arXiv subjects

Bing Zhang

Publications and source records attributed to Bing Zhang.

At least 361 records · Page 20Linked to original sources

arXiv2018

FRB 121102: A repeatedly combed neutron star by a nearby low-luminosity accreting super-massive black hole

The origin of fast radio bursts (FRBs) remains mysterious. Recently, the only repeating FRB source, FRB 121102, was reported to possess an extremely large and variable rotation measure (RM). The inferred magnetic field strength in the burst environment is comparable to that in the vicinity of the super-massive black hole Sagittarius A* of our Galaxy. Here we show that all the observational properties of FRB 121102 (including the high RM and its evolution, the high linear polarization degree, an invariant polarization angle across each burst and other properties previously known) can be interpreted within the cosmic comb model invoking a regular neutron star whose magnetosphere is sporadically and marginally combed by a variable outflow from a nearby low-luminosity accreting super-massive black hole in the host galaxy. We propose three falsifiable predictions of the model and discuss other FRBs within the context of the cosmic comb model as well as the challenges encountered by other repeating FRB models in light of the new observations.

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arXiv2018

The allowed parameter space of a long-lived neutron star as the merger remnant of GW170817

Limited by the sensitivities of the current gravitational wave (GW) detectors, the central remnant of the binary neutron star (NS) merger associated with GW170817 remains an open question. Considering the relatively large total mass, it is generally proposed that the merger of GW170817 would lead to a shortly lived hypermassive NS or directly produce a black hole (BH). There is no clear evidence to support or rule out a long-lived NS as the merger remnant. Here we utilize the GW and electromagnetic (EM) signals to comprehensively investigate the parameter space that allows a long-lived NS to survive as the merger remnant of GW170817. We find that for some stiff equations of state, the merger of GW170817 could, in principle, lead to a massive NS, which has a millisecond spin period. The post-merger GW signal could hardly constrain the ellipticity of the NS. If the ellipticity reaches $10^{-3}$, in order to be compatible with the multi-band EM observations, the dipole magnetic field of the NS ($B_p$) is constrained to the magnetar level of $\sim10^{14}$ G. If the ellipticity is smaller than $10^{-4}$, $B_p$ is constrained to the level of $\sim10^{10}-10^{12}\,$G. These conclusions weakly depend on the adoption of equations of state.

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arXiv2018

A more stringent constraint on the mass ratio of binary neutron star merger GW170817

Recently, the LIGO-Virgo collaboration reported their first detection of gravitational wave (GW) signals from a low mass compact binary merger GW170817, which is most likely due to a double neutron star (NS) merger. With the GW signals only, the chirp mass of the binary is precisely constrained to $1.188^{+0.004}_{-0.002}~\rm{M_{\odot}}$, but the mass ratio is loosely constrained in the range $0.4-1$, so that a very rough estimation of the individual NS masses ($0.86~{\rm M_{\odot}}<M_1<1.36~\rm{M_{\odot}}$ and $1.36~{\rm M_{\odot}}<M_2<2.26~\rm{M_{\odot}}$) was obtained. Here we propose that if one can constrain the dynamical ejecta mass through performing kilonova modeling of the optical/IR data, by utilizing an empirical relation between the dynamical ejecta mass and the mass ratio of NS binaries, one may place a more stringent constraint on the mass ratio of the system. For instance, considering that the red "kilonova" component is powered by the dynamical ejecta, we reach a tight constraint on the mass ratio in the range of $0.46-0.59$. Alternatively, if the blue "kilonova" component is powered by the dynamical ejecta, the mass ratio would be constrained in the range of $0.53-0.67$. Overall, such a multi-messenger approach could narrow down the mass ratio of GW170817 system to the range of $0.46-0.67$, which gives a more precise estimation of the individual NS mass than pure GW signal analysis, i.e. $0.90~{\rm M_{\odot}}<M_1<1.16~{\rm M_{\odot}}$ and $1.61~{\rm M_{\odot}}<M_2<2.11~{\rm M_{\odot}}$.

