arXiv subjects

Jin-Jun Geng

Publications and source records attributed to Jin-Jun Geng.

At least 55 records · Page 3Linked to original sources

arXiv2023

Core Collapse Supernova Explosions in Active Galactic Nucleus Accretion Disks

Astrophysical events that occur in active galactic nucleus (AGN) disks are believed to differ significantly from the ordinary in the interstellar medium. We show that stars located in the outer region of the AGN disk would explode near the original migration starting points instead of being accreted by the central supermassive black hole due to the effect of viscosity. AGN disks provide a dense environment for supernova (SN) explosions, which inevitably involve ejecta-disk interactions. In this paper, we investigate the light curves (LCs) of core-collapse SN exploded in AGN disks. In addition to the fundamental energy source of $^{56} \mathrm{Ni}$--$^{56} \mathrm{Co}$--$^{56} \mathrm{Fe}$ decay reaction powering the SN LCs, the forward-reverse shock produced during interactions may contribute significantly to the observed flux. If the stellar winds manage to create a cavity surrounded by a shell near the star before the SN explosion, the ejecta-winds-disk configurations are expected. We present various SN LCs from different types of progenitors and find that the SN LCs are dominated by the radiation of ejecta-disk interaction-induced shocks. The resulting SNe in the AGN disk is a promising transient source for UV and optical band detection by the Neil Gehrels Swift Observatory (Swift), the Ultraviolet Explorer (UVEX) and wide field survey telescopes such as Ultraviolet Transient Astronomy Satellite (ULTRASAT), Wide Field Survey Telescope (WFST) and Legacy Survey of Space and Time (LSST) at the Vera C. Rubin Observatory. These detections could aid in the investigation of AGN discs and the associated high-energy transient occurrences.

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arXiv2023

An analytic derivation of the empirical correlations of gamma-ray bursts

Empirical correlations between various key parameters have been extensively explored ever since the discovery of gamma-ray bursts (GRBs) and have been widely used as standard candles to probe the Universe. The Amati relation and the Yonetoku relation are two good examples that enjoyed special attention. The former reflects the connection between the peak photon energy (Ep) and the isotropic $γ$-ray energy release (Eiso), while the latter links Ep with the isotropic peak luminosity (Lp), both in the form of a power-law function. Most GRBs are found to follow these correlations well, but a theoretical interpretation is still lacking. Some obvious outliers may be off-axis GRBs and may follow correlations that are different from those at the on-axis. Here we present a simple analytical derivation for the Amati relation and the Yonetoku relation in the framework of the standard fireball model, the correctness of which is then confirmed by numerical simulations. The off-axis Amati relation and Yonetoku relation are also derived. They differ markedly from the corresponding on-axis relation. Our results reveal the intrinsic physics behind the radiation processes of GRBs, and they highlight the importance of the viewing angle in the empirical correlations of GRBs.

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arXiv2022

Gravitationally Lensed Orphan Afterglows of Gamma-Ray Bursts

The cosmological nature of gamma-ray bursts (GRBs) implies that a small portion of them could be gravitationally lensed by foreground objects during their propagation. The gravitational lensing effect on the GRB prompt emission and on-axis afterglows has been discussed, and some candidates have been found in the literature. In this work, considering the high detection rate of GRB orphan afterglows in future wide-field survey era, we investigate the gravitationally lensed orphan afterglows in view of three lens models, i.e., the point-mass model, the singular isothermal sphere model, and the Chang-Refsdal model. The structure of the GRB jet itself is also incorporated in calculating the lensed afterglow light curves. It is found that lensed optical/X-ray orphan afterglows in principle could be diagnosed through their temporal characteristics, and the optical band is the best band to observe the galaxy-lensed orphan afterglows. Moreover, the event rate for galaxy-lensed orphan afterglows is estimated to be $\lesssim$ 1.8 $\text{yr}^{-1}$ for the whole sky. If most orphan afterglows could be identified (from other transients in the survey data), the optimistic detection rates of the 2.5m Wide Field Survey Telescope of China and 8.4m Vera Rubin Observatory Legacy Survey of Space and Time for galaxy-lensed orphan afterglows in the optical band are $\lesssim$ 0.01$\sim$0.02 $\text{yr}^{-1}$ and $\lesssim$ 0.04$\sim$0.08 $\text{yr}^{-1}$, respectively.

