arXiv subjects

Yun-Wei Yu

Publications and source records attributed to Yun-Wei Yu.

At least 73 records · Page 4Linked to original sources

arXiv2020

The jet structure and the intrinsic luminosity function of short gamma-ray bursts

The joint observation of GW170817 and GRB 170817A indicated that short gamma-ray bursts (SGRBs) can originate from binary neutron star mergers. Moreover, some SGRBs could be detected off-axis, while the SGRB jets are highly structured. Then, by assuming an universal angular distribution of the jet emission for all SGRBs, we re-produce the flux and redshift distributions of the cosmological SGRBs detected by {\it Swift} and {\it Fermi}. For self-consistency, this angular distribution is simultaneously constrained by the luminosity and event rate of GRB 170817A. As a result, it is found that the universal jet structure of SGRBs could approximately have a two-Gaussian profile. Meanwhile, the intrinsic luminosity function (LF) of the on-axis emission of the jets can be simply described by a single power law with a low-luminosity exponential cutoff. The usually discovered broken-power-law apparent LF for relatively high luminosities can naturally result from the coupling of the intrinsic LF with the angular distribution of the jet emission, as the viewing angles to the SGRBs are arbitrarily distributed.

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arXiv2020

Kilonova Emission From Black Hole-Neutron Star Mergers. I. Viewing-Angle-Dependent Lightcurves

In this paper, we present a numerical method to study the predicted lightcurves as a function of viewing angle. We extrapolate the fitting formulae for the mass and velocity of tidal dynamical ejecta across a wide range of mass ratio validated with 66 simulations and use them in the calculations of the kilonova lightcurves. The calculated peak luminosity of a BH-NS merger kilonova is typically about a few times $10^{41}\ {\rm erg\ s^{-1}}$, which is always $\lesssim4.5\times10^{41}\ {\rm erg\ s^{-1}}$. This corresponds to the AB absolute magnitudes fainter than $\sim -15\ {\rm mag}$ in optical and $\sim -16\ {\rm mag}$ in infrared. Since the projected photosphere area of the dynamical ejecta is much larger than that of the disk wind outflows, the dynamical ejecta usually contribute to the majority of the kilonova emission from BH-NS mergers. The fitted blackbody temperature and the shape of the observed multi-band lightcurves are insensitive to the line of sight. The peak time of the observed multi-band lightcurves, affected by the light propagation effect, is related to the relative motion direction between the dynamical ejecta and the observer. The observed luminosity varies with the projected photosphere area determined by the viewing angles. However, the predicted peak luminosity only varies by a factor of $\sim (2 - 3)$ (or by $\sim1\ {\rm mag}$) for different viewing angles. When the short-duration gamma-ray burst afterglow is taken into account, for an on-axis geometry, the kilonova emission is usually outshone by the afterglow emission and can be only observed in the redder bands, especially in the $K$-band at late times. Compared with GW170817/AT2017gfo, the BH-NS merger kilonovae are optically dim but possibly infrared bright. At the same epoch after the merger, the blackbody fitting temperature of the BH-NS merger kilonovae is lower than that of GW170817/AT2017gfo.

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arXiv2019

Optical and radio transients after the collapse of super-Chandrasekhar white dwarf merger remnants

Super-Chandrasekhar remnants of double white dwarf mergers could sometimes collapse into a rapidly rotating neutron star (NS), accompanying with a mass ejection of a few times $0.01M_{\odot}$. Bright optical transient emission can be produced by the ejecta due to heating by radioactivities and particularly by energy injection from the NS. Since the merger remnants before collapse resemble a star evolving from the asymptotic giant branch phase to the planetary nebula phase, an intense dusty wind is considered to be driven about several thousand years ago before the collapse and surround the remnant at large radii. Therefore, the optical transient emission can be somewhat absorbed and scattered by the dusty wind, which can suppress the peak emission and cause a scattering plateau in optical light curves. Several years later, as the ejecta finally catches up with the wind material, the shock interaction between them can further give rise to a detectable radio transient emission on a timescale of tens of days. Discovery of and observations to such dust-affected optical transients and shock-driven radio transients can help to explore the nature of super-Chandrasekhar merger remnants and as well as the density and type ratios of double white dwarf systems, which is beneficial in assessing their gravitational wave contributions.

