is not significant, although it cannot be ignored. The curvature effect exerts a strong influence on the structure and width of the precursor. The precursor starting point x1 normalized by the shock radius is roughly expressed by $x_1={\cal M}_1^{1/7}$ for a radiation pressure dominated shock, while $x_1=1.21^{({\cal M}_1-1)}$ for a gas pressure one, where ${\cal M}_1$ is the pre-shock Mach number.

Relativistic accretion disk winds driven by disk radiation are numerically examined by calculating the relativistic radiative transfer equation under a plane-parallel approximation. We first solve the relativistic transfer equation iteratively, using a given velocity field, and obtain specific intensities as well as moment quantities. Using the obtained flux, we then solve the vertical hydrodynamical equation under the central gravity, and obtain a new velocity field and the mass-loss rate as an eigenvalue. We repeat these double iteration processes until both the intensity and velocity profiles converge. We further calculate these vertical disk winds at various disk radii for appropriate boundary conditions, and obtain the mass-loss rate as a function of a disk radius for a given disk luminosity. Since in the present study we assume a vertical flow, and the rotational effect is ignored, the disk wind can marginally escape for the Eddington disk luminosity. When the disk luminosity is close to the Eddington one, the wind flow is firstly decelerated at around z ∼ r, and then accelerated to escape. For a larger disk luminosity, on the other hand, the wind flow is monotonically accelerated to infinity. Under the boundary condition that the wind terminal velocity is equal to the Keplerian speed at the disk, we find that the normalized mass-loss rate per unit area, $\skew9\hat{\skew9\dot{J}}$, is roughly expressed as $\skew9\hat{\skew9\dot{J}} \sim 3 (r_{\rm in}/r_{\rm S}) \Gamma _{\rm d} \tau _{\rm b} (r/r_{\rm S})^{-5/2}(1-\sqrt{r_{\rm in}/r})$, where rin is the disk inner radius, rS is the Schwarzschild radius of the central object, Γd is the disk normalized luminosity, τb is the wind optical depth, and r is the radial distance from the center.

Recent X-ray observations have revealed the complexity and diversity of high-mass X-ray binaries (HMXBs). This diversity challenges a classical understanding of the accretion process onto the compact objects. In this study, we reinforce the conventional concept of the nature of wind-fed accretion onto a neutron star considering the geometrical effect of radiatively accelerated wind, and re-evaluate the transported angular momentum by using a simple wind model. Our results suggest that even in an OB-type HMXB fed by stellar wind, a large amount of angular momentum could be transported to form an accretion disk due to the wind-inhomogeneity, if the binary separation is tight enough and/or stellar wind is slow. We apply our model into actual systems such as LMC X-4 and OAO 1657-415, and discuss the possibility of disk formations in these systems., Comment: 12 pages, 12 figures, accepted for publication in PASJ

International audience; We simulate an oscillating polytropic torus with constant specific angular momentum around a Schwarzschild black hole. The goal is to search for quasi-periodic oscillations in the light curve of the torus. The torus is subjected to an initial radial, vertical or diagonal (combination of radial and vertical) velocity perturbation. The hydrodynamical simulations are performed using the general relativistic magnetohydrodynamics code cosmos++ and ray traced using the general relativistic ray-tracing code gyoto. We found that a horizontal velocity perturbation triggers the radial and plus modes, whereas a vertical velocity perturbation triggers the vertical and X modes. The diagonal perturbation gives a combination of the modes triggered in the radial and vertical cases. The breathing mode is excited in all cases. We find that the radial eigenmode frequency is independent of the equation of state used. We conclude that the radial and plus modes we find in our simulations are identical to f and o_1 modes reported in Rezolla et al.