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arXiv2022

Searching for Strange Quark Matter Objects Among White Dwarfs

The ground state of matter may be strange quark matter (SQM), not hadronic matter. A whole sequence of SQM objects, ranging from strange quark stars and strange quark dwarfs to strange quark planets, can stably exist according to this SQM hypothesis. A strange dwarf has a mass similar to that of a normal white dwarf but could harbor an extremely dense SQM core (with a density as large as $\sim \rm 4\times10^{14}\,g\,cm^{-3} $) at the center so that its radius can be correspondingly smaller. In this study, we try to search for strange dwarfs among the observed "white dwarfs" by considering their difference in the mass-radius relation. Seven strange dwarf candidates are identified in this way, whose masses are in the range of $\sim 0.02$ -- $0.12 M_{\odot}$, with the radii narrowly distributed in $\sim$ 9,000 -- 15,000 km. The seven objects are LSPM J0815+1633, LP 240-30, BD+20 5125B, LP 462-12, WD J1257+5428, 2MASS J13453297+4200437, and SDSS J085557.46+053524.5. Compared with white dwarfs of similar mass, these candidates are obviously smaller in radius. Further observations with large radio/infrared/optical telescopes on these interesting candidates are solicited.

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arXiv2022

A semi-analytical solution to the forward-reverse shock hydrodynamics of the gamma-ray burst afterglow

We extend the standard model of forward-reverse shock (FS-RS) for gamma-ray burst (GRB) afterglow to more general cases. On one hand, we derive the analytical solution to the hydrodynamics of the shocks in two limiting cases, i.e., an ultra-relativistic reverse shock case and a Newtonian reverse shock case. Based on the asymptotic solutions in these two limiting cases, we constitute a semi-analytical solution for the hydrodynamics of the shocks in the generic case, covering the mildly-relativistic reverse shock case. On the other hand, we derive the evolution of the system taking into account the condition of energy conservation which is not satisfied in the standard FS-RS model. A generic solution of semi-analytical expressions is also given. In both the extended standard FS-RS model (satisfying pressure balance condition) and the model satisfying energy conservation, we find that the results in the ultra-relativistic reverse shock case and in the early stage of the Newtonian reverse shock case are different from those in the standard FS-RS model by only a factor that close to one while the same initial conditions adopted. However, the asymptotic solutions in the limiting cases are not good approximations to those in the intermediate case. Our semi-analytical results agree well with the numerical results for a large range of model parameters, and hence can be easily employed to diagnose the physical quantities of the GRB shell and circumburst environment.

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arXiv2022

Periodic repeating fast radio bursts: interaction between a magnetized neutron star and its planet in an eccentric orbit

Fast radio bursts (FRBs) are mysterious transient phenomena. The study of repeating FRBs may provide useful information about their nature due to their redetectability. The two most famous repeating sources are FRBs 121102 and 180916, with a period of 157 days and 16.35 days, respectively. Previous studies suggest that the periodicity of FRBs is likely associated with neutron star (NS) binary systems. Here we introduce a new model which proposes that periodic repeating FRBs are due to the interaction of a NS with its planet in a highly elliptical orbit. The periastron of the planet is very close to the NS so that it would be partially disrupted by tidal force every time it passes through the periastron. Fragments generated in the process could interact with the compact star through the Alfvén wing mechanism and produce FRBs. The model can naturally explain the repeatability of FRBs with a period ranging from a few days to several hundred days, but it generally requires that the eccentricity of the planet's orbit should be large enough. Taking FRBs 121102 and 180916 as examples, it is shown that the main features of the observed repeating behaviors can be satisfactorily accounted for.