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arXiv2019

X-ray transients from the accretion-induced collapse of white dwarfs

The accretion-induced collapse (AIC) of a white dwarf in a binary with a nondegenerate companion can sometimes lead to the formation of a rapidly rotating and highly magnetized neutron star (NS). The spin-down of this NS can drive a powerful pulsar wind (PW) and bring out some detectable multi-wavelength emissions. On the one hand, the PW can evaporate the companion in a few days to form a torus surrounding the NS. Then, due to the blockage of the PW by the torus, a reverse shock can be formed in the wind to generate intense hard X-rays. This emission component disappears in a few weeks' time, after the torus is broken down at its inner boundary and scoured into a very thin disk. On the other hand, the interaction between the PW with an AIC ejecta can lead to a termination shock of the wind, which can produce a long-lasting soft X-ray emission component. In any case, the high-energy emissions from deep inside the system can be detected only after the AIC ejecta becomes transparent for X-rays. Meanwhile, by absorbing the X-rays, the AIC ejecta can be heated effectively and generate a fast-evolving and luminous ultraviolet (UV)/optical transient. Therefore, the predicted hard and soft X-ray emissions, associated by an UV/optical transient, provide a clear observational signature for identifying AIC events in current and future observations (e.g., AT 2018cow).

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arXiv2019

Observational signature of a wind bubble environment for double neutron star mergers

During the in-spiral stage of a compact binary, a wind bubble could be blown into interstellar medium, if the electromagnetic radiation due to the binary orbital motion is strong enough. Therefore, short gamma-ray bursts (SGRBs) due to double neutron star mergers would in principle happen in a wind bubble environment, which can influence the propagation of the SGRB jet and the consequent afterglow emission. By calculating the dynamics and synchrotron radiation of the jet-driven external shock, we reveal that an abrupt jump could appear in the afterglow light curves of SGRBs and the observational time of the jump is dependent on the viewing angle. This light curve jump provides an observational signature to constrain the radius of the wind bubble and then the power of the binary electromagnetic radiation, by combining with gravitational wave detection.

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arXiv2019

Studying newborn neutron stars by the transient emission after stellar collapses and compact binary mergers

The formation of neutron stars (NSs), both from collapses of massive stars and mergers of compact objects, can be usually indicated by bright transients emitted from explosively-ejected material. In particular, if the newborn NSs can rotate at a millisecond period and have a sufficiently high magnetic field, then the spin-down of the NSs would provide a remarkable amount of energy to the emitting material. As a result, super-luminous supernovae could be produced in the massive stellar collapse cases, while some unusual fast evolving and luminous optical transients could arise from the cases of NS mergers and accretion-induced collapses of white dwarfs. In all cases, if the dipolar magnetic fields of the newborn NSs can be amplified to be as high as $10^{15}$ G, a relativistic jet could be launched and then a gamma-ray burst can be produced as the jet successfully breaks out from the surrounding nearly-isotropic ejected material.

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arXiv2018

What Powered the Optical Transient AT2017gfo Associated with GW170817?

The groundbreaking discovery of the optical transient AT2017gfo associated with GW170817 opens a unique opportunity to study the physics of double neutron star (NS) mergers. We argue that the standard interpretation of AT2017gfo as being powered by radioactive decays of r-process elements faces the challenge of simultaneously accounting for the peak luminosity and peak time of the event, as it is not easy to achieve the required high mass, and especially the low opacity of the ejecta required to fit the data. A plausible solution would be to invoke an additional energy source, which is probably provided by the merger product. We consider energy injection from two types of the merger products: (1) a post-merger black hole powered by fallback accretion; and (2) a long-lived NS remnant. The former case can only account for the early emission of AT2017gfo, with the late emission still powered by radioactive decay. In the latter case, both early- and late-emission components can be well interpreted as due to energy injection from a spinning-down NS, with the required mass and opacity of the ejecta components well consistent with known numerical simulation results. We suggest that there is a strong indication that the merger product of GW170817 is a long-lived (supramassive or even permanently stable), low magnetic field NS. The result provides a stringent constraint on the equations of state of NSs.