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arXiv2018

Fermi Large Area Telescope detection of gamma-ray emission from the direction of supernova iPTF14hls

The remnant of supernova explosion is widely believed to be the acceleration site of high-energy cosmic ray particles. The acceleration timescale is, however, typically very long. Here we report the detection of a variable $γ$-ray source with the Fermi Large Area Telescope, which is positionally and temporally consistent with a peculiar supernova, iPTF14hls. A quasi-stellar object SDSS J092054.04+504251.5, which is probably a blazar according to the infrared data, is found in the error circle of the $γ$-ray source. More data about the $γ$-ray source and SDSS J092054.04+504251.5 are needed to confirm their association. On the other hand, if the association between the $γ$-ray source and the supernova is confirmed, this would be the first time to detect high-energy $γ$-ray emission from a supernova, suggesting very fast particle acceleration by supernova explosions.

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arXiv2018

Relativistic Astronomy

The "Breakthrough Starshot" aims at sending near-speed-of-light cameras to nearby stellar systems in the future. Due to the relativistic effects, a trans-relativistic camera naturally serves as a spectrograph, a lens, and a wide-field camera. We demonstrate this through a simulation of the optical-band image of the nearby galaxy M51 in the rest frame of the trans-relativistic camera. We suggest that observing celestial objects using a trans-relativistic camera may allow one to study the astronomical objects in a special way, and to perform unique tests on the principles of special relativity. We outline several examples that trans-relativistic cameras may make important contributions to astrophysics and suggest that the Breakthrough Starshot cameras may be launched in any direction to serve as a unique astronomical observatory.

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arXiv2018

Gamma-ray Bursts Induced by Turbulent Reconnection

We consider a simple model for gamma-ray bursts induced by magnetic reconnection in turbulent media. The magnetic field in a jet is subject to kink instabilities, which distort the regular structure of the spiral magnetic field, drive turbulence, and trigger reconnection. The resulting reconnection takes place in a high Reynolds number medium, where turbulence is further enhanced and in turn accelerates the reconnection process. This boot-strap reconnection gives rise to bursts of reconnection events, through which the free energy of magnetic field is transformed into a gamma-ray burst. The efficiency of magnetic reconnection and magnetic energy dissipation is not constrained by microphysical properties of plasmas.

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arXiv2017

Bright "Merger-nova" Emission Powered by Magnetic wind from a New-Born Black Hole

Mergers of neutron star-neutron star (NS-NS) or neutron star-black hole (NS-BH) binaries are candidate sources of gravitational waves (GWs). At least a fraction of the merger remnant should be a stellar mass BH with a sub-relativistic ejecta. A collimated jet is launched via Blandford-Znajek mechanism from the central BH to trigger a short gamma-ray burst (SGRB). At the same time, a near-isotropic wind can be driven by the Blandford-Payne mechanism (BP). In previous work, additional energy injection to the ejecta from the BP mechanism was ignored, and radioactive decay has long been thought as the main source of energy for "kilonova" in the BH scenario. In this letter, we propose that the wind driven by BP mechanism from the new-born BH-disk can heat up and push the ejecta during the prompt emission phase or even at late time when there is mass fall-back. Such a BP-powered merger-nova could be bright at optical band even for a low-luminosity SGRB. The detection of a GW event with a merger product of BH, and accompanied by a bright merger-nova, would be a robust test of our model.

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arXiv2017

On the synchrotron spectrum of GRB prompt emission

The prompt emission spectrum of Gamma-ray bursts (GRB) is characterized by a smoothly joint broken power-law spectrum known as the Band function. The typical low-energy photon index is $\sim -1$, which poses challenge to the standard synchrotron radiation models. We investigate the electron energy spectrum as a result of the interplay among adiabatic stochastic acceleration (ASA), particle injection, and synchrotron cooling. In the ASA-dominated low-energy range, ASA enables an efficient hardening of the injected energy spectrum to approach a spectral index $-1$. In the synchrotron cooling-dominated high-energy range, the injected high-energy electrons undergo fast synchrotron cooling and have a softer photon spectrum. With the energy range of the injected electrons broadly covering both the ASA- and synchrotron cooling-dominated ranges, the resulting photon number spectrum has the low- and high-energy indices as $α_s \sim -1$ and $β_s \sim -p/2-1$, respectively. The break energy is of the order of $\sim 100 $ keV, depending on the turbulence properties.