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arXiv2022

Tidal Deformability of Strange Quark Planets and Strange Dwarfs

Strange quark matter, which is composed of u, d, and s quarks, could be the true ground of matter. According to this hypothesis, compact stars may actually be strange quark stars, and there may even be stable strange quark dwarfs and strange quark planets. The detection of the binary neutron star merger event GW170817 provides us new clues on the equation of state of compact stars. In this study, the tidal deformability of strange quark planets and strange quark dwarfs are calculated. It is found that the tidal deformability of strange quark objects is smaller than that of normal matter counterparts. For a typical 0.6 M$_\odot$ compact star, the tidal deformability of a strange dwarf is about 1.4 times less than that of a normal white dwarf. The difference is even more significant between strange quark planets and normal matter planets. Additionally, if the strange quark planet is a bare one (i.e., not covered by a normal matter curst), the tidal deformability will be extremely small, which means bare strange quark planets will hardly be distorted by tidal forces. Our study clearly proves the effectiveness of identifying strange quark objects via searching for strange quark planets through gravitational-wave observations.

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arXiv2021

Evolution Patterns of the Peak Energy in the GRB Prompt Emission

There are two different evolution patterns of the peak energy ($E_\text{p}$) exhibited during the prompt emission phase of gamma-ray bursts (GRBs), i.e., hard-to-soft and intensity-tracking, of which the physical origin remains unknown. Except for low-energy indices of GRB prompt spectra, the evolution patterns of $E_\text{p}$ may be another crucial indicator to discriminate radiation mechanisms (e.g., synchrotron or photosphere) for GRBs. We explore the parameter space to find conditions that could generate different evolution patterns of the peak energy in the framework of synchrotron radiation. We have developed a code to calculate the synchrotron emission from a simplified shell numerically, considering three cooling processes (synchrotron, synchrotron self-Compton (SSC), and adiabatic) of electrons, the effect of decaying magnetic field, the effect of the bulk acceleration of the emitting shell, and the effect of a variable source function that describes electrons accelerated in the emitting region. After exploring the parameter space of the GRB synchrotron scenario, we find that the intensity-tracking pattern of $E_\text{p}$ could be achieved in two situations. One is that the cooling process of electrons is dominated by adiabatic cooling or SSC+adiabatic cooling at the same time. The other is that the emitting region is under acceleration in addition to the cooling process being dominated by SSC cooling. Otherwise, hard-to-soft patterns of $E_\text{p}$ are normally expected. Moreover, a chromatic intensity-tracking pattern of $E_\text{p}$ could be induced by the effect of a variable source function.

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arXiv2021

The Photosphere Emission Spectrum of Hybrid Relativistic Outflow for Gamma-ray Bursts

The photospheric emission in the prompt phase is the natural prediction of the original fireball model for gamma-ray burst (GRB) due to the large optical depth ($τ>1$) at the base of the outflow, which is supported by the quasi-thermal components detected in several Fermi GRBs. However, which radiation mechanism (photosphere or synchrotron) dominates in most GRB spectra is still under hot debate. The shape of the observed photosphere spectrum from a pure hot fireball or a pure Poynting-flux-dominated outflow has been investigated before. In this work, we further study the photosphere spectrum from a hybrid outflow containing both a thermal component and a magnetic component with moderate magnetization ($σ_{0}=L_{P}/L_{\text{Th}}\sim 1-10$), by invoking the probability photosphere model. The high-energy spectrum from such a hybrid outflow is a power law rather than an exponential cutoff, which is compatible with the observed Band function in large amounts of GRBs. Also, the distribution of the low-energy indices (corresponding to the peak-flux spectra) is found to be quite consistent with the statistical result for the peak-flux spectra of GRBs best-fitted by the Band function, with similar angular profiles of structured jet in our previous works. Finally, the observed distribution of the high-energy indices can be well understood after considering the different magnetic acceleration (due to magnetic reconnection and kink instability) and the angular profiles of dimensionless entropy with the narrower core.

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arXiv2021

Is the Birth of PSR J0538+2817 Accompanied by a Gamma-ray Burst?