We study multi-wavelength variability of a bare Seyfert 1 galaxy Fairall~9 using \swift{} monitoring observations consisting of $165$ usable pointings spanning nearly two years and covering six UV/optical bands and X-rays. Fairall~9 is highly variable in all bands though the variability amplitude decreases from X-ray to optical bands. The variations in the X-ray and UV/optical bands are strongly correlated. Our reverberation mapping analysis using the {\tt JAVALIN} tool shows that the variation in the UV/optical bands lag behind the variations in the X-ray band by $\sim 2-10{\rm~days}$. These lag measurements strongly suggest that the optical/UV variations are mainly caused by variations in the X-rays, and the origin of most of the optical/UV emission is X-ray reprocessing. The observed lags are found to vary as $\tau\propto\lambda^{1.36\pm0.13}$, consistent with the prediction, $\tau\propto\lambda^{4/3}$, for X-ray reprocessing in a standard accretion disc. However, the predicted lags for an standard accretion disc with X-ray reprocessing using black hole mass ($M_{BH} \sim 2.6\times10^{8}~M_{\odot}$) estimated from the reverberation mapping of broad emission lines and accretion rate relative to the Eddington rate ($\dot{m_E} =0.02$) are shorter than the observed lags. These observations suggest that accretion disc in Fairall~9 is larger than that predicted by the standard disc model, and confirm similar findings in a few other Seyfert 1 galaxies such as NGC~5548., Comment: 22 pages, 6 figures, 3 tables, Accepted for publication in MNRAS

We report the detection of a new feature at the centre of NGC 1275 in the Perseus cluster, hosting the radio source 3C 84. This feature emerges ~ 2 mas (~ 0.8 pc) north of the central core in recent 15 and 43 GHz VLBA images, and seems to be the counterjet to a known radio jet expanding to the south of the core. Apparently, the two jets were born through an outburst around 2005. From the ratio of the apparent lengths of the two jets from the core, we found that the jet angle to the line of sight is \theta=65^\circ\pm 16^\circ, which is not much different from the angle of the outer jets generated by an activity around 1959 and constrains theories on gamma-ray emission from jets. The new northern jet has a strongly inverted spectrum in contrast with the southern jet. This suggests that the central black hole is surrounded by a subparsec-scale accretion disk with the density of >~ 10^5 cm^-3. The brightness of the counterjet suggests that the disk is highly inhomogeneous. The ambient gas density in the direction of the jet is ~ 8 cm^-3 if the current jet activity is similar to the past average.

We investigate evolution of an accretion disc in binary black hole (BBH) systems and possible electromagnetic counterparts of the gravitational waves from mergers of BBHs. Perna et al. (2016) proposed a novel evolutionary scenario of an accretion disc in BBHs in which a disc eventually becomes "dead", i.e., the magnetorotational instability (MRI) becomes inactive. In their scenario, the dead disc survives until {\it a few seconds before} the merger event. We improve the dead disc model and propose another scenario, taking account of effects of the tidal torque from the companion and the critical ionization degree for MRI activation more carefully. We find that the mass of the dead disc is much lower than that in the Perna's scenario. When the binary separation sufficiently becomes small, the mass inflow induced by the tidal torque reactivates MRI, restarting mass accretion onto the black hole. We also find that this disc "revival" happens {\it more than thousands of years before} the merger. The mass accretion induced by the tidal torque increases as the separation decreases, and a relativistic jet could be launched before the merger. The emissions from these jets are too faint compared to GRBs, but detectable if the merger events happen within $\lesssim 10$ Mpc or if the masses of the black holes are as massive as $\sim 10^5 M_{\odot}$., Comment: 8 pages, 3 figures, accepted for publication in MNRAS

We present an extended corona model based on ray-tracing simulations to investigate X-ray time lags in Active Galactic Nuclei (AGN). This model consists of two axial point sources illuminating an accretion disc that produce the reverberation lags. These lags are due to the time delays between the directly observed and reflection photons and are associated with the light-travel time between the source and the disc, so they allow us to probe the disc-corona geometry. We assume the variations of two X-ray sources are triggered by the same primary variations, but allow the two sources to respond in different ways (i.e. having different source responses). The variations of each source induce a delayed accretion disc response and the total lags consist of a combination of both source and disc responses. We show that the extended corona model can reproduce both the low-frequency hard and high-frequency soft (reverberation) lags. Fitting the model to the timing data of PG~1244+026 reveals the hard and soft X-ray sources at $\sim6r_{\text{g}}$ and $\sim11r_{\text{g}}$, respectively. The upper source produces small amounts of reflection and can be interpreted as a relativistic jet, or outflowing blob, whose emission is beamed away from the disc. This explains the observed lag-energy in which there is no soft lag at energies $, 13 pages, 15 figures, accepted for publication in MNRAS

