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The variability of active galactic nuclei (AGNs) has long been servicing as an essential avenue of exploring the accretion physics of a black hole (BH). There are two commonly used methods for analyzing AGN variability. First, the AGN variability, characterized by the structure function (SF) of a single band, can be well described by a damped random walk (DRW) process on timescales longer than ∼weeks, shorter than which departures have been reported. Second, the color variation (CV) between two bands behaves timescale dependent, raising challenges to the widely accepted reprocessing scenario. However, both the departure from the DRW process and the timescale-dependent CV are hitherto limited to AGNs, mainly quasars, at the supermassive scale. Here, utilizing the high-cadence multiwavelength monitoring on NGC 4395 harboring an intermediate-mass BH, we unveil at the intermediate-mass scale for the first time, prominent departures from the DRW process at timescales shorter than ∼hours in all three nights and bands, and plausible timescale-dependent CVs in the two longest nights of observation. Furthermore, comparing SFs of NGC 4395 to four AGNs at the supermassive scale, we suggest a new scaling relation between the timescale (τ ; across nearly 3 orders of magnitude) and the BH mass (M BH): τ ∝ M BH γ where the exponent γ is likely ≃0.6–0.8. This exponent differs from most previous measurements, but confirms a few, and is consistent with a recent theoretical prediction, suggesting a similar accretion process in AGNs across different mass scales. [ABSTRACT FROM AUTHOR]

We present a deep optical imaging campaign on the starburst galaxy NGC 4631 with CFHT/MegaCam. By supplementing the HST/ACS and Chandra/ACIS archival data, we search for the optical counterpart candidates of the five brightest X-ray sources in this galaxy, four of which are identified as ultraluminous X-ray sources (ULXs). The stellar environments of the X-ray sources are analyzed using the extinction-corrected color–magnitude diagrams and the isochrone models. We discover a highly asymmetric bubble nebula around X4 that exhibits different morphology in the H α and [O iii ] images. The [O iii ]/H α ratio map shows that the H α -bright bubble may be formed mainly via the shock ionization by the one-sided jet/outflow, while the more compact [O iii ] structure is photoionized by the ULX. We constrain the bubble expansion velocity and interstellar medium density with the MAPPINGS V code and hence estimate the mechanical power injected into the bubble as P w ∼ 5 × 1040 erg s−1 and the corresponding bubble age as ∼7 × 105 yr. Relativistic jets are needed to provide such a level of mechanical power with a mass-loss rate of ∼10−7 M ⊙ yr−1. Besides the accretion, the black hole spin is likely an additional energy source for the super-Eddington jet power. [ABSTRACT FROM AUTHOR]

Ultraluminous X-ray bursts (ULXBs) are ultraluminous X-ray flares with a fast rise (∼1 minute) and a slow decay (∼1 hour), which are commonly observed in extragalactic globular clusters. Most ULXBs are observational one-off bursts, whereas five flares from the same source in NGC 5128 were discovered by Irwin et al. In this article, we propose a neutron star (NS)â€"white dwarf (WD) binary model with super-Eddington accretion rates to explain the repeating behavior of the ULXB source in NGC 5128. With an eccentric orbit, the mass transfer occurs at the periastron where the WD fills its Roche lobe. The ultraluminous X-ray flares can be produced by the accretion column around the NS magnetic poles. On the other hand, some repeating fast radio bursts (FRBs) were also found in extragalactic globular clusters. Repeating ULXBs and repeating FRBs are the most violent bursts in the X-ray and radio bands, respectively. We propose a possible association between the repeating ULXBs and the repeating FRBs. Such an association is worth further investigation by follow-up observations on nearby extragalactic globular clusters. [ABSTRACT FROM AUTHOR]