Recently, the Five-hundred-meter Aperture Spherical radio Telescope (FAST) measured the three-dimensional velocity of PSR J0538+2817 in its associated supernova remnant S147 and found a possible spin-velocity alignment in this pulsar. Here we show that the high velocity and the spin-velocity alignment in this pulsar can be explained by the so-called electromagnetic rocket mechanism. In this framework, the pulsar is kicked in the direction of the spin axis, which naturally explains the spin-velocity alignment. We scrutinize the evolution of this pulsar and show that the pulsar kick can create a highly relativistic jet at the opposite direction of the kick velocity. The lifetime and energetics of the jet is estimated. It is argued that the jet can generate a Gamma-ray Burst (GRB). The long term dynamical evolution of the jet is calculated. It is found that the shock radius of the jet should expand to about 32 pc at present, which is well consistent with the observed radius of the supernova remnant S147 ($32.1\pm4.8$ pc). Additionally, our calculations indicate that the current velocity of the GRB remnant should be about 440 km s$^{-1}$, which is also consistent with the observed blast wave velocity of the remnant of S147 (500 km s$^{-1}$).

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arXiv2021

X-ray Plateaus in Gamma-Ray Burst Afterglows and Their Application in Cosmology

For gamma-ray bursts (GRBs) with a plateau phase in the X-ray afterglow, a so called $L-T-E$ correlation has been found which tightly connects the isotropic energy of the prompt GRB ($E_{γ,\rm{iso}}$) with the end time of the X-ray plateau ($T_{a}$) and the corresponding X-ray luminosity at the end time ($L_{X}$). Here we show that there is a clear redshift evolution in the correlation. Furthermore, since the power-law indices of $L_{X}$ and $E_{γ,\rm{iso}}$ in the correlation function are almost identical, the $L-T-E$ correlation is insensitive to cosmological parameters and cannot be used as a satisfactory standard candle. On the other hand, based on a sample including 121 long GRBs, we establish a new three parameter correlation that connects $L_{X}$, $T_{a}$ and the spectral peak energy $E_{\rm{p}}$, i.e. the $L-T-E_{\rm{p}}$ correlation. This correlation strongly supports the so-called Combo-relation established by Izzo et al. (2015). After correcting for the redshift evolution, we show that the de-evolved $L-T-E_{\rm{p}}$ correlation can be used as a standard candle. By using this correlation alone, we are able to constrain the cosmological parameters as $Ω_{m}=0.389^{+0.202}_{-0.141}$ ($1σ$) for the flat $Λ$CDM model, or $Ω_{m}=0.369^{+0.217}_{-0.191}$, $w=-0.966^{+0.513}_{-0.678}$ ($1σ$) for the flat $w$CDM model. Combining with other cosmological probes, more accurate constraints on the cosmology models are presented.

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arXiv2021

Repeating Fast Radio Bursts from Collapses of the Crust of a Strange Star

Strange stars (SSs) are compact objects made of deconfined quarks. It is hard to distinguish SSs from neutron stars as a thin crust composed of normal hadronic matter may exist and obscure the whole surface of the SS. Here we suggest that the intriguing repeating fast radio bursts (FRBs) are produced by the intermittent fractional collapses of the crust of an SS induced by refilling of accretion materials from its low-mass companion. The periodic/sporadic/clustered temporal behaviors of FRBs could be well understood in our scenario. Especially, the periodicity is attributed to the modulation of accretion rate through the disk instabilities. To account for a $\sim 16$-day periodicity of the repeating FRB source 180916.J0158+65, a Shakura-Sunyaev disk with a viscosity parameter of $α\simeq 0.004$ and an accretion rate of $\simeq 3 \times 10^{16}$~g~s$^{-1}$ in the low state is invoked. Our scenario, if favored by future observations, will serve as indirect evidence for the strange quark matter hypothesis.