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arXiv2017

Low-energy Spectra of Gamma-ray Bursts from Cooling Electrons

The low-energy spectra of gamma-ray bursts' (GRBs) prompt emission are closely related to the energy distribution of electrons, which is further regulated by their cooling processes. We develop a numerical code to calculate the evolution of the electron distribution with given initial parameters, in which three cooling processes (i.e., adiabatic, synchrotron and inverse Compton cooling) and the effect of decaying magnetic field are coherently considered. A sequence of results are presented by exploring the plausible parameter space for both the fireball and the Poynting-flux-dominated regime. Different cooling patterns for the electrons can be identified and they are featured by a specific dominant cooling mechanism. Our results show that the hardening of the low-energy spectra can be attributed to the dominance of synchrotron self-Compton cooling within the internal shock model, or to decaying synchrotron cooling within the Poynting-flux-dominated jet scenario. These two mechanisms can be distinguished by observing the hard low-energy spectra of isolated short pulses in some GRBs. The dominance of adiabatic cooling can also lead to hard low-energy spectra when the ejecta is moving at an extreme relativistic speed. The information from the time-resolved low-energy spectra can help to probe the physical characteristics of the GRB ejecta via our numerical results.

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arXiv2017

Black hole hyperaccretion inflow-outflow model. I. long and ultra-long gamma-ray bursts

Long-duration gamma-ray bursts (LGRBs) and ultra-LGRBs (ULGRBs) originate from collapsars, in the center of which a newborn rotating stellar-mass black hole (BH) surrounded by a massive accretion disk may form. In the scenario of BH hyperaccretion inflow-outflow model and Blandford-Znajek (BZ) mechanism to trigger gamma-ray bursts (GRBs), the real accretion rate to power a BZ jet is far lower than the mass supply rate from the progenitor star. The characteristics of the progenitor stars can be constrained by GRB luminosity observations, and the results exceed usual expectations. LGRBs lasting from several seconds to tens of seconds in the rest frame may originate from solar-metallicity ($Z \sim 1~ Z_\odot$, where $Z$ and $Z_\odot$ are the metallicities of progenitor stars and the Sun), massive ($M \gtrsim 34 ~M_\odot$, where $M$ and $M_\odot$ are the masses of progenitor stars and the Sun) stars or some zero-metallicity ($Z \sim 0$) stars. A fraction of low-metallicity ($Z \lesssim 10^{-2}~Z_\odot$) stars, including Population III stars, can produce ULGRBs such as GRB 111209A. The fraction of LGRBs lasting less than tens of seconds in the rest frame is more than 40$\%$, which cannot conform to the fraction of the demanded type of progenitor star. It possibly implies that the activity timescale of central engine may be much longer than the observed timescale of prompt emission phase, as indicated by X-ray late-time activities. Alternatively, LGRBs and ULGRBs may be powered by a millisecond magnetar central engine.

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arXiv2017

Multimessenger tests of the weak equivalence principle from GW170817 and its electromagnetic counterparts

The coincident detection of a gravitational-wave (GW) event GW170817 with electromagnetic (EM) signals (e.g., a short gamma-ray burst SGRB 170817A or a macronova) from a binary neutron star merger within the nearby galaxy NGC 4933 provides a new, multimessenger test of the weak equivalence principle (WEP), extending the WEP test with GWs and photons. Assuming that the arrival time delay between the GW signals from GW170817 and the photons from SGRB 170817A or the macronova is mainly attributed to the gravitational potential of the Milky Way, we demonstrate that the strict upper limits on the deviation from the WEP are $Δγ<1.4\times10^{-3}$ for GW170817/macronova and $Δγ<5.9\times10^{-8}$ for GW170817/SGRB 170817A. A much more severe constraint on the WEP accuracy can be achieved ($\sim0.9\times10^{-10}$) for GW170817/SGRB 170817A when we consider the gravitational potential of the Virgo Cluster, rather than the Milky Way's gravity. This provides the tightest limit to date on the WEP through the relative differential variations of the $γ$ parameter for two different species of particles. Compared with other multimessenger (photons and neutrinos) results, our limit is 7 orders of magnitude tighter than that placed by the neutrinos and photons from supernova 1987A, and is almost as good as or is an improvement of 6 orders of magnitude over the limits obtained by the low-significance neutrinos correlated with GRBs and a blazar flare.