Outflows are inevitably driven from the disk if the vertical component of the black hole (BH) gravity cannot resist the radiation force. We derive the mass-loss rate in the outflows by solving a dynamical equation for the vertical gas motion in the disk. The structure of a supercritical accretion disk is calculated with the radial energy advection included. We find that most inflowing gas is driven into outflows if the disk is accreting at a moderate Eddington-scaled rate (up to ∼100) at its outer edge, i.e., only a small fraction of gas is accreted by the BH, which is radiating at several Eddington luminosities, while it reaches around ten for extremely high accretion rate cases ( m ̇ ≡ M ̇ / M ̇ Edd ∼ 1000). Compared with a normal slim disk, the disk luminosity is substantially suppressed due to the mass loss in the outflows. We apply the model to the light curves of the tidal disruption events (TDEs) and find that the disk luminosity declines very slowly with time even if a typical accretion rate m ̇ ∝ t âˆ' 5 / 3 is assumed at the outer edge of the disk, which is qualitatively consistent with the observed light curves in some TDEs and helps us to understand the energy deficient phenomenon observed in the TDEs. Strong outflows from supercritical accretion disks surrounding supermassive BHs may play crucial roles in their host galaxies, which can be taken as an ingredient in the mechanical feedback models. The implications of the results on the growth of supermassive BHs are also discussed. [ABSTRACT FROM AUTHOR]

We report a single-lined white dwarfâ€"main-sequence binary system, LAMOST J172900.17+652952.8, which is discovered by the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST)’s medium-resolution time-domain surveys. The radial-velocity semi-amplitude and orbital period of the optical visible star are measured by using follow-up observations with the Palomar 200 inch telescope and light curves from the Transiting Exoplanet Survey Satellite (TESS). Thus the mass function of the invisible candidate white dwarf is derived, f (M 2) = 0.120 ± 0.003 M ⊙. The mass of the visible star is measured based on a spectral energy distribution fitting, M 1 = 0.81 âˆ' 0.06 + 0.07 M ⊙ . Hence, the mass of its invisible companion is M 2 ≳ 0.63 M ⊙. The companion ought to be a compact object rather than a main-sequence star owing to the mass ratio q = M 2/ M 1 ≳ 0.78 and the single-lined spectra. The compact object is likely to be a white dwarf if the inclination angle is not small, i ≳ 40°. By using the Galaxy Evolution Explorer (GALEX) near-UV flux, the effective temperature of the white dwarf candidate is constrained as T eff WD ≲ 12,000â€"13,500 K. It is difficult to detect white dwarfs which are outshone by their bright companions via single-epoch optical spectroscopic surveys. Therefore, optical time-domain surveys can play an important role in unveiling invisible white dwarfs and other compact objects in binaries. [ABSTRACT FROM AUTHOR]

We present LAMOST J041920.07+072545.4 (hereafter J0419), a close binary consisting of a bloated, extremely low mass pre-white dwarf (pre-ELM WD) and a compact object with an orbital period of 0.607189 days. The large-amplitude ellipsoidal variations and the evident Balmer and He i emission lines suggest a filled Roche lobe and ongoing mass transfer. No outburst events were detected in the 15 years of monitoring of J0419, indicating a very low mass transfer rate. The temperature of the pre-ELM, T eff = 5793 âˆ' 133 + 124 K , makes it cooler than the known ELMs, but hotter than most cataclysmic variable donors. Combining the mean density within the Roche lobe and the radius constrained from our spectral energy distribution fitting, we obtain the mass of the pre-ELM, M 1 = 0.176 ± 0.014 M ⊙. The joint fitting of light and radial velocity curves yields an inclination angle of i = 66.5 âˆ' 1.7 + 1.4 degrees, corresponding to a mass of the compact object of M 2 = 1.09 ± 0.05 M ⊙. The very bloated pre-ELM has a smaller surface gravity ( log g = 3.9 ± 0.01 , R 1 = 0.78 ± 0.02 R ⊙) than the known ELMs or pre-ELMs. The temperature and the luminosity ( L bol = 0.62 âˆ' 0.10 + 0.11 L ⊙ ) of J0419 are close to those of the main sequence, which makes the selection of such systems through the H-R diagram inefficient. Based on the evolutionary model, the relatively long period and small log g indicate that J0419 could be close to the “bifurcation period” in the orbital evolution, which makes J0419 a unique source to connect ELM/pre-ELM WD systems, wide binaries, and cataclysmic variables. [ABSTRACT FROM AUTHOR]

