arXiv subjects

Wei-Min Gu

Publications and source records attributed to Wei-Min Gu.

At least 73 records · Page 4Linked to original sources

arXiv2019

Self-similar solutions for finite size advection-dominated accretion flows

We investigated effects on flow variables of transonic advection-dominated accretion flows (ADAFs) for different outer boundary locations (BLs) with a changing energy constant ($E$) of the flow. We used the ADAF solutions and investigated a general power index rule of a radial bulk velocity $(\vr\propto r^{-p})$ with different BLs, but the power index with radius for a rotation velocity and sound speed is unchanged. Here, $p\geq0.5$ is a power index. This power rule gives two types of self-similar solutions; first, when $p=0.5$ gives a self-similar solution of a first kind and exists for infinite length, which has already been discovered for the ADAFs by Narayan \& Yi, and second, when $p>0.5$ gives a self-similar solution of a second kind and exists for finite length, which corresponds to our new solutions for the ADAFs. By using this index rule in fluid equations, we found that the Mach number ($M$) and advection factor ($\fadv$) vary with the radius when $p>0.5$. The local energies of the ADAFs and the Keplerian disk are matched very well at the BLs. So, this theoretical study is supporting a two-zone configuration theory of the accretion disk, and we also discussed other possible hybrid disk geometries. The present study can have two main implications with a variation of the $p$; first, one that can help with the understanding of outflows and non-thermal spectrum variations in black hole candidates, and second, one that can help with solving partial differential equations for any sized advective disk.

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arXiv2019

Could an X-ray Flare after GRB 170817A Originate from a Post-merger Slim Accretion Disc?

GRB 170817A, detected by Fermi-GBM 1.7\,s after the merger of a neutron star (NS) binary, provides the first direct evidence for a link between such a merger and a short-duration gamma-ray burst. The X-ray observations after GRB 170817A indicate a possible X-ray flare with a peak luminosity $L_{\rm peak} \sim 2\times 10^{39}\,{\rm erg\,s}^{-1}$ near day 156. Here we show that this X-ray flare may be understood based on a slim disc around a compact object. On the one hand, there exists the maximal accretion rate $\dot M_{\rm max}$ for the slim disc, above which an optically thick outflow is significant and radiation from the disc is obscured. Based on the energy balance analysis, we find that $\dot M_{\rm max}$ is in the range of $\sim 4\dot M_{\rm Edd}$ and $\sim 21\dot M_{\rm Edd}$ when the angular velocity of the slim disc is between $\rm (1/5)^{1/2}Ω_K$ and $\rm Ω_K$ (where $\dot M_{\rm Edd}$ is the Eddington accretion rate and $Ω_K$ is the Keplerian angular velocity). With $\dot M_{\rm max}$, the slim disc can provide a luminosity $\sim L_{\rm peak}$ for a compact object of $2.5 M_{\sun}$. On the other hand, if the merger of two NSs forms a typical neutrino-dominated accretion disc whose accretion rate $\dot M$ follows a power-law decline with an index $-1.8$ , then the system must pass through the outflow regime and enter the slim disc in $\sim 11-355$ days. These results imply that a post-merger slim accretion disc could account for the observed late-time $L_{\rm peak}$.

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arXiv2019

Thick-disc model to explain the spectral state transition in NGC 247

We propose the thick-disc model of Gu et al. 2016 to interpret the transition between soft ultraluminous state (SUL) and supersoft ultraluminous (SSUL) state in NGC 247. As accretion rate increases, the inner disc will puff up and act as shield to block the innermost X-ray emission regions and absorb both soft and hard X-ray photons. The absorbed X-ray emission will be re-radiated as a much softer blackbody X-ray spectrum. Hence NGC 247 shows flux dips in the hard X-ray band and transits from the SUL state to the SSUL state. The $\sim 200$s transition timescale can be explained by the viscous timescale. According to our model, the inner disc in the super-soft state is thicker and has smaller viscous timescale than in the soft state. X-ray flux variability, which is assumed to be driven by accretion rate fluctuations, might be viscous time-scale invariant. Therefore, in the SSUL state, NGC 247 is more variable. The bolometric luminosity is saturated in the thick disc; the observed radius-temperature relation can therefore be naturally explained.