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arXiv2017

Hyperaccreting Black Hole as Gamma-Ray Burst Central Engine. II. Temporal evolution of central engine parameters during Prompt and Afterglow Phases

A hyperaccreting stellar-mass black hole has been proposed as the candidate central engine of gamma-ray bursts (GRBs). The rich observations of GRBs by \textit{Fermi} and \textit{Swift} make it possible to constrain the central engine model by comparing the model predications against data. This paper is dedicated to studying the temporal evolution of central engine parameters for both prompt emission and afterglow phases. We consider two jet launching mechanisms, i.e., $ν\barν$ annihilations and the Blandford-Znajek (BZ) processe, and obtain analytical solutions to these two models. We then investigate the black hole central engine parameters, such as the jet power, the dimensionless entropy $η$, and the central engine parameter $μ_0=η(1+σ_0)$ (where $σ_0$ is the initial magnetization of the engine) at the base of the jet. The black hole may be spun up by accretion, or spun down by the BZ process, leaving imprints in GRB lightcurves. Usually, a BZ jet is more powerful and is likely responsible for the late time central engine activities. However, an initially non-spinning black hole central engine may first launch a thermal "fireball" via neutrino annihilations, and then launch a Poynting-flux-dominated jet via the BZ process. Multiple flares, giant bumps, and plateaus in GRB afterglows can be well produced as the result of late time accretion onto the black hole.

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arXiv2017

Adiabatic non-resonant acceleration in magnetic turbulence and hard spectra of gamma-ray bursts

We introduce a non-resonant acceleration mechanism arising from the second adiabatic invariant in magnetic turbulence and apply it to study the prompt emission spectra of gamma-ray bursts (GRBs). The mechanism contains both the first- and second-order Fermi acceleration, originating from the interacting turbulent reconnection and dynamo processes. It leads to a hard electron energy distribution up to a cutoff energy at the balance between the acceleration and synchrotron cooling. The sufficient acceleration rate ensures a rapid hardening of any initial energy distribution to a power-law distribution with the index $p \sim 1$, which naturally produces a low-energy photon index $α\sim -1$ via the synchrotron radiation. For typical GRB parameters, the synchrotron emission can extend to a characteristic photon energy on the order of $\sim 100$ keV.

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arXiv2017

Dispersion Measure Variation of Repeating Fast Radio Burst Sources

The repeating fast radio burst (FRB) 121102 was recently localized in a dwarf galaxy at a cosmological distance. The dispersion measure (DM) derived for each burst from FRB 121102 so far has not shown significant evolution, even though an apparent increase was recently seen with newly detected VLA bursts. It is expected that more repeating FRB sources may be detected in the future. In this work, we investigate a list of possible astrophysical processes that might cause DM variation of a particular FRB source. The processes include (1) the cosmological scale effects such as Hubble expansion and large-scale structure fluctuations; (2) the FRB local effects such as gas density fluctuation, expansion of a supernova remnant, a pulsar wind nebula, and an HII region; and (3) the propagation effect due to plasma lensing. We find that the DM variations contributed by the large-scale structure are extremely small, and any observable DM variation is likely caused by the plasma local to the FRB source. Besides mechanisms that produce decreasing DM with time, we suggest that an FRB source in an expanding supernova remnant around a nearly neutral ambient medium during the deceleration (Sedov-Taylor and snowplow) phases or in a growing HII region can introduce DM increasing. Some effects (e.g. an FRB source moving in an HII region or plasma lensing) can give either positive or negative DM variations. Future observations of DM variations of FRB 121102 and other repeating FRB sources can bring important clues for the physical origin of these sources.

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arXiv2017

The peculiar GRB 110731A: Lorentz factor, jet composition, central engine, and progenitor