We revisit the eccentric neutron star (NS)â€"white dwarf (WD) binary model for the periodic activity of fast radio burst (FRB) sources, by including the effects of gravitational-wave (GW) radiation. In this model, the WD fills its Roche lobe at the periastron and mass transfer occurs from the WD to the NS. The accreted materials can be fragmented and arrive at the NS episodically, resulting in multiple bursts through curvature radiation. Consequently, the WD may be kicked away owing to the conservation of angular momentum. To initiate the next mass transfer, the WD has to refill its Roche lobe through GW radiation. In this scenario, whether the periodic activity can show up relies on three timescales, i.e., the orbital period P orb, the timescale T GW for the Roche lobe to be refilled, and the time span T frag for all the episodic events corresponding to each mass-transfer process. Only when the two conditions T GW ≲ P orb and T frag < P orb are both satisfied, the periodic activity will manifest itself and the period should be equal to P orb. In this spirit, the periodic activity is more likely to show up for relatively long periods (P orb ≳ several days). Thus, it is reasonable that FRBs 180916 and 121102, the only two sources having been claimed to manifest periodic activity, both correspond to relatively long periods. [ABSTRACT FROM AUTHOR]

The optical light curves of quiescent black hole low-mass X-ray binaries often exhibit significant nonellipsoidal variabilities, showing the photospheric radiation of the companion star is veiled by other sources of optical emission. Assessing this “veiling” effect is critical to the black hole mass measurement. Here in this work, we carry out a strictly simultaneous spectroscopic and photometric campaign on the prototype of black hole low-mass X-ray binary A0620-00. We find that for each observation epoch, the extra optical flux beyond a pure ellipsoidal modulation is positively correlated with the fraction of veiling emission, indicating the accretion disk contributes most of the nonellipsoidal variations. Meanwhile, we also obtain a K2V spectral classification of the companion, as well as the measurements of the companion’s rotational velocity v sin i = 83.8 ± 1.9 km sâˆ'1 and the mass ratio between the companion and the black hole q = 0.063 ± 0.004. [ABSTRACT FROM AUTHOR]

The timescales of the variabilities in changing look (CL) active galactic nuclei (AGNs) are usually on the order of years to tens of years (some of them are even shorter than one year), which is much shorter than the viscous timescale of a standard thin accretion disk. This implies that the variabilities of CL AGNs cannot be reproduced by varying the mass accretion rate of the thin disk. In this work, we employ a magnetic accretion disk-outflow model to calculate the inflow time of the disk predominantly driven by magnetic outflows. In this model, most angular momentum of the gas in the disk is carried away by the outflows, and therefore its radial velocity can be substantially higher than that of a conventional viscous disk. Our calculations show that the inflow time of such a disk with outflows can be around several years to tens of years. The calculated spectra of the disk with outflows can fit the observed spectra of a CL AGN Mrk 1018 quite well both in the low and high states. The derived inflow time of such a disk with outflows is around 5 yr in the high state, while it becomes ∼20 yr in the low state, which is roughly consistent with the observations of the variabilities in Mrk 1018. [ABSTRACT FROM AUTHOR]

The existence of intermediate-mass black holes (IMBHs) in globular clusters (GCs) remains a crucial problem. Searching for IMBHs in GCs reveals a discrepancy between radio observations and dynamical modelings: the upper mass limits constrained by radio observations are systematically lower than that of dynamical modelings. One possibility for such a discrepancy is that, as we suggest in this work, there exist outflows in accretion flows. Our results indicate that, for most sources, current radio observations cannot rule out the possibility that IMBHs may exist in GCs. In addition, we adopt an relation to revisit this issue, which confirms the results obtained by the fundamental plane relation. [ABSTRACT FROM AUTHOR]

Rapid temporal variability has been widely observed in the light curves of gamma-ray bursts (GRBs). One possible mechanism for such variability is related to the relativistic eddies in the jet. In this paper, we include the contribution of the inter-eddy medium together with the eddies to the gamma-ray emission. We show that the gamma-ray emission can either lead or lag behind the observed synchrotron emission, where the latter originates in the inter-eddy medium and provides most of the seed photons for producing gamma-ray emission through inverse Compton scattering. As a consequence, we argue that the lead/lag found in non-stationary short-lived light curves may not reveal the intrinsic lead/lag of different emission components. In addition, our results may explain the lead of gamma-ray emission with respect to optical emission observed in GRB 080319B. [ABSTRACT FROM AUTHOR]