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arXiv2018

A long-lived remnant neutron star after GW170817 inferred from its associated kilonova

The successful joint observation of the gravitational wave event GW170817 and its multi-wavelength electromagnetic counterparts first enables human to witness a definite merger event of two neutron stars (NSs). This historical event confirms the origin of short-duration gamma-ray bursts (GRBs), and in particular, identifies the theoretically-predicted kilonova phenomenon that is powered by radioactive decays of $r$-process heavy elements. However, whether a long-lived remnant NS could be formed during this merger event remains unknown, although such a central engine has been suggested by afterglow observations of some short-duration GRBs. By invoking this long-lived remnant NS, we here propose a model of hybrid energy sources for the kilonova AT2017gfo associated with GW 170817. While the early emission of AT2017gfo is still powered radioactively as usually suggested, its late emission is primarily caused by delayed energy injection from the remnant NS. In our model, only one single opacity is required and an intermediate value of $κ\simeq0.97\,\rm cm^2g^{-1}$ is revealed, which could be naturally provided by lanthanide-rich ejecta that is deeply ionized by the emission from a wind of the NS. These self-consistent results indicate that a long-lived remnant NS, which must own a very stiff equation of state, had been formed during the merger event of GW170817. This provides a very stringent constraint on the strong interaction in nuclear-quark matter. It is further implied that such GW events could provide a probe of the early spin and magnetic evolutions of NSs, e.g., the burying of surface magnetic fields.

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arXiv2018

Probing the physics of newly born magnetars through observation of superluminous supernovae

The central engines of some superluminous supernovae (SLSNe) are generally suggested to be newly born fast rotating magnetars, which spin down mainly through magnetic dipole radiation and gravitational wave emission. We calculate the magnetar-powered SLSNe light curves (LCs) with the tilt angle evolution of newly born magnetars involved. We show that, depending on the internal toroidal magnetic fields ${\bar B}_{\rm t}$, the initial spin periods $P_{\rm i}$, and the radii $R_{\rm DU}$ of direct Urca (DU) cores of newly born magnetars, as well as the critical temperature $T_{\rm c}$ for $^3P_2$ neutron superfluidity, bumps could appear in the SLSNe LCs after the maximum lights when the tilt angles grow to $π/2$. The value of $T_{\rm c}$ determines the arising time and the relative amplitude of a bump. The quantity $R_{\rm DU}$ can affect the arising time and the luminosity of a bump, as well as the peak luminosity of a LC. Moreover, it is interesting that a stronger ${\bar B}_{\rm t}$ will lead to both a brighter peak and a brighter bump in a LC. While keeping other quantities unchanged, the bump in the LC disappears for the magnetar with smaller $P_{\rm i}$. We suggest that, once the SLSNe LCs with such kinds of bumps are observed, by fitting these LCs with our model, not only $B_{\rm d}$ and $P_{\rm i}$ of newly born magnetars but also the crucial physical quantities ${\bar B}_{\rm t}$, $R_{\rm DU}$, and $T_{\rm c}$ could be determined. Nonobservation of SLSNe LCs with such kinds of bumps hitherto may already put some (\textit{though very rough}) constraints on ${\bar B}_{\rm t}$, $P_{\rm i}$, $R_{\rm DU}$, and $T_{\rm c}$. Therefore, observation of SLSNe LCs may provide a new approach to probe the physics of newly born magnetars.

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arXiv2018

Compact Binary Mergers and The Event Rate of Fast Radio Bursts

Fast radio bursts (FRBs) are usually suggested to be associated with mergers of compact binaries consisting of white dwarfs (WDs), neutron stars (NSs), or black holes (BHs). We test these models by fitting the observational distributions in both redshift and isotropic energy of 22 Parkes FRBs, where, as usual, the rates of compact binary mergers (CBMs) are connected with cosmic star formation rates by a power-law distributed time delay. It is found that the observational distributions can well be produced by the CBM model with a characteristic delay time from several ten to several hundred Myr and an energy function index $1.2\lesssimγ\lesssim1.7$, where a tentative fixed spectral index $β=0.8$ is adopted for all FRBs. Correspondingly, the local event rate of FRBs is constrained to $(3-6)\times10^4f_{\rm b}^{-1}(\mathcal T/270\rm s)^{-1}(\mathcal A/2π)^{-1}\rm ~Gpc^{-3}yr^{-1}$ for an adopted minimum FRB energy of $E_{\min}=3\times10^{39}$ erg, where $f_{\rm b}$ is the beaming factor of the radiation, $\mathcal T$ is the duration of each pointing observation, and $\mathcal A $ is the sky area of the survey. This event rate, about an order of magnitude higher than the rates of NS-NS/NS-BH mergers, indicates that the most promising origin of FRBs in the CBM scenario could be mergers of WD-WD binaries. Here a massive WD could be produced since no FRB was found to be associated with a type Ia supernova. Alternatively, if actually all FRBs can repeat on a timescale much longer than the period of current observations, then they could also originate from a young active NS that forms from relatively rare NS-NS mergers and accretion-induced collapses of WD-WD binaries.

