arXiv subjects

H. Tong

Publications and source records attributed to H. Tong.

At least 19 recordsLinked to original sources

arXiv2026

On the braking index evolution of PSR B0540-69: wind braking of pulsars

The pulsar PSR B0540-69 has both a braking index measurement and spin-down state change. After its spin-down state change, it shows an increasing braking index with time. Previously, it is pointed out that the spin-down state change may be caused by an enhanced particle wind. The prediction is that its braking index will be smaller in the high spin-down state. The current measured braking index is approaching the previous prediction. The transient variation of braking index may be due to a small varying part of the particle density. The braking index evolution of PSR B0540-69 is found to be in an exponential form. Its braking index is expected to approach some steady value. Future braking index measurement may make clear the physics for the braking index evolution. It may also help to make test the particle acceleration mechanism in the pulsar magnetosphere. Finally, a phenomenological treatment of the wind braking model is presented in the appendix. It can simplify the applications of wind braking model to pulsar braking index, intermittent pulsars and PSR B0540-69.

astro-ph.HE↗

arXiv2025

Rotating twisted magnetosphere of magnetars: approximate analytical solutions

An approximate analytical solution for the rotating twisted magnetosphere of magnetars is presented. The poloidal flux is approximated by the self-similar twisted dipole field. The toroidal field is obtained by the minimum torque model. Under this approximation, it is found that: (1) The Y-point radius decreases with the increase of twist of the magnetic field. (2) The polar cap is larger for larger twist. (3) The particle outflow luminosity is larger for larger twist. (4) The maximum acceleration potential, pulse width of magnetar radio emission both increase with the twist. (5) For an untwisting magnetosphere, the physical properties evolve toward that of the normal pulsars. The above findings are consistent with previous analytical and numerical results. The larger polar cap may correspond to the hot spot during magnetar outburst. In general, a rotating twisted magnetosphere has larger open field line regions. The radio emission of magnetars and fast radio bursts may both originate in the larger and evolving open field line regions of magnetars.

astro-ph.HE↗

arXiv2025

Graph Learning Metallic Glass Discovery from Wikipedia

Synthesizing new materials efficiently is highly demanded in various research fields. However, this process is usually slow and expensive, especially for metallic glasses, whose formation strongly depends on the optimal combinations of multiple elements to resist crystallization. This constraint renders only several thousands of candidates explored in the vast material space since 1960. Recently, data-driven approaches armed by advanced machine learning techniques provided alternative routes for intelligent materials design. Due to data scarcity and immature material encoding, the conventional tabular data is usually mined by statistical learning algorithms, giving limited model predictability and generalizability. Here, we propose sophisticated data learning from material network representations. The node elements are encoded from the Wikipedia by a language model. Graph neural networks with versatile architectures are designed to serve as recommendation systems to explore hidden relationships among materials. By employing Wikipedia embeddings from different languages, we assess the capability of natural languages in materials design. Our study proposes a new paradigm to harvesting new amorphous materials and beyond with artificial intelligence.

cs.LG↗

arXiv2025

Pulsar radio emission height as a function of phase

We present an analytical treatment of the pulsar radio emission height as a function of phase based on a set of simplifying assumptions. Assuming a dipole field geometry, the emission height can be expressed as a function of phase and the impact angle. We found that: (1) The emission height is a quadratic function of the phase, given the magnetospheric geometry. The emission height is higher at the edge of the pulse profile than at the center. (2) The emission height is also a quadratic function of the impact angle. This point can be compared directly with the observations of the geodetic precessing pulsar PSR J1906+0746. (3) Larger inclination angle may imply a higher emission height, when other parameters are similar. (4) By assuming curvature radiation, or inverse Compton scattering, the Lorentz factor of the radio emitting particles can be obtained as a function of phase.