We investigate the thermal stability of optically thin, two-temperature, radiative cooling-dominated accretion disks. Our linear analysis shows that the disk is thermally unstable without magnetic fields, which agrees with previous stability analysis on the Shapiro-Lightman-Eardley disk. By taking into account the effects of magnetic fields, however, we find that the disk can be, or partly be, thermally stable. Our results may be helpful to understand the outflows in optically thin flows. Moreover, such radiative cooling-dominated disks may provide a new explanation of the different behaviors between black hole and neutron star X-ray binaries on the radio/X-ray correlation. [ABSTRACT FROM AUTHOR]

We present the effects of self-gravity on the vertical structure and neutrino luminosity of the neutrino-dominated accretion disks in cylindrical coordinates. It is found that significant changes of the structure appear in the outer region of the disk, especially for high accretion rates (e.g., ≳ 1 M☼ s–1), and thus cause the slight increase in the neutrino luminosity. Furthermore, the gravitational instability of the disk is reviewed by the vertical distribution of the Toomre parameter, which may account for the late-time flares in gamma-ray bursts and the extended emission in short-duration gamma-ray bursts. [ABSTRACT FROM AUTHOR]

We study the fluctuations of standard thin accretion disks by linear analysis of the time-dependent energy equation together with the vertical hydrostatic equilibrium and the equation of state. We show that some of the simulation results in Hirose et al., such as the time delay, the relationship of power spectra, and the correlation between magnetic energy and radiation energy, can be understood well by our analytic results. [ABSTRACT FROM AUTHOR]

Soft extended emission (EE) following initial hard spikes up to 100 s was observed with Swift/BAT for about half of known short-type gamma-ray bursts (SGRBs). This challenges the conversional central engine models of SGRBs, i.e., compact star merger models. In the framework of black-hole-neutron-star merger models, we study the roles of radial angular momentum transfer in the disk and the magnetic barrier around the black hole in the activity of SGRB central engines. We show that radial angular momentum transfer may significantly prolong the lifetime of the accretion process, which may be divided into multiple episodes by the magnetic barrier. Our numerical calculations based on models of neutrino-dominated accretion flows suggest that disk mass is critical for producing the observed EE. In the case of the mass being ∼0.8 M☼, our model can reproduce the observed timescale and luminosity of both the main and the EE episodes in a reasonable parameter set. The predicted luminosity of the EE component is lower than the observed EE within about one order of magnitude and the timescale is shorter than 20 s if the disk mass is ∼0.2 M☼. Swift/BAT-like instruments may be not sensitive enough to detect the EE component in this case. We argue that the EE component could be a probe for the merger process and disk formation for compact star mergers. [ABSTRACT FROM AUTHOR]

By taking into account the local energy balance per unit volume between the viscous heating and the advective cooling plus the radiative cooling, we investigate the vertical structure of radiation pressure-supported accretion disks in spherical coordinates. Our solutions show that the photosphere of the disk is close to the polar axis and therefore the disk seems to be extremely thick. However, the density profile implies that most of the accreted matter exists in a moderate range around the equatorial plane. We show that the well-known polytropic relation between the pressure and the density is unsuitable for describing the vertical structure of radiation pressure-supported disks. More importantly, we find that the energy advection is significant even for slightly sub-Eddington accretion disks. We argue that the non-negligible advection may help us understand why the standard thin disk model is likely to be inaccurate above ∼0.3 Eddington luminosity, which was found by some works on black hole spin measurement. Furthermore, the solutions satisfy the Solberg-Høiland conditions, which indicate the disk to be convectively stable. In addition, we discuss the possible link between our disk model and ultraluminous X-ray sources. [ABSTRACT FROM AUTHOR]

The physical nature of gamma-ray bursts (GRBs) is believed to involve an ultra-relativistic jet. The observed complex structure of light curves motivates the idea of jet precession. In this work, we study the gravitational waves of jet precession based on neutrino-dominated accretion disks around black holes, which may account for the central engine of GRBs. In our model, the jet and the inner part of the disk may precess along with the black hole, which is driven by the outer part of the disk. Gravitational waves are therefore expected to be significant from this black-hole-inner-disk precession system. By comparing our numerical results with the sensitivity of some detectors, we find that it is possible for DECIGO and BBO to detect such gravitational waves, particularly for GRBs in the Local Group. [ABSTRACT FROM AUTHOR]