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arXiv2017

A possible relation between flare activity in superluminous supernovae and gamma-ray bursts

Significant undulations appear in the light curve of a recently discovered super-luminous supernova (SLSN) SN 2015bn after the first peak, while the underlying profile of the light curve can be well explained by a continuous energy supply from a central engine, possibly the spin-down of a millisecond magnetar. We propose that these undulations are caused by an intermittent pulsed energy supply, indicating an energetic flare activity of the central engine of the SLSN. Many post-burst flares were discovered during X-ray afterglow observations of Gamma-Ray Bursts (GRBs). We find that the SLSN flares described here approximately obey the empirical correlation between the luminosity and time scale of GRB flares, extrapolated to the relevant longer time scales of SLSN flares. This confirms the possible connection between these two different phenomena as previously suggested.

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arXiv2017

A statistical study of super-luminous supernovae in the magnetar engine model and implications for their connection with gamma-ray bursts and hypernovae

By fitting the bolometric light curves of 31 super-luminous supernovae (SLSNe) with the magnetar engine model, we derive the ejecta masses and magnetar parameters for these SLSNe. The lower boundary of magnetic field strengths of SLSN magnetars can be set just around the critical field strength $B_{\rm c}$ of electron Landau quantization. In more details, SLSN magnetars can further be divided into two subclasses of magnetic fields of $\sim(1-5)B_{\rm c}$ and $\sim(5-10)B_{\rm c}$, respectively. It is revealed that these two subclasses of magnetars are just associated with the slow-evolving and fast-evolving bolometric light curves of SLSNe. In comparison, the magnetars harbored in gamma-ray bursts (GRBs) and associated hypernovae are usually inferred to have much higher magnetic fields with a lower boundary about $\sim10B_{\rm c}$. This robustly suggests that it is the magnetic fields that play the crucial role in distinguishing SLSNe from GRBs/hypernovae. The rotational energy of SLSN magnetars are found to be correlated with the masses of supernova ejecta, which provides a clue to explore the nature of their progenitors. Moreover, the distribution of ejecta masses of SLSNe is basically intermediate between those of normal core-collapse supernovae and hypernovae. This could indicate an intrinsic connection among these different stellar explosions.

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arXiv2017

Constraining the age of a magnetar possibly associated with FRB 121102

The similarity of the host galaxy of FRB 121102 with those of long gamma-ray bursts and Type I super-luminous supernovae suggests that this FRB could be associated with a young magnetar. By assuming the FRB emission to be produced within the magnetosphere, we derive a lower limit on the age of the magnetar, after which GHz emission is able to escape freely from the dense relativistic wind of the magnetar. Another lower limit is obtained by requiring the dispersion measure contributed by the electron/positron pair wind to be consistent with the observations of the host galaxy. Furthermore, we also derive some upper limits on the magnetar age with discussions on possible energy sources of the FRB emission and the recently-discovered persistent radio counterpart. As a result, some constraints on model parameters are addressed by reconciling the lower limits with the possible upper limits that are derived with an assumption of rotational energy source.

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arXiv2016

Shock breakout driven by the remnant of a neutron star binary merger: An X-ray precursor of mergernova emission

A supra-massive neutron star (NS) spinning extremely rapidly could survive from a merger of NS-NS binary. The spin-down of this remnant NS that is highly magnetized would power the isotropic merger ejecta to produce a bright mergernova emission in ultraviolet/optical bands. Before the mergernova, the early interaction between the NS wind and the ejecta can drive a forward shock propagating outwards into the ejecta. As a result, a remarkable amount of heat can be accumulated behind the shock front and the final escaping of this heat can produce a shock breakout emission. We describe the dynamics and thermal emission of this shock with a semi-analytical model. It is found that sharp and luminous breakout emission appears mainly in soft X-rays with a luminosity of $\sim10^{45}~\rm erg~s^{-1}$ at a few hours after the merger, by leading the mergernova emission as a precursor. Therefore, detection of such an X-ray precursor would provide a smoking-gun evidence for identifying NS-powered mergernovae and distinguishing them from the radioactive-powered ones (i.e., kilonovae or macronovae). The discovery of NS-powered mergernovae would finally help to confirm the gravitational wave signals due to the mergers and the existence of supra-massive NSs.