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arXiv2020

A comparison between repeating bursts of FRB 121102 and giant pulses from Crab pulsar and its applications

There are some similarities between bursts of repeating fast radio bursts (FRBs) and giant pulses (GPs) of pulsars. To explore possible relations between them, we study the cumulative energy distributions of these two phenomena using the observations of repeating FRB 121102 and the GPs of Crab pulsar. We find that the power-law slope of GPs (with fluence $\geq$130 Jy ms) is $2.85\pm0.10$. The energy distribution of FRB 121102 can be well fitted by a smooth broken power-law function. For the bursts of FRB 121102 above the break energy (1.22 $\times 10^{37}$ erg), the best-fitting slope is $2.90_{-0.44}^{+0.55}$, similar to the index of GPs at the same observing frequency ($\sim$1.4 GHz). We further discuss the physical origin of the repeating FRB 121102 in the framework of the super GPs model. And we find that the super GPs model involving a millisecond pulsar is workable and favored for explaining FRB 121102 despite that the magnetar burst model is more popular.

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arXiv2020

Self-organized Criticality in Multi-pulse Gamma-Ray Bursts

The variability in multi-pulse gamma-ray bursts (GRBs) may help to reveal the mechanism of underlying processes from the central engine. To investigate whether the self-organized criticality (SOC) phenomena exist in the prompt phase of GRBs, we statistically study the properties of GRBs with more than 3 pulses in each burst by fitting the distributions of several observed physical variables with a Markov Chain Monte Carlo approach, including the isotropic energy $E_{\rm iso}$, the duration time $T$ and the peak count rate $P$ of each pulse. Our sample consists of 454 pulses in 93 GRBs observed by the CGRO/BATSE satellite. The best-fitting values and uncertainties for these power-law indices of the differential frequency distributions are: $α^d_{E}=1.54 \pm 0.09$, $α^d_{T}=1.82_{-0.15}^{+0.14}$ and $α^d_{P}=2.09_{-0.19}^{+0.18}$, while the power-law indices in the cumulative frequency distributions are: $α^c_{E}=1.44_{-0.10}^{+0.08}$, $α^c_{T}=1.75_{-0.13}^{+0.11}$ and $α^c_{P}=1.99_{-0.19}^{+0.16}$. We find that these distributions are roughly consistent with the physical framework of a Fractal-Diffusive, Self-Organized Criticality (FD-SOC) system with the spatial dimension $S=3$ and the classical diffusion $β$=1. Our results support that the jet responsible for the GRBs should be magnetically dominated and magnetic instabilities (e.g., kink model, or tearing-model instability) lead the GRB emission region into the SOC state.

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arXiv2020

FRB 200428: an Impact between an Asteroid and a Magnetar

A fast radio burst (FRB) was recently detected to be associated with a hard X-ray burst from the Galactic magnetar SGR 1935+2154. Scenarios involving magnetars for FRBs are hence highly favored. In this work, we suggest that the impact between an asteroid and a magnetar could explain such a detection. According to our calculations, an asteroid of mass $10^{20}$ g will be disrupted at a distance of $7 \times 10^9$ cm when approaching the magnetar. The accreted material will flow along the magnetic field lines from the Alfvén radius $\sim 10^7$ cm. After falling onto the magnetar's surface, an instant accretion column will be formed, producing a Comptonized X-ray burst and an FRB in the magnetosphere. We show that all the observational features of FRB 200428 could be interpreted self-consistently in this scenario. We predict quasi-periodic oscillations in this specific X-ray burst, which can serve as an independent observational test.

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arXiv2019

"Double-tracking" Characteristic of the Spectral Evolution of GRB 131231A: Synchrotron Origin?