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arXiv2019

Accretion flows with comparable radiation and gas pressures

By taking into account photon absorption, we investigate the vertical structure of accretion flows with comparable radiation and gas pressures. We consider two separate energy equations for matter and radiation in the diffusion limit. In order to solve the set of radiation hydrodynamic equations in steady state and axisymmetric configuration, we employ self-similar technique in the radial direction. We need the reflection symmetry about the mid-plane to find gas density at the equator. For a typical solution, we assume that the gas pressure has 10-50\% portion of the total pressure. In this paper, since the radiation energy is involved directly, we are able to estimate how much energy of viscous heating is transported in the radial direction and advected towards the central object. Our results show that although the mass accretion rate does not approach the Eddington limit, the energy advection is rather high. Moreover, in a disc with greater accretion rate and less portion of gas pressure at the total pressure, more energy is advected to its center. In addition, as we expect the accretion flow becomes thicker with greater values of gas pressure. Based on Solberg-HÃžiland conditions, we notice that the flow is convectively stable in all parts of such a disc.

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arXiv2019

A possible feedback mechanism of outflows from a black hole hyperaccretion disk in the center of jet-driven iPTF14hls

iPTF14hls is an unusually bright, long-lived II-P supernova (SN), whose light curve has at least five peaks. We propose that the outflows from the black hole hyperaccretion systems in the center of the collapsars should continuously inject into the envelope. For a jet-driven core-collapsar model, the outflow feedback results in prolonging the accretion timescale and fluctuating accretion rates in our analytic solutions. Thus, the long period of luminous, varying SN iPTF14hls might originate from the choked jets, which are regulated by the feedback of the strong disk outflows in a massive core-collapsar. One can expect that jet-driven iPTF14hls may last no more than approximately 3,000 days, and the luminosity may quickly decrease in the later stages. Moreover, the double-peak light curves in some SNe might be explained by the outflow feedback mechanism.

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arXiv2019

A Method to Search for Black Hole Candidates with Giant Companions by LAMOST

We propose a method to search for stellar-mass black hole (BH) candidates with giant companions from spectroscopic observations. Based on the stellar spectra of LAMOST Data Release 6, we obtain a sample of seven giants in binaries with large radial velocity variation $ΔV_R > 80~{\rm km~s^{-1}}$. With the effective temperature, surface gravity, and metallicity provided by LAMOST, and the parallax given by {\it Gaia}, we can estimate the mass and radius of the giant, and therefore evaluate the possible mass of the optically invisible star in the binary. We show that the sources in our sample are potential BH candidates, and are worthy of dynamical measurement by further spectroscopic observations. Our method may be particularly valid for the selection of BH candidates in binaries with unknown orbital periods.

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arXiv2018

New Clues to Jet Launching: The inner disks in radio loud quasars may be more stable

Jet launching in radio loud (RL) quasars is one of the fundamental problems in astrophysics. Exploring the differences in the inner accretion disk properties between RL and radio quiet (RQ) quasars might yield helpful clues to this puzzle. We previously discovered that the shorter term UV/optical variations of quasars are bluer than the longer term ones, i.e., the so-called timescale-dependent color variation. This is consistent with the scheme that the faster variations come from the inner and hotter disk regions, thus providing a useful tool to map the accretion disk which is otherwise unresolvable. In this work we compare the UV/optical variations of RL quasars in SDSS Stripe 82 to those of several RQ samples, including those matched in redshift-luminosity-black hole mass and/or color-magnitude. We find that while both RL and RQ populations appear bluer when they brighten, RL quasars potentially show a weaker/flatter dependence on timescale in their color variation. We further find that while both RL and RQ populations on average show similar variation amplitudes at long timescales, fast variations of RL sources appear weaker/smaller (at timescales of ~ 25 -- 300 days in the observer's frame), and the difference is more prominent in the g-band than in the r-band. Inhomogeneous disk simulations can qualitatively reproduce these observed differences if the inner accretion disk of RL quasars fluctuates less based on simple toy models. Though the implications are likely model dependent, the discovery points to an interesting diagram that magnetic fields in RL quasars may be prospectively stronger and play a key role in both jet launching and the stabilization of the inner accretion disk.