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arXiv2015

Stochastic gravitational wave background from magnetic deformation of newly born magnetars

Newly born magnetars are promising sources for gravitational wave (GW) detection due to their ultra-strong magnetic fields and high spin frequencies. Within the scenario of a growing tilt angle between the star's spin and magnetic axis, due to the effect of internal viscosity, we obtain improved estimates of the stochastic gravitational wave backgrounds (SGWBs) from magnetic deformation of newly born magnetars. We find that the GW background spectra contributed by the magnetars with ultra-strong toroidal magnetic fields of 10^{17} G could roughly be divided into four segments. Most notably, in contrast to the background spectra calculated by assuming constant tilt angles χ=π/2, the background radiation above 1000 Hz are seriously suppressed. However, the background radiation at the frequency band \sim100-1000 Hz are moderately enhanced, depending on the strengths of the dipole magnetic fields. We suggest that if all newly born magnetars indeed have toroidal magnetic fields of 10^{17} G, the produced SGWBs should show sharp variations with the observed frequency at several tens to about 100 hertz. If these features could be observed through sophisticated detection of the SGWB using the proposed Einstein Telescope, it will provide us a direct evidence of the tilt angle evolutions and further some deep understandings about the properties of newly born magnetars.

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arXiv2015

Rapidly Evolving and Luminous Transients Driven by Newly Born Neutron Stars

We provide a general analysis on the properties of emitting material of some rapidly evolving and luminous transients discovered recently with the Pan-STARRS1 Medium Deep Survey. It is found that these transients are probably produced by a low-mass non-relativistic outflow that is continuously powered by a newly born, rapidly spinning, and highly magnetized neutron star. Such a system could originate from an accretion-induced collapse of a white dwarf or a merger of a neutron star-neutron star binary. Therefore, observations to these transients would be helpful for constraining white dwarf and neutron star physics and/or for searching and identifying gravitational wave signals from the mergers.

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arXiv2015

The star formation history inferred from long gamma-ray bursts with high pseudo-redshifts

By employing a simple semi-analytical star formation model where the formation rates of Population (Pop) I/II and III stars can be calculated, respectively, we account for the number distribution of gamma-ray bursts (GRBs) with high pseudo-redshifts that was derived from an empirical luminosity-indictor relationship. It is suggested that a considerable number of Pop III GRBs could exist in the present sample of Swift GRBs. By further combining the implication for the star formation history from the optical depth of the CMB photons, it is also suggested that only a very small fraction 0.6% of Pop III GRBs could have triggered the Swift BAT. These results could provide an useful basis for estimating future detectability of Pop III stars and their produced transient phenomena.

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arXiv2015

Reverse Shock Emission and Ionization Break Out Powered by Post-merger Millisecond Magnetars

There is accumulating evidence that at least a fraction of binary neutron star mergers result in rapidly spinning magnetars, with subrelativistic neutron-rich ejecta as massive as a small fraction of solar mass. The ejecta could be heated continuously by the Poynting flux emanated from the central magnetars. Such Poynting flux could become lepton-dominated so that a reverse shock develops. It was demonstrated that such a picture is capable of accounting for the optical transient PTF11agg (Wang & Dai 2013b). In this paper we investigate the X-ray and ultraviolet (UV) radiation as well as the optical and radio radiation studied by Wang & Dai (2013b). UV emission is particularly important because it has the right energy to ionize the hot ejecta at times $t\lesssim 600$ s. It is thought that the ejecta of binary neutron star mergers are a remarkably pure sample of r-process material, about which our understanding is still incomplete. In this paper we evaluate the possibility of observationally determining the bound-bound and bound-free opacities of the r-process material by timing the X-ray, UV, and optical radiation. It is found that these timings depend on the opacities weakly and therefore only loose constraints on the opacities can be obtained.

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