astro-ph.HE↗

arXiv2025

Precession for the mode change in a gamma-ray pulsar

PSR J2021+4026 is a gamma-ray pulsar having variations in its spin-down rate and gamma-ray flux. Its variations in timing and emission are correlated, e.g., a larger spin-down rate for a low gamma-ray flux. We show that the mode change in PSR J2021+4026 can be understood in the precession scenario. In the precession model, the inclination angle is modulated due to precession. At the same time, the wobble angle may decay with time. This results in damping of the precession. Combined with magnetospheric torque model and the outer gap model, the damped precession can explain: (1) when the inclination angle is larger, the spin-down rate will be larger, accompanied by a lower gamma-ray flux. (2) The variation amplitude of the gamma-ray flux and spin-down rate is smaller than previous results due to the damping of the precession. The modulation period is becoming shorter due to a smaller wobble angle. In the end, we propose that there are two kinds of modulations in pulsars. Long-term modulations in pulsars may be due to precession. Short-term modulations may be of magnetospheric origin.

astro-ph.HE↗

arXiv2024

From ultraluminous X-ray pulsar to supermassive neutron star

The formation of a $2.7\ \rm M_{\odot}$ supermassive neutron star is explored, as the possible companion of PSR J0514--4002E. Magnetars may experience super-Eddington accretion. Observationally they may manifest themselves as ultraluminous X-ray pulsars. We propose that supermassive neutron stars may be formed through ultraluminous X-ray pulsar phase, if the ultraluminous X-ray pulsar phase can last for $10^{5}$--$10^6 \ \rm yr$. The accreted material will also bury the magnetic field of the neutron star. Assuming accretion equilibrium, the final output may be a millisecond supermassive neutron star. In order for the ultraluminous X-ray pulsar phase to last long enough, a magnetic field configuration of the low magnetic field magnetar is required. The mass, magnetic field and rotational evolution of super-Eddington accreting neutron stars are rather robust against different assumptions, although many of the model details are yet to be determined.

astro-ph.HE↗

arXiv2023

On the nature of long period radio pulsar GPM J1839$-$10: death line and pulse width

Recently another long period radio pulsar GPM J1839$-$10 is reported, similar to GLEAM-X J162759.5$-$523504.3. Previously, the energy budget and rotational evolution of long period radio pulsars had been considered. This time, the death line and pulse width for neutron star and white dwarf pulsars are investigated. The pulse width is included as the second criterion for neutron star and white dwarfs pulsars. It is found that: (1) PSR J0250+5854 and PSR J0901$-$4046 etc should be normal radio pulsars. They have narrow pulse width and they lie near the radio emission death line. (2) The two long period radio pulsars GLEAM-X J162759.5$-$523504.3 and GPM J1839$-$10 is unlikely to be normal radio pulsars. Their possible pulse width is relatively large. And they lie far below the fiducial death line on the $P-\dot{P}$ diagram. (3) GLEAM-X J162759.5$-$523504.3 and GPM J1839$-$10 may be magnetars or white dwarf radio pulsars. At present, there are many parameters and uncertainties in both of these two possibilities.

astro-ph.HE↗

arXiv2023

Rotating vector model and radius-to-frequency mapping in the presence of multipole magnetic field

The rotating vector model and radius-to-frequency mapping in the presence of multipole magnetic field in pulsars and magnetars are considered. An axisymmetric potential field is assumed. It is found that: (1) The radiation beam in the case of multipole field is wider than the dipole case. This may account the increasing pulse width at higher frequency of pulsars (anti-radius-to-frequency mapping). (2) The expression for the polarization position angle is unchanged. Only the inclination angle α and phase constant ϕ_0 will change. The angle between the rotational axis and line of sight, and the position angle constant ψ_0 will not change. When fitting the varying position angle of magnetars, these constraints should be considered. The appearance and disappearance of multipole field may account for the changing slope of position angle in the radio emitting magnetar Swift J1818.0-1607. Similar but more active process in magnetar magnetospheres may account for the diverse position angle in fast radius bursts.

astro-ph.HE↗

arXiv2023

Magnetospheric physics of magnetars

Several aspects of the magnetospheric physics of magnetars are summarized, including: GeV and hard X-ray emissions of magnetars, timing behaviors during magnetar outburst (soft X-ray observations), optical/IR observations of magnetars, radio emission of magnetars, and accreting magnetars. A unified picture for pulsars and magnetars are adopted, especially wind braking of magnetars, magnetar+ fallback disk systems, twisted dipole magnetic field, and accreting low magnetic field magnetars etc. It is pointed out that magnetars are related to a broad range of astrophysical phenomena.