The jet compositions, central engines, and progenitors of gamma-ray bursts (GRBs) remain open questions in GRB physics. Applying broadband observations, including GRB prompt emission and afterglow properties derived from {\em Fermi} and {\em Swift} data, as well as from Keck host-galaxy observations, we address these questions for the peculiar, bright GRB 110731A. By using the pair-opacity method, we derive $Γ_{0}>190$ during the prompt emission phase. Alternatively, we derive $Γ_{0} \approx 580$ and $Γ_{0} \approx 154$ by invoking the early-afterglow phase within the homogeneous density and wind cases, respectively. On the other hand, nondetection of a thermal component in the spectra suggests that the prompt emission is likely powered by dissipation of a Poynting-flux-dominated jet leading to synchrotron radiation in an optically thin region. The nondetection of a jet break in the X-ray and optical bands allows us to place a lower limit on the jet opening angle $θ_{\rm j}>5.5^{\circ}$. Within a millisecond magnetar central engine scenario, we derive the period $P_{0}$ and polar magnetic field strength $B_{\rm p}$, which have extreme (but still allowed) values. The moderately short observed duration (7.3\,s) and relatively large redshift ($z=2.83$) places the burst as a "rest-frame short" GRB, so the progenitor of the burst is subject to debate. Its relatively large $f_{{\rm eff}, z}$ parameter (ratio of the 1\,s peak flux of a pseudo-GRB and the background flux) and a large physical offset from a potential host galaxy suggest that the progenitor of GRB 110731A may be a compact-star merger.

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arXiv2017

Magnetized Reverse Shock: Density-fluctuation-induced Field Distortion, Polarization Degree Reduction, and Application to GRBs

The early optical afterglow emission of several gamma-ray bursts (GRBs) shows a high linear polarization degree (PD) of tens of percent, suggesting an ordered magnetic field in the emission region. The light curves are consistent with being of a reverse shock (RS) origin. However, the magnetization parameter, $σ$, of the outflow is unknown. If $σ$ is too small, an ordered field in the RS may be quickly randomized due to turbulence driven by various perturbations so that the PD may not be as high as observed. Here we use the "Athena++" relativistic MHD code to simulate a relativistic jet with an ordered magnetic field propagating into a clumpy ambient medium, with a focus on how density fluctuations may distort the ordered magnetic field and reduce PD in the RS emission for different $σ$ values. For a given density fluctuation, we discover a clear power-law relationship between the relative PD reduction and the $σ$ value of the outflow. Such a relation may be applied to estimate $σ$ of the GRB outflows using the polarization data of early afterglows.

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arXiv2017

Neutrino-dominated accretion flows as the central engine of gamma-ray bursts

Neutrino-dominated accretion flows (NDAFs) around rotating stellar-mass black holes (BHs) are plausible candidates for the central engines of gamma-ray bursts (GRBs). NDAFs are hyperaccretion disks with accretion rates in the range of around 0.001-10 $M_\odot ~\rm s^{-1}$, which have high density and temperature and therefore are extremely optically thick and geometrically slim or even thick. We review the theoretical progresses in studying the properties of NDAFs as well as their applications to the GRB phenomenology. The topics include: the steady radial and vertical structure of NDAFs and the implications for calculating neutrino luminosity and annihilation luminosity, jet power due to neutrino-antineutrino annihilation and Blandford-Znajek mechanism and their dependences on parameters such as BH mass, spin, and accretion rate, time evolution of NDAFs, effect of magnetic fields, applications of NDAF theories to the GRB phenomenology such as lightcurve variability, extended emission, X-ray flares, kilonovae, etc., as well as probing NDAFs using multi-messenger signals such as MeV neutrinos and gravitational waves.

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arXiv2017

Searching for magnetar powered merger-novae from short GRBs

The merger of a double neutron star (NS-NS) binary may result in a rapidly rotating massive NS with an extremely strong magnetic field (i.e., a millisecond magnetar). In this case, the magnetic spin-down of the NS remnant provides an additional source of sustained energy injection, which would continuously power the merger ejecta. The thermal emission from the merger ejecta would give rise to a bright optical "magnetar-powered merger-nova". In this work, we carry out a complete search for magnetar-powered merger-nova from Swift short gamma-ray burst (SGRB) sample. We focus on short GRBs with extended emission or internal plateau, which may signify the presence of magnetars as the central engine. We eventually find three candidates of magnetar-powered merger-nova from the late observations of GRB 050724, GRB 070714B and GRB 061006. With standard parameter values, the magnetar remnant scenario could well interpret the multi-band data of all three bursts, including the extended emission and their late chromatic features in the optical and X-ray data. The peak luminosities of these merger-novae reach several times $10^{42}~{\rm erg~s^{-1}}$, more than one order of magnitude brighter than the traditional "kilo-novae" with peak luminosity of $\sim 10^{41}~{\rm erg~s^{-1}}$. Intense, multi-color late time observations of short GRBs are encouraged to identify more merger-novae in the future.

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