We investigate the conditions required for planet formation via gravitational instability (GI) and protoplanetary disk (PPD) fragmentation around M-dwarfs. Using a suite of 64 SPH simulations with $10^6$ particles, the parameter space of disk mass, temperature, and radius is explored, bracketing reasonable values based on theory and observation. Our model consists of an equilibrium, gaseous, and locally isothermal disk orbiting a central star of mass $M_*=M_{sol}/3$. Disks with a minimum Toomre $Q$ of $Q_{min} \lesssim 0.9$ will fragment and form gravitationally bound clumps. Some previous literature has found $Q_{min} < 1.3-1.5$ to be sufficient for fragmentation. Increasing disk height tends to stabilize disks, and when incorporated into $Q$ as $Q_{eff}\propto Q(H/R)^\alpha$ for $\alpha=0.18$ is sufficient to predict fragmentation. Some discrepancies in the literature regarding $Q_{crit}$ may be due to different methods of generating initial conditions (ICs). A series of 15 simulations demonstrates that perturbing ICs slightly out of equilibrium can cause disks to fragment for higher $Q$. Our method for generating ICs is presented in detail. We argue that GI likely plays a role in PPDs around M-dwarfs and that disk fragmentation at large radii is a plausible outcome for these disks., Comment: 18 pages, 15 figures

T CrB is a symbiotic recurrent nova known to exhibit active phases, characterised by apparent increases in the hot component temperature and the appearance of flickering, i.e. changes in the observed flux on the time-scale of minutes. Historical UV observations have ruled out orbital variability as an explanation for flickering and instead suggest flickering is caused by variable mass transfer. We have analysed optical and X-ray observations to investigate the nature of the flickering as well as the active phases in T CrB. The spectroscopic and photometric observations confirm that the active phases follow two periods of ~1000d and ~5000d. Flickering in the X-rays is detected and follows an amplitude-flux relationship similar to that observed in the optical. The flickering is most prominent at harder X-ray energies, suggesting that it originates in the boundary layer between the accretion disc and the white dwarf. The X-ray radiation from the boundary layer is then reprocessed by a thick accretion disc or a nebula into UV radiation. A more detailed understanding of flickering would benefit from long-term simultaneous X-ray and optical monitoring of the phenomena in symbiotic recurrent novae and related systems such as Z And type symbiotic stars., Comment: 23 pages, 12 figures, accepted for publication in MNRAS

We analysed Kepler data of two similar dwarf novae V344 Lyr and V1504 Cyg in order to study optical fast stochastic variability (flickering) by searching for characteristic break frequencies in their power density spectra. Two different stages of activity were analysed separately, i.e. regular outbursts and quiescence. Both systems show similar behaviour during both activity stages. The quiescent power density spectra show a dominant low break frequency which is also present during outburst with a more or less stable value in V344 Lyr while it is slightly higher in V1504 Cyg. The origin of this variability is probably the whole accretion disc. Both outburst power density spectra show additional high frequency components which we interpret as generated by the rebuilt inner disc that was truncated during quiescence. Moreover, V344 Lyr shows the typical linear rms-flux relation which is strongly deformed by a possible negative superhump variability., 10 pages, 9 figures, 2 tables, Accepted for publication in MNRAS

Graham et al. found a sample of active galactic nuclei (AGNs) and quasars from the Catalina Real-time Transient Survey (CRTS) that have long-term periodic variations in optical continuum, the nature of the periodicity remains uncertain. We investigate the periodic variability characteristics of the sample by testing the relations of the observed variability periods with AGN optical luminosity, black hole mass and accretion rates, and find no significant correlations. We also test the observed periods in several different aspects related to accretion disks surrounding single black holes, such as the Keplerian rotational periods of 5100~\AA\ photon-emission regions and self-gravity dominated regions and the precessing period of warped disks. These tests shed new lights on understanding AGN variability in general. Under the assumption that the periodic behavior is associated with SMBHB systems in particular, we compare the separations ($\mathscr{D}_{\bullet}$) against characteristic radii of broad-line regions ($R_{\rm BLR}$) of the binaries and find $\mathscr{D}_{\bullet}$$\approx$ 0.05 $R_{\rm BLR}$. This interestingly implies that these binaries have only circumbinary BLRs., Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letters