astro-ph.HE↗

arXiv2022

A note on the anti-glitch of magnetar SGR 1935+2154

The magnetar SGR 1935+2154 is reported to have an anti-glitch, accompanied by fast radio bursts, and transient pulsed radio emission. In the wind braking model, this triplet event tells people that (1) SGR 1935+2154 does not have a strong particle wind and can be approximated by magnetic dipole braking in the persistent state; (2) Its anti-glitch is due to an enhanced particle wind, similar to the first anti-glitch in magnetars; (3) Its transient pulsed radio emission may be due to a decreasing emission beam during the outburst; (4) The enhanced particle acceleration potential and pulsar death line may not be the dominate factor.

astro-ph.HE↗

arXiv2022

Discussions on the nature of GLEAM-X J162759.5-523504.3

The nature of the long period radio transient GLEAM-X J162759.5$-$523504.3 (GLEAM-X J1627 for short) is discussed. We try to understand both its radio emission and pulsation in the neutron star scenario, as an alternative to the white dwarf model. We think that: (1) From the radio emission point of view, GLEAM-X J1627 can be a radio-loud magnetar. (2) From the rotational evolution point of view, GLEAM-X J1627 is unlikely to be an isolated magnetar. (3) The 1091s period is unlikely to be the precession period. (4) GLEAM-X J1627 may be a radio-loud magnetar spin-down by a fallback disk. (5) The pulsar death line is modified due to the presence of a fallback disk or a twisted magnetic field. In both cases, a higher maximum acceleration potential can be obtained. This may explain why GLEAM-X J1627 is still radio active with such a long pulsation period. (6) General constraint on the neutron star magnetic field and initial disk mass are given analytically. Possible ways to discriminate between different modelings are also discussed.

astro-ph.HE↗

arXiv2022

Circular polarization of fast radio bursts in the curvature radiation scenario

The curvature radiation is applied to the explain the circular polarization of FRBs. Significant circular polarization is reported in both apparently non-repeating and repeating FRBs. Curvature radiation can produce significant circular polarization at the wing of the radiation beam. In the curvature radiation scenario, in order to see significant circular polarization in FRBs (1) more energetic bursts, (2) burst with electrons having higher Lorentz factor, (3) a slowly rotating neutron star at the centre are required. Different rotational period of the central neutron star may explain why some FRBs have high circular polarization, while others don't. Considering possible difference in refractive index for the parallel and perpendicular component of electric field, the position angle may change rapidly over the narrow pulse window of the radiation beam. The position angle swing in FRBs may also be explained by this non-geometric origin, besides that of the rotating vector model.

astro-ph.HE↗

arXiv2021

Three aspects of the radius-to-frequency mapping in fast radio bursts

We further explored the radius-to-frequency mapping in cases of FRBs. An analytical treatment of Lyutikov (2020) is presented. The frequency dependence of the drifting rate and the drifting timescale are obtained. The aberration effect and the twist of the magnetic field lines may result in drifting in both directions. For one FRB, the burst width is larger at lower frequency according to the radius-to-frequency mapping. For the FRB population, the magnetic fields of the repeaters may be larger than that of the non-repeaters. Then, according to the radius-to-frequency mapping, the burst widths of the repeaters will be wider than that of the apparent non-repeaters. If similar window function (or emission cones) like that of pulsars and magnetars is also at work in the case of FRBs, then the window function may explain the single or multiple components of FRB profiles. The radius-to-frequency mapping modeling is to some degree independent of the underlying radio emission mechanism.

astro-ph.HE↗

arXiv2021

Studying magnetic fields of ultraluminous X-ray pulsars using different accretion torques