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arXiv2018

Winds Can "Blow Up" AGN Accretion Disk Sizes

Recent multi-band variability studies have revealed that active galactic nucleus (AGN) accretion disc sizes are generally larger than the predictions of the classical thin disc by a factor of $2\sim 3$. This hints at some missing key ingredient in the classical thin disc theory: here, we propose an accretion disc wind. For a given bolometric luminosity, in the outer part of an accretion disc, the effective temperature in the wind case is higher than that in the no-wind one; meanwhile, the radial temperature profile of the wind case is shallower than the no-wind one. In presence of winds, for a given band, blackbody emission from large radii can contribute more to the observed luminosity than the no-wind case. Therefore, the disc sizes of the wind case can be larger than those of the no-wind case. We demonstrate that a model with the accretion rate scaling as $\dot{M}_0 (R/R_{\mathrm{S}})^β$ (i.e., the accretion rate declines with decreasing radius due to winds) can match both the inter-band time lags and the spectral energy distribution of NGC 5548. Our model can also explain the inter-band time lags of other sources. Therefore, our model can help decipher current and future continuum reverberation mapping observations.

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arXiv2018

An intrinsic link between long-term UV/optical variations and X-ray loudness in quasars

Observations have shown that UV/optical variation amplitude of quasars depend on several physi- cal parameters including luminosity, Eddington ratio, and likely also black hole mass. Identifying new factors which correlate with the variation is essential to probe the underlying physical processes. Combining ~ten years long quasar light curves from SDSS stripe 82 and X-ray data from Stripe 82X, we build a sample of X-ray detected quasars to investigate the relation between UV/optical variation amplitude ($σ_{rms}$) and X-ray loudness. We find that quasars with more intense X-ray radiation (com- pared to bolometric luminosity) are more variable in UV/optical. Such correlation remains highly significant after excluding the effect of other parameters including luminosity, black hole mass, Ed- dington ratio, redshift, rest-frame wavelength (i.e., through partial correlation analyses). We further find the intrinsic link between X-ray loudness and UV/optical variation is gradually more prominent on longer timescales (up to 10 years in the observed frame), but tends to disappear at timescales < 100 days. This suggests a slow and long-term underlying physical process. The X-ray reprocessing paradigm, in which UV/optical variation is produced by a variable central X-ray emission illuminating the accretion disk, is thus disfavored. The discovery points to an interesting scheme that both the X-ray corona heating and UV/optical variation is quasars are closely associated with magnetic disc turbulence, and the innermost disc turbulence (where corona heating occurs) correlates with the slow turbulence at larger radii (where UV/optical emission is produced).

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arXiv2018

A statistical analysis of the "heartbeat" behaviour of GRS 1915+105

GRS 1915+105 has been active for more than 26 years since it was discovered in 1992. There are hundreds of RXTE pointed observations on this source, and the quasi-regular flares with a slow rise and a sharp decrease (i.e. the "heartbeat" state) were recorded in more than 200 observations. The connections among the disk/corona, jet, and the disk wind at the heartbeat state have been extensively studied. In this work, we firstly perform a statistical analysis of the light curves and the X-ray spectra to investigate this peculiar state. We calculate the parameters for heartbeat cycles, including the recurrence time, the maximum and the minimum count rate, the flare amplitude, and the cumulative radiation for each cycle. The recurrence time has a bimodal distribution ranging from $\sim 20$ to $\sim 200$ s. The minimum count rate increases with increasing recurrence time; while the maximum count rate remains nearly constant around 2 Crab. Fitting the averaged spectrum for each observation, we find the strong correlations among the recurrence time, the apparent inner radius of the accretion disk (or the color correction factor), and the (nonthermal) X-ray luminosity. We suggest that the true inner edge of the accretion disk might always extend to the marginally stable orbit, while the change in corona size should result in the observed correlations.

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arXiv2018

A lower occurrence rate of bright X-ray flares in SN-GRBs than $z<1$ GRBs: evidence of energy partitions?

The occurrence rates of bright X-ray flares in z<1 gamma-ray bursts (GRBs) with or without observed supernovae (SNe) association were compared. Our Sample I: the z<1 long GRBs (LGRBs) with SNe association (SN-GRBs) and with early Swift/X-Ray Telescope (XRT) observations, consists of 18 GRBs, among which only two GRBs have bright X-ray flares. Our Sample II: for comparison, all the z<1 LGRBs without observed SNe association and with early Swift/XRT observations, consists of 45 GRBs, among which 16 GRBs present bright X-ray flares. Thus, the study indicates a lower occurrence rate of bright X-ray flares in Sample I (11.1%) than in Sample II (35.6%). In addition, if dim X-ray fluctuations are included as flares, then 16.7% of Sample I and 55.6% of Sample II are found to have flares, again showing the discrepancy between these two samples. We examined the physical origin of these bright X-ray flares and found that most of them are probably related to the central engine reactivity. To understand the discrepancy, we propose that such a lower occurrence rate of flares in the SN-GRB sample may hint at an energy partition among the GRB, SNe, and X-ray flares under a saturated energy budget of massive star explosion.