The characteristics of the spectral evolution of the prompt emission of gamma-ray bursts (GRBs), which are closely related to the radiation mechanism (synchrotron or photosphere), are still an unsolved subject. Here, by performing the detailed time-resolved spectral fitting of GRB 131231A, which has a very bright and well-defined single pulse, some interesting spectral evolution features have been found. (i) Both the low-energy spectral index $α$ and the peak energy $E_{\rm p}$ exhibit the "flux-tracking" pattern ("double-tracking" characteristics). (ii) The parameter relations, i.e., $F$ (the energy flux)-$α$, $F$-$E_{\rm p}$, and $E_{\rm p}$-$α$, along with the analogous Yonetoku $E_{\rm p}$-$L_{γ,\rm iso}$ relation for the different time-resolved spectra, show strong monotonous (positive) correlations, both in the rising and the decaying phases. (iii) The values of $α$ do not exceed the synchrotron limit ($α$= -2/3) in all slices across the pulse, favoring the synchrotron origin. We argue that the one-zone synchrotron emission model with the emitter streaming away at a large distance from the central engine can explain all of these special spectral evolution characteristics.

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arXiv2019

Statistical Study of Gamma-Ray Bursts with a Plateau Phase in the X-ray Afterglow

A plateau phase in the X-ray afterglow is observed in a significant fraction of gamma-ray bursts (GRBs). Previously, it has been found that there exists a correlation among three key parameters concerning the plateau phase, i.e., the end time of the plateau phase in the GRB rest frame ($T_{a}$), the corresponding X-ray luminosity at the end time ($L_{X}$) and the isotropic energy of the prompt GRB ($E_{γ,\rm{iso}}$). In this study, we systematically search through all the \emph{Swift} GRBs with a plateau phase that occurred between 2005 May and 2018 August. We collect 174 GRBs, with redshifts available for all of them. For the whole sample, the correlation between $L_{X}$, $T_{a}$ and $E_{γ,\rm{iso}}$ is confirmed, with the best fit relation being $L_{X}\propto T_{a}^{-1.01}E_{γ,\rm{iso}}^{0.84}$. Such an updated three-parameter correlation still supports that the central leftover after GRBs is probably a millisecond magnetar. It is interesting to note that short GRBs with duration less than 2 s in our sample also follow the same correlation, which hints that the merger production of two neutron stars could be a high mass magnetar, but not necessarily a black hole. Moreover, GRBs having an "internal" plateau (i.e., with a following decay index being generally smaller than -3) also obey this correlation. It further strengthens the idea that the internal plateau is due to the delayed collapse of a high mass neutron star into a black hole. The updated three-parameter correlation indicates that GRBs with a plateau phase may act as a standard candle for cosmology study.

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arXiv2019

Close-in Exoplanets as Candidates of Strange Quark Matter Objects

Since the true ground state of the hadrons may be strange quark matter (SQM), pulsars may actually be strange stars rather than neutron stars. According to this SQM hypothesis, strange planets can also stably exist. The density of normal matter planets can hardly be higher than 30 g cm$^{-3}$. As a result, they will be tidally disrupted when its orbital radius is less than $\sim 5.6\times10^{10} \rm \, cm $, or when the orbital period ($P_{\rm orb}$) is less than $ \sim \rm 6100\, s $. On the contrary, a strange planet can safely survive even when it is very close to the host, due to its high density. The feature can help us identify SQM objects. In this study, we have tried to search for SQM objects among close-in exoplanets orbiting around pulsars. Encouragingly, it is found that four pulsar planets (XTE J1807-294 b, XTE J1751-305 b, PSR 0636 b, PSR J1807-2459A b) completely meet the criteria of $P_{\rm orb} < \rm 6100\, s $, and are thus good candidates for SQM planets. The orbital periods of two other planets (PSR J1719+14 b and PSR J2051-0827 b) are only slightly higher than the criteria. They could be regarded as potential candidates. Additionally, we find that the periods of five white dwarf planets (GP Com b, V396 Hya b, J1433 b, WD 0137-349 b, and SDSS J1411+2009 b) are less than 0.1 days. We argue that they might also be SQM planets. It is further found that the persistent gravitational wave emissions from at least three of these close-in planetary systems are detectable to LISA. More encouragingly, the advanced LIGO and Einstein Telescope are able to detect the gravitational wave bursts produced by the merger events of such SQM planetary systems, which will provide a unique test for the SQM hypothesis.

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