The magnetic field of ultraluminous X-ray (ULX) pulsars is the key parameter to understand the nature and accretion physics. However, the typical magnetic field values in these ULX pulsars are still under debate. We used six different torque models to study the magnetic fields of ULX pulsars, to see how derived magnetic fields change with different models, and to determine which models are more suitable for ULX pulsars. We took the currently available period, period derivative, and flux data of 7 confirmed ULX pulsars, M82 X-2, ULX NGC 7793 P13, ULX NGC 5907, NGC 300 ULX1, NGC 1313 X-2, M51 ULX-7, Swift J0243.6+6124, plus one potential ULX pulsars, SMC X-3. The magnetic fields of these ULX pulsars were constrained from two physical conditions: the spin-up process and near equilibrium. We checked possible dependence of the magnetic field estimations on the different torque models. The calculations suggest the accretion torque models by Ghosh & Lamb [1], Wang [2], Kluzniak & Rappaport [3] and Campbell [4] are more likely to support magnetar models for ULX pulsars, while Lovelace, Romanova & Bisnovatyi-Kogan [5]'s model generally predicts the magnetic field in normal neutron stars. Implications of our results combined with other independent methods are also discussed, which will help us to understand the nature and rotational behavior of these ULX pulsars.

astro-ph.HE↗

arXiv2021

Rotating vector model for magnetars

The modification of the rotating vector model in the case of magnetars are calculated. Magnetars may have twisted magnetic field compared with normal pulsars. The polarization position angle of magnetars will change in the case of a twisted magnetic field. For a twisted dipole field, we found that the position angle will change both vertically and horizontally. During the untwisting process of the magnetar magnetosphere, the modifications of the position angle will evolve with time monotonously. This may explain the evolution of the position angle in magnetar PSR J1622-4950 and XTE J1810-197. The relation between the emission point and the line of sight will also change. We suggest every magnetospheric models of magnetars also calculate the corresponding changes of position angle in their models. Order of magnitude estimation formula for doing this is given. This opens the possibility to extract the magnetic field geometry of magnetars from their radio polarization observations.

astro-ph.HE↗

arXiv2020

Discovery of state transition behaviors in PSR J1124--5916

With the twelve-year long observations by {\sl Fermi}-LAT, we discover two pairs of spin-down state transitions of PSR J1124--5916, making it the second young pulsar detected to have such behaviors. PSR J1124--5916 shows mainly two states according to its spin-down rate evolution, the normal spin-down state and the low spin-down state. In about 80\% of the observation time, the pulsar is in the normal spin-down state, in which the spin-down rate decreases linearly and gives a braking index of $1.98\pm0.04$. The two transitions to the low spin-down state are in MJD 55183--55803 and MJD 56114--56398 respectively, with fractional amplitudes both $\sim0.4\%$. No significant difference between the $γ$-ray profiles of the two spin-down states is detected, which is similar to PSR B0540-69, the other young pulsar with state transition detected.

astro-ph.HE↗

arXiv2020

Magnetar outburst and spin-down glitch

The outburst and spin-down glitch of magnetars are modeled from the magnetospheric point of view. We try to discuss the following four questions: (1) Which pulsar on the period and peirod-derivative diagram are more likely to show magnetar outburst? (2) Which outburst will make the glitch that triggered the outburst to become a spin-down glitch? (3) Can we model the outburst and spin-down glitch in PSR J1119$-$6127 simultaneously? (4) Why the torque variation is delayed compared with the peak of the X-ray luminosity in 1E 1048.1$-$5937 and PSR J1119$-$6127? It is found that both the global and local twisted magnetic field will affect the radiation and timing behaviors of magnetars. Especially, the delay of torque variations may due to the combined effect of increasing twist in the j-bundle and untwisting of the global magnetosphere. A toy model is build for magnetar outburst and torque variations. It can catch the general trend of magnetar outburst: decaying flux, shrinking hot spot, and torque variations.

astro-ph.HE↗

arXiv2020

On the radiation-loud and radiation-quiet anti-glitches in magnetar 1E 2259+586

The radiation-loud and radiation-quiet anti-glitches in magnetar 1E 2259+586 are discussed together. It is shown that the 2019 radiation-quiet anti-glitch in 1E 2259+586 is not constraining, because its amplitude is 2.5 smaller than the previous radiation-loud anti-glitch. We note that magnetars with high X-ray luminosity and low rotational energy loss rate are more likely to show anti-glitches. Anti-glitchers and radio-emitters of magnetars occupy opposite corner on the luminosity vs. rotational energy loss rate diagram. By introducing the X-ray luminosity parameter and the activity parameter, we try to characterize this trend quantitatively. A general picture of magnetar anti-glitch is given.

astro-ph.HE↗