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arXiv2018

The 2D disk structure with advective transonic inflow-outflow solutions around black holes

We solved analytically viscous two-dimensional (2D) fluid equations for accretion and outflows in spherical polar coordinates ($r, θ, ϕ$) and obtained explicitly flow variables in $r-$ and $θ-$directions around black holes (BHs). We investigated global transonic advection-dominated accretion flow (ADAF) solutions in $r-$direction on an equatorial plane with using Paczyński-Wiita potential. We used radial flow variables of ADAFs with symmetric conditions on the equatorial plane, as initial values for integration in $θ-$direction. In the study of 2D disk structure, we used two-azimuthal components of viscous stress tensors namely, $τ_{\rm rϕ}$ and $τ_{\rmθϕ}$. Interestingly, we found that the whole advective disk is not participating in outflow generation and the outflows form close to the BHs. Normally, outflow strength increased with increasing viscosity parameter ($α_1$), mass-loss parameter ($s$) and decreasing gas pressure ratio ($β$). Outflow region increased with increasing $s$, $α_1$ for $τ_{\rm rϕ}$ and decreasing $α_2$ for $τ_{\rmθϕ}$. The $τ_{\rmθϕ}$ is effective in angular momentum transportation at high latitude and outflows collimation along an axis of symmetry since it changes polar velocity ($v_{\rmθ}$) of the flow. The outflow emission is also affected by the ADAF size and decreased with decreasing it. Transonic surfaces formed for both inflows ($v_{\rm r}<0$, very close to BH) and outflows ($v_{\rm r}>0$). We also explored no outflows, outflows and failed outflows regions, which mainly depend on the viscosity parameters.

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arXiv2018

Central Engine-Powered Bright X-ray Flares in Short Gamma-Ray Bursts: A Hint of Black Hole-Neutron Star Merger?

Short gamma-ray bursts may originate from the merger of double neutron stars (NS) or that of a black hole (BH) and an NS. We propose that the bright X-ray flare related to the central engine reactivity may hint a BH-NS merger, since such a merger can provide more fall-back materials and therefore a more massive accretion disk than the NS-NS merger. Based on the observed 49 short bursts with Swift/X-ray Telescope follow-up observations, we find that three bursts have bright X-ray flares, among which three flares from two bursts are probably related to the central engine reactivity. We argue that these two bursts may originate from the BH-NS merger rather than the NS-NS merger. Our suggested link between the central engine-powered bright X-ray flare and the BH-NS merger event can be checked by the future gravitational wave detections from advanced LIGO and Virgo.

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arXiv2018

First Electromagnetic Pulse Associated with a Gravitational-Wave Event: Profile, Duration, and Delay

We study the first electromagnetic pulse after the gravitational wave chirp signal, focusing on the profile and duration. It is found that the light curve, especially the steep decay (SD) phase, can be very different by adopting different viewing angle $θ_{\rm view}$ on the jet shell. For an on-axis jet with a power-law radiation spectrum, the observed flux in the SD is proportional to $t_{\rm{obs}}^{-2-β}$ with $β$ being the spectral index and $t_{\rm{obs}}$ being the observer time. Here, $t_{\rm{obs}}=0$ is set at the observed time of the jet ejected from the central engine. The SD may become steep by increasing $θ_{\rm view}$. We also study the bolometric luminosity $L$ from a jet shell with a non-power-law radiation spectrum. For an on-axis jet, $L{\propto}t_{\rm{obs}}^{-3}$ is found in the SD. However, the SD is steeper than $L{\propto}t_{\rm{obs}}^{-3}$ for the radiation from an off-axis jet. The higher value of $θ_{\rm view}$ is, the steeper of SD would be. Then, we suggest that the SD phase can be used to discriminate an off-axis jet from an on-axis jet. The reason for above behaviors is discussed. In addition, we find that the duration of first electromagnetic pulse is close to its peak time, especially for $θ_{\rm{view}}\sim20^\circ$. This result is consistent with that found in GW~170817/GRB~170817A. Thus, the jet corresponding to the prompt emission of GRB~170817A should be immediately ejected after the merger. Our results also reveal that the duration of the first electromagnetic pulse can provide the information of the time to search gravitational waves.