We present optical multi-colour photometry of V404 Cyg during the outburst from December, 2015 to January, 2016 together with the simultaneous X-ray data. This outburst occurred less than 6 months after the previous outburst in June–July, 2015. These two outbursts in 2015 were of a slow rise and rapid decay-type and showed large-amplitude (∼2 mag) and short-term (∼10 min–3 hours) optical variations even at low luminosity (0.01–0.1LEdd). We found correlated optical and X-ray variations in two ∼1 hour time intervals and obtained a Bayesian estimate of an X-ray delay against the optical emission, which is ∼30–50 s, during those two intervals. In addition, the relationship between the optical and X-ray luminosity was Lopt ∝ L0.25−0.29 X at that time. These features cannot be easily explained by the conventional picture of tran- sient black-hole binaries, such as canonical disc reprocessing and synchrotron emission related to a jet. We suggest that the disc was truncated during those intervals and that the X-ray delays represent the required time for propagation of mass accretion flow to the inner optically-thin region with a speed comparable to the free-fall velocity.

We report on the detection and subsequent X-ray monitoring of the new Galactic black hole candidate MAXI J1535-571 with the MAXI/GSC. Afterthe discovery on 2017 September 2 made independently with MAXI and the Swift/BAT, the source brightened gradually, and in a few weeks, reached the peak intensity of ~5 Crab, or ~1.6 x 10^{-7} erg cm^{-2} s^{-1} in terms of the 2--20 keV flux. On the initial outburst rise, the X-ray spectrum was described by a power-law model with a photon index of ~2, while after a hard-to-soft transition which occurred on September 18, the spectrum required a disk blackbody component in addition. At around the flux peak, the 2-8 keV and 15-50 keV light curves showed quasi-periodic and anti-correlated fluctuations with an amplitude of 10--20%, on a time scale of ~1-day. Based on these X-ray properties obtained with the MAXI/GSC, we discuss the evolution of the spectral state of this source, and give constraints on its system parameters., Comment: Submitted to Publications of the Astronomical Society of Japan on Apri 3 2018, Accepted by PASJ on July 21 2018

We present simultaneous $g'$, $R_{\rm c}$, and $I_{\rm c}$ photometry of the notable dwarf nova ER UMa during the 2011 season. Our photometry revealed that the brightness maxima of negative superhumps coincide with the bluest peaks in $g' - I_{\rm c}$ colour variations. We also found that the amplitudes of negative superhumps are the largest in the $g'$ band. These observed properties are significantly different from those observed in early and positive superhumps. Our findings are consistent with a tilted disk model as the light source of negative superhumps., 5 pages, 4 figures, accepted for PASJ letter

We report on the X-ray spectral analysis and time evolution of GRS 1739$-$278 during its 2014 outburst based on MAXI/GSC and Swift/XRT observations. Over the course of the outburst, a transition from the low/hard state to the high/soft state and then back to the low/hard state was seen. During the high/soft state, the innermost disk temperature mildly decreased, while the innermost radius estimated with the multi-color disk model remained constant at $\sim18\ (\frac{D}{8.5\ \mathrm{kpc}}) \ {(\frac{\cos i}{\cos 30^{\circ}})}^{-1/2}$ km, where $D$ is the source distance and $i$ is the inclination of observation. This small innermost radius of the accretion disk suggests that the central object is more likely to be a Kerr black hole rather than a Schwardzschild black hole. Applying a relativistic disk emission model to the high/soft state spectra, a mass upper limit of $18.3\ \mathrm{M_{\odot}}$ was obtained based on the inclination limit $i, This paper was accepted by PASJ and published on 2018 May 25