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arXiv2018

EUCLIA - Exploring the UV/optical continuum lag in active galactic nuclei. I. a model without light echoing

The tight inter-band correlation and the lag-wavelength relation among UV/optical continua of active galactic nuclei have been firmly established. They are usually understood within the widespread reprocessing scenario, however, the implied inter-band lags are generally too small. Furthermore, it is challenged by new evidences, such as the X-ray reprocessing yields too much high frequency UV/optical variations as well as it fails to reproduce the observed timescale-dependent color variations among {\it Swift} lightcurves of NGC 5548. In a different manner, we demonstrate that an upgraded inhomogeneous accretion disk model, whose local {\it independent} temperature fluctuations are subject to a speculated {\it common} large-scale temperature fluctuation, can intrinsically generate the tight inter-band correlation and lag across UV/optical, and be in nice agreement with several observational properties of NGC 5548, including the timescale-dependent color variation. The emergent lag is a result of the {\it differential regression capability} of local temperature fluctuations when responding to the large-scale fluctuation. An average speed of propagations as large as $\gtrsim 15\%$ of the speed of light may be required by this common fluctuation. Several potential physical mechanisms for such propagations are discussed. Our interesting phenomenological scenario may shed new light on comprehending the UV/optical continuum variations of active galactic nuclei.

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arXiv2018

Compact binary merger and kilonova: outflows from remnant disc

Outflows launched from a remnant disc of compact binary merger may have essential contribution to the kilonova emission. Numerical calculations are conducted in this work to study the structure of accretion flows and outflows. By the incorporation of limited-energy advection in the hyper-accretion discs, outflows occur naturally from accretion flows due to imbalance between the viscous heating and the sum of the advective and radiative cooling. Following this spirit, we revisit the properties of the merger outflow ejecta. Our results show that around $10^{-3} \sim 10^{-1} M_\odot$ of the disc mass can be launched as powerful outflows. The amount of unbound mass varies with the disc mass and the viscosity. The outflow-contributed peak luminosity is around $10^{40} \sim 10^{41}$ erg/s. Such a scenario can account for the observed kilonovae associated with short gamma-ray bursts, including the recent event AT2017gfo (GW170817).

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arXiv2018

X-ray light curve in GRB 170714A: evidence for quark star?

Two plateaus and one following bump in the X-ray light curve of GRB 170714A have been detected by the \textit{Swift}/X-Ray Telescope, which could be very meaningful for the central engine of gamma-ray bursts (GRBs), implying that the origin of this burst might be different from that of other ultra-long GRBs. We propose that merging two neutron stars into a hyper-massive quark star (QS) and then collapsing into a black hole (BH), with a delay time around $10^4$~s, could be responsible for those X-ray components. The hyper-massive QS is initially in a fluid state, being turbulent and differentially rotating, but would be solidified and release its latent heat injected into the GRB fireball (lasting about $10^3$~s during the liquid-solid phase transition). Magnetic field as high as $\sim 10^{15}$~G could be created by dynamo action of the newborn liquid QS, and a magnetar-like central engine (after solidification) supplies significant energy for the second plateau. More energy could be released during a fall-back accretion after the post-merger QS collapses to a BH, and the X-ray bump forms. This post-merger QS model might be tested by future observations, with either advanced gravitational wave detectors (e.g., advanced LIGO and VIRGO) or X-ray/optical telescopes.

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arXiv2018

A Black Hole - White Dwarf Compact Binary Model for Long Gamma-ray Bursts without Supernova Association

Gamma-ray bursts (GRBs) are luminous and violent phenomena in the universe. Traditionally, long GRBs are expected to be produced by the collapse of massive stars and associated with supernovae. However, some low-redshift long GRBs have no detection of supernova association, such as GRBs 060505, 060614 and 111005A. It is hard to classify these events convincingly according to usual classifications, and the lack of the supernova implies a non-massive star origin. We propose a new path to produce long GRBs without supernova association, the unstable and extremely violent accretion in a contact binary system consisting of a stellar-mass black hole and a white dwarf, which fills an important gap in compact binary evolution.

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